Introduction
General virology studies the nature of viruses, their structure, reproduction, biochemistry, and genetics. Medical, veterinary and agricultural virology studies pathogenic viruses, their infectious properties, develops measures for the prevention, diagnosis and treatment of diseases caused by them.
Virology solves fundamental and applied problems and is closely related to other sciences. The discovery and study of viruses, in particular bacteriophages, made a huge contribution to the formation and development of molecular biology. The branch of virology that studies the hereditary properties of viruses is closely related to molecular genetics. Viruses are not only a subject of study, but also a tool for molecular genetic research, which connects virology with genetic engineering. Viruses are pathogens of a large number infectious diseases humans, animals, plants, insects. From this point of view, virology is closely related to medicine, veterinary medicine, phytopathology and other sciences.
Having emerged at the end of the 19th century as a branch of human and animal pathology, on the one hand, and phytopathology, on the other, virology became an independent science, rightfully occupying one of the main places among the biological sciences.
Chapter 1. History of virology
1.1. Virus discovery
Virology is a young science, its history goes back a little over 100 years. Having begun its journey as the science of viruses that cause diseases in humans, animals and plants, virology is currently developing in the areas of studying the basic laws modern biology at the molecular level, based on the fact that viruses are part of the biosphere and an important factor in the evolution of the organic world.
The history of virology is unusual in that one of its subjects - viral diseases - began to be studied long before viruses themselves were discovered. The beginning of the history of virology is the fight against infectious diseases and only subsequently the gradual disclosure of the sources of these diseases. This is confirmed by the work of Edward Jenner (1749-1823) on the prevention of smallpox and the work of Louis Pasteur (1822-1895) with the causative agent of rabies.
Since time immemorial, smallpox has been the scourge of humanity, claiming thousands of lives. Descriptions of smallpox infection are found in the manuscripts of ancient Chinese and Indian texts. The first mention of smallpox epidemics on the European continent dates back to the 6th century AD (an epidemic among the soldiers of the Ethiopian army besieging Mecca), after which there was an inexplicable period of time when there were no mentions of smallpox epidemics. Smallpox began to spread across continents again in the 17th century. For example, in North America (1617-1619) in the state of Massachusetts, 9/10 of the population died, in Iceland (1707) after a smallpox epidemic, only 17 thousand remained from 57 thousand people, in the city of Eastham (1763) ) from 1331 inhabitants there are 4 people left. In this regard, the problem of combating smallpox was very acute.
A technique for preventing smallpox through vaccination, called variolation, has been known since ancient times. References to the use of variolation in Europe date back to the mid-17th century, with references to earlier use in China, to Far East, in Turkey. The essence of variolation was that the contents of pustules from patients suffering from a mild form of smallpox were introduced into a small wound on the human skin, which caused a mild disease and prevented an acute form. However, there remained a high risk of contracting a severe form of smallpox and the mortality rate among vaccinated people reached 10%. Jenner revolutionized smallpox prevention. He was the first to notice that people who had cowpox, which was mild, never subsequently suffered from smallpox. On May 14, 1796, Jenner introduced liquid from the pustules of milkmaid Sarah Selmes, who had cowpox, into the wound of James Phipps, who had never suffered from smallpox. At the site of the artificial infection, the boy developed typical pustules, which disappeared after 14 days. Then Jenner introduced highly infectious material from the pustules of a smallpox patient into the boy’s wound. The boy did not get sick. This is how the idea of vaccination was born and confirmed (from the Latin word vacca - cow). In Jenner's time, vaccination was understood as the introduction of infectious cowpox material into the human body in order to prevent smallpox. The term vaccine was applied to a substance that protected against smallpox. Since 1840, smallpox vaccine began to be obtained by infecting calves. The human smallpox virus was discovered only in 1904. Thus, smallpox is the first infection against which a vaccine was used, i.e., the first vaccine-preventable infection. Advances in vaccine prevention of smallpox have led to its worldwide eradication.
Nowadays, vaccination and vaccine are used as general terms denoting vaccination and vaccination material.
Pasteur, who essentially did not know anything specific about the causes of rabies, except for the indisputable fact of its infectious nature, used the principle of weakening (attenuation) of the pathogen. In order to weaken the pathogenic properties of the rabies pathogen, a rabbit was used, into whose brain the brain tissue of a dog that died of rabies was injected. After the death of the rabbit, its brain tissue was injected into the next rabbit, and so on. About 100 passages were carried out before the pathogen adapted to the rabbit's brain tissue. When injected subcutaneously into the dog's body, it exhibited only moderate pathogenic properties. Pasteur called such a “re-educated” pathogen “fixed”, in contrast to the “wild” one, which is characterized by high pathogenicity. Pasteur later developed a method of creating immunity, consisting of a series of injections with gradually increasing amounts of a fixed pathogen. Dog passing full course injections turned out to be completely resistant to infection. Pasteur came to the conclusion that the process of development of an infectious disease is essentially a struggle between microbes and the body's defenses. “Every disease must have its own pathogen, and we must promote the development of immunity to this disease in the patient’s body,” said Pasteur. Not yet understanding how the body produces immunity, Pasteur was able to use its principles and direct the mechanisms of this process to the benefit of humans. In July 1885, Pasteur had the opportunity to test the properties of a “fixed” rabies pathogen on a child bitten by a rabid dog. The boy was given a series of injections of an increasingly toxic substance, with the last injection containing a completely pathogenic form of the pathogen. The boy remained healthy. The rabies virus was discovered by Remlanger in 1903.
It should be noted that neither the smallpox virus nor the rabies virus were the first viruses discovered to infect animals and humans. The first place rightfully belongs to the foot-and-mouth disease virus, discovered by Leffler and Frosch in 1898. These researchers, using multiple dilutions of the filterable agent, showed its toxicity and made a conclusion about its corpuscular nature.
By the end of the 19th century, it became clear that a number of human diseases, such as rabies, smallpox, influenza, and yellow fever, are infectious, but their causative agents were not detected by bacteriological methods. Thanks to the work of Robert Koch (1843-1910), who pioneered the use of pure bacterial culture techniques, it became possible to distinguish between bacterial and non-bacterial diseases. In 1890, at the X Congress of Hygienists, Koch was forced to declare that “... with the diseases listed, we are not dealing with bacteria, but with organized pathogens that belong to a completely different group of microorganisms.” This statement by Koch indicates that the discovery of viruses was not random event. Not only the experience of working with pathogens that were incomprehensible in nature, but also an understanding of the essence of what was happening contributed to the formulation of the idea of the existence of an original group of pathogens of infectious diseases of a non-bacterial nature. It remained to experimentally prove its existence.
The first experimental evidence of the existence of a new group of pathogens of infectious diseases was obtained by our compatriot - plant physiologist Dmitry Iosifovich Ivanovsky (1864-1920) while studying mosaic diseases of tobacco. This is not surprising, since infectious diseases of an epidemic nature were often observed in plants. Back in 1883-84. The Dutch botanist and geneticist de Vries observed an epidemic of greening of flowers and suggested the infectious nature of the disease. In 1886, the German scientist Mayer, working in Holland, showed that the sap of plants suffering from mosaic disease, when inoculated, causes the same disease in plants. Mayer was sure that the culprit of the disease was a microorganism, and searched for it without success. In the 19th century, tobacco diseases caused great harm agriculture in our country. In this regard, a group of researchers was sent to Ukraine to study tobacco diseases, which, as a student at St. Petersburg University, included D.I. Ivanovsky. As a result of studying the disease described in 1886 by Mayer as mosaic disease of tobacco, D.I. Ivanovsky and V.V. Polovtsev came to the conclusion that it represents two various diseases. One of them - "grouse" - is caused by a fungus, and the other is of unknown origin. The study of tobacco mosaic disease was continued by Ivanovsky at the Nikitsky Botanical Garden under the leadership of Academician A.S. Famytsina. Using the juice of a diseased tobacco leaf, filtered through a Chamberlant candle, which retains the smallest bacteria, Ivanovsky caused a disease of tobacco leaves. Cultivation of the infected juice on artificial nutrient media did not produce results and Ivanovsky comes to the conclusion that the causative agent of the disease is of an unusual nature - it is filtered through bacterial filters and is not able to grow on artificial nutrient media. Warming the juice at 60-70 °C deprived it of infectivity, which indicated the living nature of the pathogen. Ivanovsky first called new type pathogen "filterable bacteria". Results of the work of D.I. Ivanovsky were used as the basis for his dissertation, presented in 1888, and published in the book “On Two Diseases of Tobacco” in 1892. This year is considered the year of the discovery of viruses.
For a certain period of time, in foreign publications, the discovery of viruses was associated with the name of the Dutch scientist Beijerinck (1851-1931), who also studied tobacco mosaic disease and published his experiments in 1898. Beijerinck placed the filtered juice of an infected plant on the surface of an agar, incubated and obtained bacterial colonies on its surface. After this, the top layer of agar with bacterial colonies was removed, and the inner layer was used to infect a healthy plant. The plant is sick. From this, Beijerinck concluded that the cause of the disease was not bacteria, but some liquid substance that could penetrate inside the agar, and called the pathogen “liquid living contagion.” Due to the fact that Ivanovsky only described his experiments in detail, but did not pay due attention to the nonbacterial nature of the pathogen, a misunderstanding of the situation arose. Ivanovsky’s work became famous only after Beijerinck repeated and expanded his experiments and emphasized that Ivanovsky was the first to prove the non-bacterial nature of the causative agent of the most typical viral disease of tobacco. Beijerinck himself recognized the primacy of Ivanovsky and the current priority of the discovery of viruses by D.I. Ivanovsky is recognized throughout the world.
The word VIRUS means poison. This term was also used by Pasteur to denote an infectious principle. It should be noted that at the beginning of the 19th century, all pathogenic agents were called the word virus. Only after the nature of bacteria, poisons and toxins became clear, the terms “ultravirus” and then simply “virus” began to mean “a new type of filterable pathogen.” The term “virus” took root widely in the 30s of our century.
It is now clear that viruses are characterized by ubiquity, that is, ubiquity of distribution. Viruses infect representatives of all living kingdoms: humans, vertebrates and invertebrates, plants, fungi, bacteria.
The first report related to bacterial viruses was made by Hankin in 1896. In the Chronicle of the Pasteur Institute, he stated that “... the water of some rivers of India has a bactericidal effect...”, which is no doubt related to bacterial viruses. In 1915, Twort in London, while studying the causes of lysis of bacterial colonies, described the principle of transmission of “lysis” to new cultures over a series of generations. His work, as often happens, was virtually unnoticed, and two years later, in 1917, the Canadian de Hérelle rediscovered the phenomenon of bacterial lysis associated with a filtering agent. He called this agent a bacteriophage. De Herelle assumed that there was only one bacteriophage. However, research by Barnett, who worked in Melbourne in 1924-34, showed a wide variety of bacterial viruses in physical and biological properties. The discovery of the diversity of bacteriophages has generated great scientific interest. At the end of the 30s, three researchers - physicist Delbrück, bacteriologists Luria and Hershey, working in the USA, created the so-called “Phage Group”, whose research in the field of genetics of bacteriophages ultimately led to the birth of a new science - molecular biology.
The study of insect viruses has lagged significantly behind the virology of vertebrates and humans. It is now clear that viruses that infect insects can be divided into 3 groups: insect viruses themselves, animal and human viruses for which insects are intermediate hosts, and plant viruses that also infect insects.
The first insect virus to be identified was the silkworm jaundice virus (silkworm polyhedrosis virus, called Bollea stilpotiae). As early as 1907, Provacek showed that a filtered homogenate of diseased larvae was infectious for healthy silkworm larvae, but it was not until 1947 that the German scientist Bergold discovered rod-shaped viral particles.
One of the most fruitful studies in the field of virology is Reed's study of the nature of yellow fever on US Army volunteers in 1900-1901. It has been convincingly demonstrated that yellow fever is caused by a filterable virus that is transmitted by mosquitoes and mosquitoes. It was also found that mosquitoes remained non-infectious for two weeks after absorbing infectious blood. Thus, the external incubation period of the disease (the time required for virus reproduction in an insect) was determined and the basic principles of the epidemiology of arbovirus infections (viral infections transmitted by blood-sucking arthropods) were established.
The ability of plant viruses to reproduce in their vector, an insect, was demonstrated in 1952 by Maramorosh. The researcher, using insect injection techniques, convincingly demonstrated the ability of the aster jaundice virus to multiply in its vector, the six-spotted cicada.
1.2. Stages of development of virology
The history of virology achievements is directly related to development successes methodological base research.
^ End of XIX - beginning of XX century. The main method of identifying viruses during this period was the method of filtration through bacteriological filters (Chamberlan candles), which were used as a means of separating pathogens into bacteria and non-bacteria. Using filterability through bacteriological filters, the following viruses were discovered:
1892 - tobacco mosaic virus;
1898 - foot-and-mouth disease virus;
1899 - rinderpest virus;
1900 - yellow fever virus;
1902 - fowl and sheep pox virus;
1903 - rabies virus and swine fever virus;
1904 - human smallpox virus;
1905 - canine distemper virus and vaccine virus;
1907 - dengue virus;
1908 - smallpox and trachoma virus;
1909 - polio virus;
1911 - Rous sarcoma virus;
1915 - bacteriophages;
1916 - measles virus;
1917 - herpes virus;
1926 - vesicular stomatitis virus.
30s - the main virological method used to isolate viruses and their further identification are laboratory animals (white mice - for influenza viruses, newborn mice - for Coxsackie viruses, chimpanzees - for hepatitis B virus, chickens, pigeons - for oncogenic viruses , gnotobiont piglets - for intestinal viruses, etc.). The first person to systematically use laboratory animals in the study of viruses was Pasteur, who, back in 1881, conducted research on inoculating material from rabies patients into the brain of a rabbit. Another milestone was work on the study of yellow fever, which resulted in the use of newborn mice in virological practice. The culmination of this cycle of work was the isolation by Cycles in 1948 of a group of epidemic myalgia viruses using suckling mice.
1931 - chicken embryos, which are highly sensitive to influenza, smallpox, leukemia, chicken sarcoma and some other viruses, began to be used as an experimental model for isolating viruses. And currently, chicken embryos are widely used to isolate influenza viruses.
1932 - English chemist Alford creates artificial finely porous colloidal membranes - the basis for the ultrafiltration method, with the help of which it became possible to determine the size of viral particles and differentiate viruses on this basis.
1935 - the use of the centrifugation method made it possible to crystallize the tobacco mosaic virus. Currently, centrifugation and ultracentrifugation methods (acceleration at the bottom of the tube exceeds 200,000 g) are widely used for the isolation and purification of viruses.
In 1939, an electron microscope with a resolution of 0.2-0.3 nm was used for the first time to study viruses. The use of ultrathin tissue sections and the method of negative contrasting of aqueous suspensions made it possible to study the interaction of viruses with cells and to study the structure (architecture) of virions. The information obtained using the electron microscope was significantly expanded by X-ray diffraction analysis of crystals and pseudocrystals of viruses. The improvement of electron microscopes culminated in the creation of scanning microscopes that make it possible to obtain three-dimensional images. Using electron microscopy, the architecture of virions and the features of their penetration into the host cell were studied.
During this period, the bulk of viruses were discovered. Examples include the following:
1931 - swine influenza virus and equine western encephalomyelitis virus;
1933 - human influenza virus and eastern equine encephalomyelitis virus;
1934 - mumps virus;
1936 - mouse mammary cancer virus;
1937 - tick-borne encephalitis virus.
40s. In 1940, Hoagland and his colleagues discovered that the vaccinia virus contains DNA but not RNA. It became obvious that viruses differ from bacteria not only in size and inability to grow without cells, but also in that they contain only one type of nucleic acid - DNA or RNA.
1941 - American scientist Hurst discovered the phenomenon of hemagglutination (erythrocyte gluing) using a model of the influenza virus. This discovery formed the basis for the development of methods for detecting and identifying viruses and contributed to the study of virus-cell interactions. The principle of hemagglutination is the basis of a number of methods:
^ HRA - hemagglutination reaction - used to detect and titrate viruses;
HAI - hemagglutination inhibition reaction - is used to identify and titrate viruses.
1942 - Hurst discovers the presence of an enzyme in the influenza virus, which is later identified as neuraminidase.
1949 - discovery of the possibility of culturing animal tissue cells under artificial conditions. In 1952, Enders, Weller and Robbins received the Nobel Prize for developing the cell culture method.
The introduction to virology of the cell culture method was important event, which made it possible to obtain cultured vaccines. Of the currently widely used cultural live and killed vaccines created on the basis of attenuated strains of viruses, vaccines against polio, mumps, measles and rubella should be noted.
The creators of polio vaccines are American virologists Sabin (a trivalent live vaccine based on attenuated strains of polioviruses of three serotypes) and Salk (a killed trivalent vaccine). In our country, Soviet virologists M.P. Chumakov and A.A. Smorodintsev developed a technology for the production of live and killed polio vaccines. In 1988, the World Health Assembly set WHO the goal of eradicating polio worldwide by completely stopping the circulation of wild poliovirus. To date, enormous progress has been made in this direction. The use of global vaccination against polio using “round” vaccination schemes made it possible not only to radically reduce the incidence, but also to create areas free from the circulation of wild poliovirus.
Viruses discovered:
1945 - Crimean hemorrhagic fever virus;
1948 - Coxsackie viruses.
50s. In 1952, Dulbecco developed a method for titrating plaques in a monolayer of chicken embryo cells, which introduced a quantitative aspect to virology. 1956-62 Watson, Caspar (USA) and Klug (UK) are developing general theory symmetry of viral particles. The structure of the viral particle has become one of the criteria in the virus classification system.
This period was characterized by significant advances in the field of bacteriophages:
Induction of the prophage of lysogenizing phages has been established (Lvov et al., 1950);
It has been proven that infectivity is inherent in phage DNA, and not in the protein coat (Hershey and Chase, 1952);
The phenomenon of general transduction was discovered (Zinder and Lederberg, 1952).
The infectious tobacco mosaic virus was reconstructed (Frenkel-Conrad, Williams, Singer, 1955-57), and in 1955 the polio virus was obtained in crystalline form (Shaffer, Shwerd, 1955).
Viruses discovered:
1951 - murine leukemia viruses and ECHO;
1953 - adenoviruses;
1954 - rubella virus;
1956 - parainfluenza viruses, cytomegalovirus, respiratory syncytial virus;
1957 - polyoma virus;
1959 - Argentine hemorrhagic fever virus.
The 60s and subsequent years are characterized by the flourishing of molecular biological research methods. Advances in the field of chemistry, physics, molecular biology and genetics formed the basis of the methodological base of scientific research, which began to be used not only at the level of techniques, but also entire technologies, where viruses act not only as an object of research, but also as a tool. Not a single discovery in molecular biology is complete without a viral model.
1967 - Cates and McAuslan demonstrate the presence of a DNA-dependent RNA polymerase in the vaccinia virion. IN next year RNA-dependent RNA polymerase is found in reoviruses, and then in paramyxo- and rhabdoviruses. In 1968, Jacobson and Baltimore established that polioviruses have a genomic protein connected to RNA; Baltimore and Boston established that the poliovirus genomic RNA is translated into a polyprotein.
Viruses discovered:
1960 - rhinoviruses;
1963 - Australian antigen (HBsAg).
70s. Baltimore, simultaneously with Temin and Mizutani, reported the discovery of the enzyme reverse transcriptase (revertase) in RNA-containing oncogenic viruses. It is becoming possible to study the genome of RNA viruses.
The study of gene expression in eukaryotic viruses provided fundamental information about the molecular biology of eukaryotes themselves - the existence of the cap structure of mRNA and its role in RNA translation, the presence of a polyadenylate sequence at the 3" end of mRNA, splicing and the role of enhancers in transcription were first identified in the study of animal viruses.
1972 - Berg publishes a report on the creation of a recombinant DNA molecule. A new branch of molecular biology is emerging - genetic engineering. The use of recombinant DNA technology makes it possible to obtain proteins that are important in medicine (insulin, interferon, vaccines). 1975 - Köhler and Milstein produce the first lines of hybrids producing monoclonal antibodies (MAbs). The most specific test systems for diagnosing viral infections are being developed based on mAbs. 1976 - Blumberg receives the Nobel Prize for the discovery of HBsAg. It has been established that hepatitis A and hepatitis B are caused by different viruses.
Viruses discovered:
1970 - hepatitis B virus;
1973 - rotaviruses, hepatitis A virus;
1977 - hepatitis delta virus.
80s. Development of the ideas laid down by domestic scientist L.A. Zilber's idea that the occurrence of tumors may be associated with viruses. The components of viruses responsible for the development of tumors are called oncogenes. Viral oncogenes have proven to be among the best model systems that help study the mechanisms of oncogenetic transformation of mammalian cells.
1985 - Mullis receives the Nobel Prize for the discovery of the polymerase chain reaction (PCR). This is a molecular genetic diagnostic method, which has also made it possible to improve the technology for obtaining recombinant DNA and discover new viruses.
Viruses discovered:
1983 - human immunodeficiency virus;
1989 - hepatitis C virus;
1995 - Hepatitis G virus was discovered using PCR.
1.3. Development of the concept of the nature of viruses
Answers to the questions “What are viruses?” and “What is their nature?” have been the subject of debate for many years since their discovery. In 20-30 years. no one doubted that viruses are living matter. In the 30-40s. It was believed that viruses are microorganisms, since they are able to reproduce, have heredity, variability and adaptability to changing environmental conditions, and, finally, are susceptible biological evolution, which is provided by natural and artificial selection. In the 60s, the first successes of molecular biology determined the decline of the concept of viruses as organisms. In the ontogenetic cycle of the virus, two forms are distinguished - extracellular and intracellular. The term VIRION was introduced to denote the extracellular form of the virus. The differences between its organization and the structure of cells have been established. Facts pointing to a completely different type of reproduction from cells, called disjunctive reproduction, are summarized. Disjunctive reproduction is a temporary and territorial separation of the synthesis of viral components - genetic material and proteins - from the subsequent assembly and formation of virions. It has been shown that the genetic material of viruses is represented by one of two types of nucleic acid (RNA or DNA). It is formulated that the main and absolute criterion for distinguishing viruses from all other forms of life is the absence of their own protein-synthesizing systems.
The accumulated data allowed us to come to the conclusion that viruses are not organisms, even the smallest ones, since any, even minimal organisms such as mycoplasmas, rickettsia and chlamydia have their own protein-synthesizing systems. According to the definition formulated by Academician V.M. Zhdanov, viruses are autonomous genetic structures that can function only in cells with to varying degrees dependence on cellular systems for the synthesis of nucleic acids and complete dependence on cellular protein-synthesizing and energy systems, and undergoing independent evolution.
Thus, viruses are a diverse and numerous group of non-cellular life forms that are not microorganisms, and are united in the kingdom Vira. Viruses are studied within the framework of virology, which is an independent scientific discipline, which has its own object and research methods.
Virology is divided into general and specific, and virological research into fundamental and applied. The subject of fundamental research in virology is the architecture of virions, their composition, features of the interaction of viruses with cells, methods of transfer hereditary information, molecular mechanisms of synthesis of elements and the process of their combination into a whole, molecular mechanisms of variability of viruses and their evolution. Applied research in virology is related to solving problems in medicine, veterinary medicine and phytopathology.
CHAPTER 2
^ STRUCTURAL AND MOLECULAR ORGANIZATION OF VIRUSES
In the ontogenetic cycle of the virus, two stages are distinguished - extracellular and intracellular and, accordingly, two forms of its existence - the virion and the vegetative form. A virion is a whole viral particle, mainly consisting of protein and nucleic acid, often resistant to exposure to factors external environment and adapted to transfer genetic information from cell to cell. The vegetative form of the virus exists in a single virus-cell complex and only in their close interaction.
2.1. Virion architecture
The extracellular form of the virus - the virion, designed to preserve and transfer the nucleic acid of the virus, is characterized by its own architecture, biochemical and molecular genetic characteristics. The architecture of virions refers to the ultrafine structural organization of these supramolecular formations, differing in size, shape and structural complexity. A nomenclature of terms has been developed to describe the architecture of viral structures:
A protein subunit is a single polypeptide chain arranged in a certain way.
A structural unit (structural element) is a protein ensemble of a higher order, formed by several chemically related identical or non-identical subunits.
Morphological unit is a group of protrusions (cluster) on the surface of the capsid, visible in an electron microscope. Clusters consisting of five (pentamer) and six (hexamer) protrusions are often observed. This phenomenon is called pentameric-hexamer clustering. If a morphological unit corresponds to a chemically significant formation (preserves its organization under conditions of mild disintegration), then the term capsomere is used.
Capsid is an outer protein sheath or sheath that forms a closed sphere around the genomic nucleic acid.
Core - the inner protein shell directly adjacent to the nucleic acid.
Nucleocapsid is a complex of protein with nucleic acid, which is a packaged form of the genome.
Supercapsid or peplos is the virion envelope formed by a lipid membrane of cellular origin and viral proteins.
The matrix is a protein component located between the supercapsid and the capsid.
Peplomeres and spines are superficial projections of the supercapsid.
As already noted, viruses can pass through the most microscopic pores that trap bacteria, which is why they were called filtering agents. The filterability property of viruses is due to their size, measured in nanometers (nm), which is several orders of magnitude smaller than the size of the smallest microorganisms. The sizes of viral particles, in turn, vary within relatively wide limits. The smallest simple viruses have a diameter of slightly more than 20 nm (parvoviruses, picornaviruses, phage Qβ), medium-sized viruses - 100-150 nm (adenoviruses, coronaviruses). The largest are recognized as vaccinia virus particles, whose dimensions reach 170x450 nm. The length of filamentous plant viruses can be 2000 nm.
Representatives of the Vira kingdom are characterized by a variety of forms. According to their structure, viral particles can be simple formations, but can be quite complex ensembles, including several structural elements. A conditional model of a hypothetical virion, including all possible structural formations, is presented in Figure 1.
There are two types of viral particles (VPs), which are fundamentally different from each other:
1) HFs lacking an envelope (non-enveloped or uncovered virions);
2) HF that have an envelope (enveloped or coated virions).
Rice. 1. The structure of a hypothetical virion
2.1.1. The structure of virions lacking an envelope
Three morphological types of virions lacking an envelope have been identified: rod-shaped (thread-like), isometric and club-shaped (Fig. 2). The existence of the first two types of uncovered virions is determined by the way the nucleic acid is folded and its interaction with proteins.
1. Protein subunits bind to the nucleic acid, arranged along it in a periodic manner so that it folds into a spiral and forms a structure called a nucleocapsid. This method of regular, periodic interaction between protein and nucleic acid determines the formation of rod-shaped and filamentous viral particles.
2. Nucleic acid is not associated with a protein shell (possible non-covalent bonds are very mobile). This principle of interaction determines the formation of isometric (spherical) viral particles. The protein shells of viruses that are not associated with nucleic acid are called capsids.
3. Club-shaped virions have a differentiated structural organization and consist of a number of discrete structures. The main structural elements of the virion are the isometric head and the tail. Depending on the virus, the virion structure may also contain a muff, neck, collar, tail shaft, tail sheath, basal lamina, and fibrils. The bacteriophages of the T-even series have the most complex differentiated structural organization, the virion of which consists of all of the listed structural elements.
Virions or their components may have two main types of symmetry (the property of bodies repeating their parts) - helical and icosahedral. If the components of the virion have different symmetries, then they speak of a combined type of HF symmetry. (Scheme 1).
The helical arrangement of macromolecules is described by the following parameters: the number of subunits per turn of the helix (u, the number is not necessarily an integer); the distance between subunits along the helix axis (p); spiral pitch (P); P=pu. Classic example A virus with a spiral type of symmetry is the tobacco mosaic virus (TMV). The nucleocapsid of this rod-shaped virus measuring 18x300 nm consists of 2130 identical subunits, there are 16 1/3 subunits per turn of the helix, the helix pitch is 2.3 nm.
Icosahedral symmetry is the most effective for constructing closed circuits.
Virology (from Latin vīrus - “poison” and Greek logos - word, doctrine) - the science of viruses, a branch of biology.
Virology became an independent discipline in the middle of the 20th century. It arose as a branch of pathology - pathology of humans and animals on the one hand, and phytopathology on the other. Initially, virology of humans, animals and bacteria developed within the framework of microbiology. Subsequent successes of virology are largely based on the achievements of related natural sciences - biochemistry and genetics. The object of virology research is subcellular structures - viruses. In their structure and organization, they belong to macromolecules, therefore, from the time when a new discipline took shape, molecular biology, which united various approaches to the study of the structure, functions and organization of macromolecules that determine biological specificity, virology also became integral part molecular biology. Molecular biology widely uses viruses as a research tool, and virology uses molecular biology methods to solve its problems.
History of virology
Viral diseases such as smallpox, polio, yellow fever, and variegated tulips have been known for a long time, but for a long time no one knew anything about the causes that cause them. At the end of the 19th century, when the microbial nature of a number of infectious diseases was established, pathologists came to the conclusion that many of the common diseases of humans, animals and plants could not be explained by bacterial infection.
The discovery of viruses is associated with the names of D.I. Ivanovsky and M. Beyerinck. In 1892, D.I. Ivanovsky showed that a disease of tobacco - tobacco mosaic - can be transferred from diseased plants to healthy ones if they are infected with the juice of diseased plants, previously passed through a special filter that retains bacteria. In 1898, M. Beyerinck confirmed the data of D.I. Ivanovsky and formulated the idea that the disease is not caused by a bacterium, but by a fundamentally new infectious agent, different from bacteria. He called it contagium vivum fluidum - a living liquid infectious principle. At that time, the term “virus” was used to denote the infectious beginning of any disease - from the Latin word “poison”, “poisonous beginning”. Contagium vivum fluidum began to be called a filterable virus, and later simply a “virus”. In the same year, 1898, F. Lefler and P. Frosch showed that the causative agent of foot-and-mouth disease in cattle passes through bacterial filters. Soon after, it was discovered that other diseases of animals, plants, bacteria and fungi were caused by similar agents. In 1911, P. Rous discovered a virus that causes tumors in chickens. In 1915, F. Twort, and in 1917, F. D'Herelle, independently discovered bacteriophages - viruses that destroy bacteria.
The nature of these pathogens remained unclear for more than 30 years - until the early 30s. This was explained by the fact that traditional microbiological research methods could not be applied to viruses: viruses, as a rule, are not visible under a light microscope and do not grow on artificial nutrient media.
Categories:Detailing concepts:Virology.
Other mycoplasmas pathogenic for humans.
Mycoplasma pneumonia.
Mycoplasma pneumoniae.
M. pneumoniae differs from other species by serology, as well as by characteristics such as b-hemolysis of sheep red blood cells, aerobic reduction of tetrazolium, and the ability to grow in the presence of methylene blue.
M. pneumoniae is the most common cause of nonbacterial pneumonia. Infection with this mycoplasma may also take the form of bronchitis or mild respiratory fever.
Asymptomatic infections are common. Familial outbreaks are common, and major outbreaks have occurred in the military. training centers. The incubation period is approximately two weeks.
M. pneumoniae can be isolated by culture of sputum and throat swabs, but the diagnosis is more easily made by serological methods, usually the complement fixation test. The diagnosis of mycoplasma pneumonia is helped by the empirical finding that many patients form cold agglutinins to human red blood cells of group 0.
Mycoplasmas are normally inhabitants of the reproductive tract of men and women. The most commonly encountered species is M. hominis, which is responsible for some cases of vaginal discharge, urethritis, salpingitis and pelvic sepsis. It is the most common cause of postpartum sepsis.
The microorganism can enter the mother's blood during childbirth and be localized in the joints. A group of mycoplasmas (ureaplasmas) that form tiny colonies are considered possible reason non-gonococcal urethritis in persons of both sexes. Other species are normal commensals of the oral cavity and nasopharynx.
Prevention. Comes down to saving on high level general resistance of the human body. A vaccine made from killed mycoplasmas for the specific prevention of atypical pneumonia has been obtained in the USA
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Timakov V.D., Levashev V.S., Borisov L.B. Microbiology. - M: Medicine, 1983. - 312 p.
2. Borisov L.B., Kozmin-Sokolov B.N., Freidlin I.S. Guide to laboratory classes in medical microbiology, virology and immunology / ed. Borisova L.B. – G.: Medicine, 1993. – 232 p.
3. Medical microbiology, virology and immunology: Textbook, ed. A.A. Vorobyova. – M.: Medical Information Agency, 2004. - 691 p.
4. Medical microbiology, virology, immunology / ed. L.B.Borisov, A.M.Smirnova. - M: Medicine, 1994. - 528 p.
Odessa-2009
Lecture No. 21. Subject and tasks medical virology. General characteristics of viruses
We are starting to study a new science - virology, the science of viruses. Virology is an independent science of modern natural science, occupying a vanguard position in biology and medicine, and the role and importance of virology is steadily increasing. This is due to a number of circumstances:
1. Viral diseases occupy a leading place in human infectious pathology. The use of antibiotics makes it possible to effectively solve the treatment of most bacterial diseases, while there are still no sufficiently effective and harmless drugs for the treatment of viral diseases. As the incidence of bacterial infections decreases, the proportion of viral diseases is steadily increasing. The problem of mass viral infections - respiratory and intestinal - is acute. For example, the well-known flu often takes the form of massive epidemics and even pandemics, in which a significant percentage of the world's population falls ill.
2. The viral-genetic theory of the origin of tumors and leukemia has gained recognition and is increasingly being confirmed. Therefore, we expect that the development of virology will lead to a solution to the most important problem of human pathology - the problem of carcinogenesis.
3. Currently, new viral diseases are emerging or previously known viral diseases are becoming acute, which constantly poses new challenges for virology. An example is HIV infection.
4. Viruses have become a classic model for molecular biology and molecular genetic research. Many questions of fundamental research in biology are solved using viruses; viruses are widely used in biotechnology.
5. Virology - basic science modern natural science not only because it enriches other sciences with new methods and new ideas, but also because the subject of study of virology is a qualitatively special form of organization of living matter - viruses, which are radically different from all other living beings on Earth.
2. HISTORICAL SKETCH OF THE DEVELOPMENT OF VIRUSOLOGY
The credit for the discovery of viruses and the description of their main characteristics belongs to the Russian scientist Dmitry Iosifovich Ivanovsky (1864-1920). It is interesting that Ivanovsky began his research as a 3rd year student at St. Petersburg University, when he was doing coursework in Ukraine and Bessarabia. He studied tobacco mosaic disease and found out that it was an infectious plant disease, but its causative agent did not belong to any of the then known groups of microorganisms. Later, already a certified specialist, Ivanovsky continues his research at the Nikitsky Botanical Garden (Crimea) and performs a classic experiment: he filters the juice of the leaves of the affected plant through a bacterial filter and proves that the infectious activity of the juice does not disappear.
Subsequently, the main groups of viruses were discovered. In 1898, F. Leffler and P. Frosch proved the filterability of the causative agent of foot-and-mouth disease (the foot-and-mouth disease virus affects animals and humans), in 1911, P. Raus proved the filterability of the causative agent of the tumor disease - chicken sarcoma, in 1915, F. Twort and in 1917 Mr. D'Herelle discovered phages - bacterial viruses.
This is how the main groups of viruses were discovered. Currently, more than 500 types of viruses are known.
Further progress in the development of virology is associated with the development of methods for cultivating viruses. At first, viruses were studied only when they infected sensitive organisms. A significant step forward was the development of a method for cultivating viruses in chicken embryos by Woodruff and Goodpasture in 1931. A revolution in virology was the development of a method for cultivating viruses in single-layer cell cultures by J. Enders, T. Weller, F. Robbins, and in 1948. Not without reason in 1952 This discovery was awarded the Nobel Prize.
Already in the 30s the first virological laboratories were created. Currently in Ukraine there is the Odessa Research Institute of Epidemiology and Virology named after. I.I. Mechnikov, there are virological laboratories in a number of research institutes of epidemiology, microbiology, and infectious diseases. There are virological laboratories for practical health care, which are primarily engaged in the diagnosis of viral diseases.
3. Compose the ultrastructure of viruses
First of all, it must be said that the term “virus” was introduced in scientific terminology also by L. Pasteur. L. Pasteur received his vaccine to prevent rabies in 1885, although he did not discover the causative agent of this disease - there were still 7 years left before the discovery of viruses. L. Pasteur called the hypothetical pathogen the rabies virus, which translated means “rabies poison.”
The term “virus” is used to refer to any stage of virus development - both extracellularly located infectious particles and intracellularly reproducing virus. To designate a viral particle, the term “ virion».
By chemical composition Viruses are basically similar to other microorganisms; they have nucleic acids, proteins, and some also have lipids and carbohydrates.
Viruses contain only one type of nucleic acid - either DNA or RNA. Accordingly, DNA genomic and RNA genomic viruses are isolated. Nucleic acid in the virion can contain from 1 to 40%. Typically, the virion contains only one nucleic acid molecule, often closed in a ring. Viral nucleic acids are not much different from eukaryotic nucleic acids; they consist of the same nucleotides and have the same structure. True, viruses can contain not only double-stranded, but also single-stranded DNA. Some RNA viruses may contain double-stranded RNA, although most contain single-stranded RNA. It should be noted that viruses may contain plus-strand RNA, which can act as messenger RNA, but they may also contain minus-strand RNA. Such RNA can perform its genetic function only after the complementary plus strand is synthesized in the cell. Another feature of viral nucleic acids is that in some viruses the nucleic acid is infectious. This means that if RNA without protein admixture is isolated from a virus, for example the polio virus, and introduced into a cell, a viral infection will develop with the formation of new viral particles.
Proteins are contained in viruses in an amount of 50-90%; they have antigenic properties. Proteins are part of the envelope structures of the virion. In addition, there are internal proteins associated with the nucleic acid. Some viral proteins are enzymes. But these are not enzymes that ensure the metabolism of viruses. Viral enzymes are involved in the penetration of the virus into the cell, the exit of the virus from the cell, some of them are necessary for the replication of viral nucleic acids.
Lipoids can be from 0 to 50%, carbohydrates - 0 - 22%. Lipids and carbohydrates are part of the secondary shell of complex viruses and are not virus-specific. They are borrowed by the virus from the cell and are therefore cellular.
Let us note a fundamental difference in the chemical composition of viruses - the presence of only one type of nucleic acid, DNA or RNA.
Ultrastructure of viruses- this is the structure of virions. The sizes of virions vary and are measured in nanometers. 1 nm is a thousandth of a micrometer. The smallest typical viruses (poliomyelitis virus) have a diameter of about 20 nm, the largest (variola virus) - 200-250 nm. Average viruses have sizes of 60 - 120 nm. Small viruses can only be seen in an electron microscope; large ones are at the limit of the resolution of a light microscope and are visible in a dark field of view or with special staining that increases the size of the particles. Individual viral particles visible under a light microscope are usually called elementary Paschen-Morozov bodies. E. Paschen discovered the variola virus using a special stain, and Morozov proposed a silvering method that made it possible to see even medium-sized viruses in a light microscope.
The shape of virions can be different - spherical, cuboidal, rod-shaped, sperm-like.
Each virion consists of a nucleic acid, which in viruses constitutes a “nucleon.” Compare - nucleus in eukaryotes, nucleoid - in prokaryotes. The nucleon is necessarily associated with the primary protein shell - the capsid, consisting of protein capsomers. As a result, a nucleoprotein is formed - a nucleocapsid. Simple viruses consist only of a nucleocapsid (poliomyelitis viruses, tobacco mosaic disease virus). Complex viruses also have a secondary shell - a supercapsid, which in addition to proteins also contains lipids and carbohydrates.
The combination of structural elements in the virion may be different. There are three types of symmetry of viruses - helical, cubic and mixed. Speaking about symmetry, the symmetry of the viral particles relative to the axis is emphasized.
At spiral type of symmetry individual capsomeres, visible in an electron microscope, are arranged along the nucleic acid helix so that the thread passes between two capsomeres, covering it on all sides. The result is a rod-shaped structure, such as the rod-shaped tobacco mosaic virus. But viruses with a helical type of symmetry do not necessarily have to be rod-shaped. For example, although the influenza virus has a helical type of symmetry, its nucleocapsid is folded in a certain way and is covered with a supercapsid. As a result, influenza virions are usually spherical in shape.
At cubic type symmetry, the nucleic acid folds in a certain way in the center of the virion, and capsomers cover the nucleic acid from the outside, forming a bulk geometric figure. Most often, the figure of an icosahedron, a polyhedron with a certain ratio of the number of vertices and faces, is formed. For example, polio viruses have this form. In profile, the virion has the shape of a hexagon. A more complex form of adenovirus, also of cubic type of symmetry. Long threads and fibers extend from the vertices of the polyhedron, ending in a thickening.
With a mixed type of symmetry, for example, in bacteriophages, the head with a cubic type of symmetry has the shape of an icosahedron, and the process contains a spirally twisted contractile fibril.
Some viruses have more complex structure. For example, the variola virus contains a large nucleocapsid with a helical type of symmetry, and the supercapsid is complex and contains a system of tubular structures.
Thus, viruses are quite complex. But we must note that viruses do not have cellular organization. Viruses are non-cellular creatures, and this is one of their fundamental differences from other organisms.
A few words about the stability of viruses. Most viruses are inactivated at 56 - 60 °C for 5 - 30 minutes. Viruses tolerate refrigeration well, with room temperature Most viruses are quickly inactivated. The virus is more resistant to ultraviolet radiation and ionizing radiation than bacteria. Viruses are resistant to glycerol. Antibiotics have no effect on viruses at all. Of the disinfectants, the most effective is 5% Lysol; most viruses die within 1 - 5 minutes.
4. VIRUS REPRODUCTION
Usually we do not use the term “reproduction of viruses”, but rather say “reproduction”, reproduction of viruses, since the method of reproduction of viruses is fundamentally different from the method of reproduction of all organisms known to us.
For better study the mechanism of virus reproduction, we offer you a table that is not in textbooks, but helps to understand this complex process.
stages of virus reproduction
The first, preparatory period, begins with the stage of virus adsorption on the cell. The adsorption process is carried out due to the complementary interaction of the virus attachment proteins with cellular receptors. Cellular receptors can be of glycoprotein, glycolipid, protein and lipid nature. Each virus requires specific cellular receptors.
Viral attachment proteins located on the surface of the capsid or supercapsid act as viral receptors.
The interaction between virus and cell begins with nonspecific adsorption of the virion on the cell membrane, and then specific interaction between viral and cellular receptors occurs according to the principle of complementarity. Therefore, the process of virus adsorption on a cell is a specific process. If the body does not have cells with receptors for a particular virus, then infection with this type of virus in such an organism is impossible - there is species resistance. On the other hand, if we could block this first stage of interaction between the virus and the cell, then we could prevent the development of a viral infection at a very early stage.
Stage 2 - penetration of the virus into the cell - can occur in two main ways. The first one, which was described earlier, is called viropexis. This pathway closely resembles phagocytosis and is a variant of receptor endocytosis. The viral particle is adsorbed on the cell membrane; as a result of the interaction of receptors, the state of the membrane changes, and it invaginates, as if flowing around the viral particle. A vacuole is formed, delimited by a cell membrane, in the center of which the viral particle is located.
When a virus enters through membrane fusion mutual penetration of the elements of the virus shell and the cell membrane occurs. As a result, the “core” of the virion ends up in the cytoplasm of the infected cell. This process occurs quite quickly, so it was difficult to register it on electron diffraction patterns.
Deproteinization - release of the viral genome from the supercapsid and capsid. This process is sometimes called “undressing” of virions.
Release from the membranes often begins immediately after the virion attaches to cellular receptors and continues inside the cell cytoplasm. Lysosomal enzymes take part in this. In any case, for further reproduction to occur, deproteinization of the viral nucleic acid is necessary, since without this the viral genome is not able to induce the reproduction of new virions in the infected cell.
Average reproduction period called latent, hidden, since after deproteinization the virus seems to “disappear” from the cell, it cannot be detected on electron diffraction patterns. During this period, the presence of the virus is detected only by changes in the metabolism of the host cell. The cell is rebuilt under the influence of the viral genome on the biosynthesis of the components of the virion - its nucleic acid and proteins.
First stage of the middle period, t transcription viral nucleic acids, rewriting genetic information through the synthesis of messenger RNA is a necessary process to begin the synthesis of viral components. It occurs differently depending on the type of nucleic acid.
Viral double-stranded DNA is transcribed in the same way as cellular DNA by DNA-dependent RNA polymerase. If this process is carried out in the cell nucleus (in adenoviruses), then cellular polymerase is used. If in the cytoplasm (smallpox virus), then with the help of RNA polymerase, which penetrates the cell as part of the virus.
If the RNA is minus-strand (in influenza, measles, rabies viruses), information RNA must first be synthesized on the viral RNA matrix using a special enzyme - RNA-dependent RNA polymerase, which is part of the virions and penetrates the cell along with the viral RNA. The same enzyme is also found in viruses containing double-stranded RNA (reoviruses).
Regulation of the transcription process is carried out by sequential rewriting of information from “early” and “late” genes. “Early” genes contain information about the synthesis of enzymes necessary for gene transcription and their subsequent replication. In the “late” ones there is information for the synthesis of virus envelope proteins.
Broadcast- synthesis of viral proteins. This process is completely analogous to the known scheme of protein biosynthesis. Virus-specific messenger RNA, cellular transfer RNA, ribosomes, mitochondria, and amino acids are involved. First, enzyme proteins necessary for the transcription process are synthesized, as well as for partial or complete suppression of the metabolism of the infected cell. Some virus-specific proteins are structural and are included in the virion (for example, RNA polymerase), others are non-structural, which are found only in the infected cell and are necessary for one of the processes of virion reproduction.
Later, the synthesis of viral structural proteins - components of the capsid and supercapsid - begins.
After the synthesis of viral proteins on ribosomes, their post-translational modification can occur, as a result of which the viral proteins “mature” and become functionally active. Cellular enzymes can carry out phosphorylation, sulfonation, methylation, acylation and other biochemical transformations of viral proteins. The process of proteolytic cutting of viral proteins from large-molecular precursor proteins is essential.
Replication viral genome - synthesis of viral nucleic acid molecules, reproduction of viral genetic information.
Replication of viral double-stranded DNA occurs with the help of cellular DNA polymerase in a semi-conservative manner in the same way as cellular DNA replication. Single-stranded DNA replicates through an intermediate double-stranded replicative form.
There are no enzymes in the cell that can carry out RNA replication. Therefore, such a process is always carried out by virus-specific enzymes, information about the synthesis of which is encoded in the viral genome. During the replication of single-stranded RNA genomes, an RNA strand complementary to the viral one is first synthesized, and then this newly formed RNA strand becomes the template for the synthesis of genome copies. Moreover, in contrast to the transcription process, in which often only relatively short RNA chains are synthesized, during replication a complete strand of RNA is immediately formed. Double-stranded RNA replicates similarly to double-stranded DNA, but with the help of the corresponding enzyme - RNA polymerase of viral origin.
As a result of the process of viral genome replication, funds of viral nucleic acid molecules necessary for the formation of mature virions accumulate in the cell.
Thus, the synthesis of individual components of the virion is separated in time and space, occurs in different cellular structures and in different time.
IN final period During reproduction, virions are assembled and the virus leaves the cell.
Virion assembly may occur in different ways, but it is based on the process of self-assembly of viral components transported from the sites of their synthesis to the site of assembly. The primary structure of viral nucleic acids and proteins determines the order of conformation of the molecules and their connection with each other. First, a nucleocapsid is formed due to the strictly oriented connection of protein molecules into capsomers and capsomers with nucleic acid. For simple viruses, this is where the assembly ends. The assembly of complex viruses with a supercapsid is multistage and usually ends during the process of virions leaving the cell. In this case, elements of the cell membrane are included in the supercapsid of the virus.
Exit of the virus from the cell can happen in two ways. Some viruses that lack a supercapsid (adenoviruses, picornaviruses) exit the cell in an “explosive” manner. In this case, the cell is lysed, and the virions exit the destroyed cell into the intercellular space. Other viruses that have a lipoprotein secondary envelope, for example influenza viruses, leave the cell by budding from its envelope. The cell can remain viable for a long time.
The entire virus reproduction cycle usually takes several hours. In the 4 to 5 hours that pass from the moment one molecule of viral nucleic acid enters a cell, from several tens to several hundred new virions can be formed that can infect neighboring cells. Thus, the spread of viral infection in cells occurs very quickly.
Thus, the way viruses reproduce is fundamentally different from the way all other living things reproduce. All cellular organisms reproduce by division. When viruses reproduce, individual components are synthesized in different places virus-infected cells and at different times. This method of reproduction is called “disconnected” or “disjunctive”.
It should be said that the interaction of the virus and the cell may not necessarily lead to the described result - early or delayed death of the infected cell with the production of a mass of new mature viral particles. There are three possible types of viral infection in a cell.
The first option, which we have already discussed, occurs when productive or virulent infections.
Second option - persistent infection of a virus in a cell, when there is a very slow production of new virions with their release from the cell, but the infected cell remains viable for a long time.
Finally, the third option is integrative type interaction between a virus and a cell, during which the integration of viral nucleic acid into the cellular genome occurs. This involves the physical inclusion of a viral nucleic acid molecule into the host cell chromosome. For DNA genomic viruses, this process is quite understandable; RNA genomic viruses can integrate their genome only in the form of a “provirus” - a DNA copy of viral RNA synthesized using reverse transcriptase - RNA-dependent DNA polymerase. In the case of integration of the viral genome into the cellular genome, the viral nucleic acid replicates together with the cellular one during cell division. A virus in the form of a provirus can persist in a cell for a long time due to constant replication. This process is called " virogeny».
5. CARDINAL FEATURES OF VIRUSES
However, the size of large viruses is comparable to the size of chlamydia and small rickettsia, and filterable forms of bacteria have been described. Currently, the term “filterable viruses”, which for a long time was common to refer to viruses, is practically not used. Therefore, small size is not a fundamental difference between viruses and other living beings.
Therefore, at present, the fundamental differences between viruses and other microorganisms are based on more significant biological properties, which we discussed in this lecture.
Based on the knowledge of the properties of viruses we have analyzed, we can formulate the following 5 fundamental differences between viruses from other living beings on Earth:
1. Lack of cellular organization.
2. The presence of only one type of nucleic acid (DNA or RNA).
3. Lack of independent metabolism. Metabolism in viruses is mediated through the metabolism of cells and organisms.
4. The presence of a unique, disjunctive method of reproduction.
Thus, we can give the following definition to viruses.
The achievements of modern virology are enormous. Scientists are increasingly and more deeply and successfully understanding the subtle structure, biochemical composition and physiological properties of these ultramicroscopic living beings, their role in nature, human life, animals, and plants. Oncovirology persistently and successfully studies the role of viruses in the occurrence of tumors (cancer), trying to solve this problem of the century.
TO beginning of XXI centuries described more 6 thousand viruses belonging to more than 2,000 species, 287 genera, 73 families and 3 orders. For many viruses, their structure, biology, chemical composition and replication mechanisms have been studied. The discovery and research of new viruses continues, and they continue to amaze with their diversity. So in 2003, the largest known virus, mimivirus, was discovered.
The discovery of a large number of viruses required creating their collections and museums. The largest among them are in Russia (state collection of viruses at the D.I. Ivanovsky Institute of Virology in Moscow), USA (Washington), Czech Republic (Prague), Japan (Tokyo), Great Britain (London), Switzerland (Lausanne) and Germany (Brunschweig). The results of scientific research in the field of virology are published in scientific journals and discussed at international congresses organized every 3 years (first held in 1968). In 1966, at the 9th International Congress of Microbiology, the International Committee on Taxonomy of Viruses (ICTV) was elected for the first time.
Within the framework of general, that is, molecular virology, the study of the fundamental principles of interaction between viruses and cells continues. Advances in molecular biology, virology, genetics, biochemistry and bioinformatics have shown that the importance of viruses is not limited to the fact that they cause infectious diseases.
It has been shown that the replication features of some viruses lead to the virus capturing cellular genes and transferring them into the genome of another cell - horizontal transfer of genetic information, which can have consequences both in evolutionary terms and in terms of malignant degeneration of cells.
When sequencing the genome of humans and other mammals, a large number of repeating nucleotide sequences were identified, which are defective viral sequences - retrotransposons (endogenous retroviruses), which may contain regulatory sequences that affect the expression of neighboring genes. Their discovery and study led to active discussion and research into the role of viruses in the evolution of all organisms, in particular in the evolution of humans.
A new direction in virology is ecology of viruses. Detecting viruses in nature, identifying them and estimating their abundance is a very difficult task. At present, some methodological techniques have been developed that make it possible to estimate the amount of certain groups of viruses, in particular bacteriophages, in natural samples and to trace their fate. Preliminary data have been obtained indicating that viruses have a significant impact on numerous biogeochemical processes and effectively regulate the abundance and species diversity of bacteria and phytoplankton. However, the study of viruses in this aspect has just begun, and unresolved problems There is still a lot more in this area of science.
The achievements of general virology have given a powerful impetus to the development of its applied areas. Virology has grown into a vast field of knowledge important for biology, medicine and agriculture.
Virologists diagnose viral infections in humans and animals, study their spread, and develop methods of prevention and treatment. The greatest achievement was the creation of vaccines against polio, smallpox, rabies, hepatitis B, measles, yellow fever, encephalitis, influenza, mumps, and rubella. A vaccine has been created against the papilloma virus, which is associated with the development of one type of cancer. Thanks to vaccination, smallpox has been completely eradicated. International programs for the complete eradication of polio and measles are being implemented. Methods are being developed for the prevention and treatment of hepatitis and human immunodeficiency (AIDS). Data on substances with antiviral activity are accumulating. Based on them, a number of drugs have been created for the treatment of AIDS, viral hepatitis, influenza, and diseases caused by the herpes virus.
The study of plant viruses and the characteristics of their spread throughout the plant has led to the creation of a new direction in agriculture– obtaining virus-free planting material. Meristem technologies that make it possible to grow plants free of viruses are currently used for potatoes and a number of fruit and flower crops.
Of exceptional importance at this stage is the knowledge accumulated about the structure of viruses and their genomes for the development of genetic engineering. A striking example This is the use of bacteriophage lambda to obtain libraries of cloned sequences. In addition, based on the genomes of various viruses, it has been created and continues to be created a large number of genetically engineered vectors for delivering foreign genetic information into cells. These vectors are used for scientific research, for the accumulation of foreign proteins, especially in bacteria and plants, and for gene therapy. Genetic engineering uses some viral enzymes that are now produced commercially.
Small sizes and the ability to form regular structures have opened up the prospect of using viruses in nanotechnology to produce new bioinorganic materials: nanotubes, nanoconductors, nanoelectrodes, nanocontainers, for encapsidation of inorganic compounds, magnetic nanoparticles and inorganic nanocrystals of strictly controlled sizes. New materials can be created by the interaction of regularly organized viral protein structures with metal-containing inorganic compounds. “Spherical” viruses can serve as nanocontainers for storing and delivering drugs and therapeutic genes into cells. Surface modified infectious virions and viral substructures can be used as nanotools (for example, for the purposes of biocatalysis or the production of safe vaccines).
17. Bacteriophage titer, methods for its determination. Identification of animal and plant viruses.
The bacteriophage titer is the number of active phage particles per unit volume of the material being studied. To determine the bacteriophage titer, the agar layer method is most widely used when working with bacteriophages. , proposed by A. Grazia in 1936. This method is distinguished by its ease of implementation and accuracy of the results obtained and is also successfully used for the isolation of bacteriophages.
The essence of the method is that a bacteriophage suspension is mixed with a culture of sensitive bacteria, added to a low concentration agar (“soft agar”) and layered on the surface of a previously prepared 1.5% nutrient agar in a Petri dish. Water (“hungry”) 0.6% was used as the top layer in the classical Grazia method. - nd agar. Currently, 0.7% nutrient agar is most often used for these purposes. When incubated for 6-18 hours, the bacteria multiply within the upper "soft" layer of agar in the form of many colonies, receiving nutrition from the lower layer of 1.5% nutrient agar, which is used as a substrate. The low concentration of agar in the upper layer creates a reduced viscosity, which promotes good diffusion of phage particles and their infection of bacterial cells. Infected bacteria undergo lysis, resulting in progeny phage that again infects bacteria in close proximity to them. The formation of a negative colony for T-group phages is caused by only one bacteriophage particle, and, therefore, the number of negative colonies serves as a quantitative indicator of the content of plaque-forming units in the test sample.
The culture of phage-sensitive bacteria is used in the logarithmic growth phase in minimum quantity, providing a continuous lawn of bacteria. The ratio of the number of phage particles and bacterial cells (multiplicity of infection) for each phage-bacterium system is selected experimentally so that 50-100 negative colonies are formed on one plate.
To titrate a bacteriophage, a single-layer method can also be used, which consists of adding suspensions of bacteria and bacteriophage to the surface of a plate with nutrient agar, after which the mixture is distributed with a glass spatula. However, this method is inferior in accuracy to the agar layer method and therefore is not widely used.
Technique for titration and cultivation of bacteriophages. To determine the bacteriophage titer, the initial phage suspension is sequentially diluted in a buffer solution or in broth (dilution step 10 -1). For each dilution, use a separate pipette, and the mixture is stirred vigorously. From each dilution of the suspension, the phage is “seeded” onto a lawn of sensitive bacteria E. coli B. To do this, 1 ml of the diluted phage is added to a test tube with 3 ml of “soft agar” melted and cooled to 48-50°C, after which it is added to each test tube 0.1 ml of a culture of a sensitive microorganism (E. coli B) in the logarithmic growth phase. The contents are mixed by rotating the test tube between the palms and avoiding the formation of bubbles. Then quickly pour it onto the surface of the agar (1.5%) nutrient medium in a Petri dish and distribute it evenly over it, gently shaking the dish. When titrating using the agar layer method, at least two plates of the same phage dilution should be inoculated in parallel. After the top layer has hardened, the cups are turned upside down and placed in a thermostat with a temperature of 37°C, optimal for the development of sensitive bacteria. The results are recorded after 18-20 hours of incubation.
The number of negative colonies is counted in the same way as counting bacterial colonies, and the phage titer is determined using the formula:
Where N is the number of phage particles in 1 ml of the test material; n is the average number of negative colonies per plate; D - dilution number; V is the volume of the inoculated sample, ml.
In the case when it is necessary to determine the multiplicity of infection, the titer of viable cells of E. coli B bacteria in 1 ml of nutrient broth is determined in parallel. To do this, dilute the initial suspension of bacterial cells to 10 -6 and inoculate it (0.1 ml) in parallel onto 2 cups. After incubation at 37 °C for 24 hours, the number of colonies formed on a Petri dish is counted and the cell titer is determined.
To isolate viruses from humans, animals and plants, the material under study is introduced into the body of experimental animals and plants sensitive to viruses or infects cell (tissue) cultures and organ cultures. The presence of the virus is proven by characteristic damage in experimental animals (or plants), and in tissue cultures - by damage to cells, the so-called cytopathic effect, which is recognized by microscopic or cytochemical examination. With V. and. the “plaque method” is used - observation of defects in the cell layer caused by destruction or damage to cells in areas of virus accumulation. Virions, which have a characteristic structure among different viruses, can be identified by electron microscopy. Further identification of viruses is based on the integrated use of physical, chemical and immunological methods. Thus, viruses differ in sensitivity to ether, which is associated with the presence or absence of lipids in their shells. The type of virus nucleic acid (RNA and DNA) can be determined by chemical or cytochemical methods. To identify viral proteins, serological reactions are used with sera obtained by immunizing animals with the corresponding viruses. These reactions make it possible to recognize not only types of viruses, but also their varieties. Serological research methods make it possible to diagnose by the presence of antibodies in the blood. viral infection in humans and higher animals and study the circulation of viruses among them. To identify latent (hidden) viruses of humans, animals, plants and bacteria, special research methods are used.
The human body is susceptible to all kinds of diseases and infections, and animals and plants also get sick quite often. Scientists of the last century tried to identify the cause of many diseases, but even having determined the symptoms and course of the disease, they could not confidently say about its cause. It was only at the end of the nineteenth century that the term “viruses” appeared. Biology, or rather one of its sections - microbiology, began to study new microorganisms, which, as it turned out, have been neighbors for a long time and contribute to the deterioration of his health. In order to more effectively fight viruses, a new science has emerged - virology. It is she who can tell a lot of interesting things about ancient microorganisms.
Viruses (biology): what are they?
Only in the nineteenth century did scientists discover that the causative agents of measles, influenza, foot-and-mouth disease and other infectious diseases not only in humans, but also in animals and plants are microorganisms invisible to the human eye.
After viruses were discovered, biology was not immediately able to provide answers to the questions posed about their structure, occurrence and classification. Humanity has a need for a new science - virology. Currently, virologists are working to study familiar viruses, monitor their mutations and invent vaccines that can protect living organisms from infection. Quite often, for the purpose of experiment, a new strain of the virus is created, which is stored in a “dormant” state. On its basis, drugs are developed and observations are made of their effects on organisms.
IN modern society Virology is one of the most important sciences, and the most sought-after researcher is a virologist. The profession of a virologist, according to sociologists, is becoming more and more popular every year, which well reflects the trends of our time. After all, according to many scientists, wars will soon be fought and ruling regimes established with the help of microorganisms. In such conditions, a state with highly qualified virologists may turn out to be the most resilient, and its population the most viable.
The emergence of viruses on Earth
Scientists attribute the emergence of viruses to the most ancient times on the planet. Although it is impossible to say with certainty how they appeared and what form they had at that time. After all, viruses have the ability to penetrate absolutely any living organisms; they have access to the simplest forms of life, plants, fungi, animals and, of course, humans. But viruses do not leave behind any visible remains in the form of fossils, for example. All these features of the life of microorganisms significantly complicate their study.
- they were part of the DNA and separated over time;
- they were built into the genome initially and, under certain circumstances, “woke up” and began to reproduce.
Scientists suggest that the genome of modern people contains a huge number of viruses that infected our ancestors, and now they are naturally integrated into the DNA.
Viruses: when were they discovered?
The study of viruses is a fairly new branch of science, because it is believed that it appeared only at the end of the nineteenth century. In fact, it can be said that the viruses themselves and their vaccines were unknowingly discovered by an English doctor at the end of the nineteenth century. He worked on creating a cure for smallpox, which at that time killed hundreds of thousands of people during an epidemic. He managed to create an experimental vaccine directly from the sore of one of the girls who had smallpox. This vaccination turned out to be very effective and saved more than one life.
But D.I. Ivanovsky is considered the official “father” of viruses. This Russian scientist studied diseases of tobacco plants for a long time and made an assumption about small microorganisms that pass through all known filters and cannot exist on their own.
A few years later, the Frenchman Louis Pasteur, in the process of fighting rabies, identified its causative agents and introduced the term “viruses”. An interesting fact is that the microscopes of the late nineteenth century could not show viruses to scientists, so all assumptions were made about invisible microorganisms.
Development of virology
The middle of the last century gave a powerful impetus to the development of virology. For example, the invented electron microscope finally made it possible to see viruses and classify them.
In the fifties of the twentieth century, the polio vaccine was invented, which became a salvation from this terrible disease for millions of children around the world. In addition, scientists have learned to grow human cells in a special environment, which has led to the opportunity to study human viruses in the laboratory. At the moment, about one and a half thousand viruses have already been described, although fifty years ago only two hundred similar microorganisms were known.
Properties of viruses
Viruses have a number of properties that distinguish them from other microorganisms:
- Very small sizes, measured in nanometers. Large human viruses, such as smallpox, are three hundred nanometers in size (that's only 0.3 millimeters).
- Every living organism on the planet contains two types of nucleic acids, but viruses have only one.
- Microorganisms cannot grow.
- Viruses reproduce only in a living host cell.
- Existence occurs only inside the cell; outside it, the microorganism cannot show signs of vital activity.
Virus forms
To date, scientists can confidently declare two forms of this microorganism:
- extracellular - virion;
- intracellular - virus.
Outside the cell, the virion is in a “sleeping” state; it shows no signs of life. Once in the human body, it finds a suitable cell and, only having penetrated it, begins to actively multiply, turning into a virus.
Virus structure
Almost all viruses, despite the fact that they are quite diverse, have the same structure:
- nucleic acids that form the genome;
- protein shell (capsid);
- Some microorganisms also have a membrane coating on top of the shell.
Scientists believe that this simplicity of structure allows viruses to survive and adapt to changing conditions.
Currently, virologists distinguish seven classes of microorganisms:
- 1 - consist of double-stranded DNA;
- 2 - contain single-stranded DNA;
- 3 - viruses that copy their RNA;
- 4 and 5 - contain single-stranded RNA;
- 6 - transform RNA into DNA;
- 7 - transform double-stranded DNA through RNA.
Despite the fact that the classification of viruses and their study have made great progress, scientists admit the possibility of the emergence of new types of microorganisms that differ from all those already listed above.
Types of Viral Infection
The interaction of viruses with a living cell and the method of exit from it determines the type of infection:
- Lytic
During the infection process, all viruses simultaneously exit the cell, and as a result, the cell dies. Subsequently, the viruses “settle” in new cells and continue to destroy them.
- Persistent
Viruses leave the host cell gradually and begin to infect new cells. But the old one continues its life activity and “gives birth” to new viruses.
- Latent
The virus is embedded in the cell itself, during its division it is transmitted to other cells and spreads throughout the body. Viruses can remain in this state for quite a long time. Under the necessary circumstances, they begin to actively multiply and the infection proceeds according to the types already listed above.
Russia: where are viruses studied?
In our country, viruses have been studied for quite a long time, and it is Russian specialists who are leaders in this field. The D.I. Ivanovsky Research Institute of Virology is located in Moscow, whose specialists make a significant contribution to the development of science. On the basis of the research institute, I operate research laboratories, maintain an advisory center and a department of virology.
At the same time, Russian virologists are working with WHO and expanding their collection of virus strains. Research institute specialists work in all areas of virology:
- general:
- private;
- molecular.
It is worth noting that in recent years there has been a tendency to unite the efforts of virologists around the world. Such joint work is more effective and allows serious progress in the study of the issue.
Viruses (biology as a science has confirmed this) are microorganisms that accompany all living things on the planet throughout their entire existence. Therefore, their study is so important for the survival of many species on the planet, including humans, who have more than once in history fallen victim to various epidemics caused by viruses.
