Bacteria are the oldest group of organisms currently existing on Earth. The first bacteria appeared, probably more than 3.5 billion years ago, and for almost a billion years were the only living things on our planet. Since these were the first representatives of living nature, their body had a primitive structure.
Over time, their structure has become more complex, but to this day bacteria are considered the most primitive unicellular organisms. Interestingly, some bacteria still retain the primitive features of their ancient ancestors. This is observed in bacteria that live in hot sulfur springs and anoxic silts at the bottom of reservoirs.
Most bacteria are colorless. Only a few are colored purple or green. But the colonies of many bacteria have a bright color, which is due to the release of a colored substance into the environment or pigmentation of cells.
The pioneer of the world of bacteria was Anthony Leeuwenhoek, a Dutch naturalist of the 17th century, who was the first to create a perfect magnifying glass microscope that magnifies objects 160-270 times.
Bacteria are classified as prokaryotes and are isolated into a separate kingdom - Bacteria.
Body shape
Bacteria are numerous and varied organisms. They vary in shape.
| Name of bacteria | Bacteria shape | Bacteria image |
| Cocci | Spherical | |
| Bacillus |  | Rod-shaped |
| Vibrio | Comma curved | |
| Spirillum |  | Spiral |
| Streptococci |  | Cocci chain |
| Staphylococci |  | Bunches of cocci |
| Diplococci | Two round bacteria enclosed in one slimy capsule |
Modes of movement
Among bacteria there are mobile and immobile forms. The mobile ones move due to wave-like contractions or with the help of flagella (twisted helical filaments), which consist of a special flagellin protein. There can be one or several flagella. They are located in some bacteria at one end of the cell, in others - on two or over the entire surface.
But movement is inherent in many other bacteria, which lack flagella. So, bacteria covered with mucus on the outside are capable of sliding movement.
Some aquatic and soil bacteria devoid of flagella have gas vacuoles in the cytoplasm. There can be 40-60 vacuoles in a cell. Each of them is filled with gas (presumably nitrogen). By regulating the amount of gas in the vacuoles, aquatic bacteria can submerge in the water column or rise to its surface, and soil bacteria can move in the soil capillaries.
Habitat
Due to the simplicity of organization and unpretentiousness, bacteria are widespread in nature. Bacteria are found everywhere: in a drop of even the purest spring water, in grains of soil, in the air, on rocks, in polar snows, desert sands, on the ocean floor, in oil extracted from great depths and even in hot springs with a temperature of about 80 ° C. They live on plants, fruits, in various animals and in humans in the intestines, oral cavity, on the limbs, on the surface of the body.
Bacteria are the smallest and most numerous living things. Due to their small size, they easily penetrate into any cracks, crevices, pores. They are very hardy and adapted to various conditions of existence. They tolerate drying, extreme cold, heating up to 90 ° C, without losing their viability.
There is practically no place on Earth where bacteria would not be found, but in different quantities. The living conditions of bacteria are diverse. One of them needs oxygen in the air, others do not need it and are able to live in an oxygen-free environment.
In the air: bacteria rise up to 30 km into the upper atmosphere. and more.
There are especially many of them in the soil. One year of soil can contain hundreds of millions of bacteria.
In water: in the surface layers of water in open reservoirs. Beneficial aquatic bacteria mineralize organic residues.
In living organisms: pathogenic bacteria enter the body from the external environment, but only in favorable conditions cause disease. Symbiotic lives in the digestive organs, helping to break down and assimilate food, and synthesize vitamins.
External structure
The bacterial cell is dressed in a special dense membrane - the cell wall, which performs protective and supporting functions, and also gives the bacteria a permanent characteristic shape. The cell wall of a bacterium resembles the membrane of a plant cell. It is permeable: through it, nutrients freely pass into the cell, and metabolic products go out into the environment. Often, bacteria develop an additional protective layer of mucus - a capsule - on top of the cell wall. The thickness of the capsule can be many times the diameter of the cell itself, but it can be very small. The capsule is not an obligatory part of the cell; it is formed depending on the conditions in which the bacteria enter. It prevents bacteria from drying out.
On the surface of some bacteria there are long flagella (one, two, or many) or short thin villi. The length of the flagella can be many times greater than the dimensions of the bacterial body. With the help of flagella and villi, bacteria move.
Internal structure
Inside the bacterial cell there is a dense, immobile cytoplasm. It has a layered structure, there are no vacuoles, therefore, various proteins (enzymes) and reserve nutrients are located in the very substance of the cytoplasm. Bacterial cells do not have a nucleus. In the central part of their cells, a substance is concentrated that carries hereditary information. Bacteria, - nucleic acid - DNA. But this substance is not formed into a nucleus.
The internal organization of a bacterial cell is complex and has its own specific features. The cytoplasm is separated from the cell wall by the cytoplasmic membrane. In the cytoplasm, a basic substance, or matrix, ribosomes and a small number of membrane structures that perform a variety of functions (analogs of mitochondria, endoplasmic reticulum, Golgi apparatus) are distinguished. The cytoplasm of bacterial cells often contains granules of various shapes and sizes. Granules can be composed of compounds that serve as a source of energy and carbon. Fat droplets are also found in the bacterial cell.
In the central part of the cell, a nuclear substance is localized - DNA, not delimited from the cytoplasm by a membrane. This is an analogue of the nucleus - a nucleoid. The nucleoid does not have a membrane, nucleolus and a set of chromosomes.
Meals
Bacteria have different ways of feeding. Among them are autotrophs and heterotrophs. Autotrophs are organisms that can independently form organic matter for their nutrition.
Plants need nitrogen, but they themselves cannot assimilate nitrogen from the air. Some bacteria combine nitrogen molecules in the air with other molecules to make substances available to plants.
These bacteria take up residence in the cells of young roots, resulting in thickenings called nodules on the roots. Such nodules are formed on the roots of plants of the legume family and some other plants.
The roots provide the bacteria with carbohydrates, and the bacteria provide the roots with nitrogen-containing substances that can be absorbed by the plant. Their cohabitation is mutually beneficial.
The roots of plants secrete many organic substances (sugars, amino acids, and others) that bacteria feed on. Therefore, a particularly large number of bacteria settle in the soil layer surrounding the roots. These bacteria convert dead plant residues into substances available to the plant. This layer of soil is called the rhizosphere.
There are several hypotheses about the penetration of nodule bacteria into the root tissue:
- through damage to the epidermal and crustal tissue;
- through root hairs;
- only through the young cell membrane;
- thanks to satellite bacteria that produce pectinolytic enzymes;
- by stimulating the synthesis of B-indoleacetic acid from tryptophan, which is always present in the root secretions of plants.
The process of introducing nodule bacteria into the root tissue consists of two phases:
- root hair infection;
- the process of nodule formation.
In most cases, the invaded cell actively multiplies, forms the so-called infectious filaments, and already in the form of such filaments moves into the plant tissue. Nodule bacteria released from the infection thread continue to multiply in the host tissue.
Plant cells filled with rapidly multiplying cells of nodule bacteria begin to divide rapidly. The connection of a young nodule with the root of a legume plant is carried out thanks to the vascular-fibrous bundles. During the period of functioning, the nodules are usually dense. By the time of the manifestation of optimal activity, the nodules acquire a pink color (due to the pigment leghemoglobin). Only those bacteria that contain leghemoglobin are capable of fixing nitrogen.
Nodule bacteria create tens and hundreds of kilograms of nitrogen fertilizers per hectare of soil.
Metabolism
Bacteria differ from each other in their metabolism. In some, it goes with the participation of oxygen, in others - without its participation.
Most bacteria feed on ready-made organic matter. Only a few of them (blue-green, or cyanobacteria) are capable of creating organic substances from inorganic ones. They played an important role in the accumulation of oxygen in the Earth's atmosphere.
Bacteria absorb substances from the outside, tear their molecules apart, from these parts they collect their shell and replenish their contents (this is how they grow), and unnecessary molecules are thrown out. The shell and membrane of the bacterium allows it to absorb only the necessary substances.
If the shell and membrane of the bacteria were completely impermeable, no substances would enter the cell. If they were permeable to all substances, the contents of the cell would mix with the environment - the solution in which the bacterium lives. For the survival of bacteria, a shell is needed that allows the necessary substances to pass through, but not unnecessary ones.
The bacterium absorbs nutrients in its vicinity. What happens next? If it can move independently (by moving the flagellum or pushing mucus back), then it moves until it finds the necessary substances.
If it cannot move, then it waits until diffusion (the ability of molecules of one substance to penetrate into the midst of molecules of another substance) brings the necessary molecules to it.
Bacteria, in conjunction with other groups of microorganisms, do an enormous amount of chemical work. By transforming various compounds, they receive the energy and nutrients necessary for their life. Metabolic processes, methods of obtaining energy and the need for materials for building substances in their bodies are varied in bacteria.
Other bacteria satisfy all the requirements for carbon, necessary for the synthesis of organic substances in the body, at the expense of inorganic compounds. They are called autotrophs. Autotrophic bacteria are able to synthesize organic substances from inorganic ones. Among them are distinguished:
Chemosynthesis
The use of radiant energy is the most important, but not the only way to create organic matter from carbon dioxide and water. Bacteria are known that use not sunlight as an energy source for such synthesis, but the energy of chemical bonds occurring in the cells of organisms during the oxidation of certain inorganic compounds - hydrogen sulfide, sulfur, ammonia, hydrogen, nitric acid, ferrous compounds of iron and manganese. They use the organic matter formed with the use of this chemical energy to build the cells of their body. Therefore, this process is called chemosynthesis.
The most important group of chemosynthetic microorganisms is made up of nitrifying bacteria. These bacteria live in the soil and carry out the oxidation of ammonia formed during the decay of organic residues to nitric acid. The latter, reacts with mineral compounds of the soil, turns into nitric acid salts. This process takes place in two phases.
Iron bacteria convert ferrous iron into oxide. The formed iron hydroxide settles and forms the so-called bog iron ore.
Some microorganisms exist by oxidizing molecular hydrogen, thereby providing an autotrophic way of feeding.
A characteristic feature of hydrogen bacteria is the ability to switch to a heterotrophic lifestyle when they are provided with organic compounds and in the absence of hydrogen.
Thus, chemoautotrophs are typical autotrophs, since they independently synthesize the necessary organic compounds from inorganic substances, and do not take them ready-made from other organisms, like heterotrophs. Chemoautotrophic bacteria differ from phototrophic plants by their complete independence from light as an energy source.
Bacterial photosynthesis
Some pigment-containing sulfur bacteria (purple, green), containing specific pigments - bacteriochlorophylls, are capable of absorbing solar energy, with the help of which hydrogen sulfide in their organisms breaks down and releases hydrogen atoms to restore the corresponding compounds. This process has much in common with photosynthesis and differs only in that in purple and green bacteria, hydrogen sulfide is the donor of hydrogen (occasionally, carboxylic acids), and in green plants, water. In those and others, the elimination and transfer of hydrogen is carried out due to the energy of absorbed solar rays.
This bacterial photosynthesis, which takes place without the release of oxygen, is called photoreduction. Photoreduction of carbon dioxide is associated with the transfer of hydrogen not from water, but from hydrogen sulfide:
6СО 2 + 12Н 2 S + hv → С6Н 12 О 6 + 12S = 6Н 2 О
The biological significance of chemosynthesis and bacterial photosynthesis on a planetary scale is relatively small. Only chemosynthetic bacteria play an essential role in the sulfur cycle in nature. Absorbed by green plants in the form of sulfuric acid salts, sulfur is reduced and is part of protein molecules. Further, when dead plant and animal residues are destroyed by putrefactive bacteria, sulfur is released in the form of hydrogen sulfide, which is oxidized by sulfur bacteria to free sulfur (or sulfuric acid), which forms sulfites available for the plant in the soil. Chemo- and photoautotrophic bacteria are essential in the nitrogen and sulfur cycle.
Spore formation
Spores form inside the bacterial cell. In the process of sporulation, a bacterial cell undergoes a number of biochemical processes. The amount of free water in it decreases, the enzymatic activity decreases. This ensures the resistance of spores to unfavorable environmental conditions (high temperature, high salt concentration, drying, etc.). Sporulation is characteristic only of a small group of bacteria.
Spores are optional in the life cycle of bacteria. Spore formation begins only with a lack of nutrients or the accumulation of metabolic products. Bacteria in the form of spores can be dormant for a long time. Bacterial spores can withstand prolonged boiling and very long freezing. With the onset of favorable conditions, the spore germinates and becomes viable. Bacterial spore is an adaptation to survival in adverse conditions.
Reproduction
Bacteria multiply by dividing one cell into two. Having reached a certain size, the bacterium divides into two identical bacteria. Then each of them begins to feed, grows, divides, and so on.
After cell elongation, a transverse septum is gradually formed, and then the daughter cells diverge; in many bacteria, under certain conditions, the cells after division remain linked into characteristic groups. In this case, depending on the direction of the division plane and the number of divisions, different shapes arise. Reproduction by budding occurs in bacteria as an exception.
Under favorable conditions, cell division in many bacteria occurs every 20-30 minutes. With such a rapid reproduction, the offspring of one bacterium in 5 days is able to form a mass that can fill all seas and oceans. A simple calculation shows that 72 generations can be formed in a day (720,000,000,000,000,000,000 cells). If translated into weight - 4720 tons. However, this does not happen in nature, since most bacteria quickly die under the influence of sunlight, during drying, lack of food, heating to 65-100 ° C, as a result of the struggle between species, etc.
The bacterium (1) that has absorbed enough food increases in size (2) and begins to prepare for reproduction (cell division). Its DNA (in bacteria, the DNA molecule is closed in a ring) doubles (the bacterium produces a copy of this molecule). Both DNA molecules (3,4) turn out to be attached to the wall of the bacterium and, when the bacteria elongate, diverge to the sides (5,6). The nucleotide is divided first, then the cytoplasm.
After the divergence of two DNA molecules, a constriction appears on the bacteria, which gradually divides the body of the bacterium into two parts, each of which contains a DNA molecule (7).
It happens (in the case of a hay bacillus), two bacteria stick together, and a bridge is formed between them (1,2).
Through the bridge, DNA is transported from one bacterium to another (3). Once in one bacterium, DNA molecules intertwine, stick together in some places (4), after which they exchange sections (5).
The role of bacteria in nature
The cycle
Bacteria are the most important link in the general circulation of substances in nature. Plants create complex organic substances from carbon dioxide, water and mineral salts of the soil. These substances return to the soil with dead fungi, plants and animal corpses. Bacteria break down complex substances into simple ones, which are used by plants again.
Bacteria destroy complex organic matter of dead plants and animal corpses, excretions of living organisms and various waste products. Feeding on these organic substances, saprophytic rotting bacteria turn them into humus. These are kind of orderlies of our planet. Thus, bacteria are actively involved in the cycle of substances in nature.
Soil formation
Since bacteria are widespread almost everywhere and are found in huge numbers, they largely determine the various processes that take place in nature. In autumn, leaves of trees and shrubs fall, aerial shoots of grasses die off, old branches fall off, from time to time the trunks of old trees fall. All this gradually turns into humus. In 1 cm 3. The surface layer of forest soil contains hundreds of millions of saprophytic soil bacteria of several species. These bacteria convert humus into various minerals that can be absorbed from the soil by plant roots.
Some soil bacteria are able to absorb nitrogen from the air, using it in life processes. These nitrogen-fixing bacteria live independently or settle in the roots of legumes. Having penetrated the roots of legumes, these bacteria cause the growth of root cells and the formation of nodules on them.
These bacteria release nitrogen compounds that plants use. Bacteria receive carbohydrates and mineral salts from plants. Thus, there is a close relationship between the legume plant and the nodule bacteria, which is beneficial to both one and the other organism. This phenomenon is called symbiosis.
Thanks to their symbiosis with nodule bacteria, legumes enrich the soil with nitrogen, helping to increase the yield.
Distribution in nature
Microorganisms are ubiquitous. The only exceptions are craters of active volcanoes and small areas in the epicenters of exploded atomic bombs. Neither the low temperatures of Antarctica, nor the boiling jets of geysers, nor saturated solutions of salts in salt pools, nor the strong insolation of mountain peaks, nor the severe irradiation of nuclear reactors, interfere with the existence and development of microflora. All living things constantly interact with microorganisms, being often not only their repositories, but also distributors. Microorganisms are the aborigines of our planet, actively assimilating the most incredible natural substrates.
Soil microflora
The number of bacteria in the soil is extremely high - hundreds of millions and billions of individuals per gram. There are much more of them in the soil than in water and air. The total number of bacteria in soils varies. The number of bacteria depends on the type of soil, their condition, the depth of the layers.
On the surface of soil particles, microorganisms are located in small microcolonies (20-100 cells in each). They often develop in thick clots of organic matter, on living and dying plant roots, in thin capillaries, and inside lumps.
The microflora of the soil is very diverse. There are different physiological groups of bacteria: rotting bacteria, nitrifying, nitrogen-fixing, sulfur bacteria, etc. among them there are aerobes and anaerobes, spore and non-spore forms. Microflora is one of the factors of soil formation.
The area of development of microorganisms in the soil is the area adjacent to the roots of living plants. It is called the rhizosphere, and the aggregate of microorganisms contained in it is called the rhizosphere microflora.
Microflora of reservoirs
Water is a natural environment where microorganisms grow in large numbers. Most of them enter the water from the soil. A factor that determines the number of bacteria in water, the presence of nutrients in it. The cleanest are artesian wells and spring waters. Open reservoirs and rivers are very rich in bacteria. The greatest number of bacteria is found in the surface layers of the water, closer to the coast. With increasing distance from the coast and increasing depth, the number of bacteria decreases.
Pure water contains 100-200 bacteria in 1 ml., And polluted water - 100-300 thousand and more. There are many bacteria in the bottom sludge, especially in the surface layer, where bacteria form a film. This film contains a lot of sulfur and iron bacteria, which oxidize hydrogen sulfide to sulfuric acid and thereby prevent fish from being killed. The silt contains more spore-bearing forms, while non-spore-bearing forms prevail in the water.
In terms of species composition, the microflora of water is similar to that of soil, but there are also specific forms. Destroying various wastes that have got into the water, microorganisms gradually carry out the so-called biological purification of water.
Microflora of air
The microflora of the air is less abundant than the microflora of soil and water. Bacteria rise into the air with dust, they can stay there for some time, and then settle to the surface of the earth and die from lack of nutrition or under the influence of ultraviolet rays. The number of microorganisms in the air depends on the geographic zone, terrain, season, dust pollution, etc. each speck of dust is a carrier of microorganisms. Most bacteria are in the air above industrial plants. The air in the countryside is cleaner. The cleanest air over forests, mountains, snowy spaces. The upper layers of the air contain fewer germs. The microflora of the air contains many pigmented and spore-bearing bacteria, which are more resistant than others to ultraviolet rays.
Microflora of the human body
The human body, even a completely healthy one, is always a carrier of microflora. When a person's body comes into contact with air and soil, various microorganisms settle on clothes and skin, including pathogens (tetanus sticks, gas gangrene, etc.). Most often, exposed parts of the human body are contaminated. Escherichia coli, staphylococci are found on the hands. There are over 100 types of microbes in the oral cavity. The mouth with its temperature, humidity, nutrient residues is an excellent environment for the development of microorganisms.
The stomach has an acidic reaction, so the bulk of the microorganisms in it dies. Starting in the small intestine, the reaction becomes alkaline, i.e. friendly to microbes. In the colon, the microflora is very diverse. Every adult excretes about 18 billion bacteria every day, i.e. more individuals than people on the globe.
Internal organs that do not connect with the external environment (brain, heart, liver, bladder, etc.) are usually free of microbes. Microbes enter these organs only during illness.
Bacteria in the Cycle
Microorganisms in general and bacteria in particular play an important role in biologically important cycles of substances on Earth, carrying out chemical transformations that are completely inaccessible to either plants or animals. Different stages of the cycle of elements are carried out by organisms of different types. The existence of each individual group of organisms depends on the chemical transformation of elements carried out by other groups.
The nitrogen cycle
The cyclic transformation of nitrogenous compounds plays a primary role in supplying the necessary forms of nitrogen to organisms of the biosphere for different nutritional needs. Over 90% of total nitrogen fixation is due to the metabolic activity of certain bacteria.
The carbon cycle
The biological transformation of organic carbon into carbon dioxide, accompanied by the reduction of molecular oxygen, requires the joint metabolic activity of various microorganisms. Many aerobic bacteria carry out complete oxidation of organic matter. Under aerobic conditions, organic compounds are initially degraded by fermentation, and organic end products of fermentation are oxidized further as a result of anaerobic respiration, if there are inorganic hydrogen acceptors (nitrate, sulfate, or CO 2).
The sulfur cycle
Sulfur is available for living organisms mainly in the form of soluble sulfates or reduced organic sulfur compounds.
Iron cycle
Some freshwater bodies of water contain reduced iron salts in high concentrations. In such places, a specific bacterial microflora develops - iron bacteria that oxidize reduced iron. They participate in the formation of bog iron ores and water sources rich in iron salts.
Bacteria are the most ancient organisms, appearing about 3.5 billion years ago in the Archean. For about 2.5 billion years, they dominated the Earth, forming the biosphere, participated in the formation of an oxygen atmosphere.
Bacteria are one of the simplest living organisms (other than viruses). They are believed to be the first organisms to appear on Earth.
I think you heard about bacteria at school. These are such small creatures, which are everywhere darkness, and without which we would not be able to exist. So, it turns out that among them there are giants and dwarfs. Moreover, the largest of them is the size of a mountain in comparison with the rest! This giant bacteria is called Epulopiscium. In size (up to 0.5 mm), it can be compared with a grain of salt - a huge size in the world of microscopic creatures. She can even be seen with the naked eye. This animal can reach the size of small insects and crustaceans.
Research has been conducted and published at Cornwell Academy to determine the causes of such a large size. It turned out that this bacterium stores up to 85 thousand copies of DNA. For comparison, human cells contain a maximum of 3 copies. This cute creature lives with the help of symbiosis in the digestive tract of a surgeon fish (tropical reef fish). It was discovered in 1985.
“Other bacteria also contain many copies of DNA, but their number is no more than 100-200. But this one holds a whole bank of its genetic information, ”says Asher Angert, professor of microbiology at Cornwell University.
Common bacteria are very small and simple in structure. They lack any organs (called organelles in cells) that promote cell growth, such as plant or animal cells. Bacteria feed on the absorption of nutrients through the cell membrane. Inside, nutrients are distributed "self-propelled", so the bacteria are forced to be small, otherwise the nutrients will not be able to spread throughout their volume.
But the aforementioned giant bacterium self-copies its DNA many times, and distributes copies evenly near the shell so that they receive nutrients quickly and in sufficient volume.
“Having thousands of copies of DNA distributed around the periphery makes it possible to instantly respond to external factors - temperature, irritation and others,” adds Escher Angert. Therefore, despite its large size, this bacterium instantly reacts to the attacks of predators in its world, of which there are a lot in the digestive tract of fish. Another feature of it is a special way of dividing. Most bacteria simply divide into 2 parts, but Epulopiscium grows two daughter cells inside itself, which, after its death, go outside.
But it turns out there are even bigger bacteria! An even larger species, Thiomargarita namibiensis, was discovered in 1999. It reaches a size of 0.75 mm. This creation feeds on nitrates, synthesizing organic substances from them. These giants live on the coast of Namibia, and some of their distant relatives - in the waters of the Gulf of Mexico.
Bacteria are the smallest cellular organisms, only viruses are smaller. An ordinary bacterium is 10 times smaller than a human cell, amounting to 0.5-5.0 micrometers (such can only be seen under a microscope). Thousands of bacteria of dozens of species are found, for example, in a droplet of saliva. A gram of soil contains about 40 million bacteria, and in the smallest drop of raw water there are also millions of bacteria. The planet contains (approximately, of course) 5,000,000,000,000,000,000,000,000,000,000 bacteria (30 zeros). They are the most common life form, found everywhere - from the ocean floor to the alpine snow.
ps. The photo shows the heroine of the article. The other bacteria in this photo appear as subtle small dots around.
Life on our planet began with bacteria. Scientists believe that everything will end with them. There is a joke that when the aliens studied the Earth, they could not understand who its real master is - a human or a bacillus. The most interesting facts about bacteria are summarized below.
A bacterium is a separate organism that multiplies by fission. The more favorable the habitat, the sooner it divides. These microorganisms live in all living things, as well as in water, food, rotten trees, and plants.
The list is not limited to this. Bacilli perfectly survive on objects that a person touched. For example, on a handrail in public transport, on the handle of a refrigerator, on the tip of a pencil. Interesting facts about bacteria were recently discovered from the University of Arizona. According to their observations, "dormant" microorganisms live on Mars. Scientists are confident that this is one of the proofs of the existence of life on other planets, in addition, in their opinion, alien bacteria can be "revived" on Earth.
For the first time, the microorganism was examined in an optical microscope by the Dutch scientist Anthony van Leeuwenhoek at the end of the 17th century. Currently, there are about two thousand known species of bacilli. All of them can be roughly divided into:
- harmful;
- useful;
- neutral.
At the same time, the harmful ones usually fight with the useful and neutral ones. This is one of the most common reasons for which a person is sick.
The most curious facts
In general, unicellular organisms are involved in all life processes.
Bacteria and humans
From birth, a person enters a world full of various microorganisms. Some help him survive, others cause infections and diseases.
The most curious interesting facts about bacteria and humans:
It turns out that the bacillus can both completely heal a person and destroy our species. Bacterial toxins already exist.
How did bacteria help us survive?
Here are some more interesting facts about bacteria that benefit humans:
- some types of bacilli protect a person from allergies;
- bacteria can be used to dispose of hazardous waste (for example, oil products);
- without microorganisms in the intestines, a person would not have survived.
How to tell babies about bacilli?
Babies are ready to talk about bacilli as early as 3-4 years old. In order to convey the information correctly, it is worth telling interesting facts about bacteria. For children, for example, it is very important to understand that there are good and bad microbes. That the good ones are able to turn milk into fermented baked milk. And also that they help the tummy to digest food.
Attention must be paid to evil bacteria. Tell them that they are very small, so you can't see them. That, getting into the human body, microbes quickly become a lot, and they begin to eat us from the inside.
The child must know that the evil microbe does not enter the body it is necessary:
- Wash hands after outside and before eating.
- Don't eat a lot of sweets.
- Get vaccinated.
The best way to show bacteria is through pictures and encyclopedias.
What should every student know?
It is better to talk with an older child not about microbes, but about bacteria. Interesting facts for schoolchildren are important to argue. That is, talking about the importance of washing hands, you can tell that 340 colonies of harmful bacilli live on the handles of toilets.
You can find information together about which bacteria cause tooth decay. And also tell the student that chocolate in small quantities has an antibacterial effect.
Even an elementary school student will be able to understand what a vaccine is. This is when a small amount of a virus or bacteria is introduced into the body, and the immune system defeats them. Therefore, it is so important to get vaccinated.
Already from childhood, the understanding should come that the country of bacteria is a whole world that has not yet been fully explored. And as long as there are these microorganisms, there is the human species itself.
Attempts to sequence the genome of a giant sulfur bacterium Achromatium oxaliferum gave a paradoxical result: it turned out that each bacterial cell contains not one, but many different genomes. Intracellular genetic diversity level A. oxaliferum comparable to the diversity of the multispecies bacterial community. Apparently, different chromosomes multiply in different parts of the cytoplasm, divided by large calcite inclusions into many poorly communicating compartments (compartments). An important role in maintaining internal genetic diversity is played by numerous mobile genetic elements that facilitate the transfer of genes from chromosome to chromosome. The authors of the discovery suggest that natural selection in this unique organism takes place not so much at the level of cells as at the level of individual compartments within one giant cell.
1. Mysterious bacteria
Giant sulfur bacteria Achromatium oxaliferum was discovered in the 19th century, but its biology is still mysterious - largely because achromatium cannot be cultivated in a laboratory. Achromatium cells can reach 0.125 mm in length, making it the largest freshwater bacteria (there are even larger sulfur bacteria in the seas such as Thiomargarita, which is described in the news The earliest Precambrian embryos turned out to be bacteria? , "Elements", 15.01.2007).
Achromatium oxaliferum lives in bottom sediments of freshwater lakes, where it is usually found at the border of oxygen and anoxic zones, but it also penetrates into completely anoxic layers. Other varieties (or species) of achromatium live in mineral springs and in the saline sediments of tidal marshes.
Achromatium receives energy due to the oxidation of hydrogen sulfide, first to sulfur (which is stored in the form of granules in the cytoplasm), and then to sulfates. It is capable of fixing inorganic carbon, but it can also assimilate organic compounds. It is unclear whether he is able to do only with autotrophic metabolism or whether he needs organic feeding.
A unique feature of achromatium is the presence in its cells of numerous large inclusions of colloidal calcite (Fig. 1). Why bacteria need this and what role calcium carbonate plays in its metabolism is not exactly known, although there are plausible hypotheses (V. Salman et al., 2015. Calcite-accumulating large sulfur bacteria of the genus Achromatium in Sippewissett Salt Marsh).
The cytoplasm of achromatiium huddles in the gaps between the calcite granules, which actually divide it into many communicating compartments (compartments). Although the compartments are not completely isolated, the exchange of matter between them seems to be difficult, especially since the systems of active intracellular transport are much weaker in prokaryotes than in eukaryotes.
And now it turned out that calcite granules are not the only unique feature of achromatium. And not even the most striking one. In an article published in the journal Nature Communications, German and British biologists reported paradoxical results from attempts to read the genomes of individual cells A. oxaliferum from the bottom sediments of Lake Stechlin in northeastern Germany. These results are so unusual that it is difficult to believe in them, although there is apparently no reason to doubt their reliability: the work was done methodologically very carefully.
2. Confirmation of polyploidy
Although achromatium, as already mentioned, refers to uncultivated bacteria, this inconvenience is partly offset by the giant cell size. They are clearly visible under a light microscope even at low magnifications, and they can be taken manually from sediment samples (previously passed through a filter to remove large particles). This is how the authors collected material for their research. Cells A. oxaliferum covered with an organic cover, on the surface of which a variety of cohabitants - small bacteria - swarm. The authors carefully washed all this accompanying microbiota from the selected cells in order to reduce the proportion of foreign DNA in the samples.
To begin with, the researchers dyed achromatium cells with a special fluorescent dye for DNA to understand how much genetic material is in the cell and how it is distributed. It turned out that DNA molecules are not confined to any one part of the cytoplasm, but form many (on average, about 200 per cell) local accumulations in the gaps between calcite granules (Fig. 1, b, d).
Considering everything that is known to date about large bacteria and their genetic organization, this fact is already enough to consider it proven that A. oxaliferum is a polyploid, that is, each of its cells contains not one, but many copies of the genome.
However, in hindsight, it is already clear that such a huge prokaryotic cell could not do with a single copy. It would simply not be enough to provide the entire cell with the transcripts necessary for protein synthesis.
Judging by the fact that DNA clusters differ in fluorescence brightness, these clusters most likely contain a different number of chromosomes. Here it is necessary to make a reservation that usually the entire genome of a prokaryotic cell is placed on one ring chromosome. For achromatium, this has not been proven, but it is very likely. Therefore, for the sake of simplicity, the authors use the term "chromosome" as a synonym for the term "one copy of the genome", and we will do the same.
At this stage, nothing sensational has yet been discovered. Gone are the days when everyone thought that prokaryotes always, or almost always, have only one ring chromosome in each cell. Today, many species of polyploid bacteria and archaea are already known (see, "Elements", 06/14/2016).
3. Metagenome of a multispecies community - in one cell
Miracles began when the authors began to extract DNA from selected and washed cells and to sequencing. From 10,000 cells, a metagenome was obtained (see Metagenomics), that is, many (about 96 million) short sequenced random fragments of chromosomes (reads) belonging to different individuals and collectively giving an idea of the genetic diversity of a population.
The researchers then set about sequencing the DNA from individual cells. First, fragments of the 16s-rRNA gene were isolated from 27 cells, by which it is customary to classify prokaryotes and by which the presence of one or another type of microbes in the analyzed sample is usually determined. Almost all of the isolated fragments belonged to the achromatium (that is, they roughly coincided with the 16s rRNA sequences of the achromatiium already available in the genetic databases). It follows from this that the studied DNA was not contaminated with the genetic material of any extraneous bacteria.
It turned out that every cell A. oxaliferum, unlike the vast majority of other prokaryotes, contains not one, but several different variants (alleles) of the 16s rRNA gene. It is difficult to determine the exact number of variants, because small differences can be explained by sequencing errors, and if only very different fragments are considered "different", then the question arises, how much they should differ greatly. Using the strictest criteria, it turned out that each cell contains approximately 4–8 different alleles of the 16s rRNA gene, and this is the minimum estimate, but in reality there are most likely more of them. This contrasts sharply with the situation typical for other polyploid prokaryotes, which, as a rule, have the same variant of this gene on all chromosomes of one cell.
Moreover, it turned out that the alleles of the 16s-rRNA gene present in the same cell A. oxaliferum, often form branches that are very distant from each other on the common family tree of all variants of this gene found (earlier and now) in A. oxaliferum. In other words, 16s rRNA alleles from one cell are no more related to each other than alleles taken at random from different cells.
Finally, the authors performed a total DNA sequencing from six individual cells. For each cell, approximately 12 million random fragments were read - reads. In a normal situation, this would be more than enough to use special computer programs to collect from reads, using their overlapping parts, six very high-quality (that is, read with a very high coverage, see Coverage) individual genomes.
But that was not the case: although almost all the reads undoubtedly belonged to the achromatiium (the admixture of foreign DNA was negligible), the read fragments flatly refused to be assembled into genomes. Further analysis clarified the reason for the failure: it turned out that the DNA fragments isolated from each cell, in fact, belong not to one, but to many quite different genomes. In fact, what the authors obtained from each individual cell is not a genome, but metagenom. Such sets of reads are usually obtained when analyzing not one organism, but an entire population, which also has a high level of genetic diversity.
This finding has been validated in several independent ways. In particular, dozens of genes are known that are almost always present in bacterial genomes in a single copy (single copy marker genes). These single-copy marker genes are widely used in bioinformatics to check the quality of genome assembly, estimate the number of species in metagenomic probes, and other similar tasks. So, in the genomes (or "metagenomes") of individual cells A. oxaliferum most of these genes are present in several distinct copies. As in the case of 16s rRNA, the alleles of these single-copy genes located in the same cell, as a rule, are no more related to each other than the alleles from different cells. The level of intracellular genetic diversity was found to be comparable to the level of diversity of the entire population, estimated on the basis of the metagenome of 10,000 cells.
Modern metagenomics already has methods that make it possible to isolate fragments most likely belonging to the same genome from the multitude of heterogeneous fragments of DNA found in a sample. If there are enough such fragments, then a significant part of the genome and even the complete genome can be assembled from them. It is in this way that a new supertype of Archaea, Asgardarchea, was recently discovered and characterized in detail (see. A new supertype of Archaea is described, to which the ancestors of eukaryotes belong, "Elements", 01.16.2017). The authors applied these methods to the "metagenomes" of individual cells. A. oxaliferum. This made it possible to identify in each "metagenome" 3-5 sets of genetic fragments corresponding, most likely, to individual circular genomes (chromosomes). Or rather, each such set corresponds to a whole group of similar genomes. The number of different genomes in each cell A. oxaliferum most likely more than 3-5.
The level of difference between genomes present in the same cell A. oxaliferum, roughly corresponds to the interspecies: bacteria with such a level of differences, as a rule, belong to different species of the same genus. In other words, the genetic diversity present in every single cell A. oxaliferum, comparable not even with a population, but with a multi-species community. If DNA from a single achromatium cell were analyzed by modern metagenomics methods “blindly”, not knowing that all this DNA comes from one cell, then the analysis would unequivocally show that several types of bacteria are present in the sample.
4. Intracellular gene transfer
So have A. oxaliferum discovered a fundamentally new, downright unheard-of type of genetic organization. Undoubtedly, the discovery raises a lot of questions, and first of all the question "how can this even be ?!"
We will not consider the most uninteresting option, which is that all this is the result of gross mistakes made by researchers. If so, we will soon find out about it: Nature Communications- the journal is serious, other teams will want to repeat the study, so it is unlikely that a refutation will take long. It is much more interesting to discuss the situation on the assumption that the research has been thoroughly conducted and the result is reliable.
In this case, you must first of all try to find out the reasons for what was found in A. oxaliferum unprecedented intracellular genetic diversity: how it is formed, why it is preserved, and how the microbe itself manages to survive. All these questions are very difficult.
In all the other polyploid prokaryotes studied to date (including the salt-loving archaea known to the readers of "Elements" Haloferax volcanii) all copies of the genome present in the cell, no matter how many there are, are very similar to each other. Nothing like the colossal intracellular diversity found in A. oxaliferum, they are not observed. And this is by no means an accident. Polyploidy gives prokaryotes a number of advantages, but it contributes to the uncontrolled accumulation of recessive harmful mutations, which, of course, can lead to extinction (for more details, see the news Polyploidy of eukaryotic ancestors - the key to understanding the origin of mitosis and meiosis, "Elements", 06/14/2016).
To avoid the accumulation of a mutational load, polyploid prokaryotes (and even polyploid plastids of plants) actively use gene conversion - an asymmetric variant of homologous recombination, in which two alleles do not change places, passing from chromosome to chromosome, as in crossing over, and one of the alleles is replaced by another. This leads to chromosome unification. Due to intensive gene conversion, harmful mutations are either quickly "erased" by the untainted version of the gene, or become homozygous, appear in the phenotype and are rejected by selection.
Have A. oxaliferum gene conversion and unification of chromosomes, most likely, also occur, but not on the scale of the entire cell, but at the level of individual "compartments" - the gaps between the granules of calcite. Therefore, different variants of the genome accumulate in different parts of the cell. The authors tested this by selectively staining different allelic variants of the 16s rRNA gene (see Fluorescent in situ hybridization). It turned out that the concentration of different allelic variants really differs in different parts of the cell.
However, this is still not enough to explain the highest level of intracellular genetic diversity found in A. oxaliferum... The authors see its main reason in the high rates of mutagenesis and intracellular genomic rearrangements. Comparison of fragments of chromosomes from the same cell showed that these chromosomes, apparently, live a very stormy life: they constantly mutate, rearrange and exchange sections. Have A. oxaliferum from Lake Stekhlin, the number of mobile genetic elements is sharply increased in comparison with other bacteria (including the closest relatives - achromatiums from salt marshes, in which the level of intracellular diversity, judging by preliminary data, is much lower). The activity of mobile elements contributes to frequent genomic rearrangements and the transfer of DNA sections from one chromosome to another. The authors even coined a special term for this: "intracellular gene transfer" (intracellular gene transfer, iGT), by analogy with all known horizontal gene transfer (HGT).
One of the clearest evidence of frequent rearrangements in chromosomes A. oxaliferum- a different order of genes in different versions of the genome, including within the same cell. Even in some conservative (rarely changing during evolution) operons, individual genes are sometimes located in a different sequence on different chromosomes within the same cell.
Figure 2 schematically shows the main mechanisms that, according to the authors, create and maintain a high level of intracellular genetic diversity in A. oxaliferum.
5. Intracellular selection
Frequent rearrangements, intracellular gene transfer, a high rate of mutagenesis - even if all this can at least explain the high intracellular genetic diversity (and I think that it cannot, we will talk about this below), it remains unclear how achromatiium contrives in such conditions to remain viable. After all, the vast majority of non-neutral (affecting fitness) mutations and rearrangements should be harmful! Polyploid prokaryotes already have an increased tendency to accumulate a mutational load, and if we also allow super-high rates of mutagenesis, it becomes completely incomprehensible how such a creature as achromatium can exist.
And here the authors put forward a truly innovative hypothesis. They suggest that natural selection in achromatiium acts not so much at the level of whole cells as at the level of individual compartments - weakly communicating gaps between calcite granules, in each of which, probably, its own variants of the genome multiply.
At first glance, the assumption may seem wild. But if you think about it, why not? To do this, it is enough to assume that each chromosome (or each local accumulation of similar chromosomes) has a limited "radius of action", that is, proteins encoded in this chromosome are synthesized and work mainly in its immediate vicinity, rather than being evenly mixed throughout the cell. This is most likely the way it is. In this case, those compartments where more successful chromosomes are located (containing a minimum of harmful and maximum useful mutations) will replicate their chromosomes faster, there will be more of them, they will begin to spread inside the cell, gradually displacing less successful copies of the genome from neighboring compartments. In principle, one can imagine such a thing.
6. Intracellular genetic diversity needs additional explanations
The idea of intensive intracellular selection of genomes, answering one question (why achromatium does not die out at such a high rate of mutagenesis), immediately creates another problem. The fact is that thanks to this selection, more successful (faster replicating) copies of the genome should displace less successful copies inside the cell, inevitably reducing at the same time intracellular genetic diversity. The one that we wanted to explain from the very beginning.
Moreover, it is obvious that intracellular genetic diversity should sharply decrease with each cell division. Different chromosomes sit in different compartments, therefore, during division, each daughter cell will receive not all, but only some of the genome variants available in the mother cell. This can be seen even in Fig. 2.
Intracellular selection plus genome compartmentalization are two powerful mechanisms that should reduce intrinsic diversity so rapidly that no conceivable (life-compatible) rate of mutagenesis can counter it. Thus, intracellular genetic diversity remains unexplained.
Discussing the results obtained, the authors repeatedly refer to our work, which is described in the news. Polyploidy of eukaryotic ancestors is the key to understanding the origin of mitosis and meiosis. In particular, they mention that it is very beneficial for polyploid prokaryotes to exchange genetic material with other cells. However, they believe that intercellular genetic exchange does not play a big role in the life of achromatium. This is justified by the fact that although genes for the absorption of DNA from the external environment (transformation, see Transformation) are found in the metagenome of the achromatium, there are no genes for conjugation (see Bacterial conjugation).
In my opinion, the genetic architecture of achromatium indicates not conjugation, but more radical ways of mixing the genetic material of different individuals, such as the exchange of whole chromosomes and cell fusion. Judging by the data obtained, from a genetic point of view, the cell A. oxaliferum is something like a prokaryotic plasmodium or syncytium, like those that are formed as a result of the fusion of many genetically dissimilar cells in slime molds. Recall that achromatium is an uncultivated bacterium, so it is possible that some elements of its life cycle (such as periodic cell fusion) could have escaped the attention of microbiologists.
In favor of the fact that the intracellular genetic diversity of achromatiium is formed not intracellularly, is evidenced by one of the main facts discovered by the authors, namely, that the alleles of many genes located in the same cell form branches that are far from each other on the phylogenetic tree. If the entire intracellular diversity of alleles was formed within clonally multiplying cells that do not change genes with each other, then one would expect that alleles within a cell would be more related to each other than alleles from different cells. But the authors have convincingly shown that this is not the case. In general, I would bet that there is cell fusion in the life cycle of achromatium. This appears to be the most parsimonious and plausible explanation for the colossal intracellular genetic diversity.
In the final part of the article, the authors hint that the genetic architecture of achromatium may shed light on the origin of eukaryotes. They put it this way: “ By the way, Markov and Kaznacheev suggested that, like the achromatiium from Lake Stechlin, proto-eukaryotic cells could be rapidly mutating, diversifying their chromosomes, polyploid bacteria / archaea". Quite right, but we also showed that such a creature could not have survived without intense inter-organismal genetic exchange. Hopefully, further research will shed light on the remaining unsolved mysteries of achromatiium.
Dwarfs and giants among bacteria
Bacteria are the smallest living organisms that are the most common form of life on Earth. Normal bacteria are about 10 times smaller than a human cell. Their size is about 0.5 microns, and they can only be seen with a microscope. However, it turns out that the world of bacteria also has its own dwarfs and giants. One of these giants is the bacterium Epulopiscium fishelsoni, which reaches half a millimeter in size! That is, it reaches the size of a grain of sand or grains of salt and can be seen with the naked eye.
Reproduction of Epulopiscium
Research has been carried out at Cornwall Academy to determine the causes of such a large size. As it turned out, the bacterium contains 85,000 copies of its DNA. For comparison, human cells contain only 3 copies. This cute creature lives in the digestive tract of the tropical reef fish Acanthurus nigrofuscus (surgeon fish).
Common types of bacteria are very small and primitive, they have no organs and food occurs through the shell. The nutrients are evenly distributed throughout the bacteria, so they need to be small. In contrast, Epulopiscium copies its DNA many times, evenly distributes copies along the shell, and they receive sufficient nutrition. This structure gives her the ability to instantly respond to external stimuli. Unlike other bacteria and the way they divide. If ordinary bacteria simply divide in half, then she grows two cells inside herself, which, after her death, simply go outside.
Namibian sulfur pearl
However, even this, far from a small bacterium, cannot be compared with the largest bacterium in the world which is considered Thiomargarita namibiensis, in other words "Namibian sulfur pearl" - a gram-negative marine bacterium, discovered in 1997. Not only does it consist of only one cell, but at the same time, it does not have a supporting skeleton as well as eukaryotes. The dimensions of Thiomargarita reach 0.75-1 mm, which allows you to see it with the naked eye.
By the type of metabolism, Thiomargarite is an organism that receives energy as a result of redox reactions and can use nitrate as the final object that receives electrons. The cells of the Namibian sulfuric pearl are immobile, and therefore the nitrate content can fluctuate. Thiomargarita can store nitrate in a vacuole, which occupies about 98% of the entire cell. At a low concentration of nitrate, its contents are used for breathing. Sulfides are oxidized by nitrates to sulfur, which collects in the internal environment of the bacteria in the form of small granules, which explains the pearl color of Thiomargarite.
Thiomargarita research
Recent studies have shown that Thiomargarita namibiensis may not be obligate, but an optional organism that receives energy without the presence of oxygen. She is capable of breathing oxygen if this gas is sufficient. Another distinguishing feature of this bacterium is the possibility of palintomic division, which occurs without increasing intermediate growth. This process is used by Thiomargarita namibiensis in stressful conditions caused by fasting.
The bacterium was discovered in bottom sediments of the flattened margin of the mainland, near the Namibian coast, by Heide Schultz, a German biologist and her colleagues in 1997, and in 2005, in the cold mice of the bottom of the Gulf of Mexico, a similar strain was found, which is confirmation of the wide distribution of the Namibian sulfur pearl ...
Victor Ostrovsky, Samogo.Net
