From the point of view of modern science, prokaryotes have a primitive structure. But it is precisely this "simplicity" that helps them survive in the most unexpected conditions. For example, in hydrogen sulfide sources or at nuclear test sites. Scientists have calculated that the total mass of all terrestrial microorganisms is 550 billion tons.
Bacteria are unicellular... This does not mean, however, that bacterial cells fall behind the cells of animals or plants. Microbiology already has knowledge of hundreds of thousands of microbial species. Nevertheless, representatives of science discover new types and features of them every day.
It is no wonder that for the full development of the Earth's surface, microorganisms have to take various forms:
The size of bacteria is measured in nanometers and micrometers. Their average value is 0.8 microns. But among them there are giant prokaryotes, reaching 125 microns and more. The real giants among the midgets are the spirochetes 250 microns long. Now compare with them the size of the smallest prokaryotic cell: mycoplasmas "grow" quite a bit and reach 0.1-0.15 microns in diameter.
It is worth saying that giant bacteria do not so easily survive in the environment. They find it difficult to find enough nutrients for themselves to successfully perform their function. But on the other hand, they are not easy prey for bacteria-predators, which feed on their counterparts - single-celled microorganisms, "flowing around" and eating them.
Under unfavorable environmental conditions, bacteria form a capsule. The microcapsule fits snugly against the wall. It can only be seen with an electron microscope. The macrocapsule is often formed by pathogenic microbes (pneumococci). In Klebsiella pneumonia, a macrocapsule is always found.
The capsule-like membrane is a formation that is loosely associated with the cell wall. Thanks to bacterial enzymes, the capsule-like shell is covered with carbohydrates (exopolysaccharides) of the external environment, which ensures the adhesion of bacteria with different surfaces, even completely smooth ones. For example, streptococci, entering the human body, are capable of sticking to teeth and heart valves.
The functions of the capsule are diverse:
They provide immersion and ascent. In the soil, the bacterial cell moves along the soil channels.
The entire contents of a cell, with the exception of the nucleus and the cell wall, are called cytoplasm. The liquid, structureless phase of the cytoplasm (matrix) contains ribosomes, membrane systems, mitochondria, plastids and other structures, as well as reserve nutrients. The cytoplasm has an extremely complex, fine structure (layered, granular). Many interesting details of the cell structure have been revealed with the help of an electron microscope.
The outer lipoprotein layer of the protoplast of bacteria, which has special physical and chemical properties, is called the cytoplasmic membrane. All vital structures and organelles are located inside the cytoplasm. The cytoplasmic membrane plays a very important role - it regulates the entry of substances into the cell and the release of metabolic products outside. Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes.
In addition, synthesis of some constituent parts of the cell occurs in the membrane, mainly the components of the cell wall and capsule. Finally, the cytoplasmic membrane contains the most important enzymes (biological catalysts). The ordered arrangement of enzymes on membranes allows you to regulate their activity and prevent the destruction of some enzymes by others. Ribosomes are associated with the membrane - structural particles on which protein is synthesized. The membrane consists of lipoproteins. It is strong enough and can provide the temporary existence of a cell without a shell. The cytoplasmic membrane accounts for up to 20% of the dry mass of the cell.
In electronic photographs of thin sections of bacteria, the cytoplasmic membrane appears as a continuous strand about 75A thick, consisting of a light layer (lipids) enclosed between two darker ones (proteins). Each layer is 20-30A wide. Such a membrane is called elementary.
The cytoplasm of bacterial cells often contains granules of various shapes and sizes. However, their presence cannot be considered as some kind of permanent sign of a microorganism, usually it is largely associated with the physical and chemical conditions of the environment.
Many cytoplasmic inclusions are composed of compounds that serve as a source of energy and carbon. These storage substances are formed when the body is supplied with sufficient nutrients, and, conversely, are used when the body is placed in conditions that are less favorable in terms of nutrition.
In many bacteria, granules are composed of starch or other polysaccharides - glycogen and granulose. In some bacteria, when grown in a medium rich in sugars, droplets of fat are found inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules are composed of polymetaphosphate (a storage substance containing phosphoric acid residues). Polymetaphosphate serves as a source of phosphate groups and energy for the body. Bacteria are more likely to accumulate volutin in unusual feeding conditions, such as in a sulfur-free environment. Sulfur droplets are found in the cytoplasm of some sulfur bacteria.
There is a connection between the plasma membrane and the cell wall in the form of desmoses - bridges. The cytoplasmic membrane often gives invagination - invagination into the cell. These invaginations in the cytoplasm form special membrane structures called mesosomes.
Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside such membrane sacs. These structures perform a wide variety of functions in bacteria. Some of these structures are mitochondrial analogs.
Others perform the functions of the endoplasmic reticulum or the Golgi apparatus. The photosynthetic apparatus of bacteria is also formed by invagination of the cytoplasmic membrane. After invagination of the cytoplasm, the membrane continues to grow and forms stacks, which, by analogy with plant chloroplast granules, are called thylakoid stacks. In these membranes, which often fill most of the cytoplasm of a bacterial cell, pigments (bacteriochlorophyll, carotenoids) and enzymes (cytochromes) that carry out the process of photosynthesis are localized.
Bacteria do not have such a nucleus as in higher organisms (eukaryotes), but have its analogue - the "nuclear equivalent" - the nucleoid, which is an evolutionarily more primitive form of organization of nuclear matter. It consists of one closed in a ring double-stranded DNA strand with a length of 1.1-1.6 nm, which is considered as a single bacterial chromosome, or genophore. The nucleoid in prokaryotes is not delimited from the rest of the cell by a membrane - it does not have a nuclear envelope.
The structure of the nucleoid includes RNA polymerase, basic proteins and no histones; the chromosome is fixed on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosomes. The bacterial chromosome replicates in a polyconservative way: the parental DNA double helix unwinds and a new complementary strand is assembled on the template of each polynucleotide chain. The nucleoid does not have a mitotic apparatus, and the divergence of daughter nuclei is provided by the growth of the cytoplasmic membrane.
The bacterial nucleus is a differentiated structure. Depending on the stage of development of the cell, the nucleoid can be discrete (discontinuous) and consist of separate fragments. This is due to the fact that the division of a bacterial cell in time occurs after the completion of the replication cycle of the DNA molecule and the formation of daughter chromosomes.
The nucleoid contains the bulk of the genetic information of the bacterial cell. In addition to the nucleoid, extrachromosomal genetic elements - plasmids, represented by small circular DNA molecules capable of autonomous replication, were found in the cells of many bacteria.
Plasmids are self-contained, coiled, double-stranded DNA molecules. Their mass is much less than the mass of a nucleotide. Despite the fact that hereditary information is encoded in the DNA of plasmids, they are not vital and necessary for the bacterial cell.
The cytoplasm of bacteria contains ribosomes - protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cell. Ribosomes are made up of RNA and protein. In bacteria, many ribosomes are located freely in the cytoplasm, some of them may be associated with membranes.
Ribosomes are the centers of protein synthesis in the cell. Moreover, they often join together, forming aggregates called polyribosomes or polysomes.
Inclusions are metabolic products of nuclear and non-nuclear cells. They represent a supply of nutrients: glycogen, starch, sulfur, polyphosphate (valutin), etc. Inclusions often acquire a different appearance than the color of the dye when colored. Values can be used to diagnose diphtheria bacillus.
Since a bacterium is a prokaryotic microorganism, many organelles are always absent in bacterial cells, which are inherent in eukaryotic organisms:
It is also worth remembering that bacteria do not have a cell wall, therefore, processes such as pinocytosis and phagocytosis cannot proceed.
As a special microorganism, bacteria are adapted to exist in conditions where oxygen may be absent. And the very same breathing in them occurs due to the mesosomes. It is also very interesting that green organisms are able to photosynthesize in exactly the same way as plants. But it is important to take into account that in plants the process of photosynthesis occurs in chloroplasts, and in bacteria, on membranes.
Reproduction in a bacterial cell occurs in the most primitive way. A mature cell divides in two, they reach maturity after a while, and this process is repeated. In favorable conditions, a change of 70-80 generations can occur per day. It is important to remember that bacteria, due to their structure, do not have access to such methods of reproduction as mitosis and meiosis. They are inherent only in eukaryotic cells.
It is known that the formation of spores is one of several ways of propagation of fungi and plants. But bacteria also know how to form spores, which is inherent in few of their species. They have this ability in order to survive particularly adverse conditions that can be dangerous to their lives.
Such species are known that are able to survive even in space. This can not be repeated by any living organisms. Bacteria became the progenitors of life on Earth due to their simplicity of structure. But the fact that they exist to this day shows how important they are for the world around us. With their help, people can get as close as possible to the answer to the question of the origin of life on Earth, constantly studying bacteria and learning something new.
Staphylococcus aureus (Staphylococcus aureus) is a common bacteria that affects about 30 percent of all people. In some people, it is part of the microbiome (microflora), and is found both inside the body and on the skin or mouth. While there are harmless strains of Staphylococcus aureus, others, such as Methicillin-resistant Staphylococcus aureus, pose serious health problems, including skin infections, cardiovascular disease, meningitis, and digestive diseases.
Researchers at Vanderbilt University found that staphylococcus bacteria prefer human blood over animal blood. These bacteria are partial to iron, which is found in hemoglobin found in red blood cells. Staphylococcus aureus ruptures blood cells to reach the iron inside them. It is believed that genetic variations in hemoglobin may make some people more desirable to Staphylococcus bacteria than others.
The researchers found that bacteria in the atmosphere may play a role in the production of rain and other forms of rainfall. This process begins when bacteria from plants are blown into the atmosphere. At high altitude, ice forms around them and they begin to grow. Once the frozen bacteria reach a certain growth threshold, the ice begins to melt and returns to the ground in the form of rain. The bacteria of the species Psuedomonas syringae have even been found in the center of large hail particles. They produce a special protein in cell membranes that allows them to bind water in a unique way, promoting ice formation.
Researchers have found that certain strains of acne-causing bacteria can actually help prevent acne. The bacteria that causes acne, Propionibacterium acnes, lives in the pores of our skin. When these bacteria provoke an immune response, the area on the skin swells and pimples form.
However, some strains of bacteria have been found to be less likely to cause acne. These strains may be the reason why people with healthy skin rarely get acne. By studying the genes of the Propionibacterium acnes strains collected from people with acne and healthy skin, the researchers identified a strain that was common on clear skin and rarely found on skin with acne. Future research will include attempts to develop a drug that only kills the acne-causing strains of the bacteria Propionibacterium acnes.
Who would have thought that brushing your teeth regularly could help prevent heart disease? Earlier studies have found a link between gum disease and cardiovascular disease. Now scientists have found a specific link between these diseases.
Both bacteria and humans are thought to produce certain types of proteins called stress proteins. These proteins are formed when cells experience various types of stressful conditions. When a person has a gum infection, the cells of the immune system begin to attack the bacteria. Bacteria produce stress proteins when attacked, and white blood cells also attack stress proteins.
The problem is that white blood cells cannot differentiate between stress proteins produced by bacteria and those produced by the body. As a result, the cells of the immune system also attack the stress proteins produced by the body, which causes the accumulation of white blood cells in the arteries and leads to atherosclerosis. A calcified heart is the leading cause of cardiovascular disease.
Did you know that spending time in the garden or gardening can help you learn better? According to the researchers, the soil bacterium Mycobacterium vaccae can improve learning in mammals.
Probably, these bacteria enter our body by swallowing or breathing. According to scientists, the bacterium Mycobacterium vaccae improves learning by stimulating the growth of neurons in the brain, which leads to an increase in serotonin levels and a decrease in anxiety.
The study was carried out using mice fed with live bacteria, Mycobacterium vaccae. The results showed that mice eating the bacteria moved the maze much faster and with less anxiety than mice that did not eat the bacteria. Scientists suggest that Mycobacterium vaccae plays a role in improving new challenges and reducing stress levels.
Researchers at Argonne National Laboratory have found that the bacterium Bacillus subtilis has the ability to rotate very small gears. These bacteria are aerobic, meaning they need oxygen to grow and develop. When they are placed in a solution with microbubbles of air, bacteria float in the teeth of the gear and cause it to turn in a specific direction.
It takes several hundred bacteria working in unison to start the gear. It was also found that bacteria can turn several gears connected to each other. The researchers were able to control the speed at which the bacteria turned the gears by adjusting the amount of oxygen in the solution. The decrease in the amount of oxygen led to a slowdown in bacteria. Removing oxygen causes them to stop moving completely.
Despite the apparent simplicity, bacteria are complex organisms. Bacterial cells are composed of a protoplast and a membrane.
The main structural elements of a bacterial cell are: the cell wall, the cytoplasmic membrane, the cytoplasm with inclusions, and the nucleus, called the nucleoid. Bacteria can also have additional structures: capsule, microcapsule, mucus, flagella. Many bacteria can form spores.
The cell wall is a strong, elastic structure that gives bacteria a certain shape and keeps high osmotic pressure in the wall. It participates in the process of cell division and the transport of metabolites. The bacterial cell wall contains a small amount of polysaccharides, lipids and proteins. The cell wall of bacteria performs a number of functions: it is the outer barrier of the cell, which establishes the contact of the microorganism with the environment; possessing a high degree of strength, it can withstand the internal pressure of the protoplast in a hypotonic solution.
The cytoplasmic membrane is a three-layer structure and surrounds the outer part of the bacterial cytoplasm. It is an obligatory polyfunctional structural element of the cell. The cytoplasmic membrane makes up 8-15% of the dry mass of the cell. It participates in the regulation of osmotic pressure, transport of substances and energy metabolism of the cell (due to the enzymes of the electron transport chain, ATP-ase, etc.). Oxidative enzymes and electron transport enzymes are localized on the membrane. The chemical composition of the cytoplasmic membrane is represented by a protein-lipid complex, in which proteins account for 50 - 70%, lipids - 15 - 50%. A small amount of carbohydrates is found in the cytoplasmic membrane of some bacteria. The main lipid component of the membrane is phospholipids. The protein fraction of the cytoplasmic membrane is represented by structural proteins with enzymatic activity.
The structure of the cytoplasmic membrane of bacteria includes a liquid-mosaic model of membranes. According to this model, the membrane is formed by a fluid biolayer of lipids, which includes asymmetrically located protein molecules.
The cytoplasm of bacteria occupies the bulk of the cell and consists of soluble proteins. The cytoplasm is represented by structural elements: ribosomes, inclusions, and a nucleoid. Ribosomes of prokaryotes have a sedimentation constant of 70S. The ribosome diameter is 15 - 20 nm. The number of ribosomes in a bacterial cell can be different. Thus, in the fast-growing cell of Escherichia coli, there are about 15,000 ribosomes. The process of protein biosynthesis in a cell is carried out by polysomes. Sometimes a polysome contains several tens of ribosomes.
A nucleoid (a nucleus-like formation) is the equivalent of a nucleus in bacteria. The nucleoid is located in the central zone of bacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a ball. Unlike eukaryotes, the nucleus of bacteria does not have a nuclear envelope, nucleolus, and basic proteins. Often, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring. The nucleoid is detected in a light microscope after DNA staining by Feelgen or Giemsa methods.
Some bacteria (pneumococci, etc.) form a capsule - a mucous formation firmly connected to the cell wall, which has clearly defined external boundaries. In pure cultures of bacteria, the capsule is formed less frequently. It is detected with special staining methods that create a negative contrast of the capsule substance. The capsule is composed of polysaccharides, sometimes polypeptides. The capsule is hydrophilic, prevents bacterial phagocytosis. Many bacteria form a microcapsule, a mucous formation detected by electron microscopy.
The main function of the capsule is protective. It protects the cell from the action of various kinds of unfavorable environmental factors. In many bacteria, the capsule is covered with mucus on the outside. In soil microorganisms in hot arid climates, the mucous layer protects the cell from drying out.
In the protoplast, cytoplasm, nucleus-like formations and various inclusions are distinguished.
Cytoplasm (protoplasm) has a very complex, changing chemical composition. The main chemical compounds of the cytoplasm are proteins, nucleic acids, lipids; contains a large amount of water. microbiological prokaryotic bacterial cell
The thin surface layer of the cytoplasm adjacent to the membrane, which is denser than the rest of its mass, is called the cytoplasmic membrane (Fig. 2). It is semi-permeable and plays an important role in the metabolism between the cell and the environment. The cytoplasmic membrane consists of three layers: one lipid and two, adjacent to it on both sides, protein. It contains 60-65% protein and 35-40% lipids; many enzymes are localized in it.
Modern research methods using an electron microscope have shown that the cytoplasm is inhomogeneous. In addition to a structureless, semi-liquid, viscous mass in a colloidal state, it is in places permeated with membranes; it contains microscopic structurally formed particles of various shapes and sizes. These are ribosomes rich in ribonucleic acid (RNA) scattered in the cytoplasm in the form of small grains. They are composed of about 60% RNA and 40% protein. One bacterial cell contains thousands and tens of thousands of ribosomes; they carry out the synthesis of cell proteins.
In addition to ribosomes, special membrane (lamellar) structures of various shapes called mesosomes have been found. They are formed by branching and invading the cytoplasmic membrane into the cell cavity. In the mesosomes, the processes of oxidation of organic substances, which are a source of energy, take place; here substances with a large supply of energy are synthesized, for example, adenosine triphosphoric acid (ATP). Thus, bacterial mesosomes are analogs of the mitochondria of other organisms (yeast, plants, animals).
In addition to these formations, where the most important metabolic processes of the cell take place, the cytoplasm also contains a variety of inclusions that are reserve nutrients: grains of glycogen (starch-like substance), drops of fat, granules of volutin (metachromatin), consisting mainly of polyphosphates, etc. bacteria are dyes - pigments.
The nucleus, morphologically shaped and typical for cells of other organisms (eukaryotes), is absent in bacteria.
Modern research methods have made it possible to identify in the cells of true bacteria formations similar to the nucleus, which are called nucleoids. However, the nuclear substance concentrated in certain places of the cell (more often in the center) is not delimited from the cytoplasm by the membrane and the shape of these nucleus-like structures is unstable.
Bacteria and related organisms (spirochetes, mycoplasmas, actinomycetes), as they do not have a true nucleus, are called prokaryotes (prenuclear organisms).
The bacterial cell membrane, which is often called the cell wall, is dense, has a certain elasticity and elasticity. It determines the relative constancy of the shape of the cell, serves as protection against adverse external influences and participates in the metabolism of the cell. The shell is permeable to water and low molecular weight substances. In an electron microscope, it is easily distinguishable from the cytoplasm, has a layered structure.
The chemical composition of the shell is rather complex and heterogeneous in different bacteria; its supporting framework is a complex polysaccharide-peptide called murein (from Latin murus - wall). In addition to murein, there are other components: lipids, polypeptides, polysaccharides, teichoic acids, amino acids, in particular diaminopimelic, which is absent in other organisms. The ratio of these substances in the cell membranes of different bacteria varies considerably.
The difference in the chemical composition of the cell walls of bacteria affects their ability to stain according to the Gram method. On this basis, bacteria are distinguished gram-positive (staining) and gram-negative (not staining). The shells of gram-positive bacteria contain more polysaccharides, murein and teichoic acids. The shells of gram-negative bacteria have a multilayer structure, they have a high content of lipids in the form of lipoproteins and lipopolysaccharides.
The shell of some bacteria can become slimy. The mucous layer surrounding the membrane is very thin and approaches the limit of visibility under a conventional light microscope. It can reach a considerable thickness, forming a so-called capsule. Often, the size of the capsule is much larger than the size of the bacterial cell. The mucous membranes are sometimes so strong that the capsules of individual cells merge into mucous masses, in which bacterial cells (zoogley) are interspersed. The mucous substances produced by some bacteria are not retained as a compact mass around the cell membrane, but diffuse into the environment.
The chemical composition of mucus varies from species to species, but may be the same. The composition of the nutrient medium on which bacteria develop is of great importance. Various polysaccharides (dextrans, glucans, levans), as well as nitrogen-containing substances (such as polypeptides, protein polysaccharides, etc.) are found in the composition of bacterial mucus.
The intensity of mucus formation largely depends on environmental conditions. In many bacteria, mucus production is stimulated, for example, by cultivation at low temperatures. Slime-forming bacteria, when rapidly multiplying in liquid substrates, can turn them into a continuous mucous mass. A similar phenomenon, causing significant losses, is sometimes observed in the production of sugar in sugary beet extracts. The causative agent of this defect is the bacterium Leuconostoc mesenteroides. In a short time, the sugar syrup can turn into a viscous mucous mass. Meat, sausages, cottage cheese are subject to slime; viscous can be milk, pickles of pickled vegetables, beer, wine.
Bacteria are prokaryotes (Fig. 1.2) and differ significantly from plant and animal cells (eukaryotes). They belong to unicellular organisms and consist of a cell wall, cytoplasmic membrane, cytoplasm, nucleoid (essential components of a bacterial cell). Some bacteria can have flagella, capsules, spores (optional components of a bacterial cell).
Rice. 1.2. Combined schematic representation of a prokaryotic (bacterial) cell with flagella.
1 - granules of polyoxybutyric acid; 2 - fatty droplets; 3 - sulfur inclusions; 4 - tubular thylakoids; 5 - lamellar thylakoids; 6 - bubbles; 7 - chromatophores; 8 - nucleus (nucleoid); 9 - ribosomes; 10 - cytoplasm; 11 - basal body; 12 - flagella; 13 - capsule; 14 - cell wall; 15 - cytoplasmic membrane; 16 - mesosome; 17 - gas vacuoles; 18 - lamellar structures; 19 -granules of a polysaccharide; 20 - polyphosphate granules
Rice. 1.3. Schematic representation of the monolayer structure of peptidoglycan
Rice. 1.4. Detailed structure of peptidoglycan structure Light and black short arrows indicate bonds cleaved by lysozyme (muramidase) and specific muroendopeptidase, respectively
Gram-negative bacteria with a thin layer of peptidoglycan (5-10%) in the cell wall after the action of alcohol lose gentian violet and are additionally stained pink with fuchsin. Cell walls in gram-positive and gram-negative prokaryotes differ sharply both in chemical composition (Table 1.1) and in ultrastructure (Fig. 1.5).
Rice. 1.5. Schematic representation of the cell wall in gram-positive (a) and gram-negative (b) prokaryotes: 1 - cytoplasmic membrane; 2 - peptidoglycan; 3 - periplasmic space; 4 - outer membrane; 5 - DNA
Table 1.1. The chemical composition of the cell walls of gram-positive and gram-negative prokaryotes 
The cell wall in bacteria performs mainly form-forming and protective functions, provides rigidity, forms a capsule, and determines the ability of cells to adsorb phages.
All bacteria, depending on their relationship to Gram stain, are divided into gram-positive and gram-negative.
As a result of staining, gram-positive bacteria are colored purple, gram-negative - red.
The reason for the different ratio of bacteria to Gram stain is explained by the fact that after treatment with Lugol's solution, an alcohol-insoluble iodine complex with gentian violet is formed. This complex in gram-positive bacteria, due to the weak permeability of their walls, cannot diffuse, while in gram-negative bacteria it is easily removed by washing them with ethanol and then with water.
Bacteria, completely devoid of a cell wall, are called protoplasts, they have a spherical shape, have the ability to divide, breathe, synthesize proteins, nucleic acids, and enzymes. Protoplasts are unstable structures, very sensitive to changes in osmotic pressure, mechanical influences and aeration, do not have the ability to synthesize the constituent parts of the cell wall, are not infected with bacteria viruses (bacteriophages) and do not have active mobility.
If, under the influence of lysozyme and other factors, a partial dissolution of the cell wall occurs, then bacterial cells turn into spherical bodies, called spheroplasts.
Under the influence of some external factors, bacteria are able to lose the cell wall, forming L-forms (named after the D. Lister Institute, where they were first isolated); such transformation can be spontaneous (for example, in chlamydia) or induced, for example, under the influence of antibiotics. Stable and unstable L-forms are distinguished. The former are not capable of reversion, while the latter are reversed to their original forms after the removal of the causative factor.
Figure 1.6. Plasma membrane structure Two layers of phospholipid molecules facing each other with hydrophobic poles and covered with two layers of globular protein molecules.
Enzymes that catalyze the synthesis of peptidoglycan, cell wall proteins, and their own structures are localized on the CMP. The membrane is also the site of energy conversion during photosynthesis.
The periplasm can include up to 20% of all water in the cell; some enzymes (phosphatases, permeases, nucleases, etc.) and transport proteins, carriers of the corresponding substrates, are localized in it.
Ribosomes are submicroscopic ribonucleoprotein granules with a diameter of 15-20 nm. Ribosomes contain approximately 80-85% of all bacterial RNA. Ribosomes of prokaryotes have a sedimentation constant of 70 S. They are built of two particles: 30 S (small subunit) and 50 S (large subunit) (Fig. 1.7).
Rice. 1.7. Ribosome (a) and its subunits - large (b) and small (c) Ribosomes serve as a site for protein synthesis.
In some bacteria, there are crystals of a protein nature in the cytoplasm, which have a poisonous effect on insects.
Some bacteria are able to accumulate phosphoric acid in the form of polyphosphate granules (volutin grains, metachromatic grains). They play the role of phosphate depots and are detected in the form of dense formations in the form of a ball or ellipse, located mainly at the poles of the cell. Usually, there is one granule at the poles.
A small amount of RNA and RNA polymerase is associated with DNA. DNA is coiled around a central RNA rod and is a highly ordered, compact structure. The chromosomes of most prokaryotes have a molecular weight in the range of 1-3x109, the sedimentation constant is 1300-2000 S. A DNA molecule includes 1.6x10 nucleotide pairs. Differences in the genetic apparatus of prokaryotic and eukaryotic cells determine its name: in the former, it is a nucleoid (formation similar to the nucleus), in contrast to the nucleus in the latter.
The nucleoid of bacteria contains the main hereditary information, which is realized in the synthesis of specific protein molecules. The systems of replication, repair, transcription and translation are associated with the DNA of a bacterial cell.
A nucleoid in a prokaryotic cell can be detected in stained preparations using a light or phase contrast microscope.
In many bacteria, extrachromosomal genetic elements - plasmids - are found in the cytoplasm. They are double-stranded DNA closed in rings, consisting of 1,500-40,000 base pairs and containing up to 100 genes.
The base of the flagellum is a long spiral filament (fibril), which at the surface of the cell wall transforms into a thickened curved structure - a hook and attaches to the basal granule embedded in the cell wall and the CPM (Fig. 1.8).
Rice. 1.8. Schematic model of the basal end of the E. coli flagellum based on electron micrographs of the isolated organelle
Flagella are almost entirely made up of flagellin protein with some carbohydrate and RNA content.
Rice. 1.9. Typical forms of spore-forming cells.
In terms of chemical composition, the difference between spores and vegetative cells is only in the quantitative content of chemical compounds. Spores contain less water and more lipids.
In a state of spore, microorganisms are metabolically inactive, withstand high temperatures (140-150 ° C) and exposure to chemical disinfectants, and persist in the environment for a long time.
Once in the nutrient medium, spores germinate into vegetative cells. The spore germination process includes three stages: activation, initial stage and growth stage. The activating agents that violate the state of rest include elevated temperature, acidic reaction of the environment, mechanical damage, etc. The spore begins to absorb water and, with the help of hydrolytic enzymes, destroys many of its own structural components. After the destruction of the outer layers, a period of formation of a vegetative cell begins with the activation of biosynthesis, which ends with cell division.
L.V. Timoshchenko, M.V. Chubik
The structural components of a cell are the bacterial membrane, which consists of a cell wall, a cytoplasmic membrane, and sometimes a capsule; cytoplasm; ribosomes; various cytoplasmic inclusions; nucleoid (nucleus). In addition, some types of bacteria have spores, flagella, cilia (pili, fimbria) (Fig. 2).
Cell wall obligatory formation of bacteria of most species. Its structure depends on the type and belonging.
bacteria to groups differentiated by Gram staining. The mass of the cell wall is about 20% of the dry mass of the entire cell, the thickness is from 15 to 80 nm.
Rice. 3. Diagram of the structure of a bacterial cell
1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - cytoplasm; 5 - mesosomes; 6 - ribosomes; 7 - nucleoid; 8 - intracytoplasmic membrane formations; 9 - fatty drops; 10 - polysaccharide granules; 11 - polyphosphate granules; 12 - sulfur inclusions; 13 - flagella; 14 - basal body
The cell wall has pores up to 1 nm in diameter, so it is a semipermeable membrane through which nutrients penetrate and metabolic products are released.
These substances can penetrate into the microbial cell only after preliminary hydrolytic cleavage by specific enzymes secreted by bacteria into the external environment.
The chemical composition of the cell wall is not uniform, but it is constant for a certain type of bacteria, which is used for identification. The cell wall contains nitrogenous compounds, lipids, cellulose, polysaccharides, and pectin substances.
The most important chemical component of the cell wall is the complex polysaccharide peptide. It is also called peptidoglycan, glycopeptide, murein (from lat. murus - wall).
Murein is a structural polymer composed of glycan molecules formed by acetylglucosamine and acetylmuramic acid. Its synthesis is carried out in the cytoplasm at the level of the cytoplasmic membrane.
Peptidoglycan of the cell wall of various types has a specific amino acid composition and, depending on this, a certain chemotype, which is taken into account when identifying lactic acid and other bacteria.
In the cell wall of gram-negative bacteria, peptidoglycan is represented by one layer, while in the wall of gram-positive bacteria it forms several layers.
In 1884, Gram proposed a tissue staining method that was used to stain prokaryotic cells. If, during Gram staining, the fixed cells are treated with an alcohol solution of crystal violet paint, and then with a solution of iodine, then these substances form a stable colored complex with murein.
In homo-positive microorganisms, the colored violet complex does not dissolve under the influence of ethanol and, accordingly, does not fade; when stained with fuchsin (red paint), the cells remain stained dark violet.
In gram-negative microorganisms, gentian violet is dissolved in ethanol and washed out with water, and when stained with fuchsin, the cell turns red.
The ability of microorganisms to stain with analytical dyes and by the Gram method is called tinctorial properties . They must be studied in young (18-24 hour) cultures, since some gram-positive bacteria in old cultures lose their ability to stain positively according to the Gram method.
The significance of peptidoglycan lies in the fact that thanks to it, the cell wall has rigidity, i.e. elasticity, and is the protective framework of the bacterial cell.
When peptidoglycan is destroyed, for example, under the action of lysozyme, the cell wall loses its rigidity and collapses. The content of the cell (cytoplasm), together with the cytoplasmic membrane, takes on a spherical shape, that is, it becomes a protoplast (spheroplast).
Many synthesizing and destructive enzymes are associated with the cell wall. Cell wall components are synthesized in the cytoplasmic membrane and then transported to the cell wall.
Cytoplasmic membrane is located under the cell wall and fits snugly to its inner surface. It is a semi-permeable membrane that surrounds the cytoplasm and the inner contents of the protoplast cell. The cytoplasmic membrane is the thickened outer layer of the cytoplasm.
The cytoplasmic membrane is the main barrier between the cytoplasm and the environment, violation of its integrity leads to cell death. It contains proteins (50-75%), lipids (15-45%), in many species - carbohydrates (1-19%).
The main lipid component of the membrane is phospho- and glycolipids.
The cytoplasmic membrane, with the help of enzymes localized in it, performs various functions: synthesizes membrane lipids - components of the cell wall; membrane enzymes - selectively transfer various organic and inorganic molecules and ions through the membrane, the membrane participates in the transformation of cellular energy, as well as in the replication of chromosomes, in the transfer of electrochemical energy and electrons.
Thus, the cytoplasmic membrane provides selective entry into the cell and removal from it of various substances and ions.
Derivatives of the cytoplasmic membrane are mesosomes . These are spherical structures formed when the membrane is twisted into a curl. They are located on both sides - at the site of the formation of the cell septum or next to the zone of localization of nuclear DNA.
Mesosomes are functionally equivalent to the mitochondria of cells of higher organisms. They are involved in the redox reactions of bacteria, play an important role in the synthesis of organic substances, in the formation of the cell wall.
Capsule is a derivative of the outer layer of cell clumps and is a mucous membrane that surrounds one or more microbial cells. Its thickness can reach 10 microns, which is many times greater than the thickness of the bacteria itself.
The capsule has a protective function. The chemical composition of the capsule of bacteria is different. In most cases, it consists of complex polysaccharides, mucopolysaccharides, and sometimes polypeptides.
Capsule formation is usually a specific feature. However, the appearance of the microcapsule often depends on the culture conditions of the bacteria.
Cytoplasm- a complex colloidal system with a large amount of water (80-85%), in which proteins, carbohydrates, lipids, as well as mineral compounds and other substances are dispersed.
The cytoplasm is the content of a cell surrounded by a cytoplasmic membrane. It is divided into two functional parts.
One part of the cytoplasm is in the state of a sol (solution), has a homogeneous structure and contains a set of soluble ribonucleic acids, enzyme proteins and metabolic products.
The other part is represented by ribosomes, inclusions of various chemical nature, genetic apparatus, and other intracytoplasmic structures.
Ribosomes Are submicroscopic granules, which are spherical nucleoprotein particles with a diameter of 10 to 20 nm, a molecular weight of about 2-4 million.
Ribosomes of prokaryotes consist of 60% RNA (ribonucleic acid) located in the center, and 40 % the protein that covers the outside of the nucleic acid.
Cytoplasmic inclusions are metabolic products, as well as reserve products, due to which the cell lives in conditions of a lack of nutrients.
The genetic material of prokaryotes consists of a double strand of deoxyribonucleic acid (DNA) of a compact structure located in the central part of the cytoplasm and not separated from it by a membrane. The structure of bacterial DNA does not differ from eukaryotic DNA, but since it is not separated from the cytoplasm by a membrane, the genetic material is called nucleoid or genophore... Nuclear structures are spherical or horseshoe-shaped.
Controversy bacteria are dormant, not multiplying their form. They are formed inside the cell, they are round or oval formations. Spores are formed mainly by gram-positive bacteria, rod-shaped with aerobic and anaerobic respiration in old cultures, as well as in unfavorable environmental conditions (lack of nutrients and moisture, accumulation of metabolic products in the environment, changes in pH and temperature of cultivation, presence or absence of atmospheric oxygen, etc. etc.) can switch to an alternative development program, resulting in disputes. In this case, one spore is formed in the cell. This indicates that sporulation in bacteria is an adaptation for the preservation of a species (individual) and is not a way of their reproduction. The process of sporulation occurs, as a rule, in the external environment for 18-24 hours.
A mature spore is approximately 0.1 of the maternal cell volume. Spores in different bacteria differ in shape, size, and location in the cell.
Microorganisms in which the diameter of the spore does not exceed the width of the vegetative cell are called bacilli, bacteria that have spores, the diameter of which is 1.5-2 times larger than the cell diameter, are called clostridia.
Inside the microbial cell, the spore can be located in the middle - the central position, at the end - the terminal position and between the center and the end of the cell - the subterminal position.
Flagella bacteria are locomotor organs (organs of movement), with the help of which bacteria can move at a speed of up to 50-60 microns / s. At the same time, in 1 s, bacteria overlap their body length by 50-100 times. The length of the flagella exceeds the length of bacteria by 5-6 times. The thickness of the flagella is on average 12-30 nm.
The number of flagella, their size and location are constant for certain types of prokaryotes and therefore are taken into account when identifying them.
Depending on the number and location of flagella, bacteria are divided into monotrichs (monopolar monotrichs) - cells with one flagellum at one end, lophotrichs (monopolar polytrichs) - a bundle of flagella is located at one of the ends, amphitrichs (bipolar polytrichs) - flagella are located on each of poles, peritrichous - flagella are located over the entire surface of the cell (Fig. 4) and atrichs - bacteria devoid of flagella.
The nature of the movement of bacteria depends on the number of flagella, age, culture characteristics, temperature, the presence of various chemicals and other factors. Monotrichs have the greatest mobility.
Flagella are more often found in rod-shaped bacteria; they are not vital cell structures, since there are flagella-free variants of motile bacterial species.
The bacterial organism is represented by one single cell. The forms of bacteria are varied. The structure of bacteria differs from the structure of cells of animals and plants.
The cell lacks a nucleus, mitochondria and plastids. The carrier of hereditary information, DNA, is located in the center of the cell in a folded form. Microorganisms that do not have a real nucleus are classified as prokaryotes. All bacteria are prokaryotes.
It is believed that there are over a million species of these amazing organisms on earth. To date, about 10 thousand species have been described.
A bacterial cell has a wall, a cytoplasmic membrane, a cytoplasm with inclusions, and a nucleotide. Of the additional structures, some cells have flagella, pili (a mechanism for adhesion and retention on the surface) and a capsule. Under unfavorable conditions, some bacterial cells can form spores. The average size of bacteria is 0.5-5 microns.
Rice. 1. The structure of the bacterial cell.
Rice. 2. In the photo, the structure of the bacterial wall of gram-negative bacteria (left) and gram-positive (right).
Under unfavorable environmental conditions, bacteria form a capsule. The microcapsule fits snugly against the wall. It can only be seen with an electron microscope. The macrocapsule is often formed by pathogenic microbes (pneumococci). In Klebsiella pneumonia, a macrocapsule is always found.
Rice. 3. The photo shows pneumococcus. The arrows indicate the capsule (electron diffraction pattern of an ultrathin section).
The capsule-like membrane is a formation that is loosely associated with the cell wall. Thanks to bacterial enzymes, the capsule-like shell is covered with carbohydrates (exopolysaccharides) of the external environment, which ensures the adhesion of bacteria with different surfaces, even completely smooth ones.
For example, streptococci, entering the human body, are capable of sticking to teeth and heart valves.
The functions of the capsule are diverse:
Rice. 4. Streptococci are capable of sticking to the enamel of the teeth and together with other microbes are the cause of caries.
Rice. 5. The photo shows the defeat of the mitral valve in rheumatism. The reason is streptococci.
They provide immersion and ascent. In the soil, the bacterial cell moves along the soil channels.
Rice. 6. Scheme of attachment and operation of the flagellum.
Rice. 7. In the photo there are different types of flagellar microbes.
Rice. 8. In the photo there are different types of flagellar microbes.
Rice. 9. In the photo E. coli. Flagella and drank are visible. Photo taken with a tunneling microscope (STM).
Rice. 10. The photo shows numerous pili (fimbriae) in cocci.
Rice. 11. The photo shows a bacterial cell with fimbria.
Rice. 12. The photo clearly shows a thin cell wall (CS), a cytoplasmic membrane (CPM) and a nucleotide in the center (Neisseria catarrhalis bacterium).
Rice. 13. The photo shows the structure of a bacterial cell. The structure of a bacterial cell differs from the structure of cells of animals and plants - the cell lacks a nucleus, mitochondria and plastids.
The cytoplasm is 75% water, the remaining 25% is mineral compounds, proteins, RNA and DNA. The cytoplasm is always dense and motionless. It contains enzymes, some pigments, sugars, amino acids, a supply of nutrients, ribosomes, mesosomes, granules and all sorts of other inclusions. In the center of the cell, a substance is concentrated that carries hereditary information - a nucleoid.
Granules are made up of compounds that are a source of energy and carbon.
Mesosomes are derived cells. They have different shapes - concentric membranes, vesicles, tubules, loops, etc. Mesosomes have a connection with the nucleoid. Participation in cell division and sporulation is their main purpose.
The nucleoid is analogous to the nucleus. It is located in the center of the cell. DNA is localized in it - the carrier of hereditary information in a folded form. The unwound DNA reaches a length of 1 mm. The nuclear substance of a bacterial cell does not have a membrane, a nucleolus and a set of chromosomes; it does not divide by mitosis. The nucleotide is doubled before division. During division, the number of nucleotides increases to 4.
Rice. 14. The photo shows a section of a bacterial cell. A nucleotide is visible in the central part.
Plasmids are self-contained, coiled, double-stranded DNA molecules. Their mass is much less than the mass of a nucleotide. Despite the fact that hereditary information is encoded in the DNA of plasmids, they are not vital and necessary for the bacterial cell.
Rice. 15. The photo shows a bacterial plasmid. Photo taken with an electron microscope.
The ribosomes of a bacterial cell are involved in the synthesis of protein from amino acids. Ribosomes of bacterial cells are not united into the endoplasmic reticulum, as in cells with a nucleus. It is the ribosomes that often become the "target" for many antibacterial drugs.
Inclusions are metabolic products of nuclear and non-nuclear cells. They represent a supply of nutrients: glycogen, starch, sulfur, polyphosphate (valutin), etc. Inclusions often acquire a different appearance than the color of the dye when colored. Values can be diagnosed.
The shape of a bacterial cell and its size are of great importance for their identification (recognition). The most common forms are spherical, rod-shaped and crimped.
Table 1. The main forms of bacteria.
Globular bacteria are called cocci (from the Greek coccus - grain). They are arranged one by two (diplococci), in packages, in chains and like bunches of grapes. This arrangement depends on the way the cell is dividing. The most harmful microbes are staphylococci and streptococci.
Rice. 16. In the photo there are micrococci. The bacteria are round, smooth, white, yellow and red in color. In nature, micrococci are ubiquitous. They live in different cavities of the human body.
Rice. 17. In the photo the bacteria are diplococci - Streptococcus pneumoniae.
Rice. 18. In the photo, the bacteria sarcina. Coccoid bacteria are combined into bags.
Rice. 19. In the photo there are streptococci bacteria (from the Greek "strepto" - a chain).
Arranged in chains. They are the causative agents of a number of diseases.
Rice. 20. In the photo the bacteria are "golden" staphylococci. They are arranged like "bunches of grapes". The clusters are golden in color. They are the causative agents of a number of diseases.
The spore-forming rod-shaped bacteria are called bacilli. They are cylindrical in shape. The most prominent representative of this group is the bacillus. Bacillus include plague and hemophilus influenza sticks. The ends of the rod-shaped bacteria can be pointed, rounded, chopped off, widened, or split. The shape of the sticks themselves can be correct or incorrect. They can be located one at a time, two at a time, or form chains. Some bacilli are called coccobacilli because they are round in shape. But, nevertheless, their length exceeds the width.
Diplobacilli are double sticks. Anthrax rods form long threads (chains).
Spore formation changes the shape of the bacilli. In the center of the bacillus, spores form in butyric bacteria, giving them the appearance of a spindle. In tetanus rods - at the ends of the bacilli, giving them the appearance of drumsticks.
Rice. 21. The photo shows a rod-shaped bacterial cell. Multiple flagella are visible. Photo taken with an electron microscope. Negative.
Rice. 24. In butyric bacilli, spores form in the center, giving them the appearance of a spindle. At tetanus sticks - at the ends, giving them the appearance of drumsticks.
The bend of the cell has no more than one turn. Several (two, three or more) are campylobacter. Spirochetes have a peculiar appearance, which is reflected in their name - "spira" - bend and "hate" - mane. Leptospira ("leptos" - narrow and "spine" - gyrus) are long filaments with closely spaced curls. The bacteria resemble a coiled spiral.
Rice. 27. In the photo, a spiral-shaped bacterial cell is the causative agent of "rat bite disease".
Rice. 28. In the photo, the bacteria Leptospira are the causative agents of many diseases.
Rice. 29. In the photo, the bacteria Leptospira are the causative agents of many diseases.
Corynebacteria, the causative agents of diphtheria and listeriosis, are club-shaped. This form of bacteria is given by the arrangement of metachromatic grains at its poles.
Rice. 30. In the photo there are corynebacteria.
Read more about bacteria in the articles:
Bacteria have lived on planet Earth for over 3.5 billion years. During this time they have learned a lot and have adapted to a lot. The total mass of bacteria is enormous. It is about 500 billion tons. Bacteria have mastered almost all known biochemical processes. The forms of bacteria are varied. The structure of bacteria has become quite complicated over millions of years, but even today they are considered the most simply arranged unicellular organisms.
