Cellular structure of bacteria. Structure and chemical composition of a bacterial cell

From point of view modern science Prokaryotes have a primitive structure. But it is precisely this “unpretentiousness” that helps them survive in the most unexpected conditions. For example, in hydrogen sulfide sources or at nuclear test sites. Scientists have calculated that total weight of all terrestrial microorganisms is 550 billion tons.

Bacteria have a unicellular structure. But this does not mean that bacterial cells give in to animal or plant cells. Microbiology already has knowledge about hundreds of thousands of species of microorganisms. Nevertheless, representatives of science are discovering new types and features of them every day.

It is no wonder that in order to fully colonize the Earth’s surface, microorganisms have to take on various forms:

• cocci - balls;
• streptococci – chains;
• bacilli - rods;
• vibrios - curved commas;
• spirilla - spirals.

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 Lilliputians are the 250-micron-long spirochetes. 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 it is not so easy for giant bacteria to survive in the environment. It is difficult for them to find enough nutrients to successfully perform their function. But they are not easy prey for predator bacteria, which feed on their fellow unicellular microorganisms, “flowing around” and eating them.

External structure of bacteria

Cell wall

• The cell wall of a bacterial cell is its protection and support. It gives the microorganism its own specific shape.
• The cell wall is permeable. Nutrients pass inward and metabolic products pass through it.
• Some types of bacteria produce special mucus that resembles a capsule that protects them from drying out.
• Some cells have flagella (one or more) or villi that help them move.
• Bacterial cells that appear pink when Gram stained ( gram-negative), the cell wall is thinner and multilayered. Enzymes that help break down nutrients are released.
• Bacteria that appear violet on Gram staining ( gram-positive), the cell wall is thick. Nutrients that enter the cell are broken down in the periplasmic space (the space between the cell wall and the cytoplasmic membrane) by hydrolytic enzymes.
• There are numerous receptors on the surface of the cell wall. Cell killers - phages, colicins and chemical compounds - are attached to them.
• Wall lipoproteins in some types of bacteria are antigens called toxins.
• With long-term treatment with antibiotics and for a number of other reasons, some cells lose their membranes, but retain the ability to reproduce. They acquire a rounded shape - L-shape and can persist in the human body for a long time (cocci or tuberculosis bacilli). Unstable L-forms have the ability to return to their original form (reversion).

Capsule

Under unfavorable environmental conditions, bacteria form a capsule. The microcapsule adheres tightly to the wall. It can only be seen in an electron microscope. The macrocapsule is often formed by pathogenic microbes (pneumococci). In Klebsiella pneumoniae, the macrocapsule is always found.

Capsule-like shell

The capsule-like shell is a formation loosely associated with the cell wall. Thanks to bacterial enzymes, the capsule-like shell is covered with carbohydrates (exopolysaccharides) from the external environment, which ensures the adhesion of bacteria to different surfaces, even completely smooth ones. For example, streptococci, when entering the human body, are able to stick to teeth and heart valves.

The functions of the capsule are varied:

• protection from aggressive environmental conditions,
• ensuring adhesion (sticking) to human cells,
• Possessing antigenic properties, the capsule has a toxic effect when introduced into a living organism.

Flagella

• Some bacterial cells have flagella (one or more) or villi that help them move. The flagella contain the contractile protein flagellin.
• The number of flagella can be different - one, a bundle of flagella, flagella at different ends of the cell or over the entire surface.
• Movement (random or rotational) occurs as a result rotational movement flagella.
• The antigenic properties of flagella have a toxic effect in disease.
• Bacteria that do not have flagella, when covered with mucus, are able to glide. Aquatic bacteria contain 40–60 vacuoles filled with nitrogen.

They provide diving and ascent. In the soil, the bacterial cell moves through soil channels.

Drank

• Pili (villi, fimbriae) cover the surface of bacterial cells. The villus is a helically twisted thin hollow thread of protein nature.
• General type drank provide adhesion (sticking) to host cells. Their number is huge and ranges from several hundred to several thousand. From the moment of attachment, any infectious process begins.
• Sex drank facilitate the transfer of genetic material from the donor to the recipient. Their number is from 1 to 4 per cell.

Cytoplasmic membrane

• The cytoplasmic membrane is located under the cell wall and is a lipoprotein (up to 30% lipids and up to 70% proteins).
• Different bacterial cells have different membrane lipid compositions.
• Membrane proteins perform many functions. Functional proteins are enzymes due to which the synthesis of its various components, etc. occurs on the cytoplasmic membrane.
• The cytoplasmic membrane consists of 3 layers. The phospholipid double layer is permeated with globulins, which ensure the transport of substances into the bacterial cell. If its function is disrupted, the cell dies.
• The cytoplasmic membrane takes part in sporulation.

Internal structure of bacteria

Cytoplasm

The entire contents of a cell, with the exception of the nucleus and 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). Using an electron microscope, many interesting details of the cell structure have been revealed.

The outer lipoprotoid layer of the bacterial protoplast, which has special physical and chemical properties, is called the cytoplasmic membrane. Inside the cytoplasm are all vital structures and organelles. The cytoplasmic membrane plays a very important role - it regulates the entry of substances into the cell and the release of metabolic products to the outside. Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes.

In addition, the membrane synthesizes some components cells, mainly 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 makes it possible to regulate their activity and prevent the destruction of some enzymes by others. Associated with the membrane are ribosomes - structural particles on which protein is synthesized. The membrane consists of lipoproteins. It is strong enough and can ensure the temporary existence of a cell without a shell. The cytoplasmic membrane makes up 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) sandwiched between two darker ones (proteins). Each layer is 20–30A wide. Such a membrane is called elementary.

Granules

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; it is usually largely related to 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 reserve substances are formed when the body is supplied with sufficient nutrients, and, conversely, are used when the body finds itself in conditions less favorable in terms of nutrition.

In many bacteria, granules consist of starch or other polysaccharides - glycogen and granulosa. Some bacteria, when grown in a sugar-rich medium, have droplets of fat inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules consist of polymetaphosphate (a reserve 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 under unusual nutritional conditions, such as sulfur-free media. In the cytoplasm of some sulfur bacteria there are droplets of sulfur.

Mesosomes

Between the plasma membrane and the cell wall there is a connection in the form of desmoses - bridges. The cytoplasmic membrane often gives rise to invaginations - protrusions into the cell. These invaginations form special membrane structures in the cytoplasm called mesosomes.

Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside these membrane sacs. These structures perform a variety of functions in bacteria. Some of these structures are analogues of mitochondria.

Others perform the functions of the endoplasmic reticulum or Golgi apparatus. By invagination of the cytoplasmic membrane, the photosynthetic apparatus of bacteria is also formed. 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 the bacterial cell, pigments (bacteriochlorophyll, carotenoids) and enzymes (cytochromes) that carry out the process of photosynthesis are localized.

Nucleoid

Bacteria do not have such a nucleus as 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 double-stranded DNA strand closed in a ring, 1.1–1.6 nm long, 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 lacks a nuclear envelope.

The nucleoid structures include RNA polymerase, basic proteins and lack histones; the chromosome is anchored on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosoms. The bacterial chromosome replicates in a polyconservative manner: the parent DNA double helix unwinds and a new complementary chain is assembled on the template of each polynucleotide chain. The nucleoid does not have a mitotic apparatus, and the separation of daughter nuclei is ensured by the growth of the cytoplasmic membrane.

The bacterial core is a differentiated structure. Depending on the stage of cell development, the nucleoid can be discrete (discontinuous) and consist of individual 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 are found in the cells of many bacteria - plasmids, which are small circular DNA molecules capable of autonomous replication.

Plasmids

Plasmids are autonomous molecules coiled into a ring of double-stranded DNA. Their mass is significantly 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.

Ribosomes

The cytoplasm of bacteria contains ribosomes - protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cage. Ribosomes consist of RNA and protein. In bacteria, many ribosomes are freely located in the cytoplasm, some of them may be associated with membranes.

Ribosomes are the centers of protein synthesis in the cell. At the same time, they often connect with each other, forming aggregates called polyribosomes or polysomes.

Inclusions

Inclusions are metabolic products of nuclear and non-nuclear cells. They represent a supply of nutrients: glycogen, starch, sulfur, polyphosphate (valutin), etc. Inclusions often, when painted, take on a different appearance than the color of the dye. The currency can be used to diagnose diphtheria bacillus.

What is missing in bacterial cells?

Since a bacterium is a prokaryotic microorganism, bacterial cells always lack many organelles, which are inherent in eukaryotic organisms:

• the Golgi apparatus, which helps the cell by accumulating unnecessary substances and subsequently removing them from the cell;
• plastids, contained only in plant cells, determine their color and also play a significant role in photosynthesis;
• lysosomes, which have special enzymes and help break down proteins;
• mitochondria provide cells with the necessary energy and also participate in reproduction;
• endoplasmic reticulum, which ensures transport of certain substances into the cytoplasm;
• cell center.

It is also worth remembering that bacteria do not have a cell wall, therefore processes such as pinocytosis and phagocytosis cannot occur.

Features of bacterial processes

Being special microorganisms, bacteria are adapted to exist in conditions where oxygen may be absent. But their breathing itself occurs due to mesosomes. It is also very interesting that green organisms are able to photosynthesize just like plants. But it is important to take into account that in plants the process of photosynthesis occurs in chloroplasts, while in bacteria it occurs on membranes.

Reproduction in a bacterial cell occurs in the most primitive way. A mature cell divides in two, after some time they reach maturity, and this process is repeated. Under 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 reproduction methods such as mitosis and meiosis. They are unique to eukaryotic cells.

It is known that the formation of spores is one of several ways of reproduction of fungi and plants. But bacteria are also able to form spores, which is characteristic of few of their species. They have this ability in order to worry especially not favorable conditions which may be life-threatening.

There are known species that can survive even in space conditions. This cannot be repeated by any living organism. Bacteria became the progenitors of life on Earth due to the simplicity of their structure. But the fact that they exist to this day shows how important they are to 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.

The most interesting and fascinating facts about bacteria

Staphylococcus bacteria thirst for human blood

Staphylococcus aureus is a common type of 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 in the mouth. While there are harmless strains of staph, others, such as Methicillin-resistant Staphylococcus aureus, create serious problems for health, including skin infections, cardiovascular diseases, meningitis and diseases of the digestive system.

Researchers at Vanderbilt University have discovered that staph bacteria prefer human blood over animal blood. These bacteria are partial to iron, which is contained in hemoglobin found in red blood cells. Staphylococcus aureus ruptures blood cells to get to the iron inside them. It is thought that genetic variations in hemoglobin may make some people more desirable to staph bacteria than others.

Bacteria cause rain

Researchers have discovered that bacteria in the atmosphere may play a role in the production of rain and other forms of precipitation. This process begins when bacteria from plants are carried by wind into the atmosphere. At altitude, ice forms around them and they begin to grow. Once frozen bacteria reach a certain growth threshold, the ice begins to melt and returns to the earth in the form of rain. Bacteria of the species Psuedomonas syringae have even been found in the center of large hail particles. They produce a special protein in their cell membranes that allows them to bind water in a unique way, promoting the formation of ice.

Fighting acne-causing bacteria

Researchers have found that certain strains of acne-causing bacteria may actually help prevent acne. The bacterium 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, certain 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 develop acne. By studying the genes of Propionibacterium acnes strains collected from people with acne and healthy skin, the researchers identified a strain that was common on clear skin and rare on skin with acne. Future research will include efforts to develop a drug that kills only the acne-causing strains of the bacterium Propionibacterium acnes.

Bacteria on gums can lead to heart disease

Who would have thought that regular teeth brushing could help prevent heart disease? Previous studies have found a link between gum disease and cardiovascular disease. Now scientists have found a specific connection 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 stress conditions. When a person has a gum infection, immune system cells 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 distinguish between stress proteins produced by bacteria and those produced by the body. As a result, immune system cells also attack stress proteins produced by the body, causing a buildup of white blood cells in the arteries and leading to atherosclerosis. A calcified heart is a leading cause of cardiovascular disease.

Soil bacteria improve learning

Did you know that spending time gardening or gardening can help you learn better? According to researchers, the soil bacterium Mycobacterium vaccae can improve learning in mammals.

These bacteria probably enter our body through ingestion or through breathing. Scientists suggest that the bacterium Mycobacterium vaccae improves learning by stimulating the growth of neurons in the brain, which leads to increased serotonin levels and reduced anxiety.

The study was conducted using mice that were fed live Mycobacterium vaccae bacteria. The results showed that mice that ate the bacteria moved through 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 problem solving and reducing stress levels.

Bacterial power machines

Researchers at Argonne National Laboratory have discovered that the bacterium Bacillus subtilis has the ability to turn very small gears. These bacteria are aerobic, meaning they require oxygen to grow and develop. When they are placed in a solution with micro air bubbles, the bacteria float on the gear teeth and cause it to turn in a certain direction.

It takes several hundred bacteria working in unison to start the gear turning. It was also discovered that bacteria can turn several interconnected gears. 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 oxygen caused the bacteria to slow down. Removing oxygen causes them to stop moving completely.

Structure of a bacterial cell differs significantly from the structure of a eukaryotic cell. Unlike eukaryotes, bacteria do not have a nucleus and, in most cases, any membrane-bound organelles. Their cell wall is structured completely differently than those of those eukaryotes that have a cell wall (plants and fungi). The genetic material of bacteria is also organized differently from eukaryotes: their DNA is not associated with histones, genes do not have introns and are often assembled into operons. The bacterial ribosome differs in mass and structure from the eukaryotic ribosome. Various aspects of bacterial cell structure are discussed in detail below.

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  The shapes of bacterial cells are not very diverse. Most often, bacterial cells are spherical (cocci) or rod-shaped (bacilli), some have a shape intermediate between spherical and rod-shaped and are called coccobacilli. Many bacteria have a filamentous or convoluted shape - comma-shaped (vibrios), spirals (spirilla) or elongated, twisted like a DNA helix (spirochetes). Often bacterial cells form stable combinations, such as pairs of rods (diplobacilli) or cocci (diplococci), chains of rods (streptobacilli) or cocci (streptococci), tetrads, packages of 4, 8 or more cocci (sarcinae), clusters (staphylococci). Some bacteria form rosettes, flat tablets, networks, as well as straight or branching trichomes - chains of cells tightly adjacent to each other. Bacteria are known with cells of very unusual shapes (for example, stellate), some bacteria ( Corynebacterium, Mycobacterium, Nocardia) change morphology during the life cycle. Actinobacteria form mycelium, representatives of the genus Hyphomicrobium form hyphae with buds. Cells of some bacteria (for example, Caulobacter) bear stalks and other appendages.

  Cell membrane

  Like any living cell, a bacterial cell is surrounded by a membrane, which is a lipid bilayer. The cell membrane maintains the osmotic balance of the cell, carries out various types of transport, including the secretion of proteins, is involved in the formation of the cell wall and the biosynthesis of extracellular polymers, and also receives regulatory signals from the external environment. In many cases, the cell membrane can participate in ATP synthesis due to the transmembrane electrochemical potential (proton motive force). The bacterial cell membrane is involved in the replication and separation of daughter bacterial chromosomes during cell division, as well as in the transfer of DNA through transduction or conjugation.

  In addition to lipids, bacterial membranes contain various proteins. In terms of chemical composition, bacterial cell membranes are much more diverse than the membranes of eukaryotic cells. Archaeal membrane lipids are represented by acyl- and alkyl-containing glycerolipids (including phospholipids), as well as polyisoprenoids. Unlike eukaryotes, which change the properties of the lipid backbone of the membrane by changing the ratio between phospholipids and cholesterol, bacteria change the properties of the membrane by varying the fatty acids that make up the lipids. Steroids are found extremely rarely in bacterial membranes, and instead of steroids, the membranes contain hopanoids, which are pentacyclic hydrocarbons. Hopanoids are actively involved in the regulation of the physical properties of bacterial cell membranes.

  Cell wall

  In gram-positive bacteria, a thick layer of peptidoglycan lies on top of the membrane, which forms the cell wall. In addition, the cell wall of gram-positive bacteria contains teichoic acids, which are attached to the cell surface, forming bonds with peptidoglycan. Lipoteichoic acids interact with fatty acid residues of the cell membrane. Teichoic and lipoteichoic acids are polyanions consisting of repeating units in the form of phosphorylated sugars or glycerol residues. Phosphate groups in teichoic acids can be replaced by glucuronate, resulting in the formation of teichuronic acids. Blocking the synthesis of teichoic acids leads to the death of bacteria, but the specific functions of these compounds have not been precisely established.

  Extracellular structures

  Intracellular structures

  Forms at rest

  Notes

  1. , With. 31.
  2. , With. 157-159.
  3. Young K.D. The selective value of bacterial shape. (English) // Microbiology And Molecular Biology Reviews: MMBR. - 2006. - September (vol. 70, no. 3). - P. 660-703. - DOI:10.1128/MMBR.00001-06. - PMID 16959965.[to correct ]
  4. , With. 35-36.
  5. Jiang C., Brown P. J., Ducret A., Brun Y. V. Sequential evolution of bacterial morphology by co-option of a developmental regulator. (English) // Nature. - 2014. - February 27 (vol. 506, no. 7489). - P. 489-493. - DOI:10.1038/nature12900. - PMID 24463524.[to correct ]
  6. , With. 181-182.
  7. , With. 170-177.
  8. Joseleau-Petit D. , Liébart J. C. , Ayala J. A. , D'Ari R.

  The obligatory organelles are: nuclear apparatus, cytoplasm, cytoplasmic membrane.

  Optional(minor) structural elements are: cell wall, capsule, spores, pili, flagella.

  1.In the center of the bacterial cell is nucleoid- a nuclear formation, most often represented by one ring-shaped chromosome. Consists of a double-stranded DNA strand. The nucleoid is not separated from the cytoplasm by the nuclear membrane.

  2.Cytoplasm- a complex colloidal system containing various inclusions of metabolic origin (grains of volutin, glycogen, granulosa, etc.), ribosomes and other elements of the protein synthesizing system, plasmids (extranucleoid DNA), mesosomes(formed as a result of invagination of the cytoplasmic membrane into the cytoplasm, participate in energy metabolism, sporulation, and the formation of the intercellular septum during division).

  3.Cytoplasmic membrane limits the cytoplasm on the outside, has a three-layer structure and performs a number of important functions - barrier (creates and maintains osmotic pressure), energy (contains many enzyme systems - respiratory, redox, carries out electron transfer), transport (transfer of various substances into the cell and from the cell).

  4.Cell wall- is inherent in most bacteria (except for mycoplasmas, acholeplasmas and some other microorganisms that do not have a true cell wall). It has a number of functions, primarily providing mechanical protection and a constant shape of cells; the antigenic properties of bacteria are largely associated with its presence. It consists of two main layers, of which the outer one is more plastic, the inner one is rigid.

  The main chemical compound of the cell wall, which is specific only to bacteria - peptidoglycan(mureic acids). An important characteristic for taxonomy of bacteria depends on the structure and chemical composition of the bacterial cell wall - Relation to Gram stain. In accordance with it, two large groups are distinguished: gram-positive (“gram +”) and gram-negative (“gram -”) bacteria. The wall of gram-positive bacteria after Gram staining retains the iodine complex with gentian violet(colored blue-violet), gram-negative bacteria lose this complex and the corresponding color after treatment and are colored pink due to staining with fuchsin.

  Features of the cell wall of gram-positive bacteria.

  A powerful, thick, uncomplicated cell wall, which is dominated by peptidoglycan and teichoic acids, no lipopolysaccharides (LPS), and often no diaminopimelic acid.

  
  

  Features of the cell wall of gram-negative bacteria.

  The cell wall is much thinner than that of gram-positive bacteria and contains LPS, lipoproteins, phospholipids, and diaminopimelic acid. The structure is more complex - there is an outer membrane, so the cell wall is three-layered.

  When gram-positive bacteria are treated with enzymes that destroy peptidoglycan, structures completely devoid of a cell wall appear - protoplasts. Treatment of gram-negative bacteria with lysozyme destroys only the peptidoglycan layer, without completely destroying the outer membrane; such structures are called spheroplasts. Protoplasts and spheroplasts have a spherical shape (this property is associated with osmotic pressure and is characteristic of all cell-free forms of bacteria).

  L-forms of bacteria.

  Under the influence of a number of factors that adversely affect the bacterial cell (antibiotics, enzymes, antibodies, etc.), L- transformation bacteria, leading to permanent or temporary loss of the cell wall. L-transformation is not only a form of variability, but also an adaptation of bacteria to unfavorable living conditions. As a result of changes in antigenic properties (loss of O- and K-antigens), a decrease in virulence and other factors, L-forms acquire the ability to remain for a long time ( persist) in the host’s body, maintaining a sluggish infectious process. The loss of the cell wall makes L-forms insensitive to antibiotics, antibodies and various chemotherapy drugs, the point of application of which is the bacterial cell wall. Unstable L-forms are capable reverse into classical (original) forms of bacteria that have a cell wall. There are also stable L-forms of bacteria, the absence of a cell wall and the inability to reverse into the classical forms of bacteria are genetically fixed. In a number of ways, they are very similar to mycoplasmas and other Mollicutes- bacteria that lack a cell wall as a taxonomic feature. Microorganisms belonging to mycoplasmas are the smallest prokaryotes, do not have a cell wall and, like all bacterial wallless structures, have a spherical shape.

  To the surface structures of bacteria(optional, like the cell wall), include capsule, flagella, microvilli.

  Capsule or a mucous layer surrounds the membrane of a number of bacteria. Highlight microcapsule, detected by electron microscopy in the form of a layer of microfibrils, and macrocapsule, detectable by light microscopy. The capsule is a protective structure (primarily from drying out); in a number of microbes it is a pathogenicity factor, prevents phagocytosis, and inhibits the first stages of protective reactions—recognition and absorption. U saprophytes capsules are formed in the external environment, in pathogens, more often in the host body. There are a number of methods for coloring capsules depending on their chemical composition. The capsule often consists of polysaccharides (the most common color is Ginsu), less often from polypeptides.

  Flagella. Motile bacteria can be gliding (move along a solid surface as a result of wave-like contractions) or floating, moving due to filament-like spirally curved proteins ( flagellinaceae by chemical composition) formations - flagella.

  Based on the location and number of flagella, a number of forms of bacteria are distinguished.

  1.Monotrichous - have one polar flagellum.

  2. Lophotrichs - have a polarly located bundle of flagella.

  3. Amphitrichy - have flagella at diametrically opposite poles.

  4. Peritrichous - have flagella along the entire perimeter of the bacterial cell.

  The ability for purposeful movement (chemotaxis, aerotaxis, phototaxis) in bacteria is genetically determined.

  Fimbriae or cilia- short threads, in large quantities surrounding bacterial cell, with the help of which bacteria are attached to substrates (for example, to the surface of mucous membranes). Thus, fimbriae are factors of adhesion and colonization.

  F- pili (fertility factor)- apparatus bacterial conjugation, are found in small quantities in the form of thin protein fibers.

  Endospores and sporulation.

  Sporulation- a method of preserving certain types of bacteria in unfavorable environmental conditions. Endospores are formed in the cytoplasm, are cells with low metabolic activity and high resistance ( resistance) to drying, chemical factors, high temperature and other unfavorable environmental factors. Light microscopy is often used to identify spores. according to Ozheshko. High resistance is associated with high content calcium salt of dipicolinic acid spores in the shell. The location and size of spores in different microorganisms differs, which has differential diagnostic (taxonomic) significance. The main phases of the “life cycle” of spores sporulation(includes preparatory stage, prespore stage, shell formation, maturation and dormancy) and germination, ending with the formation of a vegetative form. The process of sporulation is genetically determined.

  Unculturable forms of bacteria.

  Many species of gram-negative bacteria that do not form spores have a special adaptive state - uncultivable forms. They have low metabolic activity and do not actively reproduce, i.e. They do not form colonies on solid nutrient media and are not detected by culture. They are highly resistant and can remain viable for several years. Not detected by classical bacteriological methods, detected only using genetic methods ( polymerase chain reaction - PCR).

  The cell of prokaryotic organisms has a complex, strictly ordered structure and has fundamental features of ultrastructural organization and chemical composition.

  The structural components of a bacterial cell are divided into basic and temporary (Fig. 2). The main structures are: cell wall, cytoplasmic membrane with its derivatives, cytoplasm with ribosomes and various inclusions, nucleoid; temporary - capsule, mucous membrane, flagella, villi, endospores, formed only at certain stages of the bacterial life cycle; in some species they are completely absent.

  In a prokaryotic cell, the structures located outside the cytoplasmic membrane are called superficial (cell wall, capsule, flagella, villi).

  The term "envelope" is currently used to refer to the cell wall and capsule of bacteria or just the cell wall; the cytoplasmic membrane is not part of the envelope and refers to the protoplast.

  The cell wall is an important structural element of the bacterial cell, located between the cytoplasmic membrane and the capsule; in non-capsular bacteria it is the outer cell membrane. It is obligatory for all prokaryotes, with the exception of mycoplasmas and L-form bacteria. Performs a number of functions: protects bacteria from osmotic shock and other damaging factors, determines their shape, participates in metabolism; in many types of pathogenic bacteria it is toxic, contains surface antigens, and also carries specific receptors for phages on the surface. The bacterial cell wall contains pores that are involved in the transport of exotoxins and other bacterial exoproteins. The thickness of the cell wall is 10-100 nm, and it accounts for 5 to 50% of the dry matter of the cell.

  The main component of the bacterial cell wall is peptidoglycan, or murein (Latin murus - wall), a supporting polymer that has a network structure and forms a rigid (hard) outer framework of the bacterial cell. Peptidoglycan has a main chain (backbone) consisting of alternating N-acetyl-M-glucosamine and N-acetylmuramic acid residues connected by 1,4-glycosidic bonds, identical tetrapeptide side chains attached to N-acetylmuramic acid molecules, and short cross-peptide chains bridges connecting polysaccharide chains. The two types of bonds (glycosidic and peptide) that connect the peptidoglycan subunits give this heteropolymer a molecular network structure. The core of the peptidoglycan layer is the same in all bacterial species; Tetrapeptide protein chains and peptide (transverse) chains are different in different species.

  Based on their tinctorial properties, all bacteria are divided into two groups: gram-positive and gram-negative. In 1884, H. Gram proposed a staining method that was used to differentiate bacteria. The essence of the method is that gram-positive bacteria firmly fix the complex of gentian violet and iodine, are not subject to bleaching with ethanol and therefore do not perceive the additional dye fuchsin, remaining purple. In gram-negative bacteria, this complex is easily washed out of the cell by ethanol, and upon additional application of fuchsin, they turn red. In some bacteria, a positive Gram stain is observed only in the active growth stage. The ability of prokaryotes to be Gram stained or decolorized with ethanol is determined by the specific chemical composition and ultrastructure of their cell wall. Peptidoglycan in gram-positive bacteria is the main component of the cell wall and makes up from 50 to 90%, in gram-negative bacteria it is 1-10%. The structural microfibrils of peptidoglycan of Gram-negative bacteria are cross-linked less compactly, therefore the pores in their peptidoglycan layer are much wider than in the molecular framework of Gram-positive bacteria. With such a structural organization of peptidoglycan, the violet complex of gentian violet and iodine in gram-negative bacteria will be washed out faster.

  The cell wall of gram-positive bacteria is tightly adjacent to the cytoplasmic membrane, massive, and its thickness is in the range of 20-100 nm. It is characterized by the presence of teichoic acids; they are associated with peptidoglycan and are polymers of trihydric alcohol - glycerol or pentaatomic alcohol - ribitol, the residues of which are connected by phosphodiester bonds. Teichoic acids bind magnesium ions and participate in their transport into the cell. Polysaccharides, proteins and lipids are also found in small quantities in the cell wall of gram-positive prokaryotes.

  Rice. 2. Scheme of the structure of a prokaryotic cell:

  1 - capsule; 2 - cell wall; 3 - cytoplasmic membrane; 4 - nucleoid; 5 - cytoplasm; 6 - chromatophores; 7 - thylakoids; 8 - mesosoma; 9 - ribosomes; 10 - flagella; 11—basal body; 12 - drank; 13 - inclusion of sulfur; 14 — drops of fat; 15 — polyphosphate granules; 16 - plasmid

  The cell wall of gram-negative bacteria is multilayered, its thickness is 14-17 nm. The inner layer is peptidoglycan, which forms a thin (2 nm) continuous network surrounding the cell. Peptidoglycan contains only mesodiaminopimelic acid and no lysine. The outer layer of the cell wall - the outer membrane - consists of phospholipids, lipopolysaccharide, lipoprotein and proteins. The outer membrane contains matrix proteins, which are tightly bound to the peptidoglycan layer. One of their functions is the formation of hydrophilic pores in the membrane, through which diffusion of molecules with a mass of up to 600, sometimes 900 occurs. Matrix proteins, in addition, also act as receptors for some phages. Lipopolysaccharide (LPS) in the cell walls of Gram-negative bacteria consists of lipid A and a polysaccharide. LPS, which is toxic to animals, is called endotoxin. Teichoic acids have not been found in gram-negative bacteria.

  The structural components of the cell wall of Gram-negative bacteria are demarcated from the cytoplasmic membrane and separated by a space called the periplasm or periplasmic space.

  Protoplasts and spheroplasts. Protoplasts are forms of prokaryotes completely devoid of a cell wall, usually formed in gram-positive bacteria. Spheroplasts are bacteria with a partially destroyed cell wall. They retain elements of the outer membrane. They are observed in gram-negative bacteria and much less frequently in gram-positive bacteria. They are formed as a result of the destruction of the peptidoglycan layer by lytic enzymes, for example lysozyme, or blocking the biosynthesis of peptidoglycan with the antibiotic penicillin, etc. in an environment with the appropriate osmotic pressure.

  Protoplasts and spheroplasts have a spherical or hemispherical shape and are 3-10 times larger than the original cells. Under normal conditions, osmotic lysis occurs and they die. Under conditions of increased osmotic pressure, they are able to survive, grow and even divide for some time. When the factor that destroys peptidoglycan is removed, protoplasts, as a rule, die off, but can turn into L-forms; spheroplasts easily revert to the original bacteria, sometimes transform into L-forms or die.

  L-Forms of bacteria. These are phenotypic modifications, or mutants, of bacteria that have partially or completely lost the ability to synthesize cell wall peptidoglycan. Thus, L-forms are bacteria that are defective in the cell wall. They received their name due to the fact that they were isolated and described at the Lister Institute in England in 1935. They are formed under the influence of L-transforming agents - antibiotics (penicillin, polymyxin, bacitracin, vencomycin, streptomycin), amino acids (glycine, methionine, leucine, etc.), the enzyme lysozyme, ultraviolet and x-rays. Unlike protoplasts and spheroplasts, L-forms have relatively high viability and a pronounced ability to reproduce. In terms of morphological and cultural properties, they differ sharply from the original bacteria, which is due to the loss of the cell wall and changes in metabolic activity.

  L-forms of bacteria are polymorphic. There are elementary bodies measuring 0.2-1 microns (minimal reproductive elements), spheres - 1-5, large bodies - 5-50, threads - up to 4 microns or more. L-form cells have good developed system intracytoplasmic membranes and myelin-like structures. Due to a defect in the cell wall, they are osmotically unstable and can only be cultured in special media with high osmotic pressure; they pass through bacterial filters.

  There are stable and unstable L-forms of bacteria. The former are completely devoid of a rigid cell wall, which makes them similar to protoplasts; they extremely rarely revert to their original bacterial forms. The latter may have elements of a cell wall, in which they are similar to spheroplasts; in the absence of the factor that caused their formation, they are reverted to the original cells.

  The process of formation of L-forms is called L-transformation or L-induction. Almost all types of bacteria, including pathogenic ones (causative agents of brucellosis, tuberculosis, listeria, etc.), have the ability to undergo L-transformation.

  L-shapes are given great importance in the development of chronic recurrent infections, carriage of pathogens, their long-term persistence in the body. The transplacental invasiveness of elementary bodies of L-form bacteria has been proven.

  The infectious process caused by L-forms of bacteria is characterized by atypicality, duration of course, severity of the disease, and is difficult to treat with chemotherapy.

  The capsule is the mucous layer located above the cell wall of the bacterium. The substance of the capsule is clearly demarcated from the environment. Depending on the thickness of the layer and the strength of the connection with the bacterial cell, a macrocapsule with a thickness of more than 0.2 microns, clearly visible in a light microscope, and a microcapsule with a thickness of less than 0.2 microns, detectable only with an electron microscope or detected by chemical and immunological methods, are distinguished. The macrocapsule (true capsule) is formed by B. anlhracis, C1. perfringens, microcapsule - Escherichia coJi. The capsule is not an essential structure of the bacterial cell: its loss does not lead to the death of the bacterium. Capsuleless mutants of bacteria are known, for example the anthrax vaccine strain STI-1.

  The substance of the capsules consists of highly hydrophilic micelles, and their chemical composition is very diverse. The main components of most prokaryotic capsules are homo- or hetsropolysaccharides (entsrobacteria, etc.). In some types of bacilli, capsules are built from a polypeptide. Thus, the composition of the capsule of B. anthracis includes the D-glutamic acid polypeptide (dextrorotatory isomer). The composition of the microcapsule of mycobacterium tuberculosis of mammals includes glycopeptides presented ester trehalose and mycolic acid (cord factor).

  Capsule synthesis - difficult process and different prokaryotes have their own characteristics; It is believed that capsule biopolymers are synthesized on the outer surface of the cytoplasmic membrane and are released onto the surface of the cell wall in certain specific areas.

  There are bacteria that synthesize mucus, which is deposited on the surface of the cell wall in the form of a structureless layer of polysaccharide nature. The mucous substance surrounding the cell is often thicker than the diameter of the cell. In the saprophytic bacterium Leuconostoca, the formation of one capsule for many individuals is observed. Such accumulations of bacteria enclosed in a common capsule are called zooglea.

  The capsule is a multifunctional organelle that plays an important biological role. It is the site of localization of capsular antigens that determine the virulence, antigenic specificity and immunogenicity of bacteria. The loss of the capsule in pathogenic bacteria sharply reduces their virulence, for example, in noncapsular strains of the anthrax bacillus. Capsules ensure the survival of bacteria, protecting them from mechanical damage, drying out, infection by phages, toxic substances, and in pathogenic forms - from the action of the protective forces of the macroorganism: encapsulated cells are poorly phagocytosed. In some types of bacteria, including pathogenic ones, it promotes the attachment of cells to the substrate.

  In veterinary microbiology, detection of the capsule is used as a differential morphological sign of the pathogen when testing for anthrax.

  For coloring capsules, special methods are used - Romanovsky - Giemsa, Gins - Burri, Olt, Mikhin, etc.

  The microcapsule and mucous layer are determined by serological reactions (RA), the antigenic components of the capsule are identified using the immunofluorescence method (RIF) and RDD.

  Flagella are organelles of bacterial movement, represented by thin, long, thread-like structures of a protein nature. Their length exceeds the bacterial cell several times and is 10-20 microns, and in some spirilla it reaches 80-90 microns. The flagellum filament (fibril) is a complete spiral cylinder with a diameter of 12-20 nm. In Vibrios and Proteus, the filament is surrounded by a sheath 35 nm thick.

  The flagellum consists of three parts: a spiral filament, a hook and a basal body. The hook is a curved protein cylinder that acts as a flexible link between the basal body and the rigid filament of the flagellum. The basal body is a complex structure consisting of a central rod (axis) and rings.

  
  

  Rice. 3. Flagella:

  a - monotrichs; b - amphitrichs; c - lophotrichs; d - peritrichous

  Flagella are not vital structures of a bacterial cell: there are phase variations in bacteria, when they are present in one phase of cell development and absent in another. Thus, in the causative agent of tetanus in old cultures, cells without flagella predominate.

  Number of flagella (from I to 50 or more) and places of their localization in bacteria different types not the same, but stable for one species. Depending on this, the following groups of flagellated bacteria are distinguished: moiotrichs - bacteria with one polarly located flagellum; amphitrichous - bacteria with two polarly arranged flagella or having a bundle of flagella at both ends; lophotrichs - bacteria with a bundle of flagella at one end of the cell; peritrichs are bacteria with many flagella located on the sides of the cell or on its entire surface (Fig. 3). Bacteria that do not have flagella are called atrichia.

  Being organs of movement, flagella are typical of floating rod-shaped and convoluted forms of bacteria and are found only in isolated cases in cocci. They provide efficient movement in liquid media and slower movement on the surface of solid substrates. The speed of movement of monotrichs and lophotrichs reaches 50 μm/s, amphitrichy and peritrichs move more slowly and usually cover a distance equal to the size of their cell in 1 s.

  Bacteria move randomly, but they are capable of directed forms of movement - taxis, which are determined by external stimuli. Reacting to various environmental factors, bacteria are localized in an optimal habitat zone in a short time. Taxis can be positive and negative. It is customary to distinguish between: chemotaxis, aerotaxis, phototaxis, magnotaxis. Chemotaxis is caused by differences in the concentration of chemicals in the environment, aerotaxis by oxygen, phototaxis by light intensity, magnetotaxis is determined by the ability of microorganisms to navigate in a magnetic field.

  Identification of motile flagellar forms of bacteria is important for their identification in the laboratory diagnosis of infectious diseases.

  Pili (fimbriae, villi) are straight, thin, hollow protein cylinders 3-25 nm thick and up to 12 µm long, extending from the surface of the bacterial cell. They are formed by a specific protein - pilin, originate from the cytoplasmic membrane, are found in motile and immobile forms of bacteria and are visible only in an electron microscope (Fig. 4). On the surface of the cell there can be from 1-2, 50-400 or more pili to several thousand.

  

  Rice. 4. Drank

  There are two classes of pili: sexual pili (sexpili) and general pili, which are more often called fimbriae. The same bacterium can have pili of different natures. Sex pili appear on the surface of bacteria during the process of conjugation and perform the function of organelles through which genetic material (DNA) is transferred from donor to recipient.

  Pili of the general type are located peritrichially (Escherichia coli) or at the poles (pseudomonas); one bacterium can contain hundreds of them. They take part in the adhesion of bacteria into agglomerates, the attachment of microbes to various substrates, including cells (adhesive function), in the transport of metabolites, and also contribute to the formation of films on the surface of liquid media; cause agglutination of red blood cells.

  Cytoplasmic membrane and its derivatives. The cytoplasmic membrane (plasmolemma) is a semi-permeable lipoprotein structure of bacterial cells that separates the cytoplasm from the cell wall. It is an obligatory multifunctional component of the cell and makes up 8-15% of its dry mass. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell. Ultrathin sections in an electron microscope reveal its three-layer structure - two limiting osmiophilic layers, each 2-3 nm thick, and one osmiophobic central layer 4-5 nm thick.

  Chemically, the cytoplasmic membrane is a protein-lipid complex consisting of 50-75% proteins and 15-50% lipids. The main part of membrane lipids (70-90%) is represented by phospholipids. It is built from two monomolecular protein layers, between which there is a lipid layer consisting of two rows of regularly oriented lipid molecules.

  The cytoplasmic membrane serves as an osmotic barrier to the cell, controls the flow of nutrients into the cell and the release of metabolic products to the outside; it contains substrate-specific permease enzymes that carry out active selective transfer of organic and inorganic molecules.

  Cytoplasmic membrane enzymes catalyze the final steps in the synthesis of membrane lipids, cell wall components, capsule and exoenzymes; Oxidative phosphorylation enzymes and electron transport enzymes responsible for energy synthesis are localized on the membrane.

  During cell growth, the cytoplasmic membrane forms numerous invaginates that form intracytoplasmic membrane structures. Local membrane invaginates are called mesosomes. These structures are well expressed in gram-positive bacteria, worse in gram-negative bacteria, and poorly expressed in rickettsia and mycoplasmas.

  A connection between mesosomes and the bacterial chromosome has been established; such structures are called nucleoidosomes. Mesosomes integrated with the nucleoid take part in karyokinesis and cytokinesis of microbial cells, ensuring the distribution of the genome after the end of DNA replication and the subsequent divergence of daughter chromosomes. Mesosomes, like the cytoplasmic membrane, are centers of bacterial respiratory activity, so they are sometimes called analogues of mitochondria. However, the significance of mesosomes has not yet been fully elucidated. They increase the working surface of the membranes, perhaps performing only a structural function, dividing the bacterial cell into relatively separate compartments, which creates more favorable conditions for the occurrence of enzymatic processes. In pathogenic bacteria they ensure the transport of protein molecules of exotoxins.

  Cytoplasm is the contents of a bacterial cell, delimited by a cytoplasmic membrane. It consists of cytosol - a homogeneous fraction, including soluble RNA components, substrate substances, enzymes, metabolic products, and structural elements - ribosomes, intracytoplasmic membranes, inclusions and nucleoid.

  Ribosomes are organelles that carry out protein biosynthesis. They consist of protein and RNA, connected into a complex by hydrogen and hydrophobic bonds. Bacterial ribosomes are granules with a diameter of 15-20 nm, have a sedimentation constant of 70S and are formed from two ribonucleoprotein subunits: 30S and 50S. One bacterial cell can contain from 5000-50,000 ribosomes; through mRNA they are combined into polysome aggregates consisting of 50-55 ribosomes with high protein-synthesizing activity.

  Various types of inclusions are detected in the cytoplasm of bacteria. They can be solid, liquid or gaseous, with or without a protein membrane, and are not permanently present. A significant part of them are reserve nutrients and products of cellular metabolism. Reserve nutrients include: polysaccharides, lipids, polyphosphates, sulfur deposits, etc. Among inclusions of a polysaccharide nature, glycogen and the starch-like substance granulosa are most often found, which serve as a source of carbon and energy material. Lipids accumulate in cells in the form of granules and fat droplets; these include membrane-surrounded granules of poly-/3-hydroxybutyric acid, which sharply refract light and are clearly visible in a light microscope. Anthrax bacilli and aerobic spore-forming saprophytic bacteria are also detected. Mycobacteria accumulate waxes as reserve substances. The cells of some measles nonbacteria, spirilla and others contain volutin granules formed by polyphosphates. They are characterized by metachromasia: toluidine blue and methylene blue color them violet-red. Volutin granules play the role of phosphate depots.

  Inclusions surrounded by a membrane also include gas vacuoles, or aerosomes; they reduce the specific gravity of cells and are found in aquatic prokaryotes.

  Nucleoid is the nucleus of prokaryotes. It consists of one double-stranded DNA strand closed in a ring, 1.1-1.6 nm long, 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 lacks a nuclear envelope.

  The nucleoid structures include RNA polymerase, basic proteins and lack histones; the chromosome is anchored on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosoms. The bacterial chromosome replicates in a polyconservative manner: the parent DNA double helix unwinds and a new complementary chain is assembled on the template of each polynucleotide chain. The nucleoid does not have a mitotic apparatus, and the separation of daughter nuclei is ensured by the growth of the cytoplasmic membrane.

  The bacterial core is a differentiated structure. Depending on the stage of cell development, the nucleoid can be discrete (discontinuous) and consist of individual 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 are found in the cells of many bacteria - plasmids, which are small circular DNA molecules capable of autonomous replication.

  The general structure of a bacterial cell is shown in Figure 2. The internal organization of a bacterial cell is complex. Each systematic group of microorganisms has its own specific features buildings.

  
  
  

  Cell wall. The bacterial cell is covered with a dense membrane. This surface layer, located outside the cytoplasmic membrane, is called the cell wall (Fig. 2, 14). The wall performs protective and supporting functions, and also gives the cell a permanent, characteristic shape (for example, the shape of a rod or coccus) and represents the external skeleton of the cell. This dense shell makes bacteria similar to plant cells, which distinguishes them from animal cells, which have soft shells. Inside the bacterial cell, the osmotic pressure is several times, and sometimes tens of times, higher than in the external environment. Therefore, the cell would quickly rupture if it were not protected by such a dense, rigid structure as the cell wall.

  
  

  The thickness of the cell wall is 0.01-0.04 microns. It makes up from 10 to 50% of the dry mass of bacteria. The amount of material that makes up the cell wall changes during bacterial growth and usually increases with age.

  
  

  The main structural component of the walls, the basis of their rigid structure in almost all bacteria studied to date, is murein (glycopeptide, mucopeptide).

  
  

  ,
  ,
  

  
  

  This is an organic compound of a complex structure, which includes nitrogen-carrying sugars - amino sugars and 4-5 amino acids. Moreover, cell wall amino acids have an unusual shape (D-stereoisomers), which is rarely found in nature.

  
  

  The constituent parts of the cell wall, its components, form a complex, strong structure (Fig. 3, 4 and 5). gram-positive Using a staining method first proposed in 1884 by Christian Gram, bacteria can be divided into two groups: gram-negative And

  
  

  The chemical composition of the cell walls of gram-positive and gram-negative bacteria is different.

  
  

  In gram-positive bacteria, the composition of the cell walls includes, in addition to mucopeptides, polysaccharides (complex, high-molecular sugars), teichoic acids (complex compounds in composition and structure, consisting of sugars, alcohols, amino acids and phosphoric acid). Polysaccharides and teichoic acids are associated with the wall framework - murein. We do not yet know what structure these components of the cell wall of gram-positive bacteria form. Using electronic photographs of thin sections (layering), no gram-positive bacteria were detected in the walls. Probably all these substances are very tightly interconnected.

  
  

  The walls of gram-negative bacteria are more complex in chemical composition; they contain a significant amount of lipids (fats) associated with proteins and sugars into complex complexes - lipoproteins and lipopolysaccharides. There is generally less murein in the cell walls of gram-negative bacteria than in gram-positive bacteria. The wall structure of gram-negative bacteria is also more complex. Using an electron microscope, it was found that the walls of these bacteria are multilayered (Fig. 6).

  
  

  

  
  

  The inner layer consists of murein. Above this is a wider layer of loosely packed protein molecules. This layer is in turn covered with a layer of lipopolysaccharide. The topmost layer consists of lipoproteins.

  
  

  The cell wall is permeable: nutrients pass through it freely into the cell, and metabolic products exit into the cell. environment. Large molecules with high molecular weight do not pass through the shell.

  
  

  

  
  

  Capsule. The cell wall of many bacteria is surrounded on top by a layer of mucous material - a capsule (Fig. 7). The thickness of the capsule can be many times greater than the diameter of the cell itself, and sometimes it is so thin that it can only be seen through an electron microscope - a microcapsule.

  
  

  The capsule is not an essential part of the cell; it is formed depending on the conditions in which the bacteria find themselves. It serves as a protective cover for the cell and participates in water metabolism, protecting the cell from drying out.

  
  

  The chemical composition of capsules is most often polysaccharides. Sometimes they consist of glycoproteins (complex complexes of sugars and proteins) and polypeptides (genus Bacillus), in rare cases - of fiber (genus Acetobacter).

  
  

  Mucous substances secreted into the substrate by some bacteria cause, for example, the mucous-stringy consistency of spoiled milk and beer.

  
  

  Cytoplasm. The entire contents of a cell, with the exception of the nucleus and 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). With the help of an electron microscope, many interesting details of the cell structure have been revealed.

  
  

  ,
  

  
  

  The outer lipoprotoid layer of the bacterial protoplast, which has special physical and chemical properties, is called the cytoplasmic membrane (Fig. 2, 15).

  
  

  Inside the cytoplasm are all vital structures and organelles.

  
  

  The cytoplasmic membrane plays a very important role - it regulates the entry of substances into the cell and the release of metabolic products to the outside.

  
  

  Through the membrane, nutrients can enter the cell as a result of an active biochemical process involving enzymes. In addition, the synthesis of some cell components occurs in the membrane, mainly 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 makes it possible to regulate their activity and prevent the destruction of some enzymes by others. Associated with the membrane are ribosomes - structural particles on which protein is synthesized. The membrane consists of lipoproteins. It is strong enough and can ensure the temporary existence of a cell without a shell. The cytoplasmic membrane makes up 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) sandwiched between two darker ones (proteins). Each layer has a width of 20-30A. Such a membrane is called elementary (Table 30, Fig. 8).

  
  

  ,
  

  
  

  Between the plasma membrane and the cell wall there is a connection in the form of desmoses - bridges. The cytoplasmic membrane often gives rise to invaginations - invaginations into the cell. These invaginations form special membrane structures in the cytoplasm called mesosomes. Some types of mesosomes are bodies separated from the cytoplasm by their own membrane. Numerous vesicles and tubules are packed inside these membrane sacs (Fig. 2). These structures perform a variety of functions in bacteria. Some of these structures are analogues of mitochondria. Others perform the functions of the endoplasmic reticulum or Golgi apparatus. By invagination of the cytoplasmic membrane, the photosynthetic apparatus of bacteria is also formed. After invagination of the cytoplasm, the membrane continues to grow and forms stacks (Table 30), which, by analogy with plant chloroplast granules, are called thylakoid stacks. In these membranes, which often fill most of the cytoplasm of the bacterial cell, pigments (bacteriochlorophyll, carotenoids) and enzymes (cytochromes) that carry out the process of photosynthesis are localized.

  
  

  ,
  

  
  

  The cytoplasm of bacteria contains ribosomes, protein-synthesizing particles with a diameter of 200A. There are more than a thousand of them in a cage. Ribosomes consist of RNA and protein. In bacteria, many ribosomes are freely located in the cytoplasm, some of them may be associated with membranes.

  
  

  Ribosomes are centers of protein synthesis in the cell. At the same time, they often connect with each other, forming aggregates called polyribosomes or polysomes.

  
  

  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; it is usually largely related to 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 reserve substances are formed when the body is supplied with sufficient nutrients, and, conversely, are used when the body finds itself in conditions that are less favorable in terms of nutrition.

  
  

  In many bacteria, granules consist of starch or other polysaccharides - glycogen and granulosa. Some bacteria, when grown in a sugar-rich medium, have droplets of fat inside the cell. Another widespread type of granular inclusions is volutin (metachromatin granules). These granules consist of polymetaphosphate (a reserve 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 under unusual nutritional conditions, such as sulfur-free media. In the cytoplasm of some sulfur bacteria there are droplets of sulfur.

  
  

  In addition to various structural components, the cytoplasm consists of a liquid part - the soluble fraction. It contains proteins, various enzymes, t-RNA, some pigments and low molecular weight compounds - sugars, amino acids.

  As a result of the presence of low molecular weight compounds in the cytoplasm, a difference arises in the osmotic pressure of the cellular contents and the external environment, and this pressure may be different for different microorganisms. The highest osmotic pressure is observed in gram-positive bacteria - 30 atm; in gram-negative bacteria it is much lower - 4-8 atm.

  
  

  Nuclear apparatus. The nuclear substance, deoxyribonucleic acid (DNA), is localized in the central part of the cell.

  
  

  ,
  

  
  

  Bacteria do not have such a nucleus as higher organisms (eukaryotes), but have an analogue - a “nuclear equivalent” - nucleoid(see Fig. 2, 8), which is an evolutionarily more primitive form of organization of nuclear matter. Microorganisms that do not have a real nucleus, but have an analogue of it, are classified as prokaryotes. All bacteria are prokaryotes. In the cells of most bacteria, the bulk of DNA is concentrated in one or several places. In eukaryotic cells, DNA is located in a specific structure - the nucleus. The core is surrounded by a shell membrane.

  
  

  In bacteria, DNA is packed less tightly, unlike true nuclei; A nucleoid does not have a membrane, a nucleolus, or a set of chromosomes. Bacterial DNA is not associated with the main proteins - histones - and is located in the nucleoid in the form of a bundle of fibrils.

  
  

  Flagella. Some bacteria have appendage structures on the surface; The most widespread of them are flagella - the organs of movement of bacteria.

  
  

  The flagellum is anchored under the cytoplasmic membrane using two pairs of discs. Bacteria may have one, two, or many flagella. Their location is different: at one end of the cell, at two, over the entire surface, etc. (Fig. 9). Bacterial flagella have a diameter of 0.01-0.03 microns, their length can be many times greater than the length of the cell. Bacterial flagella consist of a protein - flagellin - and are twisted helical filaments.

  
  

  

  
  

  On the surface of some bacterial cells there are thin villi - fimbriae.

  Life of plants: in 6 volumes. - M.: Enlightenment. Edited by A. L. Takhtadzhyan, editor-in-chief, corresponding member. USSR Academy of Sciences, prof. A.A. Fedorov. 1974 .

  
  

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