Draw the structure of a bacterial cell. The internal structure of bacteria

A bacterial cell consists of a cell wall, cytoplasmic membrane, cytoplasm with inclusions, and a nucleus called a nucleoid (Fig. 3.4). There are additional structures: capsule, microcapsule, mucus, flagella, pili. Some bacteria under unfavorable conditions are able to form disputes.

Rice. 3.4

cell wall. The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids, proteins. The main component of the thick cell wall of these bacteria is a multilayer peptidoglycan (murein, mucopeptide), which makes up 40-90% of the mass of the cell wall (Fig. 3.5, 3.7). Teichoic acids (from the Greek. teichos- wall).

Rice. 3-5-

Rice. 3.6.Phase contrast microscopyL-forms

The cell wall of gram-negative bacteria includes an outer membrane linked by a lipoprotein to the underlying layer of peptidoglycan. On ultrathin sections of bacteria, the outer membrane has the form of a wavy three-layer structure similar to the inner membrane, which is called cytoplasmic (Fig. 3.5,3.8). The main component of these membranes is a bimolecular (double) layer of lipids. The inner layer of the outer membrane is represented by phospholipids, and the outer layer contains lipopolysaccharide. Lipopolysaccharide of the outer membrane consists of 3 fragments: lipid A - a conservative structure, almost the same in gram-negative bacteria; core, or rod, bark part (from lat. core- core), relatively conservative oligosaccharide structure (the most constant part of the LPS core is ketodeoxyoctonic acid); highly variable O-specific polysaccharide chain formed by repeating identical oligosaccharide sequences (0-antigen). Proteins in the matrix of the outer membrane permeate it in such a way that protein molecules called porins border hydrophilic pores through which water and small hydrophilic molecules pass.

Rice. 3-7Electron diffraction pattern of a thin section of a listeria cell- Listeriamonocytogenes(according to A. A. Avakyan, L. N. Kats. I. B. Pavlova). The cytoplasmic membrane, mesosome and nucleoid are well expressed in the form of light zones with fibrillar, filamentous DNA structures; cell wall - thick, typical of Gram-positive bacteria

Rice. 3.8. Electron diffraction pattern of an ultrathin section of a Brucella cell (Brucellamelitensis). According to A. A. Avakyan, L. N. Kats, I. B. Pavlova.

The nucleoid has the appearance of light zones with fibrillar, filamentous DNA structures; cell wall - thin, typical of Gram-negative bacteria

Between the outer and cytoplasmic membranes there is a periplasmic space, or periplasm, containing enzymes (proteases, lipases, phosphatases, nucleases, beta-lactamases) and components of transport systems.
In case of violation of the synthesis of the bacterial cell wall under the influence of lysozyme, penicillin, protective factors of the body, cells with an altered (often spherical) shape are formed: protoplasts - bacteria completely devoid of a cell wall; spheroplasts are bacteria with a partially preserved cell wall. Sphero- or protoplast-type bacteria that have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to multiply are called L-forms (Fig. 3.b). Some L-forms (unstable) when the factor that led to changes in the bacteria is removed, can reverse, "returning" to the original bacterial cell.

cytoplasmic membrane with electron microscopy of ultrathin sections, it is a three-layer membrane (2 dark layers 2.5 nm thick are separated by a light one - intermediate). In structure, it is similar to the plasmalemma of animal cells and consists of a double layer of phospholipids with embedded surface and integral proteins, as if penetrating through the membrane structure. With excessive growth (compared to the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complexly twisted membrane structures, called mesosomes (Fig. 3.7). Less complex twisted structures are called intracytoplasmic membranes.
The cytoplasm consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosomes responsible for the synthesis (translation) of proteins. Bacterial ribosomes are about 20 nm in size and have a sedimentation coefficient of 70S, in contrast to the EOB ribosomes characteristic of eukaryotic cells. Ribosomal RNA (rRNA) are conservative elements of bacteria ("molecular clock" of evolution). 16S rRNA is part of the small subunit of ribosomes, and 23S rRNA is part of the large subunit of ribosomes. The study of 16S rRNA is the basis of gene systematics, making it possible to assess the degree of relatedness of organisms. In the cytoplasm there are various inclusions in the form of glycogen granules, polysaccharides, beta-hydroxybutyric acid and polyphosphates (volutin). They are reserve substances for the nutrition and energy needs of bacteria. Volyutin has an affinity for basic dyes and is easily detected using special staining methods (for example, according to Neisser) in the form of metachromatic granules. The characteristic arrangement of granules in the lutin is revealed in the diphtheria bacillus in the form of intensively stained poles of the cell (Fig. 3.87).

Rice. 3-9 a

Rice. 3-9 b. Pure culture swabKlebsiellapneumoniae, Burri-Gypsum staining. Visible capsules - light halos around rod-shaped bacteria

Rice. 3.10.Flagella and drank Escherichia coli. Electron diffraction pattern of a bacterium deposited with a platinum-palladium alloy. Preparation of V. S. Tyurin

Nucleoid is the equivalent of the nucleus in bacteria. It 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 (Fig. 3.4, 3.7 and 3.8). The nucleus of bacteria, unlike eukaryotes, does not have a nuclear membrane, nucleolus and basic proteins (histones). Usually in
A bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring. In addition to the nucleoid, represented by one chromosome, the bacterial cell contains extrachromosomal factors of heredity in the form of covalently closed DNA rings - the so-called plasmids (see Fig. 3.4).

Capsule, microcapsule, mucus. Capsule - a mucous structure more than 0.2 microns thick, firmly associated with the bacterial cell wall and having clearly defined outer boundaries. The capsule is distinguishable in smears-imprints from pathological material (see Fig. 3.9a). In pure cultures of bacteria, the capsule is formed less frequently. It is detected with special smear staining methods (for example, according to Burri-Gins), which create a negative contrast of the capsule substances: the ink forms a dark background around the capsule (see Fig. 3.9b).
The capsule consists of polysaccharides (exopolysaccharides), sometimes polypeptides; for example, in the anthrax bacillus, it consists of polymers of D-glutamic acid. The capsule is hydrophilic and prevents phagocytosis of bacteria. The capsule is antigenic: antibodies against the capsule cause it to enlarge (capsule swelling reaction).

Many bacteria form microcapsule - mucous formation with a thickness of less than 0.2 microns, detected only with electron microscopy. Mucus should be distinguished from the capsule - mucoid exopolysaccharides that do not have clear boundaries. Slime is soluble in water. Bacterial exopolysaccharides are involved in adhesion (sticking to substrates), they are also called glycocalyx. In addition to the synthesis of exopolysaccharides by bacteria, there is another mechanism for their formation: through the action of extracellular bacterial enzymes on disaccharides. As a result, dextrans and levans are formed.

Flagella bacteria determine the mobility of the bacterial cell. Flagella are thin filaments originating from the cytoplasmic membrane and are longer than the cell itself (Fig. 3.10). The flagella are 12–20 nm thick and 3–15 µm long. They consist of 3 parts: a spiral thread, a hook and a basal body containing a rod with special discs (1 pair of discs for gram-positive and 2 pairs of discs for gram-negative bacteria). The discs of the flagella are attached to the cytoplasmic membrane and cell wall. This creates the effect of an electric motor with a motor rod that rotates the flagellum. Flagella are made up of a protein called flagellin. flagellum- flagellum), which is an H-antigen. Flagellin subunits are coiled. The number of flagella in bacteria of various species varies from one (monotrich) in Vibrio cholerae to ten or hundreds of flagella extending along the perimeter of the bacterium (peritrich) in Escherichia coli, Proteus, etc.

Rice. 3.11.Electronogram of an ultrathin section of tetanus bacillus(Clostridiumtetani) in the vegetative cell of the bacterium, a terminal spore with a multilayered membrane is formed. (According to A. A. Avakyan, L. N. Kats, I. B. Pavlova)

Lophotrichous have a bundle of flagella at one end of the cell. Amphitrichous have one flagellum or a bundle of flagella at opposite ends of the cell.

Pili (fimbriae, villi) - filamentous formations, thinner and shorter (3-10 nm x 0.3-10 microns) than flagella. Pili extend from the cell surface and consist of the pilin protein, which has antigenic activity. There are pili responsible for adhesion, i.e., for attaching bacteria to the affected cell, as well as pili responsible for nutrition, water-salt metabolism, and sexual (F-pili), or conjugation, pili. Drinks are plentiful - several hundred per cage.

However, there are usually 1-3 sex pili per cell: they are formed by the so-called "male" donor cells containing transmissible plasmids (F-, R-, Col-plasmids). Distinctive feature sex pili is the interaction with special "male" spherical bacteriophages, which are intensively adsorbed on the sex pili (Fig. 3.10).

controversy - a peculiar form of dormant firmicute bacteria, i.e. bacteria with gram-positive cell wall structure. controversy are formed under unfavorable conditions for the existence of bacteria (drying, nutrient deficiency, etc.). Inside the bacterial cell, one spore (endospore) is formed. The formation of spores contributes to the preservation of the species and is not a method of reproduction, as in fungi. spore-forming bacteria genus Bacillus have spores that do not exceed the diameter of the cell. Bacteria whose spore size exceeds the cell diameter are called clostridium, for example, bacteria of the genus Clostridium (lat. Clostridium- spindle). Spores are acid-resistant, therefore they are stained red according to the Aujeszky method or according to the Ziehl-Nielsen method, and the vegetative cell is blue (see Fig. 3.2, bacilli, clostridia).
The shape of the dispute can be oval, spherical; the location in the cell is terminal, that is, at the end of the stick (in the causative agent of tetanus), subterminal - closer to the end of the stick (in causative agents of botulism, gas gangrene) and central (in the anthrax bacillus). The spore persists for a long time due to the presence of a multi-layered shell (Fig. 3.11), calcium dipicolinate, low water content and sluggish metabolic processes. Under favorable conditions, spores germinate through 3 successive stages: activation, initiation, germination.

Prokaryotes include archaebacteria, bacteria, and blue-green algae. prokaryotes- unicellular organisms that lack a structurally formed nucleus, membrane organelles and mitosis.

Dimensions - from 1 to 15 microns. Basic forms: 1) cocci (spherical), 2) bacilli (rod-shaped), 3) vibrios (curved in the form of a comma), 4) spirilla and spirochetes (spiral twisted).

1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirilla and spirochetes.

1 - cytoplasmic membrane wound; 2 - cell wall; 3 - slime capsule; 4 - cytoplasm; 5 - chromosomal DNA; 6 - ribosomes; 7 - meso-soma; 8 - photo-synthetic membrane wounds; 9 - inclusion; 10 - burn-tiki; 11 - drinking.

The bacterial cell is surrounded by a membrane. The inner layer of the membrane is represented by a cytoplasmic membrane (1), over which there is a cell wall (2); above the cell wall in many bacteria there is a mucous capsule (3). The structure and functions of the cytoplasmic membrane of eukaryotic and prokaryotic cells do not differ. The membrane may form folds called mesosomes(7). They can have a different shape (bag-shaped, tubular, lamellar, etc.).

Enzymes are located on the surface of mesosomes. The cell wall is thick, dense, rigid, composed of mureina(main component) and others organic matter. Murein is a regular network of parallel polysaccharide chains linked together by short protein chains. Bacteria are classified according to their cell wall structure. gram-positive(stained by Gram) and gram negative(not dyed). In gram-negative bacteria, the wall is thinner, more complex, and there is a layer of lipids above the murein layer on the outside. The inner space is filled with cytoplasm (4).

The genetic material is represented by circular DNA molecules. These DNAs can be conditionally divided into "chromosomal" and plasmid. “Chromosomal” DNA (5) is one, attached to the membrane, contains several thousand genes, unlike eukaryotic chromosomal DNA, it is not linear, not associated with proteins. The area in which this DNA is located is called nucleoid. Plasmids extrachromosomal genetic elements. They are small circular DNA, not associated with proteins, not attached to the membrane, contain a small number of genes. The number of plasmids can be different. The most studied plasmids are those that carry information about drug resistance (R-factor) and are involved in the sexual process (F-factor). A plasmid that can combine with a chromosome is called episome.

In a bacterial cell, all membrane organelles characteristic of a eukaryotic cell (mitochondria, plastids, ER, Golgi apparatus, lysosomes) are absent.

In the cytoplasm of bacteria there are 70S-type ribosomes (6) and inclusions (9). Typically, ribosomes are assembled into polysomes. Each ribosome consists of a small (30S) and a large subunit (50S). The function of ribosomes is to assemble a polypeptide chain. Inclusions can be represented by lumps of starch, glycogen, volutin, lipid drops.

Many bacteria have flagella(10) and pili (fimbriae)(eleven). Flagella are not limited by a membrane, have a wavy shape and consist of spherical flagellin protein subunits. These subunits are arranged in a spiral and form a hollow cylinder with a diameter of 10–20 nm. The prokaryotic flagellum in its structure resembles one of the microtubules of the eukaryotic flagellum. The number and arrangement of flagella may vary. Pili are straight filamentous structures on the surface of bacteria. They are thinner and shorter than flagella. They are short hollow cylinders of pilin protein. Pili serve to attach bacteria to the substrate and to each other. During conjugation, special F-pili are formed, through which genetic material is transferred from one bacterial cell to another.

sporulation bacteria have a way of experiencing adverse conditions. Spores are usually formed one at a time inside the "mother cell" and are called endospores. Spores are highly resistant to radiation, extreme temperatures, desiccation, and other factors that cause vegetative cell death.

Reproduction. Bacteria reproduce asexually by dividing the "mother cell" in two. Before division, DNA replication occurs.

Rarely, bacteria have a sexual process in which recombination of genetic material occurs. It should be emphasized that bacteria never form gametes, do not merge the contents of the cells, but transfer of DNA from the donor cell to the recipient cell takes place. There are three ways of DNA transfer: conjugation, transformation, transduction.

- unidirectional transfer of the F-plasmid from the donor cell to the recipient cell in contact with each other. In this case, the bacteria are connected to each other by special F-pilae (F-fimbria), through the channels of which DNA fragments are transferred. Conjugation can be divided into the following stages: 1) F-plasmid unwinding, 2) penetration of one of the F-plasmid strands into the recipient cell through the F-pill, 3) synthesis of a complementary strand on a single-stranded DNA template (occurs as in a donor cell (F +), and in the recipient cell (F -)).

Transformation- unidirectional transfer of DNA fragments from the donor cell to the recipient cell, not in contact with each other. In this case, the donor cell either “seeds” a small fragment of DNA from itself, or the DNA enters the environment after the death of this cell. In any case, the DNA is actively absorbed by the recipient cell and integrated into its own "chromosome".

transduction- transfer of a DNA fragment from a donor cell to a recipient cell using bacteriophages.

Viruses

Viruses consist of a nucleic acid (DNA or RNA) and proteins that form a shell around this nucleic acid, i.e. are a nucleoprotein complex. Some viruses contain lipids and carbohydrates. Viruses always contain one type of nucleic acid, either DNA or RNA. Moreover, each of the nucleic acids can be both single-stranded and double-stranded, both linear and circular.

The size of viruses is 10-300 nm. Virus shape: spherical, rod-shaped, filiform, cylindrical, etc.

capsid- the shell of the virus, formed by protein subunits, stacked in a certain way. The capsid protects the nucleic acid of the virus from various influences, ensures the deposition of the virus on the surface of the host cell. Supercapsid characteristic of complex viruses (HIV, influenza viruses, herpes). Occurs during the exit of the virus from the host cell and is a modified section of the nuclear or outer cytoplasmic membrane of the host cell.

If the virus is inside the host cell, then it exists in the form of a nucleic acid. If the virus is outside the host cell, then it is a nucleoprotein complex, and this free form of existence is called virion. Viruses are highly specific; they can use a strictly defined circle of hosts for their life activity.

The following stages can be distinguished in the virus reproduction cycle.

1. Deposition on the surface of the host cell.
2. Penetration of the virus into the host cell (they can enter the host cell by: a) "injection", b) dissolution of the cell membrane by viral enzymes, c) endocytosis; Once inside the cell, the virus transfers its protein-synthesizing apparatus under its own control).
3. Embedding viral DNA into the DNA of the host cell (in RNA-containing viruses, reverse transcription occurs before this - DNA synthesis on an RNA template).
4. Transcription of viral RNA.
5. Synthesis of viral proteins.
6. Synthesis of viral nucleic acids.
7. Self-assembly and exit from the cell of daughter viruses. Then the cell either dies or continues to exist and produce new generations of viral particles.

The human immunodeficiency virus mainly infects CD 4 lymphocytes (helpers), on the surface of which there are receptors that can bind to the surface protein of HIV. In addition, HIV penetrates the cells of the central nervous system, neuroglia, and intestines. The immune system of the human body loses its protective properties and is unable to resist pathogens of various infections. The average life expectancy of an infected person is 7-10 years.

The source of infection is only a person - a carrier of the immunodeficiency virus. AIDS is transmitted sexually, through blood and tissues containing the immunodeficiency virus, from mother to fetus.

Go to lectures number 8" Core. Chromosomes»

Go to lectures number 10 The concept of metabolism. Biosynthesis of proteins"

The structure of bacteria is well studied using electron microscopy of whole cells and their ultrathin sections. A bacterial cell consists of a cell wall, cytoplasmic membrane, cytoplasm with inclusions, and a nucleus called a nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, pili (Fig. 1); some bacteria in adverse conditions are able to form spores.

cell wall - a strong, elastic structure that gives the bacteria a certain shape and, together with the underlying cytoplasmic membrane, “restrains” the high osmotic pressure in the bacterial cell. It is involved in the process of cell division and the transport of metabolites. The thickest cell wall in gram-positive bacteria (Fig. 1). So, if the thickness of the cell wall of gram-negative bacteria is about 15-20 nm, then in gram-positive bacteria it can reach 50 nm or more. The cell wall of gram-positive bacteria contains a small amount of polysaccharides, lipids, proteins.

The main component of the cell wall of these bacteria is a multilayer peptidoglycan(murein, mucopeptide), constituting 40-90% of the mass of the cell wall.

Volyutin Mesosome Nucleoid

Rice. 1. The structure of a bacterial cell.

Teichoic acids (from the Greek. teichos- wall), the molecules of which are chains of 8-50 residues of glycerol and ribitol connected by phosphate bridges. The shape and strength of the bacteria is given by the rigid fibrous structure of the multilayer peptidoglycan with cross-linked peptides. Peptidoglycan is represented by parallel molecules of glycan, consisting of repeating residues N-acetylglucosamine and N-acetylmuramic acid connected by a glycosidic bond type P (1 -> 4).

Lysozyme, being an acetylmuramidase, breaks these bonds. Glycan molecules are linked by cross peptide bonds. Hence the name of this polymer - peptidoglycan. The basis of the peptide bond of the peptidoglycan of gram-negative bacteria is tetrapeptides, consisting of alternating L- And D-amino acids.

At E. coli peptide chains are connected to each other through D- alanine of one chain and mesodiaminopimelic acid of the other.

The composition and structure of the peptide part of peptidoglycan in gram-negative bacteria are stable, in contrast to the peptidoglycan of gram-positive bacteria, the amino acids of which may differ in composition and sequence. The tetrapeptides here are connected to each other by polypeptide chains of 5 glycine residues. Gram-positive bacteria often contain lysine instead of mesodiaminopimelic acid. Phospholipid

Rice. 2. The structure of the surface structures of gram-positive (gram +) and gram-negative (gram ") bacteria.

Glycan elements (acetylglucosamine and acetylmuramic acid) and tetrapeptide amino acids (mesodiaminopimelic and L-glutamic acids, D-alanine) are a distinctive feature of bacteria, since they and the D-isomers of amino acids are absent in animals and humans.

The ability of gram-positive bacteria to retain gentian violet in combination with iodine (blue-violet color of bacteria) during Gram staining is associated with the property of multilayer peptidoglycan to interact with the dye. In addition, the subsequent treatment of a smear of bacteria with alcohol causes narrowing of the pores in peptidoglycan and thus the retention of the dye in the cell wall. Gram-negative bacteria, after exposure to alcohol, lose the dye, become discolored, and turn red when treated with fuchsin. This is due to a smaller amount of peptidoglycan (5-10% of the mass of the cell wall).

The cell wall of Gram-negative bacteria contains outer membrane, associated by means of a lipoprotein with the underlying layer of peptidoglycan (Fig. 2). The outer membrane is a wavy three-layer structure similar to the inner membrane, which is called cytoplasmic. The main component of these membranes is a bimolecular (double) layer of lipids.

The outer membrane is an asymmetric mosaic structure represented by lipopolysaccharides, phospholipids and proteins . On its outer side is lipopolysaccharide(LPS), composed of three components: lipid A, core part, or core (lat. core- core), and a 0-specific polysaccharide chain formed by repeating oligosaccharide sequences.

Lipopolysaccharide is anchored in the outer membrane by lipid A, determining the toxicity of LPS, identified therefore with endotoxin. The destruction of bacteria by antibiotics leads to the release of large amounts of endotoxin, which can lead to endotoxic shock in the patient.

From lipid A the core, or the core part of the LPS, departs. The most constant part of the LPS core is ketodeoxyoctonic acid (3-deoxy-g)-manno-2-octulosonic acid). 0 -specific chain extending from the core part of the LPS molecule, determines serogroup, serovar (a type of bacteria detected using immune serum) certain strain of bacteria. Thus, the concept of LPS is associated with ideas about the 0-antigen, which can be used to differentiate bacteria. Genetic changes can lead to changes in the biosynthesis of components LPS bacteria and the resulting L-forms.

Matrix proteins outer membrane penetrate it in such a way that protein molecules called porins, they border hydrophilic pores through which water and small molecules with a relative mass of up to 700 pass. Between the outer and cytoplasmic membranes there is a periplasmic space, or periplasm containing enzymes. In case of violation of the synthesis of the bacterial cell wall under the influence of lysozyme, penicillin, protective factors of the body and other compounds, cells with an altered (often spherical) shape are formed: protoplasts - bacteria completely devoid of a cell wall; spheroplasts - bacteria with a partially preserved cell wall. After removal of the cell wall inhibitor, such altered bacteria can reverse, i. acquire a full-fledged cell wall and restore its original shape.

Sphero- or protoplast-type bacteria that have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are able to multiply are called L-shaped(from the name of the Lister Institute). L-forms can also arise as a result of mutations. They are osmotically sensitive, spherical, flask-shaped cells of various sizes, including those passing through bacterial filters. Some L-forms (unstable) upon removal of the factor that led to changes in bacteria, can reverse, "returning" to the original bacterial cell. L-forms can form many pathogens of infectious diseases.

cytoplasmic membrane on electron microscopy of ultrathin sections, it is a three-layer membrane surrounding the outer part of the bacterial cytoplasm. In structure, it is similar to the plasmalemma of animal cells and consists of a double layer of lipids, mainly phospholipids with embedded surface and integral proteins, as if penetrating through the membrane structure. Some of them are permeases involved in the transport of substances. The cytoplasmic membrane is a dynamic structure with mobile components, therefore it is presented as a mobile fluid structure. It is involved in the regulation of osmotic pressure, transport of substances and energy metabolism of the cell (due to the enzymes of the electron transport chain, adenosine triphosphatase, etc.). With excessive growth (compared to the growth of the cell wall), the cytoplasmic membrane forms invaginates - invaginations in the form of complexly twisted membrane structures, called mesosomes. Less complex twisted structures are called intracytoplasmic membranes. The role of mesosomes and intracytoplasmic membranes has not been fully elucidated. It is even suggested that they are an artifact that occurs after the preparation (fixation) of the preparation for electron microscopy. Nevertheless, it is believed that derivatives of the cytoplasmic membrane are involved in cell division, providing energy for the synthesis of the cell wall, take part in the secretion of substances, spore formation, i.e. in processes with high energy consumption.

Cytoplasm occupies the bulk of the bacterial cell and consists of soluble proteins, ribonucleic acids, inclusions and numerous small granules - ribosome, responsible for the synthesis (translation) of proteins. Bacterial ribosomes are about 20 nm in size and have a sedimentation coefficient 70s, 3 difference from 80^-ribosomes characteristic of eukaryotic cells. Therefore, some antibiotics bind to bacterial ribosomes and inhibit bacterial protein synthesis without affecting protein synthesis in eukaryotic cells. Ribosomes of bacteria can dissociate into two subunits - 50S And 30S . In the cytoplasm there are various inclusions in the form of glycogen granules, polysaccharides, poly-p-butyric acid and polyphosphates (volutin). They accumulate with an excess of nutrients in environment and act as reserve substances for nutrition and energy needs. Volyutin has an affinity for basic dyes, has metachromasia and is easily detected using special staining methods. The characteristic arrangement of volutin grains is revealed in diphtheria bacillus in the form of intensively stained poles of the cell.

Nucleoid - bacterial equivalent of the nucleus. It 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 (histones). Usually, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring. If division is disturbed, it may contain 4 or more chromosomes. The nucleoid is detected in a light microscope after staining with DNA-specific methods: according to Feulgen or Romanovsky-Giemsa. On electron diffraction patterns of ultrathin sections of bacteria, the nucleoid has the form of light zones with fibrillar, thread-like structures of DNA associated with certain areas with the cytoplasmic membrane or mesosome involved in chromosome replication.

In addition to the nucleoid, represented by one chromosome, in the bacterial cell there are extrachromosomal factors of heredity - plasmids, which are covalently closed DNA rings.

Capsule - a mucous structure more than 0.2 microns thick, firmly associated with the bacterial cell wall and having clearly defined outer boundaries. The capsule is distinguishable in smears-imprints from pathological material. In pure cultures of bacteria, the capsule is formed less frequently. It is detected with special Burri-Gins staining methods that create a negative contrast of the capsule substances.

Usually the capsule consists of polysaccharides (exopolysaccharides), sometimes polypeptides, for example, in anthrax bacilli. The capsule is hydrophilic, it prevents phagocytosis of bacteria.

Many bacteria form microcapsule - mucous formation with a thickness of less than 0.2 microns, detected only with electron microscopy. To be distinguished from a capsule slime - mucoid exopolysaccharides that do not have clear external boundaries. Mucoid exopolysaccharides are characteristic of mucoid strains of Pseudomonas aeruginosa, often found in the sputum of patients with cystic fibrosis. Bacterial exopolysaccharides are involved in adhesion (sticking to substrates), they are also called glycocalyx. In addition to the synthesis of exopolysaccharides by bacteria, there is another mechanism for their formation: through the action of extracellular bacterial enzymes on disaccharides. As a result, dextrans and levans are formed. The capsule and mucus protect bacteria from damage and drying out, since, being hydrophilic, they bind water well and prevent the action of protective factors of the macroorganism and bacteriophages.

Flagella bacteria determine the mobility of the bacterial cell. Flagella are thin filaments originating from the cytoplasmic membrane, they are longer than the cell itself (Fig. 3). The flagella are 12–20 nm thick and 3–12 µm long. The number of flagella in bacteria of various species varies from one (monotrich) in vibrio cholerae up to ten and hundreds of flagella extending along the perimeter of the bacterium (peri-trih) in Escherichia coli, Proteus, etc. lophotrichous have a bundle of flagella at one end of the cell. amphitriches have one flagellum or a bundle of flagella at opposite ends of the cell. The flagella are attached to the cytoplasmic membrane and cell wall by special discs. Flagella are made up of a protein called flagellin. naT.flagellum- flagellum) with antigenic specificity. Flagellin subunits are coiled. Flagella are detected using electron microscopy of preparations sprayed with heavy metals, or in a light microscope after processing by special methods based on etching and adsorption of various substances, leading to an increase in the thickness of the flagella (for example, after silvering).

Rice. 3. E. coli. Electron diffraction pattern (preparation by V.S. Tyurin). 1 - flagella, 2 - villi, 3 - F-drank.

Villi, or pili (fimbria), - filiform formations (Fig. 3), thinner and shorter (3-10 nm x 0.3-10 µm) than flagella. Pili extend from the cell surface and are composed of the pilin protein. They have antigenic activity. Among the pili, the following stand out: pili responsible for adhesion, i.e. for attaching bacteria to the affected cell (drank type 1, or general type - common pili) drank, responsible for nutrition, water-salt metabolism; genital (F-drank), or conjugation pili (drank type 2). Pili of the general type are numerous - several hundred per cage. Sex pili are formed by the so-called "male" donor cells containing transmissible plasmids. (F, R, Col). There are usually 1-3 of them per cell. A distinctive feature of sex pili is interaction with special “male” spherical bacteriophages, which are intensively adsorbed on sex pili.

controversy - a peculiar form of dormant firmicute bacteria, i.e. bacteria with gram-positive cell wall structure.

Spores are formed under unfavorable conditions for the existence of bacteria (drying, nutrient deficiency, etc.). In this case, one spore is formed inside one bacterium. The formation of spores contributes to the preservation of the species and is not a method of reproduction, as in mushrooms.

Aerobic spore-forming bacteria whose spore size does not exceed the cell diameter are sometimes called bacilli. Spore-forming anaerobic bacteria, in which the spore size exceeds the cell diameter, and therefore they take the form of a spindle, are called clostridia(lat. clostridium- spindle).

Process sporulation(sporulation) goes through a series of stages, during which part of the cytoplasm and the chromosome are separated, surrounded by a cytoplasmic membrane; a prospore is formed, then a multilayer poorly permeable shell is formed. Sporulation is accompanied by intensive consumption by the prospore, and then by the emerging spore shell of dipicolinic acid and calcium ions. After the formation of all structures, the spore acquires thermal stability, which is associated with the presence of calcium dipicolinate. Sporulation, the shape and location of spores in a cell (vegetative) are a species property of bacteria, which makes it possible to distinguish them from each other. The shape of the spores can be oval, spherical, the location in the cell is terminal, i.e. at the end of the stick (the causative agent of tetanus), subterminal - closer to the end of the stick (causative agents of botulism, gas gangrene) and central (anthrax bacillus).

Mandatory and optional structural components of a bacterial cell, their functions. The difference in the structure of the cell wall of gram-positive and gram-negative bacteria. L-forms and non-culturable forms of bacteria

Bacteria are prokaryotes and differ significantly from plant and animal cells (eukaryotes). They belong to unicellular organisms and consist of a cell wall, cytoplasmic membrane, cytoplasm, nucleoid (mandatory components of a bacterial cell). Some bacteria may have flagella, capsules, spores (optional components of a bacterial cell).

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

The cell wall is an important structural element of a bacterial cell, located between the cytoplasmic membrane and the capsule; in non-capsular bacteria, this is the outer shell of the cell. Performs a number of functions: protects bacteria from osmotic shock and other damaging factors, determines their shape, participates in metabolism; in many species of pathogenic bacteria, it is toxic, contains surface antigens, and also carries specific receptors for phages on the surface. The bacterial cell wall has pores that are involved in the transport of exotoxins and other bacterial exoproteins.

The main component of the bacterial cell wall is peptidoglycan, or murein (lat. murus - wall), a support polymer that has a network structure and forms a rigid (hard) outer frame 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 transverse peptide chains. bridges linking polysaccharide chains.

According to tinctorial properties, all bacteria are divided into two groups: gram-positive and gram-negative. Gram-positive bacteria firmly fix the complex of gentian violet and iodine, do not undergo discoloration with ethanol and therefore do not perceive the additional dye fuchsin, remaining stained purple. In gram-negative bacteria, this complex is easily washed out of the cell with ethanol, and they turn red upon additional application of fuchsin. In some bacteria, a positive Gram stain is observed only in the stage of active growth. The ability of prokaryotes to stain according to the Gram method or to decolorize with ethanol is determined by the specifics of the chemical composition and ultrastructure of their cell wall. bacterial chlamydia trachoma

L-forms of bacteria 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 their cell wall. They are formed under the influence of L-transforming agents - antibiotics (penicillin, polymyxin, bacitracin, vencomycin, streptomycin), amino acids (glycine, methionine, leucine, etc.), lysozyme enzyme, ultraviolet and X-rays. Unlike protoplasts and spheroplasts, L-forms have a 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-form cells have a well-developed system of intracytoplasmic membranes and myelin-like structures. Due to a defect in the cell wall, they are osmotically unstable and can only be cultivated on 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; they very rarely reverse to their original bacterial forms. The latter may have elements of the cell wall, in which they show similarities with spheroplasts; in the absence of the factor that caused their formation, they revert to the original cells.

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

L-shapes attached great importance in the development of chronic recurrent infections, the carriage of pathogens, their long-term persistence in the body. The infectious process caused by L-forms of bacteria is characterized by atypicality, duration of the course, severity of the disease, and is difficult to respond to chemotherapy.

A capsule is a mucous layer located above the cell wall of a bacterium. The substance of the capsule is clearly delimited from the environment. The capsule is not an obligatory structure of a bacterial cell: its loss does not lead to the death of the bacterium.

The substance of the capsules consists of highly hydrophilic micelles, while their chemical composition is very diverse. The main components of most prokaryotic capsules are homo- or heteropolysaccharides (Entsrobacteria, etc.). In some species of bacilli, the capsules are built from a polypeptide.

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 cell attachment to the substrate.

Flagella are organelles of bacterial movement, represented by thin, long, filamentous structures of a protein nature.

The flagellum consists of three parts: a spiral filament, a hook, and a basal body. Hook - 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.

Flagella are not vital structures of a bacterial cell: there are phase variations of bacteria, when they are present in one phase of cell development and absent in another.

The number of flagella and their location in bacteria different types are not the same, but are stable for one species. Depending on this, the following groups of flagellated bacteria are distinguished: moiotrichous - bacteria with one polar flagellum; amphitrichous - bacteria with two polar flagella or having a bundle of flagella at both ends; lophotrichous - bacteria that have a bundle of flagella at one end of the cell; peritrichous - bacteria with many flagella located on the sides of the cell or on its entire surface. Bacteria that do not have flagella are called atrichia.

Being organs of locomotion, flagella are typical of floating rod-shaped and tortuous forms of bacteria and are found only in isolated cases in cocci. They provide efficient movement in a liquid medium and slower movement on the surface of solid substrates.

Pili (fimbria, villi) - straight, thin, hollow protein cylinders extending from the surface of the bacterial cell. They are formed by a specific protein - pilin, originate from the cytoplasmic membrane, are found in mobile and immobile forms of bacteria and are visible only in an electron microscope. On the cell surface there can be from 1-2, 50-400 or more pili to several thousand.

There are two classes of pili: sexual (sekspili) and pili of a general type, which are more often called fimbriae. The same bacterium can have pili of different nature. Sex pili appear on the surface of bacteria in the process of conjugation and act as organelles through which the transfer of genetic material (DNA) from a donor to a recipient occurs.

Pili 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 erythrocytes.

The cytoplasmic membrane (plasmolemma) is a semi-permeable lipoprotein structure of bacterial cells that separates the cytoplasm from the cell wall. It is an essential polyfunctional component of the cell. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell.

The cytoplasmic membrane is chemically a protein-lipid complex consisting of proteins and lipids. The main part of membrane lipids is represented by phospholipids. It is built from two monomolecular protein layers, between which there is a lipid layer, consisting of two rows of correctly oriented lipid molecules.

The cytoplasmic membrane serves as an osmotic barrier of 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 actively selectively transfer organic and inorganic molecules.

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

Mesosomes, like the cytoplasmic membrane, are the centers of bacterial respiratory activity; therefore, they are sometimes called analogues of mitochondria. However, the significance of mesosomes has not yet been finally elucidated. They increase the working surface of the membranes, perhaps they perform only a structural function, dividing the bacterial cell into relatively separate compartments, which creates more favorable conditions for the enzymatic processes to occur. In pathogenic bacteria, they provide the transport of protein molecules of exotoxins.

Cytoplasm - the contents of a bacterial cell, delimited by the cytoplasmic membrane. 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 synthesis. They consist of protein and RNA connected in a complex by hydrogen and hydrophobic bonds.

In the cytoplasm of bacteria, various types of inclusions are detected. They may be solid, liquid or gaseous, with or without a proteinaceous membrane, and are intermittently present. A significant part of them is reserve nutrients and products of cellular metabolism. Reserve nutrients include: polysaccharides, lipids, polyphosphates, sulfur deposits, etc. Of the inclusions of a polysaccharide nature, glycogen and a starch-like substance granulosa are more often found, which serve as a source of carbon and energy material. Lipids accumulate in cells in the form of fat granules and droplets. Mycobacteria accumulate waxes as reserve substances. The cells of some spirilla and others contain volutin granules formed by polyphosphates. They are characterized by metachromasia: toluidine blue and methylene blue stain them purple-red. Volutin granules play the role of phosphate depots. Inclusions surrounded by a membrane also include gas vacuoles, or aerosomes, they reduce the specific mass 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, 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 no histones; the chromosome is fixed on the cytoplasmic membrane, and in gram-positive bacteria - on the mesosome. The nucleoid does not have a mitotic apparatus, and the divergence of the daughter nuclei is ensured by the growth of the cytoplasmic membrane.

The bacterial nucleus is a differentiated structure. Depending on the stage of cell development, 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 is carried out 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 a bacterial cell.

In addition to the nucleoid, extrachromosomal genetic elements have been found in the cells of many bacteria - plasmids, represented by small circular DNA molecules capable of autonomous replication.

Some bacteria at the end of the period of active growth are able to form spores. This is preceded by depletion of the environment with nutrients, a change in its pH, and the accumulation of toxic metabolic products.

According to the 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 the spore state, microorganisms are metabolically inactive, withstand high temperatures (140–150 °C), exposure to chemical disinfectants, and persist in the environment for a long time. Resistance to high temperature associated with very low water content and high dipicolinic acid content. Once in the body of humans and animals, spores germinate into vegetative cells. Spores are stained by a special method, which includes preheating the spores, as well as exposure to concentrated dye solutions at high temperatures.

Many types of Gram-negative bacteria, including pathogenic ones (Shigella, Salmonella, Vibrio cholerae, etc.), have a special adaptive, genetically regulated state, physiologically equivalent to cysts, into which they can pass under the influence of adverse conditions and remain viable for up to several years. The main feature of this condition is that such bacteria do not multiply and therefore do not form colonies on a dense nutrient medium. Such non-reproducing, but viable cells are called non-culturable forms of bacteria (NFB). NFB cells in an uncultivated state have active metabolic systems, including systems for electron transfer, protein and nucleic acid biosynthesis, and retain virulence. Their cell membrane is more viscous, the cells usually take the form of cocci, have a significantly reduced size. NFBs have a higher resistance in the environment and therefore can survive in it for a long time (for example, Vibrio cholerae in a dirty water body), maintaining the endemic state of a given region (water body).

To detect NFB, molecular genetic methods (DNA--DNA hybridization, CPR) are used, as well as a simpler method of direct counting of viable cells.

For these purposes, cytochemical methods (formation of formazan) or microautoradiography can also be used. The genetic mechanisms responsible for the transition of bacteria into NS and their reversion from it are not clear.

Bacteria are microscopic single-celled organisms. The structure of a bacterial cell has features that are the reason for the separation of bacteria into a separate kingdom of the living world.

cell membranes

Most bacteria have three shells:

• cell membrane;
• cell wall;
• mucous capsule.

The cell membrane is in direct contact with the contents of the cell - the cytoplasm. She is thin and soft.

The cell wall is a dense, thicker shell. Its function is to protect and support the cell. The cell wall and membrane have pores through which the necessary substances enter the cell.

Many bacteria have a mucous capsule that performs a protective function and ensures sticking to different surfaces.

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It is thanks to the mucous membrane that streptococci (a type of bacteria) stick to the teeth and cause tooth decay.

Cytoplasm

The cytoplasm is the interior of the cell. 75% consists of water. In the cytoplasm are inclusions - drops of fat and glycogen. They are the reserve nutrients of the cell.

Rice. 1. Scheme of the structure of a bacterial cell.

Nucleoid

Nucleoid means "like a nucleus". Bacteria do not have a real, or, as they say, shaped nucleus. This means that they do not have a nuclear envelope and nuclear space, like the cells of fungi, plants and animals. DNA is located directly in the cytoplasm.

DNA functions:

• preserves hereditary information;
• implements this information by controlling the synthesis of protein molecules characteristic of this type of bacteria.

The absence of a true nucleus is the most important feature of a bacterial cell.

Organelles

Unlike plant and animal cells, bacteria do not have organelles built from membranes.

But the cell membrane of bacteria in some places penetrates the cytoplasm, forming folds, which are called the mesosome. The mesosome is involved in cell reproduction and energy exchange and, as it were, replaces membrane organelles.

The only organelle found in bacteria is the ribosome. These are small bodies that are located in the cytoplasm and synthesize proteins.

Many bacteria have a flagellum with which they move in a liquid medium.

Shapes of bacterial cells

The shape of bacterial cells is different. Bacteria in the form of a ball are called cocci. In the form of a comma - vibrios. Rod-shaped bacteria are bacilli. Spirilla look like a wavy line.

Rice. 2. Forms of bacterial cells.

Bacteria can only be seen under a microscope. The average cell size is 1-10 microns. There are bacteria up to 100 microns long. (1 µm = 0.001 mm).

sporulation

When unfavorable conditions occur, the bacterial cell enters a dormant state, which is called a spore. The reasons for the dispute may be:

• low and high temperatures;
• drought;
• lack of nutrition;
• life-threatening substances.

The transition occurs quickly, within 18-20 hours, and the cell can be in a state of spores for hundreds of years. When normal conditions are restored, the bacterium germinates from the spore in 4-5 hours and passes into the normal mode of life.

Rice. 3. The scheme of spore formation.

reproduction

Bacteria reproduce by division. The period from the birth of a cell to its division is 20-30 minutes. Therefore, bacteria are widespread on Earth.

What have we learned?

We learned that, in general terms, bacterial cells are like plant and animal cells, they have a membrane, cytoplasm, DNA. The main difference between bacterial cells is the absence of a formed nucleus. Therefore, bacteria are called pre-nuclear organisms (prokaryotes).

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