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

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

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

The cell wall is an important structural element of the bacterial cell, located between the cytoplasmic membrane and the capsule; in non-capsular bacteria, this is the outer cell membrane. 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 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.

Based on their 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, 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, positive Gram staining 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. 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 defective in the cell wall. 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 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 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; 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-forms are given great importance in the development of chronic recurrent infections, carriage of pathogens, and their long-term persistence in the body. 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 a mucous layer located above the cell wall of the bacterium. The substance of the capsule is clearly demarcated from the environment. The capsule is not an essential structure of the bacterial cell: its loss does not lead to the death of the bacterium.

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.

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.

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

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.

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.

The number of flagella and their locations in different species of bacteria are not the same, but are 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; 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 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.

Pili (fimbriae, villi) are 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 motile and immobile forms of bacteria and are visible only in an electron microscope. On the surface of the cell there can be from 1-2, 50-400 or more pili to several thousand.

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 take part in the aggregation 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.

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. Destruction of the cytoplasmic membrane leads to the death of the bacterial cell.

Chemically, the cytoplasmic membrane is 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 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.

During cell growth, the cytoplasmic membrane forms numerous invaginates that form intracytoplasmic structures of the membrane. 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.

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 they perform 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 in a complex by hydrogen and hydrophobic bonds.

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. 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 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, 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 mesosome. 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 have been found in the cells of many bacteria - plasmids, which are small circular DNA molecules capable of autonomous replication

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

In terms of chemical composition, the difference between spores and vegetative cells is only in the quantitative content of chemical compounds. Spores contain less water and more lipids.

In the spore state, microorganisms are metabolically inactive, withstand high temperatures (140-150 ° C), exposure to chemical disinfectants and persist for a long time in the environment. High temperature resistance is associated with very low water content and high dipicolinic acid content. Once in the body of humans and animals, the spores germinate into vegetative cells. Spores are painted using a special method, which includes preheating the spores, as well as exposure to concentrated paint 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 unfavorable conditions and remain viable for up to several years. The main feature of this condition is that such bacteria do not reproduce and therefore do not form colonies on a solid nutrient medium. Such non-reproducing but viable cells are called unculturable forms of bacteria (NFB). NFB cells in an uncultured state have active metabolic systems, including electron transfer systems, protein and nucleic acid biosynthesis, and retain virulence. Their cell membrane is more viscous, the cells usually take the form of cocci and are significantly reduced in size. NFBs have a higher stability in the external environment and therefore can survive in it for a long time (for example, Vibrio cholerae in a dirty reservoir), maintaining the endemic state of a given region (reservoir).

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

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

Bacteria (“stick” from ancient Greek) are a kingdom (group) of non-nuclear (prokaryotic) microorganisms, usually single-celled. Today, about ten thousand of their species are known and described. Scientists estimate that there are more than a million of them.

It can have a round, curled, rod-shaped shape. In rare cases, cubic, tetrahedral, stellate, and O- or C-shaped shapes are found. determines the abilities that a bacterial cell has. For example, depending on their shape, microorganisms have one or another degree of mobility, the ability to attach to a surface, and one or another way of absorbing nutritional compounds.

A bacterial cell includes three essential structures: a cytoplasmic membrane, ribosomes and a nucleoid.

From the membrane on the outer side there are several layers. In particular, there is a mucous membrane, capsule, and cell wall. In addition, various surface structures develop on the outside: villi, flagella. Cytoplasm and membrane are combined into the concept of “protoplast”.

A bacterial cell with all its contents is limited from the external environment by a membrane. Inside, in the homogeneous fraction of the cytoplasm, proteins, soluble RNA, substrates of metabolic reactions, and various compounds are located. The rest contains various structural elements.

Does not contain nuclear membranes or any other intracytoplasmic membranes that are not derivatives of the cytoplasmic membrane. At the same time, some prokaryotes are characterized by local “protrusions” of the main shell. These “protrusions” - mesosomes - perform various functions and divide the bacterial cell into functionally different parts.

All the data necessary for life is contained in one DNA. The chromosome that a bacterial cell includes usually has the shape of a covalently closed ring. At one point, DNA is attached to the membrane and placed in a separate, but not separated from the cytoplasm, structure. This structure is called "nucleoid". When unfolded, the bacterial chromosome is more than a millimeter long. It is usually presented in one copy. In other words, prokaryotes are almost all haploid. However, under certain specific conditions, a bacterial cell can contain copies of its chromosome.

It is of particular importance in the life of the bacterium. However, this structural element is not mandatory. In laboratory conditions, some forms of prokaryotes were obtained in which the wall was completely or partially absent. These bacteria could exist under normal conditions, but in some cases they lost the ability to divide. In nature, there is a group of prokaryotes that do not contain walls in their structure.

On the outer surface of the wall there may be an amorphous layer - a capsule. The mucous layers are separated from the microorganism quite easily; they have no connection with the cell. The covers also have a fine structure; they are not amorphous.

Reproduction of some forms of bacteria is carried out through equal-sized, binary transverse fission or budding. Different groups have different division options. For example, in cyanobacteria, reproduction occurs in a multiple way - several successive binary fissions. As a result, from four to a thousand new microorganisms are formed. They have special mechanisms through which the plasticity of the genotype is ensured, necessary for adaptation to a changing external environment and evolution.

In addition to the 5 kingdoms of living nature, there are two more super-kingdoms: prokaryotes and eukaryotes. Therefore, if we consider the systematic position of bacteria, it will be as follows:

Why are these organisms classified as a separate taxon? The thing is that a bacterial cell is characterized by the presence of certain features that leave an imprint on its life activity and interaction with other creatures and humans.

Discovery of bacteria

Ribosomes are tiny structures scattered in large numbers in the cytoplasm. Their nature is represented by RNA molecules. These granules are the material by which the degree of relationship and systematic position of a particular type of bacterium can be determined. Their function is the assembly of protein molecules.

Capsule

The bacterial cell is characterized by the presence of protective mucous membranes, the composition of which is determined by polysaccharides or polypeptides. Such structures are called capsules. There are micro- and macrocapsules. This structure is not formed in all species, but in the vast majority, that is, it is not obligatory.

What does the capsule protect the bacterial cell from? From phagocytosis by host antibodies if the bacterium is pathogenic. Or from drying out and exposure to harmful substances, if we talk about other types.

Mucus and inclusions

Also optional structures of bacteria. Mucus, or glycocalyx, is chemically a mucoid polysaccharide. It can be formed both inside the cell and by external enzymes. Highly soluble in water. Purpose: attachment of bacteria to the substrate - adhesion.

Inclusions are microgranules in the cytoplasm of various chemical natures. These may be proteins, amino acids, nucleic acids or polysaccharides.

Organoids of movement

The characteristics of a bacterial cell are also manifested in its movement. For this purpose, flagella are present, which can be in different numbers (from one to several hundred per cell). The basis of each flagellum is the protein flagellin. Thanks to elastic contractions and rhythmic movements from side to side, the bacterium can move in space. The flagellum is attached to the cytoplasmic membrane. The location may also vary between species.

Drank

Even thinner than flagella are structures that take part in:

• attachment to the substrate;
• water-salt nutrition;
• sexual reproduction.

They consist of the protein pilin, their number can reach several hundred per cell.

Similarities to plant cells

Bacterial and have one undeniable similarity - the presence of a cell wall. However, while in plants it is undeniably present, in bacteria it is not present in all species, that is, it is an optional structure.

Chemical composition of the bacterial cell wall:

• peptidoglycan murein;
• polysaccharides;
• lipids;
• proteins.

Typically, this structure has a double layer: outer and inner. Performs the same functions as plants. Maintains and defines the constant shape of the body and provides mechanical protection.

Education dispute

We have looked at the structure of a bacterial cell in some detail. It remains only to mention how bacteria can survive unfavorable conditions without losing their viability for a very long time.

They do this by forming a structure called a dispute. It has nothing to do with reproduction and only protects bacteria from unfavorable conditions. The form of disputes can be different. When normal environmental conditions are restored, the spore initiates and germinates into an active bacterium.

Bacterial cell structure

The structure of bacteria has been well studied using electron microscopy of whole cells and their ultrathin sections. A bacterial cell consists of a cell wall, a cytoplasmic membrane, cytoplasm with inclusions, and a nucleus called the nucleoid. There are additional structures: capsule, microcapsule, mucus, flagella, pili (Fig. 1); Some bacteria are capable of forming spores under unfavorable conditions.

Cell wall - a strong, elastic structure that gives the bacterium 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 transport of metabolites. The thickest cell wall is found 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, and 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.

Volutin Mesosoma 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 glycerol and ribitol residues connected by phosphate bridges. The shape and strength of bacteria is given by the rigid fibrous structure of peptidoglycan, which is multilayered and cross-linked with peptides. Peptidoglycan is represented by parallel glycan molecules consisting of repeating residues N-acetylglucosamine and N-acetylmuramic acid connected by a P-type glycosidic bond (1 -> 4).

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

U 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 is stable, in contrast to the peptidoglycan of gram-positive bacteria, the amino acids of which may differ in composition and sequence. 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 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 when stained using Gram stain (blue-violet color of bacteria) is associated with the property of multilayer peptidoglycan to interact with the dye. In addition, subsequent treatment of a bacterial smear with alcohol causes a narrowing of the pores in the peptidoglycan and thereby retention of the dye in the cell wall. After exposure to alcohol, gram-negative bacteria lose their dye, become discolored, and when treated with magenta, turn red. This is due to a smaller amount of peptidoglycan (5-10% of the cell wall mass).

The cell wall of gram-negative bacteria contains outer membrane, connected via lipoprotein to 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 there is lipopolysaccharide(LPS), consisting of three components: lipid A, core part, or core (lat. core - core), and an 0-specific polysaccharide chain formed by repeating oligosaccharide sequences.

Lipopolysaccharide is “anchored” in the outer membrane by lipid A, causing the toxicity of LPS, which is therefore identified 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 core part of the LPS, comes off. The most constant part of the LPS core is ketodeoxyoctonic acid (3-deoxy-g)-manno-2-octulosonic acid). 0 -a specific chain extending from the core part of the LPS molecule determines serogroup, serovar (a type of bacteria detected using immune serum) a specific strain of bacteria. Thus, the concept of LPS is associated with the concept of 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 porinami, 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. When the synthesis of the bacterial cell wall is disrupted 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 lacking a cell wall; spheroplasts - bacteria with a partially preserved cell wall. After removal of the cell wall inhibitor, such altered bacteria can reverse, i.e. acquire a full cell wall and restore its original shape.

Bacteria of the sphero- or protoplast type, which have lost the ability to synthesize peptidoglycan under the influence of antibiotics or other factors and are capable of reproducing, are called L-shapes(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), when the factor that led to changes in bacteria is removed, can reverse, “returning” to the original bacterial cell. L- forms can be formed by many pathogens of infectious diseases.

Cytoplasmic membrane in 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 that seem to penetrate through the structure of the membrane. Some of them are permeases involved in the transport of substances. The cytoplasmic membrane is a dynamic structure with mobile components, so it is thought of 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 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 is not fully understood. It is even suggested that they are an artifact that occurs after preparing (fixing) a specimen for electron microscopy. Nevertheless, it is believed that derivatives of the cytoplasmic membrane participate in cell division, providing energy for the synthesis of the cell wall, and take part in the secretion of substances, sporulation, 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 - ribosomes responsible for the synthesis (translation) of proteins. Bacterial ribosomes have a size of about 20 nm and a sedimentation coefficient 70S, 3 difference from 80^-ribosomes characteristic of eukaryotic cells. Therefore, some antibiotics, by binding to bacterial ribosomes, suppress bacterial protein synthesis without affecting protein synthesis in eukaryotic cells. Bacterial ribosomes can dissociate into two subunits - 50S And 30S . The cytoplasm contains various inclusions in the form of glycogen granules, polysaccharides, poly-p-butyric acid and polyphosphates (volutin). They accumulate when there is an excess of nutrients in the environment and act as reserve substances for nutrition and energy needs. Volutin 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 the diphtheria bacillus in the form of intensely stained cell poles.

Nucleoid - equivalent to 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. Unlike eukaryotes, the bacterial nucleus does not have a nuclear envelope, nucleolus, or basic proteins (histones). Typically, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring. If division is disrupted, it may contain 4 or more chromosomes. The nucleoid is detected in a light microscope after staining using DNA-specific methods: Feulgen or Romanovsky-Giemsa. In electron diffraction patterns of ultrathin sections of bacteria, the nucleoid appears as light zones with fibrillar, thread-like structures of DNA bound in certain areas to the cytoplasmic membrane or mesosome involved in chromosome replication.

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

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

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

Many bacteria form microcapsule - mucous formation less than 0.2 microns thick, detectable only by electron microscopy. It should be distinguished from a capsule mucus - 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 the 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 and are longer than the cell itself (Fig. 3). The thickness of the flagella is 12-20 nm, length 3-12 µm. The number of flagella in different species of bacteria varies from one (monotrich) cholera vibrio has up to tens and hundreds of flagella extending along the perimeter of the bacterium (peri-trich) in Escherichia coli, Proteus, etc. Lophotrichs have a bundle of flagella at one end of the cell. Amphitrichy have one flagellum or a bundle of flagella at opposite ends of the cell. Flagella are attached to the cytoplasmic membrane and cell wall by special discs. Flagella consist of a protein - flagellin (from naT.flagellum - flagellum), which has antigen specificity. Flagellin subunits are twisted in the form of a spiral. Flagella are detected using electron microscopy of preparations coated with heavy metals, or in a light microscope after treatment with 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. Escherichia coli. Electron diffraction pattern (preparation by V.S. Tyurin). 1 - flagella, 2 - villi, 3 - F-pili.

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

Controversy - a peculiar form of resting firmicute bacteria, i.e. bacteria with a gram-positive type of 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, like mushrooms.

Spore-forming aerobic bacteria in which the spore size does not exceed the diameter of the cell are sometimes called bacilli. Spore-forming anaerobic bacteria in which the spore size exceeds the diameter of the cell and therefore takes on a spindle shape 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 of prospore, and then the formation of the spore shell of dipicolinic acid and calcium ions. After the formation of all structures, the spore acquires heat resistance, 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 allows them to be distinguished 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 (causative agent of tetanus), subterminal - closer to the end of the stick (pathogens of botulism, gas gangrene) and central (anthrax bacillus).

Bacteria, despite their apparent simplicity, have a well-developed cell structure that is responsible for many of their unique biological properties. Many structural details are unique to bacteria and not found among archaea or eukaryotes. However, despite the relative simplicity of bacteria and the ease of growing individual strains, many bacteria cannot be grown in the laboratory, and their structures are often too small to study. Therefore, although some principles of bacterial cell structure are well understood and even applied to other organisms, most of the unique features and structures of bacteria are still unknown.

cell morphology

Most bacteria are either spherical in shape, the so-called coci (from the Greek word kokkos- grain or berry), or rod-shaped, the so-called bacilli (from the Latin word bacillus- stick). Some rod-shaped bacteria (vibrios) are somewhat bent, while others form spiral curls (spirochetes). All this diversity of bacterial forms is determined by the structure of their cell wall and cytoskeleton. These forms are important for bacterial function because they can influence the bacteria's ability to obtain nutrients, attach to surfaces, move, and escape from predators.

Bacteria size

Bacteria can have a wide range of shapes and sizes (or morphologies). In size, bacterial cells are typically 10 times smaller than eukaryotic cells, of course being only 0.5-5.0 µm at their largest size, although giant bacteria such as Thiomargarita namibiensis And Epulopiscium fishelsoni, can grow up to 0.5 mm in size and be visible to the naked eye. The smallest free-living bacteria are mycoplasmas, members of the genus Mycoplasma only 0.3 microns in length, approximately equal in size to the largest viruses.

Small size is important for bacteria because it results in a large surface area to volume ratio, aiding rapid transport of nutrients and excretion of waste. A low surface area to volume ratio, on the contrary, limits the metabolic rate of the microbe. The reason for the existence of large cells is unknown, although it appears that the large volume is used primarily to store additional nutrients. However, there is also a smallest size of free-living bacterium. According to theoretical calculations, a spherical cell with a diameter of less than 0.15-0.20 microns becomes incapable of independent reproduction, since it physically does not contain all the necessary biopolymers and structures in sufficient quantities. Recently, nanobacteria (and similar nanobes And ultramicrobacteria), having sizes smaller than “acceptable” ones, although the existence of such bacteria is still in question. They, unlike viruses, are capable of independent growth and reproduction, but require a number of nutrients that they cannot synthesize from the host cell.

Cell membrane structure

As in other organisms, the bacterial cell wall provides the structural integrity of the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor caused by much higher concentrations of proteins and other molecules inside the cell compared to those around it. The bacterial cell wall differs from the wall of all other organisms by the presence of peptidoglycan (role-N-acetylglucosamine and N-acetomuramic acid), which is located directly outside the cytoplasmic membrane. Peptidoglycan is responsible for the rigidity of the bacterial cell wall and, in part, for determining the shape of the cell. It is relatively porous and does not resist the penetration of small molecules. Most bacteria have cell walls (with a few exceptions, such as mycoplasma and related bacteria), but not all cell walls have the same structure. There are two main types of bacterial cell walls, gram-positive and gram-negative bacteria, which are distinguished by Gram staining.

Cell wall of gram-positive bacteria

The cell wall of Gram-positive bacteria is characterized by the presence of a very thick layer of peptidoglycan, which is responsible for the staining of gentian violet dye during the Gram staining procedure. Such a wall is found exclusively in organisms belonging to the phyla Actinobacteria (or gram-positive bacteria with high %G+C) and Firmicutes (or gram-positive bacteria with low %G+C). Bacteria in the Deinococcus-Thermus group can also stain positive for Gram stains, but contain some cell wall structures typical of Gram-negative organisms. The cell walls of Gram-positive bacteria have polyalcohols built into them called techoic acid, some of which are associated with lipids to form lipochoic acids. Because lipochoic acids covalently bind to lipids within the cytoplasmic membrane, they are responsible for linking peptidoglycan to the membrane. Techoic acid provides gram-positive bacteria with a positive electrical benefit due to phosphodiesterate bonds between the monomers of techoic acid.

Cell wall of gram-negative bacteria

Unlike Gram-positive bacteria, Gram-negative bacteria contain a very thin layer of peptidoglycan, which is responsible for the inability of cell walls to contain crystal violet dye during the Gram staining procedure. In addition to the peptidoglycan layer, gram-negative bacteria have a second, so-called outer membrane, located outside the cell wall and assembles phospholipids and lipopolysaccharide on its outer side. Negatively charged lipopolysaccharide also provides the cell with a negative electrical charge. The chemical structure of the outer membrane lipopolysaccharide is often unique to individual strains of bacteria and is often responsible for the reaction of antigens with members of those strains.

outer membrane

Like any phospholipid bilayer, the outer membrane is quite impermeable to all charged molecules. However, protein channels (dip) present in the outer membrane allow the passive transport of many ions, sugars and amino acids across the outer membrane. Thus, these molecules are present in the periplasmic, the layer between the outer and cytoplasmic membranes. The periplasmic contains a layer of peptidoglycan and many proteins that are responsible for hydrolysis and reception of extracellular signals. It is read that periplasma is gel-like, not liquid, due to its high protein and peptidoglycan content. Signals and vital substances from the periplasmic membrane enter the cell cytoplasm using transport proteins in the cytoplasmic membrane.

Bacterial cytoplasmic membrane

The bacterial cytoplasmic membrane is composed of a bilayer of phospholipids, and therefore has all the general functions of the cytoplasmic membrane, acting as a permeability barrier to most molecules and enclosing transport proteins that regulate the transport of molecules into cells. In addition to these functions, energy cycling reactions also occur on bacterial cytoplasmic membranes. Unlike eukaryotes, bacterial membranes (with some exceptions, such as mycoplasmas and methanotrophs) generally do not contain sterols. However, many bacteria contain structurally related compounds, called hopanoids, that presumably perform the same function. Unlike eukaryotes, bacteria can have a wide variety of fatty acids in their membranes. Along with the typical saturated and unsaturated fatty acids, bacteria may contain fatty acids with additional methyl, hydroxy or even cyclic groups. The relative proportions of these fatty acids can be adjusted by the bacterium to maintain optimal membrane fluidity (for example, during changes in temperature).

Surface structures of bacteria

Villi and fimbriae

Villi and fimbriae (pili, fimbriae)— oriental in structure the surface structures of bacteria. At first these terms were introduced separately, but now similar structures are classified as types I, IV and sex villi, but many other types remain unclassified.

Genital villi are very long (5-20 microns) and are present on the bacterial cell in small quantities. They are used to exchange DNA during bacterial conjugation.

Type I villi or fimbriae are short (1-5 microns), extend from the outer membrane in many directions, and are tubular in shape, present in many members of the phylum Proteobacteria. These fibers are usually used to attach to surfaces.

Type IV villi or fimbriae are of medium length (about 5 microns), located at the poles of bacteria. Type IV villi help to attach to surfaces (for example, during the formation of biofilms), or to other cells (for example, animal cells during pathogenesis). Some bacteria (for example, Myxococcus) use type IV villi as a mechanism of movement.

S-layer

On the surface, outside the peptidiglycan layer or outer membrane, there is often a protein S-layer. Although the function of this layer is not fully known, it is believed that this layer provides chemical and physical protection to the cell surface and may serve as a macromolecular barrier. It is also believed that S-layers may have other functions, for example, they can serve as pathogenicity factors in Campylobacter and contain external enzymes in Bacillus stearothermophilus.

Capsules and mucus

Many bacteria secrete extracellular polymers outside their cell walls. These polymers are usually composed of polysaccharides and sometimes proteins. Capsules are relatively impermeable structures that cannot be dyed with many dyes. They are generally used to attach bacteria to other cells or non-living surfaces when forming biofilms. They have a varied structure from a disorganized mucous layer of cellular polymers to highly structured membrane capsules. Sometimes these structures are involved in protecting cells from engulfment by eukaryotic cells, such as macrophages. Also, mucus secretion has a signaling function for slow-moving bacteria and is possibly used directly for the movement of bacteria.

flagella

Perhaps the most easily recognized extracellular structure of a bacterial cell is the flagella. Bacterial flagella are filamentous structures that actively rotate around their axis using a flagellar motor and are responsible for the movement of many bacteria in a liquid environment. The location of the flagella depends on the type of bacteria and there are several types. Cell flagella are complex structures consisting of many proteins. The filament itself is composed of flagellin (FlaA), which forms a tubular-shaped filament. The basal motor is a large protein complex that spans the cell wall and both membranes (if present), forming the rotational motor. This motor moves due to the electrical potential on the cytoplasmic membrane.

secretion systems

In addition, specialized secretion systems are located in the cytoplasmic membrane and cell membrane, the structure of which depends on the type of bacterium.

Internal structure

Compared to eukaryotes, the intracellular structure of a bacterial cell is somewhat simpler. Bacteria contain almost no membrane organelles like eukaryotes. Of course, the chromosome and ribosomes are the only easily visible intracellular structures found in all bacteria. Although some groups of bacteria contain complex, specialized intracellular structures, a few of them are discussed below.

Cytoplasm and cytoskeleton

The entire interior of a bacterial cell within the inner membrane is called the cytoplasm. The homogeneous fraction of the cytoplasm, containing a set of soluble RNA, proteins, products and substrates of metabolic reactions, is called cytosol. The other part of the cytoplasm is represented by various structural elements, including the chromosome, ribosomes, bacterial cytoskeleton and others. Until recently, it was believed that bacteria do not have a cytoskeleton, but now orthologues or even homologs of all types of eukaryotic filaments have been found in bacteria: microtubules (FtsZ), actin (MreB and ParM) and intermediate filaments (Crestentin). The cytoskeleton has many functions, often responsible for cell shape and intracellular transport.

Bacterial chromosome and plasmids

Unlike eukaryotes, the bacterial chromosome is not located in the inner part of the membrane-bounded nucleus, but is located in the cytoplasm. This means that the transfer of cellular information through the processes of translation, transcription and replication occurs within the same compartment and its components can interact with other structures of the cytoplasm, in particular ribosomes. The bacterial chromosome is not packaged using histones like eukaryotes, but instead exists as a compact, supercoiled structure called a nucleoid. Bacterial chromosomes themselves are circular, although there are examples of linear chromosomes (for example, in Borrelia burgdorferi). Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids, which often encode individual proteins that are beneficial but of little importance to the host bacterium. Plasmids can be easily acquired or lost by bacteria and can be transferred between bacteria as a form of horizontal gene transfer.

Ribosomes and protein complexes

In most bacteria, numerous intracellular structures are ribosomes, the site of protein synthesis in all living organisms. Bacterial ribosomes are also somewhat different from eukaryotic and archaeal ribosomes and have a sedimentation constant of 70S (as opposed to 80S in eukaryotes). Although the ribosome is the most abundant intracellular protein complex in bacteria, other large complexes are sometimes observed using electron microscopy, although in most cases their purpose is unknown.

internal membranes

One of the main differences between a bacterial cell and a eukaryotic cell is the absence of a nuclear membrane and, often, the absence of membranes at all within the cytoplism. Many important biochemical reactions, such as energy cycle reactions, occur due to ionic gradients across membranes, creating a potential difference like a battery. The lack of internal membranes in bacteria means that these reactions, such as electron transfer in electron transport chain reactions, occur across the cytoplasmic membrane, between the cytoplasm and the periplasm. However, in some photosynthetic bacteria there is a developed network of cytoplasmic photosynthetic membranes derived from them. In purple bacteria (eg. Rhodobacter) they have retained a connection with the cytoplasmic membrane, which is easily detected on sections under an electron microscope, but in cyanobacteria this connection is either difficult to find or lost in the process of evolution.

granules

Some bacteria form intracellular granules to store nutrients such as glycogen, polyphosphate, sulfur, or polyhydroxyalkanoates, giving the bacteria the ability to store these substances for later use.

gas vesicles

Gas vesicles are spindle-shaped structures found in some floating bacteria that provide buoyancy to the cells of these bacteria, reducing their overall density. They consist of a protein shell that is very impermeable to water but penetrable to most gases. By adjusting the amount of gas present in its gas vesicles, the bacterium can increase or decrease its overall density and thus move up or down within the water column, maintaining itself in an environment optimal for growth.

Carboxysomes

Carboxysomes are intracellular structures found in many autotrophic bacteria, such as Cyanobacteria, nitrous bacteria and Nitrobacteria. These are protein structures that resemble viral particles in morphology, and contain the carbon dioxide fixation enzymes in these organisms (especially ribulose bisphosphate carboxylase/oxygenase, RuBisCO, and carbonic anhydrase). It is believed that the high local concentration of enzymes together with the rapid conversion of bicarbonate to carbonic acid by carbonic anhydrase allows faster and more efficient fixation of carbon dioxide than is possible within the cytoplasm.

Such structures are known to contain coenzyme B12-containing glycerol dehydratase, a key enzyme in the fermentation of glycerol to 1,3-propanediol in some members of the family Enterobacteriaceae (e.g. Salmonella).

Magnetosomes

A well-known class of membrane organelles in bacteria that more closely resemble eukaryotic organelles but may also be associated with the cytoplasmic membrane are magnetosomes, present in magnetotactic bacteria.

Bacteria on the farm

With the participation of bacteria, fermented milk products (kefir, cheese) and otsotic acid are obtained. Certain groups of bacteria are used to produce antibiotics and vitamins. Used for pickling cabbage and tanning leather. And in agriculture, bacteria are used for the production and storage of green animal feed.

It's a pity on the farm

Bacteriaii can spoil food. By settling in products, they produce toxic substances for both humans and animals. If the serum and poisoned drugs are NOT applied in a timely manner, a person may die! Therefore, be sure to wash vegetables and fruits before eating!

Spores and inactive forms of bacteria

Some bacteria of the phylum Firmicutes are capable of forming endospores, allowing them to withstand extreme environmental and chemical conditions (for example, gram-positive Bacillus, Anaerobacter, Heliobacterium And Clostridium). In almost all cases, one endosprora is formed, so this is not a reproductive process, although Anaerobacter can form up to seven endospores per cell. Endospores have a central nucleus composed of cytoplasm containing DNA and ribosomes, surrounded by a layer of plug and protected by an impenetrable and rigid membrane. Endospores do not exhibit any metabolism and can withstand extreme physicochemical pressures, such as high levels of ultraviolet radiation, gamma radiation, detergents, disinfectants, heat, pressure and drying. In this inactive state, these organisms, in some cases, can remain viable for millions of years and survive even in outer space. Endospores can cause diseases, for example anthrax can be caused by inhalation of endospores Bacillus anthracis.

Methane-oxidizing bacteria in the genus Methylosinus also form spores that are resistant to drying, the so-called exospores, because they are formed by budding at the end of the cell. Exospores do not contain diaminopicolinic acid, a characteristic component of endospores. Cysts are other inactive, thick-walled structures formed by members of the genera Azotobacter, Bdellovibrio (bdelocysts), And Myxococcus (myxospores). They are resistant to drying and other harmful effects, but to a lesser extent than endopores. When cysts form, representatives Azotobacter, cell division ends with the formation of a thick multilayered wall and membrane surrounding the cell. Filamentous Actinobacteria form reproductive spores of two categories: conditioniospores, which are chains of spores formed from mycelium-like threads, and sporangiospores, which are formed in specialized sacs, sporangia.

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