Bacteria are made up of one cell. Morphology of microorganisms

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 residues of N-acetyl-M-glucosamine and N-acetylmuramic acid 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 sharply differ 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 (pathogens of brucellosis, tuberculosis, listeria, etc.), have the ability to L-transformation.

L-forms are of 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 connecting 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 the places of their localization in bacteria of different species are not the same, but they 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 arise 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 entry 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. It consists of cytosol - a homogeneous fraction, including soluble RNA components, substrate substances, enzymes, metabolic products, and structural elements - ribosomes, intracytoplasmic membranes, inclusions and a 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. High temperature resistance is associated with a very low water content and a high content of dipicolinic acid. 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 species 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.

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 plasma membrane 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 participate 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 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, 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 the environment and serve 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 membrane, 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), - filamentous 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 common 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 the 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 (causative agent of tetanus), subterminal - closer to the end of the stick (causative agents of botulism, gas gangrene) and central (anthrax bacillus).

Despite their apparent simplicity, bacteria are complex organisms. Bacterial cells are composed of a protoplast and a membrane.

The main structural components of a bacterial cell are: the cell wall, the cytoplasmic membrane, the cytoplasm with inclusions, and the nucleus, called the nucleoid. Bacteria can also have additional structures: capsule, microcapsule, mucus, flagella. Many bacteria are able to form spores.

The cell wall is a strong, elastic structure that gives bacteria a certain shape and restrains high osmotic pressure in the wall. It is involved in the process of cell division and the transport of metabolites. The cell wall of bacteria contains a small amount of polysaccharides, lipids and proteins. The cell wall of bacteria performs a number of functions: it is the outer barrier of the cell, establishing contact between the microorganism and the environment; having a high degree of strength, it withstands the internal pressure of the protoplast in a hypotonic solution.

The cytoplasmic membrane is a three-layer structure and surrounds the outer part of the bacterial cytoplasm. It is an essential polyfunctional structural element of the cell. The cytoplasmic membrane makes up 8-15% of the dry mass of the cell. It is involved in the regulation of osmotic pressure, the transport of substances and the energy metabolism of the cell (due to the enzymes of the electron transport chain, ATPase, etc.). Oxidative enzymes and electron transport enzymes are localized on the membrane. The chemical composition of the cytoplasmic membrane is represented by a protein-lipid complex, in which proteins account for 50-70%, lipids - 15-50%. A small amount of carbohydrates was found in the cytoplasmic membrane of some bacteria. Phospholipids are the main lipid component of the membrane. The protein fraction of the cytoplasmic membrane is represented by structural proteins with enzymatic activity.

The fluid-mosaic model of membranes belongs to the structure of the cytoplasmic membrane of bacteria. According to this model, the membrane is formed by a fluid biolayer of lipids, which includes asymmetrically arranged protein molecules.

The cytoplasm of bacteria occupies the bulk of the cell and consists of soluble proteins. The cytoplasm is represented by structural elements: ribosomes, inclusions and nucleoid. Ribosomes of prokaryotes have a sedimentation constant of 70S. Ribosome diameter is 15 - 20 nm. The number of ribosomes in a bacterial cell can be different. Thus, in a rapidly growing Escherichia coli cell, there are about 15,000 ribosomes. The process of protein biosynthesis in a cell is carried out by polysomes. Sometimes a polysome contains several dozen ribosomes.

Nucleoid (formation similar to the nucleus) is the equivalent of the nucleus in bacteria. The nucleoid is located in the central zone of bacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a coil. Unlike eukaryotes, the bacterial nucleus does not have a nuclear envelope, nucleolus, or major proteins. Often a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring. The nucleoid is detected under a light microscope after DNA staining using Feulgen or Giemsa methods.

Some bacteria (pneumococci, etc.) form a capsule - a mucous formation, firmly associated with the cell wall, with clearly defined outer boundaries. In pure cultures of bacteria, the capsule is formed less frequently. It is detected with special staining methods that create a negative contrast of the capsule substance. The capsule consists of polysaccharides, sometimes polypeptides. The capsule is hydrophilic and prevents phagocytosis of bacteria. Many bacteria form a microcapsule - a mucous formation that is detected by electron microscopy.

The main function of the capsule is protective. It protects the cell from the action of various kinds of adverse environmental factors. In many bacteria, the capsule is covered with mucus on the outside. In soil microorganisms in a hot arid climate, the mucous layer protects the cell from drying out.

In the protoplast, cytoplasm, nucleus-like formations and various inclusions are distinguished.

Cytoplasm (protoplasm) has a very complex, changing chemical composition. The main chemical compounds of the cytoplasm are proteins, nucleic acids, lipids; contains a large amount of water. microbiological prokaryote bacterial cell

The thin surface layer of the cytoplasm adjacent to the membrane, denser than the rest of its mass, is called the cytoplasmic membrane (Fig. 2). It is semipermeable and plays an important role in the exchange of substances between the cell and the environment. The cytoplasmic membrane consists of three layers: one lipid layer and two protein layers adjacent to it on both sides. It contains 60-65% protein and 35-40% lipids; it contains many enzymes.

Modern research methods using an electron microscope have shown that the cytoplasm is inhomogeneous. In addition to the structureless semi-liquid, viscous mass, which is in a colloidal state, it is penetrated by membranes in places; it contains microscopic structurally formed particles of various shapes and sizes. These are ribosomes rich in ribonucleic acid (RNA) scattered in the cytoplasm in the form of small grains. They are about 60% RNA and 40% protein. One bacterial cell contains thousands and tens of thousands of ribosomes; they carry out the synthesis of cell proteins.

In addition to ribosomes, special membrane (lamellar) structures of various shapes, called mesosomes, have been found. They are formed by branching and invagination of the cytoplasmic membrane into the cell cavity. In mesosomes, the processes of oxidation of organic substances, which are a source of energy, take place; here substances with a large supply of energy are synthesized, for example, adenosine triphosphoric acid (ATP). Bacterial mesosomes are thus analogues of mitochondria of other organisms (yeast, plants, animals).

In addition to these formations, where the most important metabolic processes of the cell take place, the cytoplasm also contains a variety of inclusions that are reserve nutrients: grains of glycogen (a starch-like substance), fat drops, granules of volutin (metachromatin), consisting mainly of polyphosphates, etc. In the cells of some bacteria are coloring substances - pigments.

The nucleus, morphologically formed and typical of cells of other organisms (eukaryotes), is absent in bacteria.

Modern research methods have made it possible to identify formations similar to the nucleus in the cells of true bacteria, which are called nucleoids. However, the nuclear substance concentrated in certain places of the cell (more often in the center) is not delimited from the cytoplasm by a membrane, and the shape of these nucleus-like structures is not constant.

Bacteria and organisms close to them (spirochetes, mycoplasmas, actinomycetes) as they do not have a true nucleus are called prokaryotes (pre-nuclear organisms).

The shell of bacterial cells, which is often called the cell wall, is dense, has a certain elasticity and elasticity. It determines the relative constancy of the shape of the cell, serves as protection against adverse external influences, and participates in the metabolism of the cell. The shell is permeable to water and low molecular weight substances. In an electron microscope, it is easily distinguishable from the cytoplasm, has a layered structure.

The chemical composition of the shell is quite complex and heterogeneous in different bacteria; its supporting frame is a complex polysaccharide peptide called murein (from Latin murus - wall). In addition to murein, there are other components: lipids, polypeptides, polysaccharides, teicic acids, amino acids, in particular diaminopimelic, which is absent in other organisms. The ratio of these substances in the cell membranes of different bacteria varies considerably.

The difference in the chemical composition of the cell membranes of bacteria affects their ability to stain according to the Gram method. On this basis, bacteria are distinguished between gram-positive (staining) and gram-negative (not staining). The shells of gram-positive bacteria contain more polysaccharides, murein and teichoic acids. The shells of gram-negative bacteria have a multilayer structure, they contain a high content of lipids in the form of lipoproteins and lipopolysaccharides.

The shell of some bacteria can be mucilaginous. The mucous layer surrounding the shell is very thin and approaches the limit of visibility under a conventional light microscope. It can also reach a considerable thickness, forming the so-called capsule. Often the size of the capsule is much larger than the size of the bacterial cell. The mucus of the membranes is sometimes so strong that the capsules of individual cells merge into mucous masses, in which bacterial cells (zoogleys) are interspersed. Mucous substances produced by some bacteria are not retained in the form of a compact mass around the cell membrane, but diffuse into the environment.

The chemical composition of mucus is different in individual species, but may be the same. Of great importance is the composition of the nutrient medium on which bacteria develop. Various polysaccharides (dextrans, glucans, levans), as well as nitrogen-containing substances (such as polypeptides, protein polysaccharides, etc.) were found in the composition of bacterial mucus.

The intensity of mucus formation largely depends on environmental conditions. In many bacteria, mucus formation is stimulated, for example, by cultivation at low temperatures. Mucus-forming bacteria, when rapidly multiplying in liquid substrates, can turn them into a continuous slimy mass. A similar phenomenon, causing significant losses, is sometimes observed in the production of sugar in sugary extracts from beets. The causative agent of this defect is the bacterium leukonostoc (Leuconostoc mesenteroides). In a short time, sugar syrup can turn into a viscous slimy mass. Meat, sausages, cottage cheese are subjected to mucus; viscous can be milk, brines of pickled vegetables, beer, wine.

Sizes - from 1 to 15 microns. Basic forms:

Forms of bacteria:

mesosomes

mureina gram-positive(stained by Gram) and gram negative

nucleoid. Plasmids episome.

Many bacteria have flagella(10) and pili (fimbriae)

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sporulation

Reproduction.

Conjugation

Transformation

transduction

Viruses

Virus sizes are 10–300 nm. Virus shape:

capsid Supercapsid

virion

The structure of bacterial cells

The first bacteria appeared probably more than 3.5 billion years ago and for nearly a billion years were the only living beings on our planet. Currently, they are ubiquitous and determine various processes occurring in nature.

The shape and size of bacteria

Bacteria are single-celled microscopic organisms. They have the form of sticks, balls, spirals. Some species form clusters but several thousand cells. The length of rod-shaped bacteria is 0.002-0.003 mm. Therefore, even with a microscope, individual bacteria are very difficult to see. However, they are easy to spot with the naked eye when they develop in large numbers and form colonies. Under laboratory conditions, colonies of bacteria are grown on special media containing the necessary nutrients.

The bacterial cell, like the cells of plants, fungi and animals, is covered with a plasma membrane. But unlike them, a dense cell wall is located on the outside of the membrane. It consists of a durable substance and performs both protective and supporting functions, giving the cell a permanent shape. Through the cell membrane, nutrients freely pass into the cell, and unnecessary substances go into the environment. Often, an additional protective layer of mucus is produced on top of the cell membrane in bacteria - a capsule.

What is the structure of bacterial cells, or is everything as simple as it seems

The nuclear substance, unlike the nucleus, is not separated from the cytoplasm. Due to the absence of a formed nucleus and other structural features of the cell, all bacteria are combined into a separate kingdom of living nature - the kingdom of Bacteria.

Distribution of bacteria and their role in nature

Bacteria are the most common living things on Earth. They live everywhere: in water, air, soil. Bacteria are able to live even where other organisms cannot survive: in hot springs, in the ice of Antarctica, in underground oil fields, and even inside nuclear reactors. Each bacterial cell is very small, but the total number of bacteria on earth is enormous. This
associated with a high rate of bacterial growth. Bacteria perform a variety of functions in nature.

The role of bacteria in the formation of fuel minerals is great. For millions of years, they decomposed the remains of marine organisms and land plants. As a result of the vital activity of bacteria, deposits of oil, natural gas, and coal were formed.

The structure of a bacterial cell

Sizes - 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).

Forms of bacteria:
1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirilla and spirochetes.

The structure of a bacterial cell:
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 other organic substances. 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 10–20 nm in diameter. 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.

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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.

Conjugation- 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 chains into the recipient cell through the F-pill, 3) synthesis of a complementary chain 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 "excretes" a small fragment of DNA from itself, or the DNA enters the environment after the death of this cell.

Bacteria cell. Structure

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.

Virus sizes are 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 release 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 structure of a bacterial cell

Sizes - 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).

Forms of bacteria:
1 - cocci; 2 - bacilli; 3 - vibrios; 4-7 - spirilla and spirochetes.

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

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.

The structure of a bacterial cell: features. What is the structure of a bacterial cell?

These subunits are arranged in a spiral and form a hollow cylinder 10–20 nm in diameter. 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.

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

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

Viruses

Virus sizes are 10–300 nm. Virus shape: spherical, rod-shaped, filiform, cylindrical, etc.

The shape and size of bacteria

The structure of a bacterial cell

The bacterial cell, like the cells of plants and animals, is covered with a plasma membrane. But unlike them, a dense cell wall is located on the outside of the membrane. It consists of a durable substance and performs both protective and supporting functions, giving the cell a permanent shape. Through the cell membrane, nutrients freely pass into the cell, and unnecessary substances go into the environment. Often, an additional protective layer of mucus is produced on top of the cell membrane in bacteria - a capsule.

On the surface of the cell membrane of some bacteria there are outgrowths - long flagella (one, two or more) or short thin villi. They help bacteria move around. In the cytoplasm of a bacterial cell there is a nuclear substance - a nucleoid, which carries hereditary information. The nuclear substance, unlike the nucleus, is not separated from the cytoplasm. Due to the absence of a formed nucleus and other features, all bacteria are combined into a separate kingdom of living nature - the kingdom of Bacteria.

Distribution of bacteria and their role in nature

Bacteria are the most common living things on Earth. They live everywhere: in water, air, soil. Bacteria are able to live even where other organisms cannot survive: in hot springs, in the ice of Antarctica, in underground oil fields, and even inside nuclear reactors. Each bacterial cell is very small, but the total number of bacteria on earth is enormous. This
associated with a high rate of bacteria. Bacteria perform a variety of functions in nature.