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

According to scientists, bacteria are over 3.5 billion years old. They existed on Earth long before the advent of highly organized organisms. Being at the origins of life, bacterial organisms received an elementary structure according to the prokaryotic type, characterized by the absence of a formed nucleus and nuclear membrane. One of the factors that influenced the formation of their biological properties is the shell of bacteria (cell wall).

The bacterial wall is designed to perform several fundamental functions:

be the skeleton of a bacterium;
give it a certain shape;
communicate with the external environment;
protect from the harmful effects of environmental factors;
participate in the division of a bacterial cell that does not have a nucleus and a nuclear envelope;
hold antigens and various kinds of receptors on its surface (typical for gram-negative bacteria).

Certain types of bacteria have an outer capsule, which is durable and serves to maintain the integrity of the microorganism for a long time. In this case, the shell in bacteria is an intermediate form between the cytoplasm and the capsule. Some bacteria (for example, leuconostoc) have the peculiarity of encapsulating several cells in one capsule. This is called a zoogel.

The chemical composition of the capsule is characterized by the presence of polysaccharides and a large amount of water. The capsule may also allow the bacterium to attach itself to a particular object.

How easily a substance penetrates through the shell depends on the degree of its absorption by the bacterium. Molecules with long chain sections, which are resistant to biodegradation, have a high penetration probability.

What is a shell?

The bacterial membrane consists of lipopolysaccharides, proteins, lipoproteins, teichoic acids. The main component is murein (peptidoglycan).

The thickness of the cell wall can be different and reach 80 nm. The surface is not continuous, it has pores of various diameters through which the microbe receives nutrients and releases its waste products.

The significance of the outer wall is evidenced by its significant weight - it can vary from 10 to 50% of the dry mass of the entire bacterium. The cytoplasm can protrude, changing the external relief of the bacterium.

From above, the shell can be covered with cilia or flagella can be located on it, which consist of flagellin, a specific substance of a protein nature. For attachment to the bacterial membrane, flagella have special structures - flat discs. Bacteria with one flagellum are called monotrichous, those with two flagella are called amphitriches, those with a bunch are called lophotrichs, and those with many bunches are called peritrichs. Microorganisms that do not have flagella are called atrichia.

The cell wall has an inner part that begins to form after the completion of cell growth. Unlike the outer, it consists of a much smaller amount of water and has greater elasticity and strength.

The process of synthesis of the walls of microorganisms begins inside the bacterium. To do this, it has a network of polysaccharide complexes that alternate in a certain sequence (acetylglucosamine and acetylmuramic acid) and are linked by strong peptide bonds. The assembly of the wall is carried out outside, on the plasma membrane, where the shell is located.

Since the bacterium does not have a nucleus, it does not have a nuclear envelope.

The shell is an unstained thin structure, which cannot even be seen without special staining of the cells. For this, plasmolysis and a darkened field of view are used.

Gram stain

To study the detailed structure of the cell in 1884, Christian Gram proposed a special method for its coloring, which was later named after him. Gram stain divides all microorganisms into Gram-positive and Gram-negative. Each species has its own biochemical and biological properties. Different coloration is also due to the structure of the cell wall:

Gram positive Bacteria have a massive shell that includes polysaccharides, proteins and lipids. It is durable, the pores have a minimum size, the paint used for coloring penetrates deeply and is practically not washed out. Such microorganisms acquire a blue-violet color.
Gram negative bacterial cells have certain differences: their wall thickness is less, but the shell has two layers. The inner layer consists of peptidoglycan, which has a looser structure and wide pores. The Gram stain washes out easily with ethanol. The cell becomes discolored. In the future, the technique provides for the addition of a contrasting red dye, which stains the bacteria red or pink.

The proportion of gram-positive microbes that are harmless to humans is much higher than gram-negative ones. To date, three groups of gram-negative microorganisms that cause disease in humans have been classified:

cocci (streptococci and staphylococci);
non-spore-forming forms (corynebacteria and listeria);
spore-forming forms (bacilli, clostridia).

Characteristics of the periplasmic space

Between the bacterial wall and the cytoplasmic membrane is the periplasmic space, which consists of enzymes. This component is an obligatory structure; it makes up 10-12% of the dry mass of the bacterium. If the membrane is destroyed for some reason, the cell dies. Genetic information is located directly in the cytoplasm, not separated from it by the nuclear envelope.

Regardless of whether the microbe is gram-positive or gram-negative, it is the osmotic barrier of the microorganism, the transporter of organic and inorganic molecules deep into the cell. A certain role of the periplasm in the growth of the microorganism has also been proven.

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

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

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

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

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

Nucleoid - bacterial equivalent of the nucleus. It is located in the central zone of bacteria in the form of double-stranded DNA, closed in a ring and tightly packed like a ball. Unlike eukaryotes, the nucleus of bacteria does not have a nuclear envelope, nucleolus, and basic proteins (histones). Usually, a bacterial cell contains one chromosome, represented by a DNA molecule closed in a ring. If division is disturbed, it may contain 4 or more chromosomes. The nucleoid is detected in a light microscope after staining with DNA-specific methods: according to Feulgen or Romanovsky-Giemsa. On electron diffraction patterns of ultrathin sections of bacteria, the nucleoid has the form of light zones with fibrillar, thread-like structures of DNA associated with certain areas with the cytoplasmic membrane or mesosome involved in chromosome replication.

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

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

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

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

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

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

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

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

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

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

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

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 membrane 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.

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. it
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 “seeds” 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 exit of the virus from the host cell and is a modified section of the nuclear or outer cytoplasmic membrane of the host cell.

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

The 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.

Bacteria (grains)

The shape and structure of bacterial cells

Bacteria are the most ancient group of living organisms, having dimensions that most often do not exceed 0.5 microns. Their structure can only be seen under an electron microscope (Fig. 2.1). Bacteria do not have mitochondria, lysosomes, Golgi complex, endoplasmic reticulum. They have no plastids, no formalized nucleus, and the nuclear substance (DNA) is represented by one ring-shaped chromosome (nucleoid), located directly in the cytoplasm, but attached to the cytoplasmic membrane at one point. There are many ribosomes in the cytoplasm, in which protein synthesis proceeds intensively. Most bacteria are colorless, but some are green or purple. Bacteria are the most common organisms in nature, they are classified as prokaryotes, i.e. prenuclear organisms.

Rice. 2.1. Bacterium

The shape of bacteria is varied. Some of them look like single balls - cocci, which can pair up - diplococci, four - tetracocci, form chains - streptococci. Accumulations of cocci have the form of packages - sarcina or bunch of grapes - staphylococci. Some bacteria are elongated in the form of rods - bacilli, others are bent in the form of a comma - vibrios, or several times along the entire length - spirilla (Fig. 2.2).

Rice. 2.2. Bacteria cell shapes:

1 - cocci; 2, 3 - diplococci; 4 - streptococci; 5 - tetracocci; 6 - staphylococci; 7 - sarcins; 8, 9 - bacilli; 10 - chains of bacilli; 11 - vibrios; 12 - spirilla; 13 - flagellated, 14 - ciliated

Many bacteria have organelles of movement- one or more flagella. Bacteria that do not have flagella, but are covered with mucus on the outside, are also capable of gliding movement. Some water and soil bacteria, in particular cyanobacteria, can rise and fall by regulating the amount of gas in the gas vacuoles present in the cytoplasm.

The bacterial cell is covered with a membrane cytoplasmic membrane and cell wall(Fig. 2.3). The membrane is made up of proteins and lipids. It is semi-permeable and ensures selective entry of substances into the cell and release of decay products into the environment. On the surface of the invaginations of the cytoplasmic membrane inside the bacterium, called mesosomes, there are oxidative enzymes that take part in the respiration process. Such invaginations of the membrane play the role of mitochondria and some other organelles that are absent in the bacterial cell. In bacteria capable of photosynthesis (cyanobacteria, green bacteria, etc.), photosynthetic pigments are localized on the mesosomes.

Rice. 2.3. Scheme of the structure of a bacterial cell:

1 - ribosomes; 2 - cell membrane; 3 - mucous capsule; 4 - nucleoid; 5 - cell wall; 6 - flagellum; 7 - mesosome

The cell wall is also permeable to nutrients and waste products. It has a strong lattice of mureins (peptidoglycans), gives the bacteria a certain shape and protects it from environmental influences. In some bacteria, the cytoplasmic membrane and cell wall take part in the formation of another, outer layer of the membrane - capsules. A capsule is a semi-liquid mucous mass that covers the outside of the cell wall. It performs a protective function.

The bacterial cell as a whole is arranged quite simply. It is separated from the external environment by a cytoplasmic membrane and filled with cytoplasm, in which there is a nucleoid zone, including a circular DNA molecule, from which the transcribed mRNA can “hang”, to which, in turn, ribosomes are attached, synthesizing protein on its matrix simultaneously with the process of synthesizing the protein itself. matrices. At the same time, DNA can be associated with proteins that carry out its replication and repair. Bacterial ribosomes are smaller than eukaryotic ones and have a sedimentation coefficient of 70S. They, like eukaryotic ones, are formed by two subunits - a small one (30S), which includes 16S rRNA, and a large one - 50S, including 23S and 5S rRNA molecules.

The photograph obtained with the help of transmission microscopy (Fig. 1) clearly shows a bright zone in which the genetic apparatus is located and the processes of transcription and translation take place. Ribosomes are visible as small granular inclusions.

Most often, in a bacterial cell, the genome is represented by only one DNA molecule, which is closed in a ring, but there are exceptions. Some bacteria may have more than one DNA molecule. For example, Deinococus radiodurans, a bacterium known for its phenomenal resistance to radiation and the ability to safely withstand a dose of radiation 2,000 times the lethal dose for humans, has two copies of its genomic DNA. Bacteria are known to have three or four copies. Some species of DNA may not be closed in a circle, and some Agrobacterium contain one circular and one linear DNA.

In addition to the nucleoid, the genetic material can be present in the cell in the form of additional small circular DNA molecules - plasmids. Plasmids replicate independently of the nucleoid and often contain genes that are useful for the cell, giving the cell, for example, resistance to antibiotics, the ability to assimilate new substrates, the ability to conjugate, and much more. Plasmids can be transferred both from the mother cell to the daughter cell, and by horizontal transfer they can be transferred from one cell to another.

A bacterial cell is most often surrounded not only by a membrane, but also by a cell wall, and according to the type of device of the cell wall, bacteria are divided into two groups - gram-positive and gram-negative.

The cell wall of bacteria is formed by peptidoglycan - murein. At the molecular level, the murein layer is a network formed by molecules of N-acetylglucosamine and N-acetylmuramic acid, cross-linked into long chains by β-1-4-glycosidic bonds, adjacent chains, in turn, are connected by transverse peptide bridges (Fig. 2) . So it turns out one big network surrounding the cell.

Gram-positive bacteria have a thick cell wall that sits on top of a membrane. Murein is cross-linked with another type of molecules - teichoic and lipoteichoic (if they are connected to membrane lipids) acids. It is believed that these molecules give the cell wall elasticity under transverse compression and tension, acting as springs. Because the murein layer is thick, it stains easily with the Gram method: the cells appear bright purple because the dye (gentian or methyl violet) gets stuck in the cell wall layer.

In gram-negative bacteria, the murein layer is very thin (cyanobacteria are an exception), therefore, when stained by Gram, the violet dye is washed out, and the cells are stained in the color of the second dye (Fig. 3).

The cell wall of gram-negative bacteria is covered on top with another, outer, membrane attached to the peptidoglycan by lipoproteins. The space between the cytoplasmic membrane and the outer membrane is called the periplasm. The outer membrane contains lipopolyproteins, lipopolysaccharides (LPS), as well as proteins that form hydrophilic pores. The components of the outer membrane are often responsible for the interaction of the cell with the external environment. It contains antigens, phage receptors, molecules involved in conjugation, etc.

Since Gram-positive and Gram-negative cells differ in the structure of the integument (Fig. 4, top), the apparatus that anchors the flagellum in the cell integument also differs (Fig. 4, bottom).

The flagellum of gram-positive bacteria is anchored in the membrane by two protein rings (S-ring and M-ring) and driven by a system of proteins that, consuming energy, make the filament spin. In gram-negative bacteria, in addition to this design, there are two more rings that additionally fix the flagellum in the outer membrane and cell wall.

The bacterial flagellum itself consists of the flagellin protein, the subunits of which are connected into a helix that has a cavity inside and forms a thread. The thread is flexibly attached to the anchoring and torsion apparatus using a hook.

In addition to flagella, there may be other outgrowths on the surface of bacterial cells - pili. These are protein villi that allow bacteria to adhere to various surfaces (increasing the hydrophobicity of the cell) or take part in the transport of metabolites and the conjugation process (F-pili).

A bacterial cell usually does not contain any membrane structures inside, including vesicles, but there may be various kinds of inclusions (reserve lipids, sulfur) and gas bubbles surrounded by a protein membrane. Without a membrane, a cell can store polysaccharide molecules, cyanophycin (as a nitrogen depot), and can also contain carboxysomes - vesicles containing the enzyme RuBisCO, which is necessary for fixing carbon dioxide in the Calvin cycle.

In microbiology, this term refers to a nutrient that can be taken up by a microorganism.

This name of the groups comes from the name of the doctor G.K. Gram, who developed a method for staining bacterial cell walls, which makes it possible to distinguish between cells with different types of cell wall structure.

Ribulose bisphosphate carboxylase/oxygenase