Bacterial cell structure

The cytoplasm of most bacteria is surrounded by membranes: the cell wall, the cytoplasmic membrane and the capsular (mucous) layer. These structures take part in the metabolism, food products enter through the cell membranes and metabolic products are removed. They protect the cell from the action of harmful environmental factors, largely determine the surface properties of the cell (surface tension, electric charge, osmotic state, etc.). These structures in a living bacterial cell are in constant functional interaction.

Cell wall... The bacterial cell is separated from the external environment by the cell wall. Its thickness is 10-20 nm, its mass reaches 20-50% of the cell mass. This is a complex multifunctional system that determines the constancy of the cell shape, its surface charge, anatomical integrity, the ability to adsorb phages, participation in immune reactions, contact with the external environment and protection from adverse external influences. The cell wall is elastic and strong enough to withstand an intracellular pressure of 1-2 MPa.

The main components of the cell wall are peptidoglycans(glycopeptides, mucopeptides, mureins, glycosaminopeptides), which are found only in prokaryotes. A specific heteropolymer peptidoglycan consists of alternating N-acetylglucose-amine and N-acetylmuramic acid residues connected by β-1-4-glycosidic bonds, diaminopimelic acid (DAP), D-glutamic acid, L- and D-alanine in the ratio 1: 1: 1: 1: 2. The glycosidic and peptide bonds that unite the subunits of peptidoglycans give them the structure of a molecular network or bag. Teichoic acids, polypeptides, lipopolysaccharides, lipoproteins, etc. are also included in the network of murein in the cell wall of prokaryotes. The cell wall is rigid, and it is this property that determines the shape of the bacterial wall. The cell wall has tiny pores through which metabolic products are transported.

Gram stain... Most bacteria, depending on their chemical composition, are divided into two groups. This property was first noticed in 1884 by the Danish physicist H. Gram. The essence is that when bacteria are stained with gentian violet (crystal violet, methyl violet, etc.) in some bacteria, paint with iodine forms a compound that is retained by the cells when they are treated with alcohol. Such bacteria are colored blue-violet and are called gram-positive (Gr +). The discolored bacteria are gram-negative (Gr -), they are painted over with a contrasting paint (magenta). Gram stain is diagnostic, but only for prokaryotes with a cell wall.

In terms of structure and chemical composition, gram-positive bacteria differ significantly from gram-negative ones. In gram-positive bacteria, the cell wall is thicker, homogeneous, amorphous, contains a lot of murein, which is associated with teichoic acids. In gram-negative bacteria, the cell wall is thinner, layered, contains little murein (5-10%), teichoic acids are absent.

Table 1.1 Chemical composition of Gr + and Gr-bacteria

Microbiology: lecture notes Tkachenko Ksenia Viktorovna

1. Features of the structure of the bacterial cell. Main organelles and their functions

Differences between bacteria and other cells

1. Bacteria are classified as prokaryotes, that is, they do not have a separate nucleus.

2. The cell wall of bacteria contains a special peptidoglycan - murein.

3. The bacterial cell lacks the Golgi apparatus, endoplasmic reticulum, mitochondria.

4. The role of mitochondria is played by mesosomes - invaginations of the cytoplasmic membrane.

5. There are many ribosomes in a bacterial cell.

6. Bacteria can have special organelles of movement - flagella.

7. The size of bacteria ranges from 0.3-0.5 to 5-10 microns.

According to the shape of the cells, bacteria are subdivided into cocci, rods and crimped ones.

In a bacterial cell, there are:

1) main organelles:

a) nucleoid;

b) cytoplasm;

c) ribosomes;

d) cytoplasmic membrane;

e) cell wall;

2) additional organelles:

b) capsules;

c) villi;

d) flagella.

The cytoplasm is a complex colloidal system consisting of water (75%), mineral compounds, proteins, RNA and DNA, which are part of the organelles of the nucleoid, ribosomes, mesosomes, inclusions.

Nucleoid is a nuclear substance atomized into the cytoplasm of a cell. Has no nuclear membrane, nucleoli. It localizes DNA, represented by a double-stranded helix. Usually closed in a ring and attached to the cytoplasmic membrane. Contains about 60 million base pairs. This is pure DNA, it does not contain histone proteins. Their protective function is performed by methylated nitrogenous bases. The nucleoid encodes the basic genetic information, i.e. the cell genome.

Along with the nucleoid, the cytoplasm can contain autonomous circular DNA molecules with a lower molecular weight - plasmids. They also encoded hereditary information, but it is not vital for the bacterial cell.

Ribosomes are 20 nm ribonucleoprotein particles consisting of two subunits - 30 S and 50 S. Ribosomes are responsible for protein synthesis. Before the start of protein synthesis, these subunits are combined into one - 70 S. Unlike eukaryotic cells, the ribosomes of bacteria are not combined into the endoplasmic reticulum.

Mesosomes are derivatives of the cytoplasmic membrane. Mesosomes can be in the form of concentric membranes, vesicles, tubules, in the form of a loop. Mesosomes are associated with a nucleoid. They are involved in cell division and sporulation.

Inclusions are metabolic products of microorganisms, which are located in their cytoplasm and are used as reserve nutrients. These include inclusions of glycogen, starch, sulfur, polyphosphate (volutin), etc.

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

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

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

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

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

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

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

Features of the cell wall of gram-positive bacteria.

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

Features of the cell wall of gram-negative bacteria.

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

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

L-forms of bacteria.

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

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

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

Flagella. Motile bacteria can be gliding (moving on a hard surface as a result of wave-like contractions) or floating, moving due to filamentous spirally curved proteins ( flagelline by chemical composition) formations - flagella.

According to the location and number of flagella, a number of forms of bacteria are distinguished.

1.Monotrichs have one polar flagellum.

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

3.Amphitrichs - have flagella at diametrically opposite poles.

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

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

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

F- drank (fertility factor)- apparatus conjugation of bacteria, are found in small quantities in the form of thin proteinaceous villi.

Endospores and sporulation.

Spore formation- a method of preserving certain types of bacteria in adverse environmental conditions. Endospores are formed in the cytoplasm, are cells with low metabolic activity and high resistance ( resistance) to drying, the action of chemical factors, high temperature and other undesirable environmental factors. Light microscopy often uses a spore detection method. by Ozheshko... High resistance is associated with a high content of calcium salt of dipicolinic acid in the shell of a dispute. The location and size of spores in different microorganisms is different, which has differential diagnostic (taxonomic) significance. The main phases of the "life cycle" of spores sporulation(includes the preparatory stage, the pre-dispute stage, the formation of the shell, maturation and dormancy) and germination ending in the formation of a vegetative form. The process of sporulation is genetically determined.

Non-cultivated forms of bacteria.

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

The overall structure of the bacterial cell is quite simple. It is separated from the external environment by a cytoplasmic membrane and filled with cytoplasm, in which the nucleoid zone is located, which includes 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 synthesis process itself matrices. Simultaneously, DNA can be associated with proteins that carry out its replication and repair. The ribosomes of bacteria are smaller than eukaryotic ones and have a sedimentation coefficient of 70S. They, like eukaryotic ones, are formed by two subunits - small (30S), which includes 16S rRNA and large - 50S, which includes 23S and 5S rRNA molecules.

The photograph obtained using transmission microscopy (Fig. 1) clearly shows the light 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 have more than one DNA molecule. For example, Deinococus radiodurans, a bacterium known for its phenomenal resistance to radiation and able to safely withstand a dose of radiation 2,000 times the lethal dose to humans, has two copies of its genomic DNA. Bacteria are known to have three or four copies. In some species, the DNA may not be closed in a ring, and some Agrobacterium contain one circular and one linear DNA.

In addition to the nucleoid, genetic material can be presented in the cell in the form of additional small circular DNA molecules - plasmids. Plasmids replicate independently of the nucleoid and often contain genes useful for the cell that give the cell, for example, antibiotic resistance, 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 a peptidoglycan - murein. At the molecular level, the murein layer is a network formed by molecules of N-acetylglucosamine and N-acetylmuramic acid, linked together into long chains by β-1-4-glycosidic bonds, adjacent chains, in turn, are connected by transverse peptide bridges (Fig. 2) ... This creates one large net that surrounds the cell.

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

In gram-negative bacteria, the murein layer is very thin (with the exception of cyanobacteria), therefore, when staining according to 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 by another outer membrane attached to the peptidoclycan with lipoproteins. The space between the cytoplasmic membrane and the outer membrane is called the periplasm. The outer membrane contains lipopolyproteins, lipopolysaccharides (LPS), and 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 the structure of integuments differs in gram-positive and gram-negative cells (Fig. 4, top), the apparatus that anchors the flagellum in the cell integuments is also different (Fig. 4, bottom).

The flagellum of gram-positive bacteria is fixed in the membrane by two protein rings (S-ring and M-ring) and is set in motion by a system of proteins, which, consuming energy, make the thread spin. In addition to this structure, gram-negative bacteria have two more rings that additionally fix the flagellum in the outer membrane and cell wall.

The flagellum itself in bacteria consists of the flagellin protein, the subunits of which are connected into a spiral, which has a cavity inside and forms a filament. The thread is flexibly attached to the anchoring and twisting apparatus by means of a hook.

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

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

In microbiology, this term means a nutrient that can be assimilated by a microorganism.

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

Ribulose bisphosphate carboxylase / oxygenase

The first bacteria appeared, probably more than 3.5 billion years ago, and for almost a billion years were the only living things 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 are in the form of sticks, balls, spirals. Some species form clusters of 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, bacterial colonies are grown on special media containing the necessary nutrients.

Bacterial cell structure

The bacterial cell, like the cells of plants and animals, is covered with a plasma membrane. But in contrast to 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 are released into the environment. Often on top of the cell membrane, bacteria develop an additional protective layer of mucus - 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. With their help, bacteria move. 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 lack of a formed nucleus and other features, all bacteria are combined into a separate kingdom of living nature - the kingdom of bacteria.

The spread of bacteria and their role in nature

Bacteria are the most abundant living creatures on Earth. They live everywhere: in water, air, soil. Bacteria can 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 the high rate of bacteria. Bacteria perform a wide 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.