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1. Growth and reproduction of bacteria

Bacterial growth is an increase in bacterial cell size without increasing the number of individuals in the population.

Reproduction of bacteria is a process that ensures an increase in the number of individuals in a population. Bacteria are characterized by a high reproduction rate.

Growth always precedes reproduction. Bacteria reproduce by transverse binary fission, in which two identical daughter cells are formed from one mother cell.

The process of bacterial cell division begins with the replication of chromosomal DNA. At the point of attachment of the chromosome to the cytoplasmic membrane (replicator point), an initiator protein acts, which causes rupture of the chromosome ring, and then despiralization of its threads occurs. The threads unwind, and the second thread attaches to the cytoplasmic membrane at the pro-replicator point, which is diametrically opposite to the replicator point. Due to DNA polymerases, an exact copy of each strand is completed along the matrix. Doubling of genetic material is a signal for doubling the number of organelles. In the septal mesosomes, a septum is being built that divides the cell in half.

Double-stranded DNA is helicalized, twisted into a ring at the point of attachment to the cytoplasmic membrane. This is a signal for the cells to disperse along the septum. Two daughter individuals are formed.

On solid nutrient media, bacteria form clusters of cells - colonies, varying in size, shape, surface, color, etc. On liquid media, bacterial growth is characterized by the formation of a film on the surface of the nutrient medium, uniform turbidity or sediment.

The reproduction of bacteria is determined by the generation time. This is the period during which cell division occurs. The duration of generation depends on the type of bacteria, age, composition of the nutrient medium, temperature, etc.

Phases of bacterial cell reproduction on a liquid nutrient medium:

1) initial stationary phase; the amount of bacteria that has entered the nutrient medium and remains in it;

2) lag phase (resting phase); duration – 3–4 hours, bacteria adapt to the nutrient medium, active cell growth begins, but there is no active reproduction yet; at this time the amount of protein and RNA increases;

3) phase of logarithmic reproduction; the processes of cell reproduction in the population are actively underway, reproduction prevails over death;

4) maximum stationary phase; bacteria reach maximum concentration, i.e. the maximum number of viable individuals in the population; the number of dead bacteria is equal to the number of bacteria formed; there is no further increase in the number of individuals;

5) phase of accelerated death; the processes of death prevail over the process of reproduction, since the nutrient substrates in the environment are depleted. Toxic products and metabolic products accumulate. This phase can be avoided if you use the flow cultivation method: metabolic products are constantly removed from the nutrient medium and nutrients are replenished.

The term “growth” when applied to microorganisms means an increase in the size of an individual, and “reproduction” means an increase in the number of individuals in a population. As a microbial cell grows, its volume increases much faster than its surface, so the distribution of nutrients in the cell's cytoplasm becomes less efficient and the cell divides. Before dividing, DNA molecules double. Each daughter cell receives a copy of the mother's DNA.

The rate of reproduction of different microbes grown under the same conditions is different. For most bacteria, the generation period (the time elapsed) organisms can use a large set of oxidizable organic compounds, most often glucose. Energy is obtained from these compounds as a result of their oxidation or, more precisely, the donation of electrons by them.

The set of biochemical processes, as a result of which the energy necessary for the life of a cell is released, is called respiration, or biological oxidation. In relation to microorganisms, they speak of anaerobic and aerobic types of respiration.

Between two successive cell divisions) is on average 15-30 minutes; for example for coli—15–17 min, pathogens typhoid fever- 23 min, Corynebacterium diphtheria - 34 min. Mycobacterium tuberculosis divides more slowly - once every 18 hours, spirochetes - every 10 hours.

Reproduction methods various groups microorganisms are not the same: bacteria, rickettsia, spirochetes reproduce by transverse division into two equal individuals. Gram-positive bacteria divide by forming a septum growing from the periphery to the center. In Mycobacterium tuberculosis, a transverse septum is formed inside the cell, then it splits into two layers and the cell is divided into two parts. Both the cytoplasmic membrane and the cell wall take part in the formation of the septum. Apparently, the mesosome, closely associated with the cytoplasmic membrane, takes an active part in the process of bacterial division. Gram-negative bacteria and rickettsia become thinner in the center and are divided into two individuals by the constriction. Reproduction of nodule bacteria and Fraicisella occurs through the formation of a bud, which is smaller in size than the original cell. Bacteria also have a process of conjugation - a temporary connection of two individuals.

The growth of bacteria and spirochetes is not always accompanied by their division. Bile salts, soaps, penicillin, ultraviolet rays delay cell division, resulting in the formation long threads much bigger size than the original cells.
When bacteria are introduced into a nutrient medium, their growth and reproduction phases are distinguished, which are determined by the availability of available food sources and the accumulation of toxic metabolic products (Fig. 21).

The first phase - latent (lag phase) - corresponds to the adaptation of bacteria to new living conditions. During this period, the bacteria adapt to the nutrient medium and their growth is not observed.

The second phase is logarithmic growth (exponential), when bacteria grow vigorously, increase in size, and upon reaching a certain size begin to divide into two daughter cells. Division during this period occurs at a constant rate. The average generation (or doubling) time for each bacterial species is different. At this time, bacteria extract nutrients from the environment, as a result of which metabolic products begin to accumulate in it.

The third phase is stationary growth, during which the number of organisms in the culture remains constant all the time. During this period, the amount of nutrients in the nutrient medium decreases significantly, and the accumulation of metabolic products increases. Living conditions for microorganisms are becoming less and less favorable. The duration of the stationary phase varies among bacteria.

The fourth phase is death, when bacterial cells become fewer and fewer and die. At the end of this phase, the number of dying bacteria begins to prevail over the number of viable cells. Complete death of microbes in a culture can occur after several weeks or months, depending on the type of microbe, the reaction of the environment and other factors.

Protozoa can reproduce by transverse division, constriction into two equal individuals - amoebas, and by longitudinal division - trypanosomes, lamblia, balantidia. Before dividing into two individuals, balantidia can exchange their nuclei - micronuclei (the process of conjugation); the malarial plasmodium has an asexual and sexual development cycle.

Viruses multiply (reproduce) only inside a living cell of the host.

The virus reproduction process consists of several stages:

1) penetration of the virus into the cell;

2) intracellular reproduction;

3) maturation of the virus and the formation of outer shells in some viruses; 4) isolation of the virus from the cell.

The process of virus penetration into a sensitive cell begins with its adsorption on the surface of a cell that has specific viral receptors. The process of releasing nucleic acid from the capsid and outer shells begins in the cytoplasmic membrane of the cell and ends in the cytoplasm (influenza virus, vaccinia).

The phase of intracellular reproduction of the virus, or its reproduction, usually begins with the processes of suppression of cellular macromolecular synthesis. All energy systems of the cell, its enzymes, RNA, ribosomes begin to work to reproduce the virus. The affected cell supplies the virus with nucleotides to build nucleic acids, amino acids - for proteins. Replication (English replicate - copy, repeat) of viral RNA is carried out with the help of enzymes - polymerases, and the RNA molecule of the virus itself serves as the matrix. In DNA-containing viruses, specific RNA is synthesized on a DNA matrix in the cell nucleus, which then determines the synthesis of viral DNA and protein. Viral proteins are synthesized in cell ribosomes.

The maturation of the viral particle, the enclosure of the viral nucleic acid in the capsid, occurs in the nucleus of the affected cell (herpesviruses, adenoviruses) or in the cytoplasm (smallpox viruses, rhabdoviruses, picornaviruses). The formation of outer shells in myxoviruses and togaviruses occurs when passing through the cytoplasmic membrane of the host cell. The herpes virus is part of its outer shell receives by passing through the membrane of the cell nucleus.

Isolation of a virus from a cell can occur in different ways. Myxoviruses and togaviruses, as they mature, can be secreted by the cell for hours without damaging it. The polio virus (which does not have an outer shell) forms quickly in a cell, remains in it for a long time and is released instantly, in the form of an outbreak. The end result interactions between the virus and the host cell can result in rapid destruction and death of the cell. Sometimes viruses can be present in a cell for a long time without causing its death, and remain in an infinite number of cellular generations - latent viruses. In some cases, the virus can be destroyed by the cell without visible consequences for her (abortive viral infection).

Bacterial growth- This is an increase in the number, mass and size of the entire microbial cell, starting immediately after its division. Growth is inextricably linked with reproduction.

Reproduction in bacteria– process of self-reproduction of a microbial cell. It begins by dividing the nucleoid DNA into two daughter strands, each of which is then completed by a complementary strand, while the formation of two daughter cells occurs simultaneously (semi-conservative method). Bacteria multiply transverse division a sharp increase in the number of cells in the population, the process is repeated at regular intervals (from several minutes to several days), being an individual genetic characteristic of the microbial species. During division, either two identical cells or two asymmetrical (polymorphic) cells can be formed.

Bacteria are characterized by a high rate of reproduction on various nutrient media, which is characterized by generation time. This is the time between two cell divisions, passing from the moment the cell appears to the moment of division (for example, the generation time of E. coli is 20 minutes, the causative agent of tuberculosis is 14 hours). The rate of reproduction depends on the type of bacteria and cultivation conditions ( chemical composition nutrient medium, its state of aggregation, pH, temperature, aeration, gas composition, presence of nutrients and growth stimulants, etc.). When bacteria multiply on solid nutrient media, they form colonies- the offspring of one cell, visually determined on (or in) a nutrient medium. Isolated Colonies are clusters of microbes of the same species, and, as a rule, of the same clone.

Appearance colonies in some bacteria it can be very peculiar, being typical for some microorganisms. For example, pathogen colonies anthrax compared to the “mane of a lion” or “the head of a jellyfish”, colonies of the plague pathogen look like a “lace scarf”, etc.

To characterize colonies growing on nutrient media, a number of standard parameters are used - macroscopic characteristic .

According to the shape of the colony There are regular - round, or irregular - amoeboid and rhizoid, reminiscent of intertwined tree roots. Depending on their size, colonies are distinguished as pinpoint (diameter less than 1 mm), small (diameter I - 2 mm), medium (diameter 2 - 4 mm) and large (diameter 4 - 6 mm or more).

Color determined by the type of pigment (white, yellow, red, etc. - Fig. 25 - appendix). Pigmented colonies, for example, are found in Staphylococcus (white, lemon yellow or golden), in Sarcinus the pigment color is yellow, in bacteria of the genus Serratia red, in yeast-like fungi Candida albicans white. Many pathogenic bacteria do not produce pigment; their colonies are transparent or opalescent.

By consistency Colonies of bacteria are often soft, mucous or dense, crumbly. By character of the regions There are smooth edges in the form of a clearly defined line and uneven edges - scalloped and wavy. Surface Colonies can be matte or shiny with gloss, dry or wet, smooth or rough. Smooth colonies are designated by the letter S (smooth), rough colonies by the letter R (rough).

When growing bacteria on a liquid nutrient medium, a sequential change of individual phases in the reproduction of the bacterial population is observed (Fig. 9):

1. Initial phase (lag phase). Cell reproduction does not occur; microbes adapt to the nutrient medium, increase in size, accumulate enzymes, and DNA replication begins. At the end of the phase, the slow proliferation of microbes begins.

2. Exponential phase (log phase) characterized maximum speed reproduction, while the number of bacteria increases exponentially.

3. Stationary phase, in which there is a balance between the number of newly formed cells and the number of dead cells.

4. Dying phase. During this phase, cell death occurs.

The amount of biomass is determined by its dry mass, as well as the content of bacterial nitrogen, protein, DNA, and phosphorus.

The vital activity of bacteria is characterized by growth- the formation of structural and functional components of the cell and the increase in the bacterial cell itself, as well as reproduction-- self-reproduction, leading to an increase in the number of bacterial cells in the population.

Bacteria multiply by binary fission in half, less often by budding. Actinomycetes, like fungi, can reproduce by spores. Actinomycetes, being branching bacteria, reproduce by fragmentation of filamentous cells. Gram-positive bacteria divide by ingrowth of synthesized division septa into the cell, and gram-negative bacteria by constriction, as a result of the formation of dumbbell-shaped figures, from which two identical cells are formed.

Cell division is preceded by replication of the bacterial chromosome according to a semi-conservative type (the double-stranded DNA strand opens and each strand is completed by a complementary strand), leading to doubling of the DNA molecules of the bacterial nucleus - the nucleoid.

DNA replication occurs in three stages: initiation, elongation, or chain growth, and termination.

Reproduction of bacteria in a liquid nutrient medium. Bacteria seeded in a certain, unchanging volume of the nutrient medium, multiplying, consume nutrients, which subsequently leads to the depletion of the nutrient medium and the cessation of bacterial growth. Cultivation of bacteria in such a system is called batch cultivation, and the culture is called batch culture. If the cultivation conditions are maintained by the continuous supply of fresh nutrient medium and the outflow of the same volume of culture fluid, then such cultivation is called continuous, and the culture is called continuous.

When bacteria are grown on a liquid nutrient medium, bottom, diffuse or surface (in the form of a film) growth of the culture is observed. The growth of a periodic culture of bacteria grown on a liquid nutrient medium is divided into several phases, or periods:

• 1. lag phase;
• 2. logarithmic growth phase;
• 3. phase of stationary growth, or maximum concentration

bacteria;

4. phase of bacterial death.

These phases can be depicted graphically in the form of segments of a bacterial reproduction curve, reflecting the dependence of the logarithm of the number of living cells on the time of their cultivation.

Lag phase-- the period between the sowing of bacteria and the beginning of reproduction. The duration of the lag phase is on average 4-5 hours. At the same time, the bacteria increase in size and prepare to divide; the amount of nucleic acids, proteins and other components increases.

Logarithmic (exponential) growth phase is a period of intense bacterial division. Its duration is about 5-6 hours. optimal conditions bacterial growth can divide every 20-40 minutes. During this phase, bacteria are most vulnerable, which is explained by the high sensitivity of the metabolic components of an intensively growing cell to inhibitors of protein synthesis, nucleic acids, etc.

Then comes the stationary growth phase, at which the number of viable cells remains unchanged, amounting to maximum level(M-concentration). Its duration is expressed in hours and varies depending on the type of bacteria, their characteristics and cultivation.

The death phase completes the bacterial growth process., characterized by the death of bacteria under conditions of depletion of sources of nutrient medium and accumulation of bacterial metabolic products in it. Its duration ranges from 10 hours to several weeks. The intensity of bacterial growth and reproduction depends on many factors, including the optimal composition of the nutrient medium, redox potential, pH, temperature, etc.

Reproduction of bacteria on a solid nutrient medium. Bacteria growing on dense nutrient media form isolated round-shaped colonies with smooth or uneven edges (S- and R-forms), of varying consistency and color, depending on the pigment of the bacteria.

Water-soluble pigments diffuse into the nutrient medium and color it. Another group of pigments is insoluble in water, but soluble in organic solvents. And finally, there are pigments that are insoluble neither in water nor in organic compounds.

The most common pigments among microorganisms are carotenes, xanthophylls and melanins. Melanins are insoluble black, brown or red pigments synthesized from phenolic compounds. Melanins, along with catalase, superoxide mutase and peroxidases, protect microorganisms from the effects of toxic oxygen peroxide radicals. Many pigments have antimicrobial, antibiotic-like effects.