Cells, like any living organism, are born, live and die. The growth and reproduction of bacteria is very fast, they could capture the entire living space on the planet, if not for their fragility and limiting factors (temperature, acidity level, lack of food, etc.). Under favorable conditions, cell doubling takes an average of about half an hour. However, in critical situations, some types of microorganisms (spore-forming bacteria) are able to form spores and “hibernate” for a rather long period.
The rapid multiplication of bacteria has its pros and cons. The use of microorganisms in biotechnology (yeast, lactic acid, nitrogen-fixing organisms, molds, etc.) is aimed at improving the quality of life. However, the uncontrolled growth of disease-causing (pathogenic) microbes is dangerous for humans. A person's own microflora can also harm health. In medicine, there is the concept of bacterial overgrowth syndrome, in which the number of opportunistic microbes in the human body increases dramatically, which poses a threat to health.
Cell growth and reproduction are two different processes. Growth is understood as an increase in cell mass due to the formation of all cellular structures. Reproduction is the increase in the number of cells in a colony. There are binary fission, budding and genetic recombination (a process resembling sexual reproduction).
Most prokaryotic (non-nuclear) cells, to which all bacteria belong, reproduce by splitting in two (binary fission). In this way, for example, lactic acid bacteria multiply. The process begins with the duplication of the bacterial chromosome (a DNA molecule that replaces the nucleus) and proceeds in several stages:
The result is two identical organisms.
Individual microorganisms divide by budding, but this is rather an exception to the general rule. The process consists in the formation of a short protrusion at one of the poles of the cell, into which one of the halves of the separated nucleoid (DNA molecules with genetic information) “drifts”. Then the protrusion grows and separates from the mother cell.
There is another option that resembles sexual reproduction - genetic recombination. In this case, an exchange of genetic information takes place and the result is a cell containing the genes of its parents. There are three ways to transfer genetic information:
Thus, cells identical to each other are obtained only as a result of binary fission and budding. During genetic recombination, the cell undergoes changes, developing new properties and receiving other functions.
In nutrient media, the growth and reproduction of bacteria take place in several stages, which differ in the amount of food available and the accumulation of waste products:
The rate of kinetic growth of a bacterial colony largely depends on the type of bacteria, the composition of nutrient media, the number of seeded (introduced into the medium) cells, the age of the culture, the method of respiration, and a number of other factors. For example, for the reproduction of lactic acid bacteria, it is important to maintain temperatures in a rather narrow range (25-30⁰С) and a certain level of acidity of the medium (pH). For the reproduction of aerobic and anaerobic cells, the presence or absence of oxygen for respiration becomes a decisive factor, and spore-forming cells need a sufficient amount of food.
For study (medicine, microbiology) and use (industry), bacterial cultures are grown on artificial nutrient media, which are divided according to consistency, origin and purpose:
There are bacteria that require special conditions. For example, anaerobic microorganisms (both spore-forming and non-spore-forming) are cultivated under anaerobic conditions (without oxygen). For aerobic cells, oxygen becomes a decisive factor in reproduction. Facultative anaerobes are able to change the way they breathe depending on the conditions. The spore-forming aerobic organisms used to produce probiotics are very sensitive to reduced nutrition and quality. Spore-forming anaerobes require the complete absence of oxygen. The basic principle of the cultivation of microorganisms is the creation of favorable conditions (nutrition, respiration, temperature), which sometimes presents certain difficulties.
So, for the cultivation of anaerobes, the deep seeding method is used, i.e., the culture of bacteria is introduced into the depth of a dense nutrient medium, chemical substances that absorb oxygen are added to the growth atmosphere, or air is pumped out, replacing it with an inert gas. In the case of spore-forming bacteria, an inhibitor of protein synthesis is introduced into the nutrient medium, thereby stopping the spore formation process.
Cultivation refers to the artificial cultivation of cells under controlled conditions. The ultimate goal is to obtain a biological product from bacteria or with the help of bacteria. Such drugs can be therapeutic, diagnostic, prophylactic. There are several cultivation methods:
One of the most important industrial preparations is the culture of lactic acid bacteria, which are used for the preparation of lactic ferment, sauerkraut, forage ensiling, and the production of a blood plasma substitute. To obtain a guaranteed end result, the resulting quality of lactic acid bacteria must be strictly controlled.
You need an appropriate nutrient medium and a preparation with a pure culture of lactic acid bacteria grown in the laboratory. Further, the cultivation process is left until the onset of the third phase (equilibrium), after which you can begin to collect the "harvest" of lactic acid bacteria.
Not always the growth of bacterial cells is beneficial, an excessive increase in the population of bacteria in the human body can be hazardous to health. Violation of the qualitative and quantitative composition of the intestinal microflora is called the clinical syndrome of bacterial overgrowth. Doctors say that it is not entirely correct to use the term "dysbacteriosis" to describe this process. The fact is that the number of anaerobic bacteria (bifidobacteria) useful for the body is actually decreasing, but the number of conditionally pathogenic cells (for example, aerobic Escherichia coli) is increasing.
Various bacteria live in different parts of the gastrointestinal tract. In the small intestine, as you move, the composition of the microflora and the number of microorganisms gradually change. Aerobic (growing in an oxygen environment) species of bacteria gradually give way to anaerobic (oxygen-free environment). In the clinical syndrome of overgrowth, the bacterial spectrum shifts towards gram-negative (most pathogenic), facultative aerobic and anaerobic organisms.
As you approach the large intestine, the number of anaerobic bacteria (bifidobacteria and bacteroids) increases. The main representatives of the anaerobic microflora - bifidobacteria - are responsible for the synthesis of proteins, B vitamins, various acids and other substances necessary for life. Aerobic microorganisms (E. coli) produce a number of vitamins and acids involved in digestion and supporting immunity.
Lactic acid bacteria are another representative of the intestinal microflora. They belong to microaerophilic organisms, i.e. one of the factors for the growth and reproduction of lactic acid bacteria is oxygen, but in very small quantities. These microorganisms are responsible for regulating the acidity of the gastrointestinal tract, thereby inhibiting the growth of putrefactive bacteria.
Each type of bacteria performs its own, clearly defined function. With overgrowth syndrome, the fecal microflora that normally lives in the large intestine (E. coli or anaerobic cells) enters the small intestine. The quantitative and qualitative composition of the bacterial microflora changes, the performance of certain functions slows down or becomes impossible. There are conditions for the growth and reproduction of pathogenic bacteria.
The criterion for the development of bacterial overgrowth syndrome can be:
Clinical manifestations of bacterial overgrowth syndrome are easily confused with other diseases, often they overlap each other, completely distorting the picture. In such cases, it is possible to make a diagnosis only with the help of special tests aimed at identifying the overgrowth syndrome, which determine not only the number, but also the species of bacteria. This approach will allow you to select the necessary medications to correct the composition of the microflora.
Clinical symptoms of the disease:
Antibacterial drugs are used to treat overgrowth syndrome. In the future, to restore the microflora, probiotic and prebiotic preparations will be needed.
A wide variety of bacterial cells (autotrophs and heterotrophs, aerobic and anaerobic, spore-forming and non-spore-forming, etc.) dictates certain conditions for their reproduction. The basic principle of cultivation on an industrial scale is the strict control of environmental conditions and growth rates. In nature, there are rarely ideal environments for the development of microorganisms. Otherwise, bacteria would have filled all available space long ago.
Bacteria, like all living organisms, reproduce. This happens most often by simple transverse division in various planes. In this case, various combinations of cells are formed: paired connections, single cells, clusters, chains, packages, etc.
Some microorganisms reproduce by sporulation (actinomycetes and fungi) and budding (yeast), some microorganisms reproduce sexually, but most of them reproduce asexually (vegetatively). Under favorable conditions, reproduction proceeds with extraordinary speed - every 20-30 minutes, the mother bacterial cell divides into two daughter cells. The daughter cell eventually becomes the mother cell and also divides. Thus, the division of bacteria goes exponentially. If such a division proceeded unhindered, then in 48 hours one bacterium could give rise to hundreds of billions of cells, and in five days such a mass that would fill the basins of all the seas and oceans. However, this does not happen, because microorganisms are affected by various environmental factors.
Cell division is preceded by a uniform increase in total nitrogen, RNA and protein in the cytoplasm. Then DNA replication (doubling) occurs. In a dividing cell, hydrogen bonds are broken between the DNA helices and single daughter DNA helices are formed (Fig. 25).
Rice. 25. The process of binary fission of rod-shaped prokaryotes
1 - formation of single strands of DNA;
2 - doubling (replication) of DNA;
3 - stretching of the cell;
Partition formation;
4 - the end of the formation of the septum and the formation of a convex cell wall;
5 - cell division.
Immediately after DNA replication, cell elongation and the formation of a transverse septum begin due to two layers of the cytoplasmic membrane protruding towards each other. Most often, a septum is formed in the middle of the mother cell, as a result of which the daughter cells are approximately the same size. Between the layers of the partition is the formation of the cell wall.
The single helix of DNA in new cells serves as a template for creating a second helix, resulting in the formation of a DNA double helix with restored hydrogen bonds and the formation of a new nucleoid.
In the process of reproduction, one of the halves of the cell constantly retains flagella. At the final stage of bacterial reproduction, flagella also grow in the other half.
The growth and reproduction of microorganisms depends on various environmental factors and species characteristics. Observation of the development of microorganisms cultivated in a liquid nutrient medium in closed tanks shows that biomass growth requires an energy source, the presence of components necessary for biomass synthesis, the absence of inhibitors in the medium that inhibit cell growth, and maintaining the necessary physicochemical conditions in the medium. . Under these conditions, the growth of microorganisms can be conditionally divided into several successive phases or periods (Fig. 26):
Rice. 26. A typical growth curve of a population of microorganisms 1 - lag phase;
2 - phase of accelerated growth; 3 - phase of logarithmic (exponential) growth;
4 - growth retardation phase; 5 - phase of stationary growth; 6 - phase of aging and dying.
1. lag phase (English lag - delay) - the period between the sowing of bacteria and the start of reproduction. During this period, the bacterial culture adapts to the nutrient medium. It manifests itself in the accumulation of the optimal amount of the necessary enzymes, in the inactivation of some inhibitor present in the medium, in the germination of spores, etc. Under favorable conditions, the bacteria increase in size and prepare for division. The lag phase can last from 10 minutes to several hours, but on average it is 4-5 hours.
2. The phase of accelerated growth is observed after the lag phase and is characterized by an increase in the rate of division of microorganisms and the accumulation of biomass.
3. The phase of logarithmic or exponential growth is the period of the most intensive division of bacteria. Bacteria divide every 20-40 minutes. During this phase, bacteria are especially vulnerable, which is explained by the high sensitivity of growing cells to environmental factors. The duration of exponential growth depends on the concentration of nutrients in the substrate and averages 5-6 hours.
4. The deceleration phase is the transitional period from exponential growth to the stationary growth phase. During this phase, there is a depletion of nutrients in the substrate and the accumulation of metabolic products in it, which reduces the intensity of reproduction of microorganisms.
5. The stationary growth phase is caused by gradual depletion of the medium, accumulation of lytic enzymes in it, chemical inhibition of microbial cell growth by metabolic products. This phase differs from the previous one in the increased resistance of bacteria to many chemical and physical factors. By the beginning of this phase, the number of viable cells reaches its maximum level and remains at this maximum for several hours, depending on the type of microorganisms and the characteristics of their cultivation. At the end of this phase, some microorganisms experience the process of sporulation.
6. The final phase of the reproduction process - the phase of aging and death - is characterized by the death of bacteria due to the depletion of the nutrient medium and the accumulation of metabolic products in it. Autolysis of microorganisms is observed as an extreme manifestation of cell instability after growth stops. This phase can last from a few hours to several weeks.
Growth is an increase in the number of chemical components of a microbial cell. To characterize the growth of microorganisms, the concept of bacterial mass is used, which is expressed by the density of bacteria (dry weight per 1 ml). The reproduction of microbes is described by the number of bacteria, reflecting the concentration of cells in 1 ml. There is no strict proportionality between the increase in the number of bacteria and the bacterial mass. This is explained by the fact that not all cells in the bacterial population are viable - some of them are dead, some are at different stages of destruction. Participating in the creation of the bacterial mass, such cells do not participate in the further reproduction of bacteria. Bacteria reproduce by direct division. In this case, a constriction is formed or the cytoplasmic membrane begins to grow inward, perpendicular to the longitudinal axis of the cell with the formation of a disk-cell plate.
This plate can sometimes be incomplete and has a hole in the center of which connects both sister cells. Subsequently, a side wall grows into the cell plate, which forms a transverse partition dividing the cell plate into two parts, each of which goes to one of the formed cells. The central opening, not separated by a transverse septum or plate, is called plasmodesmos. Plasmodesmos plays a role in connecting the cells of some bacteria into long chains or groups. In addition to the above, the process of division of bacterial cells can occur by ligation. The number of bacterial cells in the process of reproduction increases exponentially. For most bacteria, the generation time is 20 - 30 minutes.
Growth and reproduction of bacteria manifest themselves differently depending on the cultivation conditions. On dense nutrient media, a manifestation of the growth and reproduction of bacteria is the appearance of colonies, which are visually distinguishable clusters of bacterial cells. Colonies are characterized by a set of specific features, on the basis of which pure cultures of bacteria can be identified. These signs include: sizes (large, medium, small, microscopic); shape (round, flattened, etc.); color, depending on the formation of pigments by bacteria; surface (convex, flat, matte, shiny, etc.); the nature of the edges (smooth, rough, etc.); consistency (homogeneous, pasty, mucous, etc.); transparency (transparent, cloudy).
During periodic cultivation in bacteria, a successive change in growth phases is observed, which is reflected by the growth curve (Fig. 54).
The growth process begins with a growth retardation phase, or lag phase. During this period, intense metabolic activity of bacteria occurs, the result of which is the preparation of the cell for rapid reproduction. The phase begins with the introduction of bacteria into the medium. Its duration depends on the age of the sown culture (it is longer when the old culture is introduced), the composition of the medium, temperature and other factors.
Growth of a bacterial cell. The increase in the biomass of cellular protoplasm resulting from the synthesis of plastic material in the process of nutrition is called growth. Microbes grow rapidly, reaching within a short time the limit of their physiological maturity.
Cell division. The cell, which has reached a certain mature age, begins to divide, and in the nutrient medium, an increase in the bacterial population of the Ї culture is simultaneously observed.
Cell division is preceded by the formation of a cytoplasmic membrane, which usually forms in the middle of a bacterial cell. During cell division, DNA replication occurs. In this case, hydrogen bonds are broken and two chains (helices) of DNA are formed, each of which is present in daughter cells. Then single-stranded DNAs are joined by hydrogen bonds and double-stranded DNAs reappear with genetic information.
Cell division is considered complete when the newly formed cells are separated by a cytoplasmic septum.
Reproduction - binary fission of bacteria, rickettsiae, protozoa, etc. In this case, two new individuals are formed, endowed with the genetic information of the mother cell. This method of reproduction is called integral, and viruses reproduce in a disjunctive way, that is, by separate synthesis of their components - nucleic acid and protein in the host cell.
Bacterial cells divide faster in the initial stages of population growth. In the later stages, division proceeds more slowly, some of the mother cells die, and various inclusions appear in certain types of bacteria.
In favorable conditions, the rate of reproduction of bacteria is very high. Every 15-20 minutes, two individuals are obtained from one. According to the calculations of some researchers, if a microbe produces two individuals in just one hour, then in a day the number of microbial cells reaches 16.5 million. which can fill the pools of all seas and oceans.
With such reproduction, microbial cells could cover the entire surface of not only the seas and oceans, but also the continents. Consequently, the life of people on our planet would be impossible. However, in the reproduction of microbes there is no absolute law of geometric progression. Their growth and reproduction are adversely affected by the antagonistic relationships of microorganisms, depletion of the nutrient medium, oxygen deficiency, and the accumulation of toxic waste products of microbes. These factors prevent continuous cell division.
Bayle's long-term studies and observations have established that in a liquid nutrient medium in a certain volume, the maximum development of cells with a limiting number occurs. Within 24 hours under the same conditions, the concentration of cells in 1 ml of the liquid medium is established: for E. coli and paratyphoid B bacteria - 1.5 billion, Grigoriev's dysentery bacteria - Shiga and staphylococci - 300 billion, typhoid bacillus - 800 billion. The given numerical expression is accepted call M-k he concentration (M maximum) of microbes. Under normal growing conditions, M-cocentration of cells is the limit of microbial accumulation. It is interesting to note that if such a quantity of microbes is inoculated into a fresh nutrient medium that is equal to the M-concentration, then the number of cells does not increase, and if the number of cultivated microbes exceeds the M-concentration, the extra ones die.
Reproduction of bacteria in a population. To understand the patterns of reproduction of microbes in a population, pure cultures are studied. However, microbes in natural and artificial conditions are found in associations. A bacterial population is a collection of bacteria that multiply in a certain volume of a liquid medium in a test tube, flask, etc. With continuous growth of bacteria on the surface of a dense nutrient medium in a test tube, the totality of all cells in it is considered to be a single population. In the case of the growth of isolated colonies, each of them can be considered a separate population, since they do not communicate with each other.
When microbes are grown on dense nutrient media, certain features of their growth are revealed, i.e., colonies appear, representing the offspring of one or more cells. The appearance of colonies, their shape, color, transparency, size and other properties are the distinguishing features for each type of bacteria. A number of bacterial species that have flagella on agar give a continuous growth covering the entire surface of the dish (Proteus vulgaris). Sporogenic species differ in the nature of the colonies, forming opaque colonies with a matte surface.
The growth of bacterial cells in liquid nutrient media is characterized by uniformity, which is not the case on dense nutrient media. However, even with this method, some features of bacterial growth can be observed. Species that form dry colonies on dense media give a variety of sediments in a clear broth. Species that form soft and moist colonies generally give a homogeneous growth, evenly disturbing the nutrient medium.
For a culture growing in a liquid medium, aeration is of great importance. It is known that in test tubes or flasks, only the upper layers of the liquid come into contact with atmospheric air, and because of this, some obligate aerobes, for example, Mycobacterium tuberculosis, Vibrio cholerae, etc., accumulate on the surface, forming a delicate film.
Phases of growth of a bacterial population. In 1918 Buchanan, studying the characteristics of bacterial reproduction, proposed a curve indicating the number of cells in each period of time. The dynamics of bacterial reproduction is characterized by the following phases, indicated by Roman numerals.
In the initial phase (segment I), bacteria adapt to new living conditions from the moment they are sown on a nutrient medium. In this phase, bacteria do not multiply. The duration of the initial phase is 1-2 hours.
The initial phase of reproduction (section II) is characterized by increased metabolic processes, growth rate and cell division. However, bacterial growth is slow. The duration of this phase is 2 hours.
In the logarithmic phase (segment III), accelerated cell growth and division are observed. In this phase of maximum reproduction, morphological, cultural, biochemical, antigenic, and virulent properties typical of each bacterial species are formed. The duration of the phase is 5-6 hours.
The deceleration phase (segment IV) occurs after the active growth and reproduction of bacterial cells. By this time, the conditions in the environment are changing; poisonous metabolic products accumulate, the supply of nutrients decreases, the pH of the medium does not correspond to the individual needs of individual microbes, hydrogen acceptors are consumed, the release of energy and the rate of cell division slow down, the generation time decreases, the number of dying cells increases. The duration of the phase is 2 hours.
The stationary phase (segment V) is characterized by a constant concentration of living cells in the nutrient medium. Moderate multiplication of cells does not lead to an increase in microbial mass. In this phase, a balance is established between the number of dying and emerging cells. The duration of phase Ї is 2 hours.
The death acceleration phase (segment VI) is characterized by an imbalance between reproduction and accelerated cell death. This phase lasts 4-5 hours.
In the phase of logarithmic death (segment VII), massive cell death occurs at a constant rate. The duration of the phase is about 5 hours.
The phase of decreasing the rate of death (segment VIII) is characterized by the fact that the surviving bacteria go into a dormant state.
The phases of bacterial reproduction in time depend on the type of bacteria, the quality of the nutrient medium, its concentration, temperature and aeration. Therefore, the duration of each phase is indicated approximately. Under optimal conditions, cell division in a number of individuals occurs at different times, for example, Escherichia coli divide after 15-20 minutes, typhoid bacteria - 20-25 minutes, streptococci - 25-30 minutes, Mycobacterium tuberculosis - 18 - 20 hours.
The duration of the cell death phase is related to the species characteristics of bacteria. The death period of pneumococcus lasts 2
3 days, and E. coli Ї months. In the dying stage, the cells stain weakly, and some of them do not perceive dyes. In addition, the forms of bacteria, their biochemical activity and antigenic properties change.
The term "growth" when applied to microorganisms means an increase in the size of an individual, and "reproduction" - an increase in the number of individuals in a population. With the growth of a microbial cell, its volume increases much faster than the surface, so the distribution of nutrients in the cytoplasm of the cell becomes less efficient and the cell divides. Before dividing it, the DNA molecules are duplicated. 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 (time elapsed) rhoorganisms can use a wide range 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 totality of biochemical processes, as a result of which the energy necessary for the life of the 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 Escherichia coli - 15-17 minutes, causative agents of typhoid fever - 23 minutes, corynebacteria diphtheria - 34 minutes. Mycobacterium tuberculosis divides more slowly - once in 18 hours, spirochetes - in 10 hours.
The methods of reproduction in different groups of microorganisms are not the same: bacteria, rickettsia, spirochetes reproduce by transverse division into two equivalent individuals. Gram-positive bacteria divide by forming a septum that grows 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, which is 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 by constriction into two individuals. Reproduction of nodule bacteria and fraicisella occurs by the formation of a kidney, which, in size, is smaller than the original cell. In bacteria, there is also 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 of long filaments much larger than the original cells.
When bacteria are introduced into a nutrient medium, the phases of their growth and reproduction 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 conditions of existence. During this period, the bacteria adapt to the nutrient medium, their growth is not observed.
The second phase is logarithmic growth (exponential), when the bacteria grow vigorously, increase, and upon reaching a certain size, they begin to divide into two daughter cells. Division during this period occurs at a constant rate. The average generation (or doubling) time for each type of bacteria is different. At this time, bacteria extract nutrients from the medium, 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 is different for different bacteria.
The fourth phase is death, when bacterial cells become smaller and they die. At the end of this phase, the number of dying bacteria begins to prevail over the number of viable cells. The complete death of microbes in culture can occur in a few weeks or months, depending on the type of microbe, the reaction of the environment, and other factors.
The simplest can reproduce by transverse division, constriction into two equivalent individuals - amoeba and longitudinal division - trypanosomes, giardia, balantidia. Before dividing into two individuals, balantidia can exchange their nuclei - micronuclei (conjugation process), malarial plasmodium has an asexual and sexual development cycle.
Viruses reproduce (reproduce) only inside a living cell of the host organism.
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 cell surface, which has specific viral receptors. The process of nucleic acid release from the capsid and outer membranes begins already 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 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 for building 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 a matrix. In DNA-containing viruses, specific RNA is synthesized on the DNA template in the cell nucleus, which then determines the synthesis of viral DNA and protein. Virus proteins are synthesized in the ribosomes of the cell.
The maturation of a viral particle, the conclusion of a viral nucleic acid in a capsid, occurs in the nucleus of the affected cell (herpesviruses, adenoviruses) or in the cytoplasm (variola group viruses, rhabdoviruses, picornaviruses). The formation of outer shells in myxoviruses, togaviruses occurs when passing through the cytoplasmic membrane of the host cell. The herpes virus receives part of its outer shell by passing through the membrane of the cell nucleus.
Isolation of the virus from the 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) is formed in the cell quickly, remains in it for a long time and is released instantly, in the form of a flash. The end result of the interaction between the virus and the host cell can be 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 cell generations - latent viruses. In some cases, the virus can be destroyed by the cell without visible consequences for it (abortive viral infection).
The term "growth" means an increase in the cytoplasmic mass of an individual cell or group of bacteria as a result of the synthesis of cellular material (eg, protein, RNA, DNA). Having reached a certain size, the cell stops growing and begins to multiply.
The reproduction of microbes means their ability to self-reproduce, to increase the number of individuals per unit volume. Otherwise, we can say: reproduction is an increase in the number of individuals of a microbial population.
Bacteria reproduce mainly by simple transverse fission (vegetative reproduction), which occurs in different planes, with the formation of diverse combinations of cells (grape bunch - staphylococci, chains - streptococci, pairs - diplococci, bales, packets - sarcins, etc.). The division process consists of a number of successive stages. The first stage begins with the formation of a transverse septum in the middle part of the cell (Fig. 6), which initially consists of a cytoplasmic membrane that divides the cytoplasm of the mother cell into two daughter cells. In parallel with this, a cell wall is synthesized, which forms a full-fledged partition between two daughter cells. In the process of bacterial division, an important condition is the replication (doubling) of DNA, which is carried out by DNA polymerase enzymes. When DNA is duplicated, hydrogen bonds are broken and two strands of DNA are formed, each of which is located in daughter cells. Further, daughter single-stranded DNA restore hydrogen bonds and again form double-stranded DNA.
DNA replication and cell division occurs at a certain rate inherent in each type of microbe, which depends on the age of the culture and the nature of the nutrient medium. For example, the growth rate of Escherichia coli ranges from 16 to 20 minutes; Mycobacterium tuberculosis divides only after 18-20 hours; a mammalian tissue culture cell takes 24 hours. Consequently, bacteria of most species reproduce almost 100 times faster than tissue culture cells.
Types of bacterial cell division. 1. Cell division is ahead of division, which leads to the formation of "multicellular" rods and cocci. 2. Synchronous cell division, in which the division and division of the nucleoid are accompanied by the formation of unicellular organisms. 3. Nucleoid division precedes cell division, causing the formation of multinucleoid bacteria.
Separation of bacteria, in turn, occurs in three ways: 1) breaking division, when two individual cells, repeatedly breaking at the junction, break the cytoplasmic bridge and repel each other, thus forming chains (anthrax bacilli); 2) sliding separation, in which, after cell division, cells separate and one of them slides over the surface of the other (separate forms of Escherichia); 3) secant separation, when one of the divided cells describes an arc of a circle with its free end, the center of which is the point of its contact with another cell, forming a Roman five or cuneiform (corynebacterium diphtheria, l hysteria).
Phases of development of a bacterial population. Theoretically, it is assumed that if bacteria are provided with conditions for a continuous influx and progressive increase in the mass of fresh nutrient medium and the outflow of excretory products, then reproduction will increase logarithmically, and death arithmetically.
The general pattern of growth and reproduction of a bacterial population is usually shown graphically in the form of a curve that reflects the dependence of the logarithm of the number of living cells on time. A typical growth curve has an S-shape and allows you to distinguish between several growth phases that follow each other in a certain sequence:
1. Initial (stationary, latent, or resting phase). Represents the time from the moment of sowing bacteria on a nutrient medium to their growth. In this phase, the number of living bacteria does not increase, and may even decrease. The duration of the initial phase is 1-2 hours.
2. Reproduction delay phase. During this phase, bacterial cells grow intensively but multiply weakly. The period of this phase takes about 2 hours and depends on a number of conditions: the age of the culture (young cultures adapt faster than old ones); biological characteristics of microbial cells (bacteria of the intestinal group are characterized by a short period of adaptation, for Mycobacterium tuberculosis - a long one); usefulness of the nutrient medium, growing temperature, CO2 concentration, pH, degree of aeration of the medium, redox potential, etc. Both phases are often combined with the term “lag-phase” (eng. lag - lag, delay).
3. Logarithmic phase. In this phase, the rate of cell reproduction and the increase in the bacterial population are maximum. The generation period (lat. generatio - birth, reproduction), i.e., the time elapsed between two successive divisions of bacteria, at this stage will be constant for a given species, and the number of bacteria will begin to double exponentially. This means that at the end of the first generation, two cells are formed from one cell, at the end of the second generation, both bacteria divide and form four, eight of the obtained four, etc. Therefore, after n generations, the number of cells in the culture will be 2n. The duration of the logarithmic phase is 5-6 hours.
4. Phase of negative acceleration. The rate of reproduction of bacteria ceases to be maximum, the number of dividing individuals decreases, and the number of dead increases (duration is about 2 hours). One of the possible reasons slowing down the reproduction of bacteria is the depletion of the nutrient medium, i.e., the disappearance from it of substances specific to a given bacterial species.
5. Stationary phase of the maximum. In it, the number of new bacteria is almost equal to the number of dead ones, i.e., an equilibrium occurs between the dead cells and the newly formed ones. This phase lasts 2 hours.
6. Phase of death acceleration. It is characterized by a progressive superiority of the number of dead cells over the number of newly born ones. It lasts about 3 hours.
7. Phase of logarithmic death. Cell death occurs at a constant rate (lasting about 5 hours).
8. Phase of decrease in the rate of death. The cells that remain alive go into a state of rest.
