The term "growth" refers to the increase in the cytoplasmic mass of an individual cell or group of bacteria as a result of the synthesis of cellular material (for example, protein, RNA, DNA). Having reached a certain size, the cell stops growing and begins to multiply.
The reproduction of microbes means their ability to reproduce themselves, to increase the number of individuals per unit volume. In other words, we can say: reproduction is an increase in the number of individuals in a microbial population.
Bacteria reproduce predominantly by simple transverse division (vegetative propagation), which occurs in different planes, with the formation of diverse combinations of cells (a bunch of grapes - staphylococci, chains - streptococci, compounds in pairs - diplococci, bales, bags - sarcina, etc.). The division process consists of a number of successive stages. The first stage begins with the formation of a transverse partition in the middle part of the cell (Fig. 6), initially consisting 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, forming a full-fledged partition between the 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 doubles, hydrogen bonds are broken and two DNA helices are formed, each of which is located in the daughter cells. Next, the daughter single-stranded DNAs restore hydrogen bonds and again form double-stranded DNAs.
DNA replication and cell division occur at a certain speed 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 E. coli ranges from 16 to 20 minutes; in Mycobacterium tuberculosis, division occurs only after 18-20 hours; Mammalian tissue culture cells require 24 hours. Consequently, bacteria of most species multiply almost 100 times faster than tissue culture cells.
Types of bacterial cell division. 1. Cell division precedes division, which leads to the formation of “multicellular” rods and cocci. 2. Synchronous cell division, in which the division and fission of the nucleoid are accompanied by the formation of single-celled organisms. 3. Nucleoid division precedes cell division, causing the formation of multinucleoid bacteria.
The separation of bacteria, in turn, occurs in three ways: 1) breaking separation, when two individual cells, repeatedly breaking at the junction, break the cytoplasmic bridge and repel each other, and chains are formed (anthrax bacilli); 2) sliding separation, in which after division the cells separate and one of them slides over the surface of the other (individual forms of Escherichia); 3) secant division, when one of the divided cells with its free end describes an arc of a circle, the center of which is the point of its contact with another cell, forming a Roman quinque or cuneiform (Corynebacterium diphtheria, l hysteria).
Phases of bacterial population development. 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 is S-shaped and allows one to distinguish several growth phases that follow each other in a certain sequence:
1. Initial (stationary, latent, or resting phase). It represents the time from the moment bacteria are inoculated on a nutrient medium until they grow. During 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 rapidly but reproduce weakly. The period of this phase takes about 2 hours and depends on a number of conditions: the age of the crop (young crops adapt faster than old ones); biological characteristics of microbial cells (bacteria of the intestinal group are characterized by a short period of adaptation, while mycobacterium tuberculosis is characterized by a long period); the 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” (English lag - lag, delay).
3. Logarithmic phase. In this phase, the rate of cell reproduction and increase in bacterial population is maximum. The generation period (Latin 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 double exponentially. This means that at the end of the first generation, two bacteria are formed from one cell, at the end of the second generation, both bacteria, dividing, form four, eight are formed from the resulting four, etc. Consequently, after n generations, the number of cells in the culture will be equal to 2n. The duration of the logarithmic phase is 5-6 hours.
4. Negative acceleration phase. The rate of bacterial reproduction ceases to be maximum, the number of dividing individuals decreases, and the number of deaths increases (duration about 2 hours). One of the possible reasons that slow down the proliferation of bacteria is the depletion of the nutrient medium, that is, the disappearance from it of substances specific to a given bacterial species.
5. Stationary maximum phase. In it, the number of new bacteria is almost equal to the number of dead ones, i.e., an equilibrium occurs between dead cells and newly formed ones. This phase lasts 2 hours.
6. Acceleration phase of death. 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. Logarithmic death phase. Cell death occurs at a constant rate (duration about 5 hours).
8. Phase of decreasing rate of death. The surviving cells go into a state of rest.
In order to study microorganisms, determine the etiological factors of infectious diseases, deal with issues of prevention and treatment of infectious diseases and solve many other issues related to microorganisms, it is necessary to have them in sufficient quantities, and this means creating all the conditions for the normal growth and reproduction of microorganisms.
The term “reproduction” of microbes means their ability to reproduce themselves and increase the number of individuals.
Reproduction of microorganisms occurs through transverse division, budding, spore formation, and reproduction.
Microbial growth means an increase in the mass of microbes as a result of the synthesis of cellular material and the reproduction of all cellular components and structures.
Bacteria, spirochetes, actinomycetes, fungi, rickettsia, mycoplasmas, protozoa, and chlamydia are said to reproduce, while viruses and phages (microbial viruses) reproduce.
The reproduction of microorganisms follows certain patterns. The rate of division of microorganisms is different, it depends on the type of microbe, the age of the culture, the characteristics of the natural and artificial nutrient medium, temperature, carbon dioxide concentration and many other factors.
During the process of reproduction, microorganisms at various stages undergo morphological and physiological changes (in shape, size, colorability, biochemical activity, sensitivity to physical and chemical factors, etc.).
Microorganisms exhibit age-related variability, i.e. individuals change at different stages of growth, maturation and aging. These changes are observed in the normal cycle of individual development of a microorganism, which depends on the nature of the organism, the complexity of its structure and the sequence of development.
Bacteria have the simplest development cycle among microorganisms. They reproduce by simple transverse division in different planes. Depending on this, cells can be arranged randomly, in clusters, chains, packages, in pairs, fours, etc.
A characteristic feature of bacteria that distinguishes them from numerous animals and plants is their extraordinary rate of reproduction.
Each bacterial cell undergoes division on average within half an hour, which is due to increased metabolism and the speed at which nutritional material enters the cell.
The factor inhibiting the proliferation of bacteria is the depletion of the nutrient substrate and the poisoning of the environment with decay products.
Bacteria have eight main phases of reproduction.
1. The initial stationary phase, which is a period of one to two hours from the moment the bacteria are inoculated onto the nutrient medium. Reproduction does not occur in this phase
2. The phase of delayed reproduction (lag phase), during which the reproduction of bacteria occurs very slowly, and their growth rate increases. The duration of the second phase is about two hours.
3. The phase lasts five to six hours. The third phase is characterized by a maximum rate of division and a decrease in cell size.
4. Negative acceleration phase (lasts about two hours). The rate of bacterial reproduction decreases, the number of dividing cells decreases.
5. Stationary phase, lasting about two hours. The number of new bacteria is almost equal to the number of dead individuals.
6. Acceleration phase of cell death (lasts about three hours).
7. Logarithmic cell death phase (lasts about five hours), in which cell death occurs at a constant rate
8. Phase of decreasing rate of death. The surviving individuals go into a state of rest.
The duration of the reproductive phases is not constant. It may vary depending on the type of microorganisms and cultivation conditions.
The development cycle of coccoid bacteria boils down to cell growth and its subsequent division. Rod-shaped asporogenous bacteria grow at a young age, reach a maximum size, then divide into two daughter cells, which repeat the same cycle. In bacilli and clostridia, the development cycle includes, under certain conditions, the process of sporulation.
Spirochetes and rickettsiae, like bacteria, reproduce by binary fission.
Among mycoplasmas, all elementary bodies of spherical or ovoid shape have the ability to reproduce. During the development process, several thread-like outgrowths appear on the elementary body, in which spherical bodies are formed. Gradually, the threads become thinner and chains with clearly defined spherical bodies are formed. Then the filaments are divided into fragments and the spherical bodies are released.
Reproduction of some mycoplasmas occurs by budding daughter cells from larger spherical bodies. Mycoplasmas multiply by transverse division if the processes of mycoplasma division occur synchronously with the replication of the nucleoid DNA. When synchrony is disturbed, filamentous polynucleoid forms are formed, which subsequently divide into coccoid cells.
Actinomycetes and fungi have two different stages of development: the stage of vegetative growth, which is characterized by the formation of mycelium, and the stage of formation of spores that form on spore carriers.
An important feature of actinomycetes and fungi is the significant variety of methods of their reproduction. They are characterized by vegetative, asexual and sexual reproduction.
Vegetative propagation is carried out by dividing hyphae into fragments with the subsequent formation of individual rod-shaped and coccoid cells.
Asexual reproduction occurs vegetatively (growth of fragments of hyphae or their individual cells) and with the help of more or less specialized reproductive organs (spores and conidia). The most common, asexual, way of reproduction is manifested in the formation of exogenous and endogenous spores. Exospores or conidia are formed at the ends of the fruiting hyphae, but are enclosed inside a common sac - the sporangium. Hyphae bearing sporangia are called sporangiophores. Sporangiophores can be straight, wavy, or spiral.
Sexual reproduction occurs with the help of special organs - ascospores, basidiospores, the formation of which is preceded by the sexual process. According to their biological purpose, spores of actinomycetes and fungi are dormant, serve to preserve the species for a certain period, and serve for rapid reproduction.
Spores of actinomycetes and fungi are formed by each individual in large quantities, since, unlike bacterial spores, they mainly serve the purpose of reproduction. They are less resistant to environmental factors than bacterial spores.
In protozoa, as well as in actinomycetes and fungi, along with reproduction by division, there is also a sexual process.
Chlamydia, viruses and phages have unique development cycles.
Reproduction of chlamydia begins with the penetration of elementary bodies into sensitive tissue cells through endocytosis. These bodies in the cell vacuole turn into vegetative forms called initial or reticular bodies, which have the ability to divide. Reticular bodies have a lamellar cell wall, and in the cytoplasm there are loosely located nuclear fibrils and numerous ribosomes. After repeated division, the reticular bodies turn into intermediate forms, from which a new generation of elementary bodies develops. The entire development cycle of chlamydia lasts 40–48 hours and ends with the formation of a microcolony of chlamydia in the cytoplasm of the host cell.
After the vacuole wall ruptures and the host cell is completely destroyed, the chlamydia microcolonies, once outside the whole cell, disintegrate into independent elementary bodies, and the cycle of chlamydia penetration into the cell followed by their reproduction is repeated.
Virus reproduction is characterized by a sequence of individual stages.
1. Adsorption stage. Virions are adsorbed on the surface structures of the cell. In this case, the interaction of complementary structures of the virion and the cell, called receptors, occurs.
2. Stage of penetration of the virion into the host cell. The ways in which viruses enter cells sensitive to them are not the same. Many virions enter the cell by pinocytosis, when the resulting pinocytic vacuole “pulls” the virion into the cell. Some viruses enter the cell directly through its membrane.
3. The stage of destruction of the outer shell and capsid of the virion with the help of proteolytic enzymes of the host cell. In some virions, the process of destruction of their shell begins at the adsorption stage, in others - in the pinocytic vacuole, in others - directly in the cytoplasm of the cell with the participation of the same proteolytic enzymes.
4. Stage of synthesis of viral proteins and replication of nucleic acids. After complete or partial release of the viral nucleic acid, the process of synthesis of viral proteins and replication of nucleic acids begins.
5. Stage of assembly or morphogenesis of the virion. The formation of virions is possible only under the condition of a strictly ordered connection of viral structural polypeptides and their nucleic acid, which is ensured by the self-assembly of protein molecules around the nucleic acid. In some viruses this process occurs in the cytoplasm, in others - in the nucleus of the host cell. In complex viruses that have an outer envelope, further assembly occurs in the cytoplasm during their exit from the cell.
6. Stage of release of virions from the host cell. A number of complex viruses emerge from the host cell, while the cells remain viable for some time and then die. Simple virions leave the cell through holes formed in its shell; the host cell dies without maintaining viability for some time.
In some cases, reproduction of virions in cells can occur over many months and even years. Viruses are released through the cell membrane. When such cells divide, virions are transferred to daughter cells, which in turn begin to produce viral particles.
There are three types of interaction between a virus and a cell: productive, abortive and virogenic.
Productive the type of interaction is the formation of new virions.
Abortive the type of interaction can suddenly be interrupted during the stage of viral nucleic acid replication or viral protein synthesis, or virion morphogenesis.
Virogenic the type is characterized by the incorporation (integration) of viral nucleic acid into cell DNA, which ensures synchronous replication of viral and cellular DNA.
During phage reproduction, its adsorption on the cell surface also occurs (stage 1) as a result of the interaction of amino groups of proteins localized in the peripheral part of the phage tail process and negatively charged carboxyl groups on the surface of the bacterial cell.
There are reversible and irreversible phases of adsorption. The reversible phase is characterized by the fact that fixed phages can be separated from the cell by vigorous stirring or by sharply reducing the ion concentration. The released phages retain their viability.
During the second irreversible phase of adsorption, the phage is not separated from the body of the microbial cell. The adsorption process lasts several minutes. Under the influence of an enzyme located in the tail process of the phage, a hole is formed in the body of the microbial cell at the site of attachment of the phage, through which the phage DNA penetrates into the cell. The phage shell remains outside (stage 2).
Some phages introduce their nucleic acid into the cell without prior mechanical damage to the cell wall. During the latent period that follows the penetration of the phage nucleic acid into the cell, the biosynthesis of phage nucleic acid and phage capsid proteins occurs.
The synthesis of enzymes necessary for the replication of phage nucleic acid and phage structural proteins occurs (stage 3).
In the fourth stage, hollow phage particles are filled with phage nucleic acid and mature phages are formed. Phage morphogenesis occurs.
At the end of the latent period, lysis of infected microbial cells occurs and mature phage particles are released (stage 5).
It is believed that phage adsorption lasts 40 minutes, the latent period is 75 minutes. The entire cycle of interaction between a phage and a microbial cell lasts a little more than three hours.
The introduction of a phage into a microbial cell is not always accompanied by its lysis. Often, the interaction of a phage with a microbial cell leads to the formation of lysogenic cultures.
Based on the nature of interaction with the microbial cell, temperate and virulent phages are distinguished. The state of lysogeny is caused by temperate phages. Lysogenic microbial cells are resistant to virulent phages. Virulent phages cause the formation of new phages and lysis of the microbial cell.
Reproduction of microorganisms is an increase in the concentration of microorganisms per unit volume of the environment, aimed at preserving the species.
Microorganisms are characterized by:
variety of reproduction methods;
switching from one method of reproduction to another;
possibility of simultaneous use of several methods;
high reproduction rate.
Methods of propagation of microorganisms
I. Sexual withreproduction method observed only in eukaryotes.
II. Asexual methods of reproduction.
Equal area binary transverse division (simple division, isomorphic division, mitosis) observed in most unicellular microorganisms (bacteria, rickettsia, protozoa, yeast), as a result, two new daughter full-fledged individuals are formed, endowed with the genetic information of the mother cell, symmetrical in relation to the longitudinal and transverse axis, the mother cell itself disappears.
Moreover, in most Gram+ bacteria, division occurs through the synthesis of a transverse septum running from the periphery to the center (Fig. 63A). The cells of most Gram bacteria divide by cell constriction (the cell becomes thinner in the middle) (Fig. 63B).
Budding (unequal binary fission) observed in representatives of the genera Francisella And Mycoplasma and yeast-like fungi. During budding, the mother cell gives rise to a daughter cell: at one of the poles of the mother cell, a small outgrowth (bud) is formed, which increases in size during growth. Gradually, the bud reaches the size of the mother cell, after which it separates. The kidney CS is completely synthesized anew (Fig. 63B). During the budding process, symmetry is observed only in relation to the longitudinal axis. There are morphological and physiological differences between mother and daughter cells. The new daughter cell adapts better to changing conditions.
Fragmentation of filamentous forms characteristic of the genus Actinomyces And Mycoplasma.
Exospore formation typical for Streptomycetes, yeast-like and mold fungi.
A special development cycle Tia observed in Chlamydia. Only vegetative forms of chlamydia (reticular or initial bodies) are capable of dividing in the cells of the macroorganism. Their cycle, consisting of several divisions, ends with the formation of intermediate forms, from which elementary bodies are formed, giving rise to vegetative forms. After destruction of the vacuole wall and the host cell, the elementary bodies are released and the cycle repeats. The cycle lasts 40–48 hours.
Multiple division described for one group of unicellular cyanobacteria. Multiple fission is based on the principle of equal-area binary fission. The difference is that in this case, after binary fission, the resulting daughter cells do not grow, but they undergo division again (Fig. 63D).
Multiple fission (schizogony) also described in protozoa (malarial plasmodia): the nuclear material is divided into many nucleoli, surrounded by areas of cytoplasm, resulting in the formation of many daughter cells.
Mechanism and phases of simple division
A. Growth to a certain degree of maturity. Cell growth is not unlimited and after reaching a certain size, the bacterial cell begins to divide. During division, cell growth slows down and begins again after division.
B. Karyokinesis ( DNA replication and d division of the nucleoid). A signal comes from the mature cytoplasm that activates the initiator gene on the DNA. Microorganisms, under the influence of the initiator gene, synthesize an initiator protein, which acts on the replicator gene - a special section of DNA from which DNA doubling and division into two strands begins.
The division of a DNA molecule (replication) occurs according to a semi-conservative mechanism and normally always precedes cell division. DNA replication begins at the point of attachment of the circular chromosome to the CPM, where the enzymatic apparatus responsible for replication is localized.
The mechanism of DNA replication is expressed in the breaking of hydrogen bonds between its two polynucleotide chains, their unwinding and the synthesis of new chains with a complementary sequence of bases along each old strand using DNA polymerase. After divergence into daughter cells along one old and one new polynucleotide chain, hydrogen bonds are restored between them and semi-conservative double-stranded DNA is formed.
Normally, there is a certain temporal relationship between chromosome replication and bacterial cell division. Exposure to various chemicals and physical factors, leading to the suppression of DNA replication, also stops cell division. However, under certain conditions, the connection between both processes can be broken, and cells are able to divide in the absence of DNA synthesis.
B. Cytokinesis (cell division). In parallel with the replication of DNA molecules, membrane synthesis occurs next to the mesosome, in the area of contact of DNA with the CPM. The formation of a septum leads to cell division. The moment that initiates cell division is the end of DNA replication. This leads to the separation of daughter DNA molecules and the formation of separate chromosomes. The newly formed daughter cells separate from each other.
Inhibition of membrane synthesis before the end of replication leads to disruption of the division process: the cell stops dividing and grows in length. In some bacteria, the formation of a septum does not lead to cell division: multilocular cells are formed.
D. Divergence of the resulting daughter cells occurs as a result of lysis of the middle layer of the CS. If, after repeated division in one plane, the cells do not diverge, chains of rod-shaped (Bacillus) or spherical(Streptococcus) cells or paired cells(Neisseria) . Cell separation is possible with the separation of one of the cells by moving along the surface of another, as a result of which the bacteria are located randomly (Escherichia). If, during separation, one of the daughter cells, without breaking away from the division point, moves along an arc, a V-shaped form (Corynebacterium, Bifidobacterium). After binary fission and divergence of cells in several planes, cell clusters of various shapes: bunches (Staphylococcus), packages (Sarcina) (Fig. 65). If nucleoid division precedes cell division, polynucleoid microorganisms. Under the influence of unfavorable external factors (bile salts, UV rays, surfactants, antibiotics), cell division can stop while its growth continues. In this case, the formation of elongated filamentous cells.
Rice. 65. Division of cocci
Generation period- the time interval during which the number of bacteria doubles. The rate of reproduction of microorganisms and the generation period depend on the type of microorganism, the size and properties of the inoculum, the composition of the nutrient medium, its pH, aeration, incubation temperature, and other factors. Under favorable conditions, many microorganisms divide within 15–30 minutes (E. coli, S. typhi). In fastidious microorganisms, division occurs within 45–90 minutes (Streptococcus, Corynebacterium) and even after 18 hours (M. tuberculosis).
The term "growth" refers to the increase in the cytoplasmic mass of an individual cell or group of bacteria as a result of the synthesis of cellular material (for example, protein, RNA, DNA). Having reached a certain size, the cell stops growing and begins to multiply.
The reproduction of microbes means their ability to reproduce themselves, to increase the number of individuals per unit volume. In other words, we can say: reproduction is an increase in the number of individuals in a microbial population.
Bacteria reproduce predominantly by simple transverse division (vegetative propagation), which occurs in different planes, with the formation of diverse combinations of cells (a bunch of grapes - staphylococci, chains - streptococci, compounds in pairs - diplococci, bales, bags - sarcina, etc.). The division process consists of a number of successive stages. The first stage begins with the formation of a transverse partition in the middle part of the cell (Fig. 6), initially consisting 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, forming a full-fledged partition between the 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 doubles, hydrogen bonds are broken and two DNA helices are formed, each of which is located in the daughter cells. Next, the daughter single-stranded DNAs restore hydrogen bonds and again form double-stranded DNAs.
DNA replication and cell division occur at a certain speed 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 E. coli ranges from 16 to 20 minutes; in Mycobacterium tuberculosis, division occurs only after 18-20 hours; Mammalian tissue culture cells require 24 hours. Consequently, bacteria of most species multiply almost 100 times faster than tissue culture cells.
Types of bacterial cell division. 1. Cell division precedes division, which leads to the formation of “multicellular” rods and cocci. 2. Synchronous cell division, in which the division and fission of the nucleoid are accompanied by the formation of single-celled organisms. 3. Nucleoid division precedes cell division, causing the formation of multinucleoid bacteria.
The separation of bacteria, in turn, occurs in three ways: 1) breaking separation, when two individual cells, repeatedly breaking at the junction, break the cytoplasmic bridge and repel each other, and chains are formed (anthrax bacilli); 2) sliding separation, in which after division the cells separate and one of them slides over the surface of the other (individual forms of Escherichia); 3) secant division, when one of the divided cells with its free end describes an arc of a circle, the center of which is the point of its contact with another cell, forming a Roman quinque or cuneiform (Corynebacterium diphtheria, l hysteria).
Phases of bacterial population development. 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 is S-shaped and allows one to distinguish several growth phases that follow each other in a certain sequence:
1. Initial (stationary, latent, or resting phase). It represents the time from the moment bacteria are inoculated on a nutrient medium until they grow. During 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 rapidly but reproduce weakly. The period of this phase takes about 2 hours and depends on a number of conditions: the age of the crop (young crops adapt faster than old ones); biological characteristics of microbial cells (bacteria of the intestinal group are characterized by a short period of adaptation, while mycobacterium tuberculosis is characterized by a long period); the 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” (English lag - lag, delay).
3. Logarithmic phase. In this phase, the rate of cell reproduction and increase in bacterial population is maximum. The generation period (Latin 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 double exponentially. This means that at the end of the first generation, two bacteria are formed from one cell, at the end of the second generation, both bacteria, dividing, form four, eight are formed from the resulting four, etc. Consequently, after n generations, the number of cells in the culture will be equal to 2n. The duration of the logarithmic phase is 5-6 hours.
4. Negative acceleration phase. The rate of bacterial reproduction ceases to be maximum, the number of dividing individuals decreases, and the number of deaths increases (duration about 2 hours). One of the possible reasons that slow down the proliferation of bacteria is the depletion of the nutrient medium, that is, the disappearance from it of substances specific to a given bacterial species.
5. Stationary maximum phase. In it, the number of new bacteria is almost equal to the number of dead ones, i.e., an equilibrium occurs between dead cells and newly formed ones. This phase lasts 2 hours.
6. Acceleration phase of death. 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. Logarithmic death phase. Cell death occurs at a constant rate (duration about 5 hours).
8. Phase of decreasing rate of death. The surviving cells go into a state of rest.
№ 10 Growth and reproduction of bacteria. Reproduction phases.
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 a branching tankteria, 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, resulting in the formation of dumbbell-shaped figures from which two identical cells are formed.
Cell division is preceded byreplication 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 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 batch culture of bacteria grown in 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 of bacteria;
4.
bacterial death phase.
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 phaseis a period of intense bacterial division. Its duration is about 5-6 hours. Under optimal growth conditions, bacteria 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, constituting the 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 -shape), 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 dismutase and peroxidases, protect microorganisms from the effects of toxic oxygen peroxide radicals. Many pigments have antimicrobial, antibiotic-like effects.