Growth and reproduction of bacteria. Reproduction phases

The growth of a bacterial cell should be understood as an increase in the mass of its cytoplasm, which occurs as a result of the synthesis of cellular material during nutrition. The growth of a bacterial population goes through 4 stages: 1) lag phase, 2) exponential or logarithmic phase, 3) stationary phase, 4) dying phase.

LAG PHASE (4 -5 hours) Occurs after the seed is introduced into the medium. This is the period of adaptation of bacteria to the nutrient medium, when differential activation of exo- and endoenzymes occurs for the subsequent implementation of the enzyme-substrate reaction. With stable DNA content, there is a sharp increase in bacterial protein and RNA.

LAG PHASE (4 -5 hours) The duration of the lag phase is usually short, measured in hours and depends on the type of bacteria, the multiplicity of inoculation on a given medium, the state of the culture, the temperature used for cultivation, and the composition of the nutrient medium. In the absence of visible signs of growth in the lag phase, an increase in biomass occurs, as a result of which the size of the bacterial cell increases several times.

LAG PHASE (4 -5 hours) Having reached a certain size, having “accumulated” the required amount of protein, RNA and DNA, activating exo- and endoenzymes, the bacterial cell begins to actively divide. Bacteria reproduce by transverse cell division.

LOGARITHMIC GROWTH PHASE (5 - 6 hours) This is the phase of reproduction, carried out through the binary division of the mother cell into two daughter cells. “The chain reaction of progressively accelerating binary fission of bacterial cells leads to a rapid increase in bacterial mass in the nutrient medium, intensive consumption of its energy substrate and accumulation of bacterial metabolic products.

STATIONARY GROWTH PHASE As a result, the environment becomes increasingly unfavorable for further growth and reproduction of bacteria. During the stationary phase, the rate of reproduction remains constant. Depending on the type of bacteria being cultivated, it can last for a long time, after which the fourth stage occurs -

DYING PHASE The dying phase is characterized by the progressive death of bacterial cells in a logarithmic manner. The duration of this phase ranges from 48 hours to several weeks.

The nature of bacterial growth on liquid nutrient media is different - diffuse turbidity of the nutrient medium, - formation of a film or sediment (bottom growth), - growth in the form of a “ball of cotton wool”. The growth pattern on liquid nutrient medium is used to differentiate bacteria.

Nutrient media For the cultivation of bacteria in laboratory conditions, artificial nutrient media of various compositions are used. Conventional or simple nutrient media (meat peptone agar, meat peptone broth) are used for initial crops (primary). Complex media include selective and differential diagnostic nutrient media.

Nutrient media Elective media ensure the growth of only a certain type of microorganisms, while the accompanying microflora is suppressed by special additives. Differential diagnostic nutrient media are used to study the biochemical properties of microorganisms and make it possible to differentiate bacteria by enzymatic activity.

CLASSIFICATION OF MICROORGANISMS As new species of bacteria were studied and identified, each newly created classification reflected the level of development of science. The classification of microorganisms, that is, the systematization of all known species, was based on a number of characteristics:

Sequence of determining a microorganism I. Which kingdom does it belong to - prokaryotes or eukaryotes II. Which of the main categories does it belong to: 1. Gram-negative eubacteria that have cell walls. 2. Gram-positive eubacteria with cell walls. 3. Eubacteria lacking cell walls. 4. Archaebacteria.

A total of 35 groups of microorganisms are known III. Which group within 4 categories does the microorganism belong to: 1. Spirochetes 2. Aerobic /microaerophilic, motile, spiral-shaped/, vibroid, gram-negative bacteria. 3. Non-motile gram-negative, curved bacteria. 4. Gram-negative, anaerobic, microaerophilic rods and cocci.

I. gram-negative eubacteria with a cell wall 5. Facultative anaerobic, gram-negative rods. 6. Gram-negative, anaerobic, straight, curved and spiral rods. 7. Bacteria that carry out the dissimilatory reduction of sulfate or sulfur. 8. Anaerobic gram-negative cocci. 9. Rickettsia and chlamydia.

I. gram-negative eubacteria with a cell wall 10. Anoxygenic phototrophic bacteria. 11. Oxygenic phototrophic bacteria. 12. Aerobic chemolithotrophic bacteria. 13. Budding and/or outgrowth-forming bacteria. 14. Bacteria with covers. 15. Non-photosynthetic gliding bacteria that do not form fruiting bodies. 16. Sliding bacteria forming fruiting bodies.

II. Gram-positive eubacteria with cell walls. 1. Gram-positive cocci. 2. Gram-positive rods and cocci that form endospores. 3. Gram-positive rods that do not form spores and are of regular shape. 4. Gram-positive rods of irregular shape that do not form spores. 5. Mycobacteria. 6. Actinomycetes.

IV. Archaebacteria. 1. Methanogens. 2. Sulfate-reducing archaea. 3. Extremely halophilic archaebacteria. 4. Archaebacteria lacking a cell wall. 5. Extreme thermophiles and hyperthermophiles metabolizing S

Sequence for identifying a microorganism IV. What genus does the microorganism belong to? V. What family does the microorganism belong to? VI. What type of microorganism is it?

Construction of the taxonomic name of the microorganism. 1. KINGDOM 2. CATEGORY. 3. GROUP. 4. ROD. 5. FAMILY. 6. VIEW

Advantages of the modern classification of microorganisms The phylogenetic systematization created to date has all the advantages and disadvantages of a classification based on one characteristic. The advantages include the almost complete identity of the results obtained in various laboratories around the world. To establish species identity, they also began to additionally evaluate the degree of DNA-DNA homology using type strains.

Disadvantages of the existing classification of microorganisms. The disadvantage of the existing classification is that it does not provide an idea of ​​the functions of bacteria. Therefore, the creation of a phenotypic or functional classification is now of great importance for practical microbiologists. To quickly determine the taxonomic position of microorganisms, use the Bergey Determinant. This reference publication is constantly updated with new groups of isolates and is periodically reprinted. Now the 11th edition is current.

Formation of a modern classification of microorganisms. At the present stage, identification of the phylogenetic position of prokaryotes, including uncultivated ones, is being developed on the basis of nucleotide sequences of 16 S r RNA. Improved sequencing and data processing techniques have made this approach practically no alternative to determining the genus of new organisms. The description of new bacterial taxa has occurred at a very rapid pace in the last 50 years, thanks to advances in the study of anaerobes.

The difference between classification and identification In addition to classifications, in microbiology there are schemes for identifying isolated bacterial cultures. To build an identification scheme, select characteristics of microorganisms that are easy to determine, and for classification, often require the use of complex methods. In this case, the identification scheme should include a small number of characteristics, and for taxonomic determination, the classification uses as many characteristics as possible.

THANK YOU FOR YOUR ATTENTION. YOU HAVE LISTENED LECTURE No. 3 ON MICROBIOLOGY ON THE TOPIC: “GROWTH AND REPRODUCTION OF MICROORGANISMS. EVOLUTION AND CLASSIFICATION OF MICROORGANISMS".

Growth and reproduction of bacteria. Mechanism and speed of reproduction. Phases of microbial reproduction.

Parameter name	Meaning
Article topic:	Growth and reproduction of bacteria. Mechanism and speed of reproduction. Phases of microbial reproduction.
Rubric (thematic category)	Culture

1. Concepts of bacterial growth and reproduction

2. Bacterial population

3.Colonies

1 . For microbiological diagnostics, study of microorganisms and for biotechnological purposes microorganisms are cultivated on artificial nutrient media.

Under bacterial growth understand increase in cell mass without changing their number in the population as a result of the coordinated reproduction of all cellular components and structures.
Posted on ref.rf
Increasing the number of cells in a population of microorganisms denoted by the term "reproduction". It is characterized generation time(the time interval during which the number of cells doubles) and such a concept as bacteria concentration(number of cells in 1 ml).

In contrast to the mitotic division cycle in eukaryotes, the reproduction of most prokaryotes (bacteria) occurs by binary fission, and actinomycetes - budding. However, all prokaryotes exist in a haploid state, since the DNA molecule is represented in the cell in the singular.

2. When studying the process of bacterial reproduction, it is extremely important to consider that bacteria always exist in the form of more or less numerous populations, and development bacterial population in a liquid nutrient medium in batch culture can be considered as closed system.

There are 4 phases in this process:

‣‣‣ 1st - initial, or lag phase or delayed reproduction phase - characterized the beginning of intensive cell growth, But the rate of their division remains low;

‣‣‣ 2nd - logarithmic or log phase, or exponential phase, - characterized a constant maximum rate of cell division and a significant increase in the number of cells in the population;

‣‣‣ 3rd - stationary phase - comes when the number of cells in the population stops increasing. This is due to the fact that an equilibrium occurs between the number of newly formed and dying cells. The number of living bacterial cells in a population per unit volume of nutrient medium in the stationary phase is denoted as M-concentration. This indicator is a characteristic feature for each type of bacteria;

‣‣‣ 4th - die-off phase (logarithmic death) - characterized a predominance in the population of the number of dead cells and a progressive decrease in the number of viable cells in the population. The cessation of growth in the number (reproduction) of a population of microorganisms occurs due to the depletion of the nutrient medium and/or the accumulation of metabolic products of microbial cells in it. For this reason, by removing metabolic products and/or replacing the nutrient medium, regulating the transition of the microbial population from the stationary phase to the dying phase, it is possible to create open biological system seeking to eliminate dynamic equilibrium at a certain level of population development.

This process of growing microorganisms is commonly called flow cultivation (continuous culture). Growth in a continuous culture makes it possible to obtain large masses of bacteria during flow cultivation in special devices (chemostats and turbidistats) and is used in the production of vaccines, as well as in biotechnology for the production of various biologically active substances produced by microorganisms.

To study metabolic processes throughout the cell division cycle, it is also possible to use synchronous cultures - such bacterial cultures, all members of the population of which are in one phase of the cycle. This is achieved using special cultivation methods.

In this case, after several simultaneous divisions, the synchronized cell suspension gradually switches back to asynchronous division, so that the number of cells subsequently increases not stepwise, but continuously.

3. When cultivated on solid nutrient media, bacteria form colonies - a cluster of bacteria of the same species visible to the naked eye, which is most often the offspring of one cell.

Colonies of bacteria of different species are different:

‣‣‣ shape;

‣‣‣ size;

‣‣‣ transparency;

‣‣‣ color;

‣‣‣ height;

‣‣‣ the nature of the surface and edges;

‣‣‣ consistency.

The nature of the colonies - one of the taxonomic characteristics of bacteria.

44. Definition and essence of the concepts “biosphere” and “biocenosis”. Modern ideas about the evolution of microbes.

In nature, microorganisms inhabit almost any environment (soil, water, air) and are much more widespread than other living beings. Due to the variety of mechanisms for recycling food and energy sources, as well as pronounced adaptation to external influences, microorganisms can live where other forms of life cannot survive.

The natural habitats of most organisms are water, soil and air. The number of microorganisms living on plants and in animal bodies is much smaller. The wide distribution of microorganisms is associated with the ease of their spread through air and water; in particular, the surface and bottom of freshwater and salt water bodies, as well as several centimeters of the top layer of soil, are replete with microorganisms that destroy organic matter. A smaller number of microorganisms colonize the surface and some internal cavities of animals (for example, the gastrointestinal tract, upper respiratory tract) and plants.

In habitat areas, microorganisms form biocenoses[from Greek bios, life, + koinos, community] - complex associations with specific and often unusual relationships. Each microbial community in a specific biocenosis forms specific autochthonous microorganisms[from Greek autos yours, + chthon, country, locality], that is, microbes inherent in a specific area.

Symbiosis[from Greek symbiosis, cohabitation] - the long-term coexistence of microorganisms in long-lived communities. The relationship in which a microorganism is located outside the cells of the host (larger organism) is known as ectosymbiosis: when localized inside cells, it is known as endosymbiosis. Typical ectosymbiotic microbes - Escherichia coli bacteria genera Bacteroides And Bifidobacterium, Proteus vulgaris, as well as other representatives of intestinal microflora. As an example of endosymbiosis, we can consider plasmids that provide, for example, bacterial resistance to drugs. Symbiotic relationships are also divided according to the benefits received by each partner.

Mutualism[from lat. mutuus, mutual] - mutually beneficial symbiotic relationship. Thus, microorganisms produce biologically active substances necessary for the host body (for example, B vitamins). At the same time, endo- and ectosymbionts living in macroorganisms are protected from unfavorable environmental conditions (drying and extreme temperatures) and have constant access to nutrients. Of all the types of mutualism, the most surprising is the cultivation of some fungi by insects (beetles and termites). On the one hand, this contributes to a wider distribution of fungi, on the other - o\

mj provides a constant source of nutrients for the larvae. This is reminiscent of human cultivation of useful plants and microorganisms.

Commensalism- a type of symbiosis in which only one partner benefits (without causing “visible” harm to the other); microorganisms involved in such relationships are commensals [from lat. cell-, s, + mensa, table; literally - table mates]. Commensal microorganisms colonize the skin and cavities of the human body (for example, the gastrointestinal tract), without causing “visible” harm; their totality is normal microbial flora (natural microflora). Typical ectosymbiotic commensal organisms are Escherichia coli, bifidobacteria, staphylococci, and lactobacilli. Many commensal bacteria belong to opportunistic microflora and are capable, under certain circumstances, of causing diseases of the macroorganism (for example, when they are introduced into the bloodstream during medical procedures).

Growth and reproduction of bacteria. Mechanism and speed of reproduction. Phases of microbial reproduction. - concept and types. Classification and features of the category "Growth and reproduction of bacteria. Mechanism and speed of reproduction. Phases of microbial reproduction." 2017, 2018.

Bacterial growth – This is an increase in bacterial cell size without increasing the number of individuals in the population. Cell growth is not unlimited. After reaching a critical size, the cell undergoes division.

Reproduction of bacteria - 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. In most gram-positive bacteria, division occurs through the synthesis of a transverse septum running from the periphery to the center. The cells of most gram-negative bacteria divide by constriction.

The process of bacterial cell division begins with the replication of chromosomal DNA. Replication begins in one selected region, called origin, which has a specific nucleotide sequence. One or two replication forks may occur here. More than 20 enzymes are involved in the replication process. Since bacterial DNA is double-stranded, it must be divided before replication. This process involves the enzymes helicase, which unwinds the double helix, and topoisomerase, which prevents the formation of secondary curls. SSB protein binds to single-stranded DNA, preventing it from refolding into a double helix. As a result, a replication fork is formed. The synthesis of new DNA chains is carried out by the enzyme DNA polymyrase. To carry out the polymerization reaction of nucleotides on the template of the parent chain, polymerase requires a primer. The primer is a short RNA nucleotide chain, complementary to the template strand, with a free 3/- end. After the DNA strand has begun to be synthesized, the RNA primer is removed. Since the DNA strands in a duplex are antiparallel, the direction of unwinding of the double strand coincides only with the direction of DNA synthesis on one template, which is called the leading one. On the complementary strand, DNA is synthesized in short fragments, which are subsequently stitched into one strand by DNA ligases. The bacterial DNA replication process continues until all the DNA is doubled.

When bacteria are added to the nutrient medium, they grow and multiply until the content of one of the necessary components of the medium reaches a minimum, after which growth and reproduction stop. If we do not add nutrients and do not remove end products of metabolism, we get a statistical bacterial culture.

Bacterial reproduction phases:

1. Initial(lag phase) covers the period of time from the moment the bacteria are inoculated until the start of their growth. Its duration averages 2-5 hours and depends on the composition of the nutrient medium.

2. Exponential(logarithmic) phase. Characterized by a constant maximum rate of cell division. This speed depends on the type of bacteria and the nutrient medium. The time it takes for cells to double is called generation time. This time varies from several minutes to several hours.

3. Stationary phase occurs when the number of cells stops increasing. With a decrease in the concentration of nutrients in the nutrient medium, a decrease in the partial pressure of oxygen, and the accumulation of toxic metabolic products, the growth rate of bacteria decreases. The duration of the stationary phase is several hours and depends on the type of bacteria.

4. Dieback phase occurs due to the accumulation of acidic metabolic products or as a result of autolysis under the influence of its own enzymes. The duration of this phase ranges from ten hours to several weeks.

3.2. Nutrient media, principles of their classification, requirements for nutrient media, conditions for cultivating microorganisms.

The basis of bacteriological work is nutrient media, often determining the results of the study by their quality.

Basic requirements for nutrient media:

1. Culture media must contain all the nutrients necessary to feed the microbe, i.e. have nutritional value.

2. Have sufficient humidity

3. Have an optimal pH (7.2-7.6) acidity of the environment.

4. Be isotonic (NaCl concentration 0.87%), for halophilic bacteria the salt concentration is 1% or higher.

5. Have an optimal electronic potential, indicating the content of dissolved oxygen in the medium. It should be high for aerobes and low for anaerobes.

6. Be transparent so that bacterial growth is visible, especially in liquid media.

7. Be sterile (so that there are no other bacteria).

To prepare nutrient media, products of animal origin (meat, fish, blood, eggs, milk) and products of plant origin (potatoes) are used. Synthetic nutrient media composed of chemical compounds are also used.

The source of nitrogen for bacteria is simple ammonium compounds, amino acids or peptones; carbon source – sugar, polyhydric alcohols, organic acids. The need of bacteria for inorganic elements is satisfied by salts added to the nutrient medium: NaCl, KN 2 PO 4, K 2 HPO 4.

Depending on the consistency, nutrient media can be: liquid, semi-liquid and dense. The density of the medium is achieved by adding agar. Agar is a polysaccharide obtained from algae. It melts at a temperature of 100 oC, cools at a temperature of 45-50 oC. For semi-liquid media, agar is added at a concentration of 0.5%, for dense media - 1.5-2%. Liquid media do not contain agar-agar.

The composition of nutrient media can be simple and complex. Simple media include peptone water, meat peptone broth, meat peptone agar, Hottinger agar. Complex ones are simple ones with the addition of an additional nutritional component (sugar, whey, bile broths, blood, whey, yolk-salt agars, Keith-Tarozzi, Wilson-Blair medium).

Depending on the purpose of the environment, they are divided into:

1. General purpose – for cultivating most bacteria (meat peptone agar, blood agar).

2. Special purpose:

a) elective environments– these are the media on which a specific microorganism grows. For example, alkaline agar with a pH of 9 is used to isolate Vibrio cholerae.

b) With enrichment steps– these are environments that stimulate the growth of a particular microorganism, inhibiting the growth of others. For example, magnesium and selenite media stimulate the growth of bacteria of the genus Salmonella, inhibiting the growth of E. coli.

c) differential diagnostic environments serve to study the enzymatic activity of bacteria (Hiss medium).

d) combined nutrient media combine an selective medium that suppresses the growth of accompanying flora and a differential diagnostic medium (Ploskirev’s medium for isolating Shigella, bismuth-sulfite agar for Salmonella). Both of these media inhibit the growth of E. coli.

To differentiate prototrophic and auxotrophic bacteria, selective media are used. Prototrophs grow on a minimal medium containing only salts and carbohydrates, since they themselves are able to synthesize the metabolites they need for development. Auxotrophs need media containing certain amino acids, vitamins, i.e. growth factors.

Preparation of nutrient media is one of the most important and difficult areas of bacteriological work.

Currently, the medical industry has organized the production of canned media. Dry nutrient media are contained in plastic jars with tight-fitting lids to ensure an airtight seal.

Bacteria cultivation conditions:

1. Availability of a complete nutrient medium.

2. A certain cultivation temperature (optimal temperature 37 0 C).

3. A certain cultivation atmosphere. Strict aerobes require oxygen, so they grow well on the surface of agar Petri dishes or in a thin top layer of liquid medium. For the growth of aerobes in the deep layer of a liquid medium, it is necessary to continuously mix or shake the nutrient media so that oxygen is distributed throughout the entire volume of the medium. For facultative anaerobes, the same methods are used. Microaerophiles multiply at reduced partial pressure of oxygen. The CO 2 concentration should be 1-5%. To do this, special CO 2 incubators are used or the crops are placed in desiccators in which a hot candle is installed. For the growth of obligate anaerobes, it is necessary to exclude access to oxygen. To do this, add oxygen-reducing substances (thioglycolic acid) to the nutrient media, regenerate liquid nutrient media from air oxygen by boiling them, use oxygen absorbers by placing them in hermetically sealed “gaspack” containers, and use anaerostats.

4. Cultivation time (18-48 hours). For the cultivation of Mycobacterium tuberculosis (3-4 weeks).

5. Lighting. Light is required to grow phototrophic bacteria.

In industrial conditions, to obtain biomass of bacteria or fungi in order to obtain antibiotics, vaccines, and diagnostic drugs, cultivation is carried out in apparatus (fermenters) with strict adherence to the optimal parameters for growth and propagation of crops.

Nutrition of bacteria.

Nutrition refers to the processes of entry and exit of nutrients into and out of cells. Nutrition primarily ensures cell reproduction and metabolism.

Essential nutrients include: carbon, oxygen, hydrogen, nitrogen, phosphorus, potassium, magnesium, calcium. In addition to organogens, microelements are necessary. They provide enzyme activity. These are zinc, manganese, molybdenum, cobalt, copper, nickel, tungsten, sodium, chlorine.

Bacteria have a variety of sources of nutrients.

Depending on the source of carbon, bacteria are divided into: 1) autotrophs (use inorganic substances - CO 2); 2) heterotrophs (use organic C-hexoses, polyhydric alcohols, amino acids);

Nutrition processes must provide the energy needs of the bacterial cell. Based on energy sources, microorganisms are divided into: 1) phototrophs - the source is solar energy; 2) chemotrophs - obtain energy through redox reactions; 3) chemolithotrophs - use inorganic compounds; 4) chemoorganotrophs - use organic substances.

Medical microbiology studies bacteria that are heterochemoorganotrophs.

Bacterial growth factors are vitamins, amino acids, purine and pyrimidine bases, the presence of which accelerates growth. Among bacteria there are: 1) prototrophs (able to synthesize the necessary substances themselves); 2) auxotrophs (need growth factors).

Microorganisms assimilate nutrients in the form of small molecules, so proteins, polysaccharides and other biopolymers can serve as sources of nutrition only after they are broken down by exoenzymes into simpler compounds.

Paths for the entry of metabolites and ions into the microbial cell: I. Passive transport (without energy costs): simple diffusion; 2) facilitated diffusion (along a concentration gradient, with the help of carrier proteins). II. Active transport (with energy consumption, against a concentration gradient; in this case, the substrate interacts with a carrier protein on the surface of the cytoplasmic membrane).

There are modified versions of active transport - the transfer of chemical groups. Phosphorylated enzymes act as carrier proteins, so the substrate is transported in a phosphorylated form. This transfer of a chemical group is called translocation.

Concepts of bacterial growth and reproduction

For microbiological diagnostics, the study of microorganisms and for biotechnological purposes, microorganisms are cultivated on artificial nutrient media.

Under bacterial growth understand an increase in cell mass without changing their number in a population as a result of the coordinated reproduction of all cellular components and structures. An increase in the number of cells in a population of microorganisms is designated by the term "reproduction". It is characterized by generation time (the time interval during which the number of cells doubles) and such a concept as bacterial concentration (the number of cells in 1 ml).

In contrast to the mitotic division cycle in eukaryotes, the reproduction of most prokaryotes (bacteria) occurs by binary fission, and that of actinomycetes by budding. Moreover, all prokaryotes exist in a haploid state, since the DNA molecule is represented in the cell in the singular.

Bacterial population

When studying the process of bacterial reproduction, it is necessary to take into account that bacteria always exist in the form of more or less numerous populations, and the development bacterial population in a liquid nutrient medium in batch culture can be considered as a closed system. There are 4 phases in this process:

• 1st - initial, or lag phase or delayed reproduction phase- characterized by the beginning of intensive cell growth, but the rate of their division remains low;
• 2nd - logarithmic or log phase, or exponential phase,- characterized by a constant maximum rate of cell division and a significant increase in the number of cells in the population;
• 3rd - stationary phase- occurs when the number of cells in a population stops increasing. This is due to the fact that an equilibrium occurs between the number of newly formed and dying cells. The number of living bacterial cells in a population per unit volume of nutrient medium in the stationary phase is designated as M-concentration. This indicator is a characteristic feature for each type of bacteria;
• 4th - die-off phase (logarithmic death)- characterized by a predominance in the population of the number of dead cells and a progressive decrease in the number of viable cells of the population. The cessation of growth in the number (reproduction) of a population of microorganisms occurs due to the depletion of the nutrient medium and/or the accumulation of metabolic products of microbial cells in it. Therefore, by removing metabolic products and/or replacing the nutrient medium, regulating the transition of the microbial population from the stationary phase to the dying phase, it is possible to create an open biological system that tends to eliminate dynamic equilibrium at a certain level of population development.

This process of growing microorganisms is called flow cultivation (continuous culture). Growth in a continuous culture makes it possible to obtain large masses of bacteria during flow cultivation in special devices (chemostats and turbidistats) and is used in the production of vaccines, as well as in biotechnology for the production of various biologically active substances produced by microorganisms.

To study metabolic processes throughout the cell division cycle, it is also possible to use synchronous cultures- such bacterial cultures, all members of the population of which are in the same phase of the cycle. This is achieved using special cultivation methods.

However, after several simultaneous divisions, the synchronized cell suspension gradually switches back to asynchronous division, so that the number of cells no longer increases stepwise, but continuously.

Colonies

When cultivated on solid nutrient media, bacteria form colonies- a cluster of bacteria of the same species visible to the naked eye, which is most often the offspring of one cell.

Colonies of bacteria of different species differ:

• shape;
• size;
• transparency;
• color;
• height;
• the nature of the surface and edges;
• consistency.

The nature of the colonies is one of the taxonomic characteristics of bacteria.

The intense processes of anabolism and catabolism in the cell lead to rapid cell growth.

Bacterial growth is an orderly increase in the number and size of all components of the cell, subject to the presence of all the necessary chemical elements, which leads to an increase in its mass. Nutrient substrates must contain these elements in a metabolically accessible form. Cell growth is not unlimited. After reaching a critical size, the cell undergoes division or reproduction.

Most bacteria divide by transverse binary fission or cytokinesis. In most gram-positive bacteria, division occurs through the synthesis of a transverse septum running from the periphery to the center. The cells of most gram-negative bacteria divide by constriction. The division process is repeated at approximately equal intervals of time (from several minutes to several days), which is an individual genetic characteristic of the microbial species. As a result of reproduction, the number of cells in the population sharply increases.

Reproduction or reproduction in bacteria is the division of supercoiled nucleoid DNA into two daughter strands, each of which is further completed by a complementary strand and the formation of two daughter cells simultaneously occurs (semi-conservative method).

Reproduction is characterized generation time(the time interval during which the number of cells doubles) and such a concept as bacteria concentration(number of cells in 1 ml).

When bacteria are introduced into a nutrient medium, they grow and multiply until the content of any of the necessary components of the medium reaches a minimum, after which growth and reproduction stop. If throughout this entire time we do not add nutrients and do not remove the final products of metabolism, then we get static bacterial culture. A static (batch) culture of bacteria behaves like a multicellular organism, with genetic growth limitation. If we construct a graph with time on the abscissa axis and the number of cells on the ordinate axis, we obtain a curve describing the dependence of the number of cells formed on the time of reproduction, which is called growth curve.

Growth curve of bacteria in a nutrient medium. On this curve, several phases can be distinguished, replacing each other in a certain sequence (Fig. 11):

1. Initial - lag phase(English) lag- to lag behind). Covers the period of time between the inoculation of bacteria and the start of reproduction. Its duration is on average 2-5 hours and depends on the composition of the nutrient medium and the age of the crop being sown. During the lag phase, bacterial cells adapt to new cultivation conditions and inducible enzymes are synthesized.

2. Exponential (logarithmic) phase. Characterized by a constant maximum rate of cell division, a phase of geometric growth with a sharp increase in the population of microorganisms (2 in degree n). The rate of reproduction depends on the type of bacteria and the nutrient medium. The cell doubling time is called generation time, which varies depending on the type of bacterial culture: in bacteria of the genus Pseudomonas it is equal to 14 minutes, and Mycobacterium 18 - 24 hours. The size of the cells and the protein content in them remain constant during the exponential phase. The bacterial culture in this phase consists of standard cells.

Rice. 11. Phases of bacterial reproduction

3. Stationary phase(phase of equilibrium of reproduction and death of microbial cells). Occurs when the number of cells stops increasing. Since the growth rate depends on the concentration of nutrients, when the content of nutrients in the nutrient medium decreases, the growth rate also decreases. A decrease in growth rate also occurs due to the high density of bacterial cells, a decrease in the partial pressure of oxygen, and the accumulation of toxic metabolic products. The duration of the stationary phase is several hours and depends on the type of bacteria and the characteristics of their cultivation.

4. Dieback phase or death - a decrease in population size due to a decrease and lack of conditions for the reproduction of microorganisms. Occurs due to the accumulation of acidic metabolic products or as a result of autolysis under the influence of its own enzymes. The duration of this phase ranges from ten hours to several weeks.

This dynamics is typical for periodic crops with gradual depletion of nutrients and accumulation of metabolites. The constant presence of the bacterial population in the logarithmic growth phase is observed in a continuous culture, which is achieved by gradual dosing of nutrients, control of the density of the bacterial suspension and removal of metabolites. This process of growing microorganisms is called flow cultivation (continuous culture). Growth in a continuous culture makes it possible to obtain large masses of bacteria during flow cultivation in special devices (chemostats and turbidistats) and is used in the production of vaccines, as well as in biotechnology for the production of various biologically active substances produced by microorganisms.