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 in the process of 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 has been introduced into the medium. This is the period of adaptation of bacteria to culture medium when differential activation of exo- and endoenzymes occurs for the subsequent implementation of an enzyme-substrate reaction. With a stable DNA content, there is a sharp increase in bacterial protein and RNA.
LAG-PHASE (4-5 hours) The duration of the lag-phase, as a rule, is short, measured in hours and depends on the type of bacteria, the multiplicity of sowing on a given medium, the state of the culture, the temperature used for growing, and the composition of the nutrient medium. In the absence of visible manifestations 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 endo-enzymes, bacterial cell starts sharing. Reproduction of bacteria occurs by transverse cell division.
LOGARITHMIC GROWTH PHASE (5 - 6 hours) This is the phase of reproduction carried out by the binary division of the mother cell into two daughter cells. " Chain reaction progressively accelerating binary fission of bacterial cells leads to a rapid increase in the 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 more and more unfavorable for the further growth and reproduction of bacteria. During the stationary phase, the rate of reproduction remains constant. Depending on the type of cultivated bacteria, it can last a long time, after which the fourth stage begins -
DIE PHASE The death phase is characterized by the progressive death of bacterial cells in a logarithmic manner. The duration of this phase is 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 (near-bottom growth), - growth in the form of a "ball of cotton". The growth pattern on a 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. Ordinary or simple nutrient media (meat-peptone agar, meat-peptone broth) are used for initial crops (primary). Complex ones include elective and differential-diagnostic nutrient media.
Nutrient media Elective environments 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 types of bacteria were studied and isolated, 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 features:
The sequence of determining the microorganism I. To which kingdom does it belong - prokaryote or eukaryote II. Which of the main categories does: 1. Gram-negative eubacteria with cell walls. 2. Gram-positive eubacteria with cell walls. 3. Eubacteria lacking cell walls. 4. Archaebacteria.
A total of 35 groups of microorganisms III are known. Which group within 4 categories does the microorganism belong to: 1. Spirochetes 2. Aerobic / microaerophilic, mobile, spiral /, 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 dissimilation 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 sheaths. 15. Non-photosynthetic gliding bacteria that do not form fruiting bodies. 16. Gliding 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. Regularly shaped, non-spore-forming Gram-positive rods. 4. Irregularly shaped Gram-positive rods that do not form spores. 5. Mycobacteria. 6. Actinomycetes.
IV. Archaebacteria. 1. Methanogens. 2. Sulfate-reducing archaea. 3. Extremely halophilic archaebacteria. 4. Archaebacteria deprived cell wall. 5. Extreme thermophiles and hyperthermophiles metabolizing S
The sequence of determination of the 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 modern classification microorganisms The phylogenetic systematization created to date has all the advantages and disadvantages of a classification based on a single feature. The advantages include the almost complete identity of the results obtained in various laboratories around the world. To establish the species, they also began to additionally assess 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 give an idea of the functions of bacteria. So now great importance for practical microbiologists has the creation of a phenotypic or functional classification. To quickly determine the taxonomic position of microorganisms, use the Burgee's Key. This reference book is constantly updated with new groups of isolates and is periodically reprinted. The 11th edition is currently up to date.
Formation of modern classification of microorganisms. On the present stage identification of the phylogenetic position of prokaryotes, including non-cultivated ones, is being developed on the basis of the nucleotide sequences of 16 S-r RNA. The improved technique of sequencing and data processing has made this approach practically uncontested in determining the generic affiliation of new organisms. The description of new taxa of bacteria in the last 50 years has been very fast, 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, such signs of microorganisms are chosen that are easy to determine, and for classification, complex methods often require the use. In this case, the identification scheme should include a small number of characters, and for taxonomic definition, the classification uses as many characters as possible.
THANK YOU FOR YOUR ATTENTION. YOU HAVE ATTENDED LECTURE No. 3 ON MICROBIOLOGY ON THE TOPIC: “GROWTH AND REPRODUCTION OF MICROORGANISMS. EVOLUTION AND CLASSIFICATION OF MICROORGANISMS".
Cells, like any living organism, are born, live and die. The growth and reproduction of bacteria is very fast, they could capture all the 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 various process. 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 cell lengthens;
- the outer shell "grows" inward and forms a transverse partition (constriction);
- two new (daughter) cells diverge in different directions.
The result is two identical organisms.
Individual microorganisms divide by budding, but this is rather an exception to 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:
- conjugation - direct transfer (not exchange) of a part of DNA upon contact from one bacterium to another (the process goes only in one direction);
- transduction - the transfer of a DNA fragment using a bacteriophage (bacteria virus);
- transformation is the absorption of the genetic information of dead or destroyed cells from environment.
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.
Speed and phases of growth of microorganisms
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 first phase (latent) is determined by the factors of adaptation to the nutrient medium. At this time, microorganisms are just getting used to new conditions. Bacterial growth is not observed.
- The second phase (exponential) is characterized by an increase in geometric progression(exponential increase). During this period, bacterial cells actively grow using all available food (maximum growth rate). Having reached a certain size, the bacterium begins to divide, and the process of reproduction proceeds at a constant rate, since food supplies are still sufficient. As a result of the increased rate of growth and reproduction, waste products (toxins) accumulate in the environment. Towards the end of the phase, the growth rate begins to decrease.
- The third phase is characterized by stationary growth, i.e. the number of "newborn" cells coincides with the number of dead ones. The curve of growth and reproduction in this segment no longer rises. The growth rate slows down. For some time, the total number of bacteria in the nutrient medium remains unchanged. However, due to the appearance of new "family members", nutrient reserves are reduced, and the toxicity of the environment increases. This process worsens the living conditions of the entire colony.
- The fourth phase - the death of microorganisms - occurs as a result of a catastrophic decrease in food and an increase in the toxicity of the environment. The number of living organisms is steadily decreasing, eventually, there are fewer viable cells than their dead counterparts.
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.
Conditions for growing microbes in artificial environments
For study (medicine, microbiology) and use (industry), bacterial cultures are grown on artificial nutrient media, which are divided according to consistency, origin and purpose:
- liquid, semi-liquid and dense (solid) artificial media;
- animal, vegetable or synthetic media (chemically pure compounds in a strictly defined concentration);
- conventional (universal), differential (different types of bacteria), special, selective or enrichment media (suppressing the growth of unwanted microbes).
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 cultivating 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 of microorganisms
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:
- The stationary method is characterized by the constancy of the environment, there is no intervention in the process. However, with this method of cultivation in liquid nutrient media, anaerobic organisms give an insignificant yield.
- The submerged culture method is used in industry to grow bacterial biomass. For this purpose, special containers are used. Growth factors are maintaining temperature and supplying nutrients to liquid media. In addition, if necessary, mixing or oxygen supply (for respiration of aerobic bacteria) is carried out.
- The flow media method (industrial cultivation) is based on the constant maintenance of the culture in the exponential growth phase. This is achieved by the continuous supply of nutrients and the removal of toxic waste products from cells. This technology makes it possible to achieve the maximum yield of various biologically active substances (antibiotic preparations, vitamins, etc.).
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.
bacterial overgrowth syndrome
Not always the growth of bacterial cells is beneficial, an excessive increase in the population of bacteria in the human body can be dangerous 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.
Clinical criteria for the disease
The criterion for the development of bacterial overgrowth syndrome can be:
- indigestion, decreased immunity, changes in the acidity of the stomach;
- violation of the integrity of the intestinal tract;
- consequences of surgical intervention;
- diseases of the gastrointestinal tract;
- stress;
- uncontrolled intake of antibiotic drugs.
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:
- diarrhea and flatulence appear at an early stage of the disease;
- bloating and spasmodic pain;
- fatigue, weakness;
- fast weight loss.
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.
Growth and reproduction
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 division (vegetative reproduction), which occurs in different planes, with the formation of diverse combinations of cells (grape bunch - staphylococci, chains - streptococci, pairs - diplococci, bales, packages - 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. During the division of bacteria 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; in mycobacterium tuberculosis, division occurs 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.
The process of reproduction of microbial culture on a non-replaceable medium proceeds unevenly. It defines four main phases.
1. The initial phase (lag phase), or resting phase. At this time, the culture adapts to the nutrient medium. In the microbial cell, the content of RNA increases, and with its help, the synthesis of the necessary enzymes occurs.
2. Exponential (logarithmic) phase characterized by a maximum increase in cells in culture, it goes exponentially (1, 2.4, 8, 16, 256, etc.). At this time, the majority of young and biologically active cells are in the medium. At the end of the phase, when the environment is depleted, the substances necessary for this microbe disappear, the amount of oxygen decreases, and the metabolic products increase - the growth of the culture slows down. The curve gradually assumes a horizontal direction.
3. stationary phase, or period of maturity, graphically represents a line running parallel to the x-axis. There comes a balance between the number of newly formed and dead cells. The amount of medium decreases, the density of cells in the population increases, the toxic effect of metabolic products increases - all this causes cell death.
4. Dying phase. In this phase, not only a decrease, but also a change in cells is observed. Degraded forms appear, as well as spores. After a few weeks or months, the culture dies. This happens because toxic waste products not only inhibit, but also kill microbial cells.
Thus, thanks to the processes of metabolism, the vital activity of the microbial cell is maintained. Aerobes need oxygen to breathe, while anaerobes use nitrate and sulfate respiration and fermentation. Microorganisms digest organic and inorganic substances from the external environment, oxidizing which receive the necessary energy and plastic elements. The result is cell growth. Having reached the necessary stage of maturity, the cell reproduces by simple division. In the course of their life activity, microorganisms gradually consume nutrients, releasing their metabolites into the environment, thereby changing the composition of the environment and making it unsuitable for life.
Bacterial activity is characterized by growth- the formation of structural and functional components of the cell and the increase in the bacterial cell itself, as well as reproduction- self-reproduction, leading to an increase in the number of bacterial cells in the population.
bacteria multiply by binary fission in half, less often by budding. Actinomycetes, like fungi, can reproduce by spores. Actinomycetes, being branching bacteria, reproduce by fragmentation of filamentous cells. Gram-positive bacteria divide by growing the synthesized division partitions into the cell, and gram-negative bacteria divide by constriction, as a result of the formation of dumbbell-shaped figures, from which two identical cells are formed.
Cell division preceded replication of the bacterial chromosome according to a semi-conservative type (the double-stranded DNA chain opens and each strand is completed by a complementary strand), leading to the 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. The cultivation of bacteria in such a system is called periodic cultivation, and the culture is called periodic. 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 growing bacteria on a liquid nutrient medium, near-bottom, diffuse, or surface (in the form of a film) culture growth is observed. The growth of a periodic culture of bacteria grown on a liquid nutrient medium is divided into several phases, or periods:
1. lag phase;
2. phase of logarithmic growth;
3. stationary growth phase, or maximum concentration
bacteria;
4. phase of bacterial death.
These phases can be depicted graphically as segments of the bacterial reproduction curve, which reflects the dependence of the logarithm of the number of living cells on the time of their cultivation.
Lag phase- the period between sowing bacteria and the beginning of reproduction. The duration of the lag phase is on average 4-5 hours. At the same time, bacteria increase in size and prepare for division; the amount of nucleic acids, protein and other components increases.
Logarithmic (exponential) growth phase is a period of intensive division of bacteria. 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 a rapidly growing cell to inhibitors of protein synthesis, nucleic acids and 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 process of bacterial growth, characterized by the death of bacteria in conditions of depletion of the sources of the nutrient medium and the accumulation of metabolic products of bacteria in it. Its duration varies from 10 hours to several weeks. The intensity of growth and reproduction of bacteria depends on many factors, including the optimal composition of the nutrient medium, redox potential, pH, temperature, etc.
Reproduction of bacteria on a dense nutrient medium. Bacteria growing on dense nutrient media form isolated round-shaped colonies with even or uneven edges (S- and R-forms), of different consistency and color, depending on the bacterial pigment.
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 cismutase, and peroxidases, protect microorganisms from the effects of toxic oxygen peroxide radicals. Many pigments have antimicrobial, antibiotic-like effects.
Reproduction of bacteria by fission is the most common method of increasing the size of the microbial population. After division, bacteria grow to their original size, which requires certain substances (growth factors).
The methods of reproduction of bacteria are different, but for most of their species, a form of asexual reproduction is inherent in the division method. Bacteria rarely reproduce by budding. Sexual reproduction of bacteria is present in a primitive form.
Rice. 1. In the photo, a bacterial cell is in the division stage.
The genetic apparatus of bacteria
The genetic apparatus of bacteria is represented by a single DNA - the chromosome. DNA is closed in a ring. The chromosome is located in a nucleotide that does not have a membrane. The bacterial cell contains plasmids.
Nucleoid
The nucleoid is analogous to the nucleus. It is located in the center of the cell. DNA is localized in it - the carrier of hereditary information in a folded form. The untwisted DNA reaches a length of 1 mm. The nuclear substance of a bacterial cell does not have a membrane, a nucleolus and a set of chromosomes, and is not divided by mitosis. Before division, the nucleotide is doubled. During division, the number of nucleotides increases to 4.
Rice. 2. In the photo, a bacterial cell on a cut. A nucleotide is visible in the central part.
Plasmids
Plasmids are autonomous molecules folded into a ring of double-stranded DNA. Their mass is much less than the mass of a nucleotide. Despite the fact that hereditary information is encoded in the DNA of plasmids, they are not vital and necessary for a bacterial cell.
Rice. 3. The photo shows a bacterial plasmid.
Division stages
After reaching a certain size inherent in an adult cell, division mechanisms are launched.
DNA replication
DNA replication precedes cell division. Mesosomes (folds of the cytoplasmic membrane) hold DNA until the process of division (replication) is completed.
DNA replication is carried out with the help of DNA polymerase enzymes. During replication, hydrogen bonds in 2-stranded DNA are broken, as a result of which two daughter single-stranded ones are formed from one DNA. Subsequently, when the daughter DNA has taken its place in the separated daughter cells, they are restored.
As soon as DNA replication is completed, a constriction appears as a result of synthesis, dividing the cell in half. First, the nucleotide undergoes division, then the cytoplasm. Cell wall synthesis completes division.
Rice. 4. Scheme of bacterial cell division.
Exchange of DNA segments
In hay bacillus, the DNA replication process is completed by the exchange of 2 DNA segments.
After cell division, a bridge is formed, along which the DNA of one cell passes into another. The two DNAs then intertwine. Some stretches of both DNA stick together. At the sites of adhesion, DNA segments are exchanged. One of the DNA goes back to the first cell along the jumper.
Rice. 5. Variant of DNA exchange in hay bacillus.
Types of bacterial cell divisions
If cell division is ahead of the division process, then multicellular rods and cocci are formed.
With synchronous cell division, two full-fledged daughter cells are formed.
If a nucleotide divides faster than the cell itself, then multinucleotide bacteria are formed.
Ways to separate bacteria
Division by breaking
Division by breaking is characteristic of anthrax bacilli. As a result of this division, the cells break at the joints, breaking the cytoplasmic bridges. Then they repel each other, forming chains.
sliding separation
With sliding separation after division, the cell separates and, as it were, slides over the surface of another cell. This method separation is characteristic of some forms of Escherichia.
split split
With a split division, 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, listeria).
Rice. 6. In the photo, rod-shaped bacteria forming chains (anthrax rods).
Rice. 7. In the photo, a sliding method for separating Escherichia coli.
Rice. 8. Splitting method for separating corynebacteria.
View of bacterial clusters after division
Accumulations of dividing cells have a variety of shapes, which depend on the direction of the division plane.
globular bacteria arranged one at a time, two at a time (diplococci), in bags, in chains, or like bunches of grapes. Rod-shaped bacteria - in chains.
Spiral bacteria- chaotic.
Rice. 9. The photo shows micrococci. They are round, smooth, white, yellow and red in color. Micrococci are ubiquitous in nature. They live in different cavities of the human body.
Rice. 10. In the photo, diplococcus bacteria - Streptococcus pneumoniae.
Rice. 11. Sarcina bacteria in the photo. Coccoid bacteria are combined into packets.
Rice. 12. In the photo, streptococcus bacteria (from the Greek "streptos" - a chain). Arranged in chains. They are the causative agents of a number of diseases.
Rice. 13. In the photo, the bacteria are "golden" staphylococci. Arranged like "bunch of grapes". The clusters have a golden color. They are the causative agents of a number of diseases.
Rice. 14. In the photo, the convoluted bacteria of leptospira are the causative agents of many diseases.
Rice. 15. In the photo, rod-shaped bacteria of the genus Vibrio.
bacterial division rate
The rate of division of bacteria is extremely high. On average, one bacterial cell divides every 20 minutes. Within only one day, one cell forms 72 generations of offspring. Mycobacterium tuberculosis divides slowly. The whole process of division takes them about 14 hours.
Rice. 16. The photo shows the process of streptococcus cell division.
Sexual reproduction of bacteria
In 1946, scientists discovered sexual reproduction in a primitive form. In this case, gametes (male and female germ cells) are not formed, however, some cells exchange genetic material ( genetic recombination).
Gene transfer occurs as a result of conjugations— unidirectional transfer of a part of genetic information in the form plasmid upon contact between bacterial cells.
Plasmids are small DNA molecules. They are not associated with the chromosome genome and are able to duplicate autonomously. Plasmids contain genes that increase the resistance of bacterial cells to adverse environmental conditions. Bacteria often pass these genes on to each other. The transfer of gene information to bacteria of another species is also noted.
In the absence of a true sexual process, it is conjugation that plays a huge role in the exchange of useful traits. This transfers the ability of bacteria to exhibit drug resistance. For humanity, the transmission of antibiotic resistance between disease-causing populations is especially dangerous.
Rice. 17. In the photo, the moment of conjugation of two Escherichia coli.
Phases of development of a bacterial population
When sowing on a nutrient medium, the development of the bacterial population goes through several phases.
Initial phase
The initial phase is the period from the moment of sowing to their growth. On average, the initial phase lasts 1-2 hours.
Reproductive delay phase
This is the phase of intensive growth of bacteria. Its duration is about 2 hours. It depends on the age of the culture, the period of adaptation, the quality of the nutrient medium, etc.
logarithmic phase
In this phase, the peak of the rate of reproduction and increase in the bacterial population is noted. Its duration is 5 - 6 hours.
Phase of negative acceleration
In this phase, a decline in the rate of reproduction is noted, the number of dividing bacteria decreases and the number of dead bacteria increases. The reason for the negative acceleration is the depletion of the nutrient medium. Its duration is about 2 hours.
Stationary maximum phase
In the stationary phase, an equal number of dead and newly formed individuals is noted. Its duration is about 2 hours.
Accelerated Death Phase
In this phase, the number of dead cells progressively increases. Its duration is about 3 hours.
Phase of logarithmic death
In this phase, bacterial cells die off at a constant rate. Its duration is about 5 hours.
Decreasing phase
In this phase, the remaining living bacterial cells go into a dormant state.
Rice. 18. The figure shows the growth curve of a bacterial population.
Rice. 19. In the photo, a colony of Pseudomonas aeruginosa is blue-green, a colony of micrococci is yellow, a colony of Bacterium prodigiosum is blood-red, and a colony of Bacteroides niger is black.
Rice. 20. The photo shows a colony of bacteria. Each colony is the offspring of a single cell. In a colony, the number of cells is in the millions. a colony grows in 1-3 days.
Division of magnetically sensitive bacteria
In the 1970s, bacteria living in the seas were discovered that had a sense of magnetism. Magnetism allows these amazing creatures to follow lines magnetic field Earth and find sulfur, oxygen and other substances that are so necessary for it. Their "compass" is represented by magnetosomes, which consist of a magnet. When dividing, magnetically sensitive bacteria divide their compass. In this case, the constriction during division becomes clearly insufficient, so the bacterial cell bends and makes a sharp fracture.
Rice. 21. The photo shows the moment of division of a magnetically sensitive bacterium.
Bacteria growth
At the beginning of bacterial cell division, two DNA molecules diverge to different ends of the cell. Next, the cell is divided into two equal parts, which are separated from each other and increase to the original size. The rate of division of many bacteria is on average 20-30 minutes. Within only one day, one cell forms 72 generations of offspring.
The mass of cells in the process of growth and development quickly absorbs nutrients from the environment. This is facilitated by favorable environmental factors - temperature, a sufficient amount of nutrients, the necessary pH of the environment. Aerobic cells require oxygen. For anaerobes, it is dangerous. However, the unlimited reproduction of bacteria in nature does not occur. Sunlight, dry air, lack of food, high ambient temperature and other factors have a detrimental effect on the bacterial cell.
Rice. 22. In the photo, the moment of cell division.
growth factors
Bacterial growth requires certain substances (growth factors), some of which are synthesized by the cell itself, and some come from the environment. All bacteria have different requirements for growth factors.
The need for growth factors is a constant feature, which makes it possible to use it for identification of bacteria, preparation of nutrient media and use in biotechnology.
Bacterial growth factors (bacterial vitamins) - chemical elements, most of which are water-soluble B vitamins. This group also includes hemin, choline, purine and pyrimidine bases, and other amino acids. In the absence of growth factors, bacteriostasis occurs.
Bacteria use growth factors in minimal amounts and unchanged. Row chemical substances of this group are part of cellular enzymes.
Rice. 23. In the photo, the moment of division of a rod-shaped bacterium.
The most important bacterial growth factors
- Vitamin B1 (thiamine). Takes part in carbohydrate metabolism.
- Vitamin B2 (riboflavin). Takes part in redox reactions.
- Pantothenic acid is an integral part of coenzyme A.
- Vitamin B6 (pyridoxine). Takes part in the metabolism of amino acids.
- Vitamins B12(cobalamins are substances containing cobalt). They take an active part in the synthesis of nucleotides.
- Folic acid. Some of its derivatives are part of enzymes that catalyze the synthesis of purine and pyrimidine bases, as well as some amino acids.
- Biotin. Participates in nitrogen metabolism, and also catalyzes the synthesis of unsaturated fatty acids.
- Vitamin PP(a nicotinic acid). Participates in redox reactions, the formation of enzymes and the metabolism of lipids and carbohydrates.
- Vitamin H(paraaminobenzoic acid). It is a growth factor for many bacteria, including those inhabiting the human intestine. Folic acid is synthesized from para-aminobenzoic acid.
- Gemin. It is an integral part of some enzymes that take part in oxidation reactions.
- Choline. Takes part in the reactions of lipid synthesis of the cell wall. It is a supplier of the methyl group in the synthesis of amino acids.
- Purine and pyrimidine bases(adenine, guanine, xanthine, hypoxanthine, cytosine, thymine and uracil). Substances are needed mainly as components of nucleic acids.
- Amino acids. These substances are the constituents of cell proteins.
Need for growth factors of some bacteria
Auxotrophs to ensure life, they need the supply of chemicals from outside. For example, clostridia are unable to synthesize lecithin and tyrosine. Staphylococci need the intake of lecithin and arginine. Streptococci need the intake of fatty acids - components of phospholipids. Corynebacteria and Shigella need nicotinic acid intake. Staphylococcus aureus, pneumococcus and brucella need vitamin B1 intake. Streptococci and tetanus bacilli - in pantothenic acid.
Prototrophs independently synthesize the necessary substances.
Rice. 24. Different environmental conditions affect the growth of bacterial colonies in different ways. On the left - stable growth in the form of a slowly expanding circle. On the right - rapid growth in the form of "shoots".
Studying the need of bacteria for growth factors allows scientists to obtain a large microbial mass, which is so necessary in the manufacture of antimicrobials, sera and vaccines.
Read more about bacteria in the articles:
Reproduction of bacteria is a mechanism for increasing the number of microbial populations. Bacterial division is the main mode of reproduction. After division, the bacteria should reach the size of adults. Bacteria grow by rapidly absorbing nutrients from their environment. Growth requires certain substances (growth factors), some of which are synthesized by the bacterial cell itself, and some come from the environment.
By studying the growth and reproduction of bacteria, scientists are constantly discovering the beneficial properties of microorganisms, the use of which in Everyday life and in production is limited only by their properties.