Three elements of growth and reproduction. Mastering the conditions, influencing factors and laws of bacterial growth and reproduction is of great significance to clinical medicine and basic research.
1. Conditions for bacterial growth and reproduction
1. Adequate nutrition: There must be sufficient nutrients to provide necessary raw materials and sufficient energy for the metabolism and growth of bacteria.
2. Suitable temperature: The temperature limit for cell growth is -7℃～90℃. Various types of bacteria have different requirements for temperature. They can be divided into psychrophiles, the most suitable growth temperature is (10℃～20℃); Mesophiles, 20℃～40℃; Thermophiles (Thermophiles) It grows best at 56℃～60℃. The pathogenic bacteria are all mesophilic bacteria, and the optimum temperature is the body temperature of the human body, which is 37°C. Therefore, the laboratory generally uses 37°C to cultivate bacteria.
Some mesophilic bacteria can also grow and multiply at low temperatures. For example, in a refrigerator at 5°C, Staphylococcus aureus grows slowly to release toxins. Therefore, eating food stored in the refrigerator overnight can cause food poisoning.
3. Proper pH: During the metabolism of bacteria, the activity of enzymes can only be exerted within a certain pH range. The optimal pH for most pathogens is neutral or weakly alkaline (pH 7.2 to 7.6). The pH of human blood and tissue fluid is 7.4, and bacteria can easily survive. The gastric juice is acidic, and most bacteria can be killed. Individual bacteria grow well under alkaline conditions. For example, Vibrio cholerae grows best at pH 8.4 to 9.2; some bacteria have an acidic pH optimum, such as Mycobacterium tuberculosis (pH 6.5 to 6.8) and Lactobacillus (pH 5). .5). In the process of bacterial metabolism, sugars are decomposed to produce acid, and the pH decreases, which affects the growth of bacteria. Therefore, buffers should be added to the culture medium to keep the pH stable.
4. Necessary gas environment: The presence of oxygen is related to growth. Some bacteria can only grow under aerobic conditions; some can only grow under anaerobic conditions; while most pathogens can only grow under aerobic and anaerobic conditions Both can survive. Generally, CO2 is needed in bacterial metabolism, but the CO2 produced by most bacteria's own metabolism can meet the demand. Some bacteria, such as meningococcus, require a higher concentration of CO2 (5-10%) during the initial isolation, otherwise the growth is poor or even unable to grow.
2. The way and speed of bacterial growth and reproduction
The growth and reproduction of bacteria includes the regular growth of the components of the bacteria and the increase in the number of bacteria.
Bacteria reproduce asexually in a simple way of two divisions, and its outstanding feature is the extremely fast reproduction speed. The necessary time for bacteria to divide and multiply is called Generation time. The generation time of bacteria is determined by the type of bacteria and is affected by environmental conditions. The generation time of bacteria is generally 20-30 minutes. Some bacteria are slower, such as Mycobacterium tuberculosis. The generation time is 18-20 hours, and Treponema pallidum is 33 hours.
(1) Growth and reproduction of individual bacteria
Bacteria generally reproduce asexually by a simple two-division method, and individual bacteria such as Mycobacterium tuberculosis occasionally reproduce by branching. Under suitable conditions, most bacteria reproduce extremely fast, and it only takes 20 to 30 minutes to divide once. Cocci can divide from different planes and form different arrangements after division. Bacteria divide along the horizontal axis. When the bacteria divide, the bacteria cells first enlarge and the chromosomes are duplicated. In Gram-positive bacteria, the bacterial chromosome is connected to the midvalence body. When the chromosome is replicated, the midvalence body is also divided into two, each moving to both ends, pulling the copied chromosome to the side of the cell. . Then the cell membrane in the middle of the cell sinks from the outside to the inside and gradually stretches to form a diaphragm. At the same time, the cell wall also grows inward and becomes the cell wall of the two progeny cells. Finally, due to the action of peptidoglycan hydrolase, the covalent bond of the cell wall peptidoglycan is broken and split into two cells. Gram-negative bacteria have no mediators and their chromosomes are directly connected to the cell membrane. The new chromosomes produced by duplication are attached to a nearby point, forming a new cell membrane between the two points, separating the two clusters of chromosomes on both sides. Finally, the cell wall invades along the diaphragm, and the whole cell divides into two daughter cells.
(2) The growth and reproduction law of bacterial population
The speed of bacteria reproduction is amazing. The generation time of Escherichia coli is 20 minutes. Based on this calculation, one cell can multiply to 2 million after 8 hours under optimal conditions, and it can exceed 1 billion after 10 hours. After 24 hours, the number of bacteria multiplies can be huge To the extent that it is difficult to calculate the data. But in fact, due to the consumption of nutrients, the accumulation of toxic products and the change of environmental pH during bacterial reproduction, bacteria can never keep the original speed of infinite proliferation. After a certain period of time, the active proliferation of bacteria gradually slows down and the bacteria die Increasingly, the rate of viable bacteria decreases.
After connecting a certain number of bacteria to the appropriate medium, study the law of bacterial growth. Taking the culture time as the abscissa and the logarithm of the number of viable bacteria in the culture as the ordinate, a growth curve can be obtained (Figure 3-1).
Figure 3-1 Growth curve of bacteria
The growth and reproduction of bacterial populations can be divided into four stages:
1. Lag phase: After the bacteria are inoculated into the culture medium, they have a short process of adapting to the new environment (the unadapted may die due to the conversion). In this period, the curve is flat and stable, because bacteria multiply very little. The length of the sluggish period varies with the factors, the amount of inoculated bacteria, the bacterial age and the nutrients, etc., and it is generally 1 to 4 hours. During this period, the size of the bacteria increases, and the metabolism is active, for the division and proliferation of bacteria to synthesize, and reserve sufficient enzymes, energy and intermediate metabolites.
2. Logarithmic phase (Logarithmicphase): also known as exponential phage. The number of viable bacteria on the growth curve increased linearly during this period. Bacteria grow extremely fast in a stable geometric progression, which can last from several hours to several days (depending on the culture conditions and bacterial generation time). The morphology, staining, and biological activity of the bacteria in this period are very typical, and they are sensitive to the effects of external environmental factors. Therefore, it is best to study the characteristics of bacteria in this period. Antibiotics have the best effect on bacteria during this period.
3. Stationary phase: The total number of growing bacteria in this phase is in a flat phase, but the viability of the bacterial population changes greatly. Due to the depletion of nutrients in the culture medium, the accumulation of toxic products (organic acid, H2O2, etc.), the pH drop and other unfavorable factors, the bacterial reproduction speed gradually decreases, and the relative bacterial death number begins to gradually increase. During this period, the bacterial proliferation and death numbers gradually balance. The morphology, staining, and biological activity of bacteria can change, and corresponding metabolites such as exotoxin, endotoxin, antibiotics, and spores can be produced.
4. Decline phase: As the stable phase develops, bacterial reproduction becomes slower and slower, and the number of dead bacteria increases significantly. There is an inverse relationship between the number of viable bacteria and the culture time. During this period, the bacteria grow and swell or decay abnormally, and even the bacteria are autolyzed, making it difficult to identify their shape. Physiological and metabolic activities tend to stagnate. Therefore, it is difficult to identify bacteria on old cultures.
The growth and reproduction of bacteria in the body and in nature is affected by the body's immune factors and environmental factors in many ways, and there will be no typical growth curve like in the culture medium. Grasp the law of bacterial growth, study and control the growth of pathogenic bacteria, discover and cultivate bacteria useful to humans.