There are four stages in the growth and reproduction of bacteria: slow stage, logarithmic stage, stable stage and declining stage. Lag period: also called adjustment period. After the bacteria are inoculated into the culture medium, they have a short adaptation process to the new environment (those who are not adapted can die because of replanting). The curve of this period is flat and stable, because there is little bacterial reproduction. The length of lag period varies with varieties, inoculation amount, age and nutritional components, and is generally 1 ~ 4 hours. During this period, the bacteria increased in size and were active in metabolism, which reserved sufficient enzymes, energy and intermediate metabolites for the division, proliferation and synthesis of bacteria. Logarithmic period: also known as exponential period. On the growth curve of this period, the number of viable bacteria increased linearly. Bacteria grow rapidly in a stable geometric progression, which can last for several hours to several days (depending on culture conditions and bacterial generations). The morphology, dyeing and biological activity of bacteria in this period are typical and sensitive to external environmental factors, so it is best to study the bacterial characteristics in this period. Antibiotics have the best effect on bacteria in this period. Stable period: the total number of growing flora is in a gentle stage, but the vitality of bacterial population changes greatly. Due to unfavorable factors, such as the consumption of nutrients in the culture medium, the accumulated pH value of toxic products (organic acids, H2O2, etc.) decreases. ), the rate of bacterial reproduction gradually decreased, and the relative number of bacterial deaths began to increase gradually. At this stage, the number of bacterial proliferation and death is gradually balanced. It can change the morphology, dyeing and biological activity of bacteria and produce corresponding metabolites such as exotoxin, endotoxin, antibiotics and spores. Decline period: With the development of stable period, bacterial reproduction becomes slower and slower, and the number of dead bacteria increases obviously. The number of living bacteria is inversely proportional to the culture time. During this period, bacteria become longer, swell or rot abnormally, and even autolyse, so it is difficult to identify their shapes. Physiological and metabolic activities tend to stagnate. Therefore, it is difficult to identify bacteria on old cultures.