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Causes of Microbial Strain Decline
Created on 2024.05.05
Product Introduction:
Microorganisms are inevitably subject to certain genetic variations due to long-term exposure to external influences. It is the existence of these variations that allows biological evolution to progress. However, due to the presence of mutations, strains may experience a deterioration of excellent production traits, loss of genetic markers, and changes in typical characteristics during the cultivation or preservation process, a phenomenon known as strain decline. For example, due to strain decline, Bacillus thuringiensis may produce smaller spores and parasporal crystals in the new generation of bacterial bodies; the loss of flagellar antigen in preserved Salmonella strains leads to non-coagulation of H polyvalent serum, etc.
The fundamental cause of strain decline is the negative mutation of related genes. If the gene controlling yield undergoes a negative mutation, it is manifested as a decrease in yield; if the gene controlling spore production undergoes a negative mutation, the ability to produce spores decreases. Spontaneous mutations occur during the strain's subculturing and propagation process. Although the probability of spontaneous mutations is very low (generally 10^-10 to 10^-6), especially for a specific gene, the mutation frequency is even lower. However, due to the high metabolic and reproductive capacity of microorganisms, as the number of generations increases, the number of declining cells will continue to increase, gradually becoming dominant in terms of quantity, and eventually becoming a declined strain.
In addition, there are several other reasons for strain decline.
- Phenotypic lag can also cause strain decline. For instance, during the process of mutagenesis breeding, it is often found that the yield of a certain strain is higher in the initial screening, but the yield decreases during the secondary screening.
- Plasmid loss leading to strain decline is more common in antibiotic production, as the synthesis of many antibiotics is controlled by plasmids. When the bacterial cells, due to spontaneous mutations or external conditions (such as high temperature), cause the plasmid controlling the yield to fall off or the replication of nuclear DNA and plasmid to be inconsistent, that is, the DNA replication rate exceeds that of the plasmid, after several generations, some cells will no longer have the plasmid that plays a decisive role in yield. As the number of such cells continuously increases and becomes dominant, the strain shows signs of decline.
- Continuous subculturing is an important reason that accelerates strain decline. On the one hand, the more times the subculturing is performed, the higher the chance of spontaneous mutations (especially negative mutations); on the other hand, the more times the subculturing is performed, the faster the number of individual declining cells in the population increases and becomes dominant, causing the population phenotype to decline.
- Unfavorable cultivation and preservation conditions are another important reason for accelerating strain decline. Poor cultivation conditions such as nutrient composition, temperature, humidity, pH value, aeration volume, etc., and preservation conditions such as nutrition, water content, temperature, oxygen, etc., not only can induce the emergence of declining cells but also promote the rapid proliferation of declining cells, significantly exceeding the number of normal cells in quantity, leading to strain decline.
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