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Why do strains decline? Causes and preventive measures for the decline of common strains

What is bacterial decline?

In the process of culture or preservation, due to spontaneous mutation, the deterioration of some original excellent production characters and the loss of genetic markers are known as strain recession.The decline of strain is not a sudden occurrence, but a gradual process from quantitative change to qualitative change.The main cause of strain decline is negative mutations in the genes involved.Reasonable breeding, selection of appropriate medium, creation of good culture conditions, the use of effective preservation methods can effectively prevent the degradation of strains.

The commonly used methods for the preservation of bacterial species are inclined low-temperature preservation, liquid paraffin preservation, filter paper preservation, sand preservation, liquid nitrogen freezing preservation, freeze-drying preservation.

The decline of strain is not a sudden occurrence, but a gradual process from quantitative change to qualitative change.At the beginning, only a few cells in the population had spontaneous mutation (generally negative change), which did not change the performance of the population strain.After successive generations, the negative variable individuals in the group reach a certain number and develop into the dominant group, thus making the whole group show serious decline.

What causes a strain to decline?

The main cause of strain decline is negative mutations in the genes involved.If the negative mutation of the gene controlling the yield occurs, the yield will be reduced.The ability to produce spores decreases if negative mutations occur in the gene that controls spore production.Spontaneous mutation occurs in the transplanting process.Although spontaneous mutations are rare (typically 10-6 to 10-9), they are especially rare for a particular gene.However, due to the extremely high metabolic reproductive capacity of microorganisms, with the increase in the number of passage times, the number of declining cells will continue to increase, gradually dominant in number, and eventually become a declining strain.

Phenotypic delay causes strain decay

Phenotypic delay can also cause strain decay.For example, in the process of mutation breeding, it is often found that the yield of a certain strain is higher in the initial screening, but decreased in the rescreening.

Plasmid shedding causes strain decay

Plasmid shedding leads to the decline of bacteria in the production of antibiotics, and the synthesis of many antibiotics is controlled by plasmid.When strain cells due to spontaneous mutation or external conditions such as high temperature, the control output of plasmid loss or nuclear DNA and plasmid replication, namely DNA replication at a faster rate than plasmid, after multiple batches, some cells don't have decide on yield of plasmid, such cells constantly improve to achieve advantage, the strains of a recession.

Consecutive batches

Continuous passage is an important reason to accelerate the decline of strain.On the one hand, the more passage times, the higher the probability of spontaneous mutation (especially negative mutation).On the other hand, the more passage times, the faster the number of individual decaying cells in the population increases and occupies the dominant position, resulting in the decline of the population phenotype.

Inappropriate culture and preservation conditions

Improper culture and preservation conditions are another important reason for accelerating the decline of strains.Poor culture conditions such as nutrients, temperature, humidity, pH value, ventilation and preservation conditions such as nutrition, water content, temperature, oxygen, not only will induce the emergence of recessionary cells, but also promote the rapid proliferation of recessionary cells, greatly more than the number of normal cells, causing bacterial decline.

What measures should be taken to prevent the decline of bacteria?

Reasonable breeding

Mononuclear cells should be used in selecting and breeding strains, and multicellular cells should be avoided.The variety and dose of mutagens should be reasonably selected or the mutation sites should be increased to reduce the recovery of separation.After mutagenesis treatment, sufficient post-culture and separation and purification were carried out to ensure the pure extraction of preserved species.These can effectively prevent the degradation of bacteria.

Select the appropriate medium

It has been found that cultivating "5406" antimicrobial bacteria, Streptomyces flaviformis, in the root juice medium of old alfalfa can prevent its degeneration.Adding molasses, aspartic acid, glutamine, 5-nucleotide or mannitol into the culture medium of gibberellus producing bacteria -- gibberellus tengana can also prevent the degradation of bacteria.Also has made a selection of relatively poor nutrition medium preservation medium, such as easy to use the medium the proper limits of added sugar source such as glucose, because mutations are mostly through the growth of strains, when medium rich nutrition, strains in vigorous growth condition, high metabolism, provides good condition for mutation, greatly increases the chances of the degradation of strains.

Create good cultivation conditions

In production practice, the creation and discovery of a suitable condition for the growth of the original species can prevent the degradation of the species, such as low temperature, drying, hypoxia and so on.In the cultivation of aspergillus oryzae 3.942, measures were taken to change the culture temperature (from 20-30℃ to 33-34℃) to prevent the degradation of its spore producing ability.

Control the passage times

There are spontaneous mutations in microorganisms, and mutations occur during reproduction.Therefore, unnecessary transplanting and passage should be avoided as far as possible, and necessary passage should be reduced to the lowest level to reduce the probability of spontaneous outbreak.The more times a strain is passed, the higher the chance of mutation, and thus the more likely it is to degenerate.This requires strict control of the number of transfer and passage of bacterial species in both laboratory and production practice, and an appropriate interval of transfer and passage should be established according to different preservation methods.If at the same time use slope preservation and other preservation methods (vacuum freeze-dried preservation, sand pipe, liquid nitrogen preservation, etc.), in order to extend the preservation time of bacteria.

Different cell types were used for transplanting

In some microorganisms, such as actinomycetes and molds, the cells of their bacteria often contain several nuclei or even heterokaryotes, so the inoculation with mycelium will appear impure and decline, while the spores are generally mononuclear, when inoculated with it, no such phenomenon occurs.In practice, it is found that aspergillus oryzae is easy to degenerate if passing by conidia, while ascospore is not easy to degenerate if passing by transmigration.In addition, some people use cotton pellets that have been sterilized to lightly dip "5406" spores for slant transplanting, which avoids the access of mycelia and thus achieves the effect of preventing degradation.

Note: some new preservation methods, such as porcelain bead preservation, can also significantly extend the preservation period.


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