A common experimental method used in biology labs around the world affects mutations in unexpected ways
To keep bacteria alive in the laboratory for many generations, scientists must regularly supply them with food. When the bacteria have used up the nutrients, a small fraction of the culture is usually diluted into fresh nutrient solution and the cycle repeats — a method known as serial passage. In a theoretical study, we determined that this very method significantly reduces the impact of mutations.
Genetic mutations occur randomly in bacterial populations. If a mutation provides a competitive advantage, such as an increased rate of reproduction, there is a chance for the mutants to displace the original cells and completely take over the population. Some scientists observe this process to learn about the mechanisms of evolution, whereas, for others, the establishment of mutations is a nuisance because it might disrupt a function the bacteria are carrying out. The new theory shows not only that some mutations are eliminated simply by the fact that the serial passage protocol is used. The mathematical description also predicts that the suppression of mutations happens to different extents depending on the advantage the mutation confers, which may distort the picture of evolution extracted from such experiments. The results will now allow scientists to account for these effects, while, at the same time, providing a strategy to limit the effect of mutations if they are undesired.
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