Bacterial competition in food shortage situations prevents mutants from surviving

Publicado em 10 março 2021

A study conducted by researchers from the University of São Paulo in Brazil shows that competition for nutrients and the lack of cooperation between bacteria of the species Escherichia coli in the same population and in food shortage situations prevent mutants better adapted to the environment from thrive except for those who organize in small groups. The phenomenon masks the emergence of new bacterial variants and makes the mutation rate appear lower than it actually is.

Mutants are constantly emerging and accumulating from one generation to the next. The mutation frequency determines the development of a particular species. Understanding the origin of mutations is also important to explain biological processes. In the case of bacteria, for example, it helps to explain the possible evolutionary divergence of a pathogen in an epidemic or resistance to antibiotics.

Published in an article about the study in the journal BMC Biology The researchers compare what happens in E. coli colonies to the “tragedy of the community,” a term used by both economists and ecologists to indicate the problems that arise when individuals are to their detriment Community seek personal gain in the destruction of public goods or natural resources.

“In times of food shortages, bacteria do not interact for mutual benefit in order to keep the colony growing. We have found that even the presence of a few people who are able to utilize the available food sources does not prevent this lack of cooperation from putting them at risk a small number of mutants can multiply and form new colonies, “said Beny Spira, professor at the university’s Biomedical Sciences Institute (ICB-USP) and last author of the article.

The finding explains the longstanding question of why the frequency of mutants that can split certain nutrients (break them down into molecules that can be metabolized) is always much lower in practice than in theory.

The research on mutant frequency masking is part of a project supported by the São Paulo Research Foundation (FAPESP) through a regular research grant, a doctoral grant, and a grant for scientific initiation.

Tragedy of the community

In contrast to wild-type E. coli, bacteria with PHO-constitutive mutations overexpress the enzyme alkaline phosphatase and can therefore cleave glycerol-2-phosphate (G2P) to release phosphate and glycerol, an important source of carbon, especially in nutrient-scarce situations. PHO stands for phosphate. The “PHO-Regulon” helps plant cells to survive and thrive in the environment despite nutrient scarcity and phosphate deficiency.

The frequency of PHO-constitutive colonies on G2P-selective plates (cell culture dishes) is exceptionally low. “If we measure the actual frequency of mutants, we find that in a population of 100 billion bacteria there are tens of thousands of PHO-constitutive mutants. Yet only 50 to 100 manage to multiply and produce new colonies with the mutation that gives G2P the ability to split, “said Spira.

The mutant, he explained, is able to express large amounts of alkaline phosphatase, which is divided in the cell periplasm (a region between the inner cytoplasmic and outer membrane). So when G2P is broken down and glycerin is produced, the nutrient can be stored by the bacteria or released into the external environment, where it is instantly ingested by the many wild-type (non-mutated) bacteria nearby.

However, the glycerol released by mutants is insufficient for colonies of wild-type bacteria to grow. Around 20,000 bacteria can be found near each mutant. “The few mutants that manage to reproduce are those that cooperate with each other to form clusters and exchange glycerol. Otherwise, nutrients will run out, mutants will be inhibited and the population will die out,” said Spira.

Wild-type and mutant bacteria usually compete for insufficient glycerin. “Mutants do not reproduce due to the scarcity of this nutrient, which gives the impression that there are no mutations, and drastically reduces the incidence of PHO-constitutive mutants in the population,” he said.

The inhibition of PHO-constitutive mutants is therefore an example of the mutation frequency that is masked by the competition between mutants and their ancestral wild-type cells. “The example shows that cases resembling a ‘tragedy of the community’ can occur in different settings and should be taken into account when estimating mutation rates,” he said.

More information:
Henrique Iglesias Neves et al., Competition for Nutrients Masks the True Abundance of Mutant Bacteria, BMC Biology (2020). DOI: 10.1186 / s12915-020-00913-1

Quote: Bacterial competition in food shortage situations prevents the survival of mutants (2021, March 10), which was found on March 10, 2021 from scarcity.html were retrieved

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