Microbial Sex Rapidly Disseminates Ecologically Important Genes

    Horizontal gene transfer, the scientific moniker for bacterial sex, is a fundamental part of microbial evolution, as it allows key genetic loci, such as antibiotic resistance, to spread through diverse microbial communities. However, since the event is relatively infrequent, scientists have often wondered why gene transfer events have been able to exert such strong effects.

    Now, researchers at the University of Oxford have been able to demonstrate through mathematical modeling that the secret to horizontal gene transfer is migration—as the movement between communities of microbes rapidly increases the odds of various bacterial species being able to swap DNA elements and adopt new traits.

    “It is well known that bacteria are able to swap little pieces of DNA, which is crucial for them to be able to evolve and adapt to new environments, including responding to antibiotics. It’s different to sex in humans, but the effect—swapping genetic material—is similar,” explained senior author Kevin Foster, Ph.D., professor of evolutionary biology at the University of Oxford. “However, sex in bacteria is a very rare event, with only one cell among millions swapping DNA. And in theory, any resistant strain will rapidly divide and take over the community, shutting down any opportunity to share the resistance gene with others. But it does keep happening, and genes are often able to hop through diverse groups of different bacteria. Until now, the mystery has been why.”

    The findings from this study were published recently in Nature Communications through an article entitled “Migration and horizontal gene transfer divide microbial genomes into multiple niches.”

    The Oxford researchers were able to show that horizontal sweeps through bacterial populations can occur without the use of negative frequency-dependent selection. Moreover, once they analyzed all of their data, the investigators found that migration was a key factor, providing for the highest rates of horizontal gene transfer for ecologically important traits that were under positive natural selection.

    “Our model investigates the conditions necessary for this bacterial sex to keep taking place—how does a function like antibiotic resistance keep hopping between bacteria?” noted lead author Rene Niehus, a graduate student in Dr. Foster’s laboratory. “What we found was that the missing ingredient was migration. Previous work ignored that these communities are open, and our model shows that this very high immigration rate among bacteria gives a huge opportunity for different microbes to meet and swap DNA, even though it’s a rare event when taken in isolation.”

    Migration between communities of bacteria can take place anywhere, from the human body to soil and while antibiotic resistance is a good example of a beneficial trait passed horizontally between microbes, it could also involve being able to survive in an environmental toxin or on a particular nutrient.

    “The key point is that a bacterial system with continual immigration of strains will allow traits like antibiotic resistance to spread much more easily between different species of bacteria,” Niehus added. “Our model offers a theoretical framework for understanding the processes behind this spread.”