Bacterial Gene Transfer Found to Influence Fitness

Researchers in Cologne, Germany have demonstrated that bacteria can gain survival benefits from incorporating genetic material from their peers. Termed horizontal gene transfer, the authors also report on a high-throughput system that may make it possible to predict whether a particular transfer would contribute to the bacteria’s fitness. The work was part of a larger project focused on the predictability of evolution. 

A single bacterial species has only a relatively small genome and reproduces only by copying its own genetic information—making its adaptability is limited. In community with other species, however, bacteria can acquire genetic material from their neighbors and incorporate it into their own genome.

In a new study published in The ISME Journal, the team at the University of Cologne’s Institute of Biological Physics set out to determine whether such acquisitions actually improve bacteria’s chances of survival, and, if so, under what conditions.

By generating a series of hybrids of the bacterium Bacillus subtilis and related bacterial species by horizontal gene transfer and testing the resulting bacteria for their adaptability to environmental factors such as temperature and food supply, the authors found that they can predict whether gene transfer accelerates adaptation to certain growth conditions.

Professor Dr. Berenike Maier and colleagues generated a large number of hybrids between the ‘hay bacillus’ Bacillus subtilis and related Bacillus species by means of horizontal gene transfer and determined their fitness, which is defined by how fast a hybrid reproduces compared to the unmodified bacterium (parent strain). Rapid growth means that one bacterial species can displace another and thus achieve dominance of its own descendants.

To test their adaptability, the researchers determined the fitness of the hybrids under different environmental conditions, such as elevated temperature or different food sources. “We obtained a different distribution of fitness effects for each condition. The important question now was whether we could predict from this if horizontal gene transfer would increase adaptability under a particular environmental condition,” says Isabel Rathmann, one of the study’s lead authors.

To test these predictions, the scientists designed a high-throughput evolution experiment in collaboration with the research group of Dr. Tobias Bollenbach, a professor at the University’s Institute of Biological Physics and member of the Faculty of Mathematics and Natural Sciences. The experiment generated populations of different hybrids that competed with each other for nutrients under different growth conditions.

The use of a high-throughput system made it possible to observe hundreds of such populations over 450 bacterial generations. Initial results indicated that, using the distribution of fitness effects, predictions can be made about the fitness effects of gene transfer. “We found that under most growth conditions, there were some hybrids that were better adapted than the parent strain. This result suggests that a shared gene pool could help bacteria adapt to certain environmental conditions. However, there are also conditions under which gene transfer does not confer an advantage,” said Mona Förster, another of the study’s lead authors.

Going forward, the researchers plan to use the method to specifically predict when horizontal gene transfer will accelerate bacterial adaptation.