Crowded house.
Once they settled on a glass slide (top), it didn't take long for these two microbes (red and green) to become much more intermingled (bottom).

Susse Kirkelund Hansen / Technical University of Denmark

Group Housing Helps Communities Thrive

A crowded space may not seem like the best environment to thrive, but two soil bacteria make it work. A new experiment shows how the two species rapidly evolve to coexist the in cramped quarters, establishing a booming community there. Figuring out why these microbes settle down together will help researchers understand species interactions within complex, macroscopic communities, such as tropical forests, says Paul Rainey, an evolutionary geneticist at the University of Auckland, New Zealand.

Microbial mats called biofilms are everywhere. The plaque on your teeth is actually a community of hundreds of microbes, and the gunk that clogs your drain is home to many, many more. These kinds of dense, rich agglomerations are also a prime testing ground for evolutionary biologists who want to understand how species interact. Rainey, Susse Kirkelund Hansen from the Technical University of Denmark, and colleagues looked to see what happened when two species of the soil bacteria Acinetobacter and Pseudomonas putida were put in the same place at the same time.

The team evaluated the physiology and fitness of the species under various living conditions. In some experiments, the researchers put the microbes in a 300-milliliter container with constant water circulation; in other experiments, the microbes were in a 4-milliliter-long chamber with a glass cover. A key aspect was that P. putida depended on Acinetobacter for food. Most of the time, the only nutrient the researchers provided was benzyl alcohol, an organic compound that Acinetobacter can process, but that P. putida can not. Instead, P. putida must rely on a metabolized form of benzyl alcohol--benzyoate--which is produced by Acinetobacter.

The researchers tracked the growth and morphological changes of the two species. Not much happened in the circulating container, and the P. putida could only survive there if there were high concentrations of the benzyl alcohol for the Acinetobacter to use. But on the glass slide, a new community blossomed. At first the two species were not intermingled, existing in separate clumps and aggregations. But within 10 days, the P. putida had overgrown the Acinetobacter clumps.

This new interaction was heritable, as it took just a day for the 10-day-old P. putida to re-establish its intimacy with Acinetobacter when the researchers placed the two species together in a new flow chamber. By introducing mutations into different parts of the P. putida genome, Kirkelund Hansen discovered a genetic change that had made P. putida glue itself much more tightly to its underlying host, even when low oxygen supplies would have otherwise forced it to detach. In these close-knit quarters, the total growth of two organisms doubled, and therefore, the community was more productive, the team reports in the 1 February issue of Nature.

"This is really exciting work, demonstrating community evolution on rapid time scales," says Simon Levin, a theoretical ecologist at Princeton University in New Jersey. It shows "the potential of biofilms for addressing fundamental questions in evolutionary ecology."

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