Researchers for the first time have tracked the specific genetic mutations -- occurring over just a few generations -- that allow bacteria to respond to environmental changes, they report online in Nature today (November 4).

Image of Pseudomonas fluorescens Image: Courtesy of Hubertus Beaumont

"We showed how evolution happens in real time," said Hubertus Beaumont, a biologist from Leiden University in the Netherlands and first author on the study.

Studies have shown bacteria and other organisms can switch back and forth between phenotypes to better survive in new environments. For instance, Beaumont said, many bacteria switch their surface antigens when invading a host, so they can avoid being attacked, and certain desert plants are programmed to germinate seeds at random time intervals, increasing their chances of encountering rain. "This bet-hedging strategy is very simple, but captures the essence of evolution." Beaumont said. "Natural selection in these uncertain environments causes an organism to evolve protective traits." Exactly how such phenotypic adaptability emerges, however, was unknown.

In order to observe how bet-hedging evolves, Beaumont and his colleague Paul Rainey at Massey University in New Zealand observed Pseudomonas fluorescens, a common rod-shaped bacterium, in a new type of environment. They already knew that the bacteria grow well in a test tube that's shaken manually or in an incubator that allows oxygen to circulate in the culture. So instead of shaking the test tube, a condition to which the bacteria are well-adapted, the researchers watched the bacteria grow in non-shaken test tubes.

As expected, some of the bacteria adapted to the novel environment, forming colonies with an advantageous "wrinkly" morphology as opposed to the ancestral bacteria, which grew smoothly. The team identified these new colony types in the test tube and transferred them to fresh tubes, repeating this process 15 times to select for the new variations. Eventually, the bacteria evolved the ability to rapidly switch their phenotypes between the "wrinkly" and "smooth" cell-types to prepare themselves to cope with the different environments.

"I find these results really intriguing," said Martin Ackermann, an environmental microbiologist at Eidgenössische Technische Hochschule Zurich (or ETH Zurich) in Switzerland who was not involved in the research. "It is amazing to see that phenotypic switching can evolve so rapidly, in the course of a just a few rounds of selection. As far as I know, this has not been observed before."

Beaumont and Rainey then sequenced the evolved bacterial genome and found all the mutations that had arisen and that might have contributed to this new trait. The team identified nine mutations distinguishing bet-hedgers from their ancestors. They pinpointed one specific mutation as the one which allows the phenotype to switch back and forth between different morphologies, while the other mutations, they found, were essential for growing the new type of bacteria.

"It's yet another beautiful study at multiple levels from Paul Rainey's team," said Richard Lenski, a microbial ecologist at Michigan State University who didn't participate in the research, in an email. "This is a neat demonstration that the evolution of bet-hedging was contingent on other mutations that had occurred earlier in the lineage." These earlier changes improved the fitness of the bacteria at each stage.

"The results thus suggest that phenotypic switching is a strategy that can readily evolve," and may capture the earliest evolutionary solutions to life in fluctuating environments, Ackerman wrote in an email.


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