To survive hostile environments, an organism often has to acquire new traits. But the rules of evolution appear to restrict how many such characteristics it can optimize at once. In a new study, researchers say they found that some bacteria make a genetic trade-off: the microbes involved were able to develop only one of two new traits and selected the one that best helped them thrive in a given setting.

The results could provide a model for studying how infectious microbes become resistant to antibiotics. “We want to understand the rules, if there are rules, for how organisms adapt,” says senior study author Seppe Kuehn, a biological physicist at the University of Illinois at Urbana-Champaign. “If we can, maybe there's a chance to make great breakthroughs in terms of treatment.”

David Fraebel, a graduate student in Kuehn's laboratory, grew Escherichia coli in either a nutrient-rich or nutrient-poor growth medium and measured how quickly the microbes spread. A mathematical model predicted that the fastest-spreading microbes would be those that combined two traits: swimming speed and growth rate. But instead the microbes chose just one trait: in the nutrient-rich environment, those that migrated farthest had opted for speedy swimming. In contrast, in the nutrient-poor medium, the fastest reproducers won out.

By comparing the DNA sequences of the more evolved microbes with those of their ancestors, Fraebel found that the fastest movers had acquired one mutation, whereas the quickest reproducers had acquired a different one. None of the surviving organisms had both.

The finding, reported in eLife, suggests the fittest bacteria selected one “evolutionary path or the other,” Kuehn says. Such compromises may be one of the many genetic tools organisms use to survive when confronting a challenge in their environment.