A Game for Three Microbes

AMSTERDAM--Microbes can play rock-paper-scissors just like children, a new study has shown. Under the right circumstances, three strains of Escherichia coli can co-exist because each has a strategy that beats one competitor but loses to the other. Researchers think this kind of situation may be much more common in the real world than was once thought, and it may contribute to biological diversity.

Ecologists often depict the rivalry between organisms vying for the same niche as all-or-nothing struggle: there are clear winners and losers. But more recently, some ecologists have suggested that such final reckonings are rare, and competition in the real world keeps going. They say the separation of organisms in space often allows them to co-exist.

At the International Symposium on Microbial Ecology here on 28 August, Stanford University ecologist Brendan Bohannan presented a striking example. Bohannan, grad student Benjamin Kerr, and their colleagues used three strains of E. coli. Strain C grows slowly but is armed with a toxin. Strain S is susceptible to the toxin. Strain R is resistant to the toxin, but grows more slowly than S, although faster than C. When the strains were grown in contiguous hexagonal patches on a petri dish, something akin to rock-paper-scissors developed: C killed adjacent patches of S with its toxin, fast-growing S colonized patches of R, and R overran patches of slowpoke C. All three strains persisted in a waltz of circular competition.

The elegant balance broke down, however, when the three strains were grown in a shaking flask. Without home turf defended by growing borders, all of strain S fell victim to strain C's toxin. Then R won by outgrowing C. The bottom line? "Simply allowing for spatial interactions is sufficient for diversity," says ecologist Paul Rainey of Oxford University in the U.K. And biological games like rock-paper-scissors may be under way in all ecosystems, Rainey says. If so, studying them could help solve one of ecology's enduring puzzles: nature's staggering biodiversity.

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Posted in Biology