In evolutionary terms, “non-transitive” competitive effects—those without strict competitive hierarchies in which the biggest, best, or meanest win—have been shown to underlie processes such as the diversity of mating tactics observed in side-blotched lizards. A paper published in the March 25 issue of Nature suggests a similar dynamic exists between competing strains of Escherichia coli in the intestines of their host mice, but others remain to be convinced.

Benjamin Kirkup and Margaret A. Riley at the Department of Ecology and Evolutionary Biology, Yale University, built on in vitro and in silico studies of antibiotic-mediated antagonism between strains of E. coli. Some strains (C) produce colicins, narrow spectrum antibiotics that kill other, colicin-sensitive (S) strains. In turn, S strains outcompete colicin-resistant (R) strains, which close the circle by outcompeting C. This is analogous to the game of rock–paper–scissors, no one strategy can prevail over the other two: scissors cut paper,...

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