For bacteria, assassination can breed cooperation

WASHINGTON — Bacteria assassinating each other when crowded together ironically can favor the evolution of cooperation.

When a Vibrio cholerae bacterium jostles neighbors in crowds on crab shells, it fires a spring-loaded toxin injection. Siblings with the same immunity genes don’t die, but genetically different strains of V. cholerae can succumb.

In both laboratory battles and computer simulations, these neighbor-to-neighbor harpoonings over time can separate a random mix of strains into a patchy landscape of same-strain clumps. The change from all mixed up to irregular patches works like a separation process of phases of metals (called the Model A order-disorder transition) and has not been reported before in living things, William Ratcliff of Georgia Institute of Technology in Atlanta said August 5 at the 2nd American Society for Microbiology Conference on Experimental Microbial Evolution.

This resulting clumpydistribution, despite its murderous origin, favors the rise of cooperation, such as secreting substances useful to a whole community, said Georgia Tech colleague Brian Hammer. In an analysis of more than 400 strains of bacteria from 26 genera, the more elaborate the weapons made by microbial strains are, the more of a strain’s genes were devoted to secretions.

Cooperative patches of bacteria are of interest because they form scary defenses like biofilms. They also matter to biologists trying to weed or reseed the microflora of the gut.