Bees swarm, wolves mark territories, and dolphins recognize their family pod. Complex social behaviors, while common in the animal kingdom, seem far beyond the ken of a single-celled bacterium. But certain bacteria not only cluster with relatives and swarm to expand their colony, researchers found they will also form distinct territorial boundaries when they encounter other species or strains. How do such simple creatures deploy these complex behavioral tactics? The microbiologists studied the social behavior of the bacterium Proteus mirabilis and then isolated a handful of genes that allow the bacterial colonies to distinguish between "self" and "non-self"—an essential skill for any gang that wants to keep others off its turf. They published their findings in the July 11^th, 2008 issue of the journal Science.

Karine Gibbs and Peter Greenberg of the University of Washington, Seattle, and their colleague Mark Urbanowski of the University of Iowa, Iowa City, mapped mutations within a six-gene stretch of P. mirabilis DNA that codes for the ability to identify self. Swarms with a mutation within this locus, called id, will muster the troops and form a boundary against relatives lacking the mutation. In other words, even though they shared all other genes with the parent colony, the original form of a single gene within the id locus told the mutants, "Those are foreign cells. Keep out!"

Gibbs and her colleagues speculate that other bacteria may employ similar self-recognition genes and that the system may prevent competition or superinfection of the bacterial host by more than one strain. The researchers don''t yet know what the cells produce to halt encroaching swarms, but it is unlikely to be a toxin since they don''t find dead cells littering the battlefields between the colonies. It seems the simple bacteria have trumped more complex social beings like us with a kinder, gentler approach to social conflict.

K.A. Gibbs, E.P. Greenberg, and M.L. Urbanowski. Genetic Determinants of Self Identity and Social Recognition in Bacteria," Science, Vol. 321, pp. 256-259, July 11, 2008. DOI: 10.1126/science.1156499