The Japanese bobtail squid and luminescent marine bacteria live in blissful harmony: the bacteria have a home in the squid's light organ while producing for the squid rippling patterns of light to confuse prey or predators. The way bacteria form this symbiotic relationship may now shed light of another kind, on human disease.

Only some strains of the bacterium Vibrio fischeri will colonise squid, while other bacteria of the same species live happily in fish but avoid squid. Mark Mandel and colleagues at the University of Wisconsin in Madison report this week that a single gene makes all the difference: fish bacteria will colonise squid if given a gene carried by their squid-friendly kin (Nature, DOI: link).

Intriguingly, that gene is the one that enables the bacteria to form a biofilm, the tightly woven matrix of "slime" which allows bacterial colonies to behave in many ways like a single organism. "The biofilm might be critical for adhering to the light organ, or telling the host that the correct symbiont has arrived," says Mandel.

Biofilms also seem to be important in another kind of bacterial invasion of animals: disease. Some normally harmless lung bacteria can turn into a nasty infection in humans by forming a biofilm, for example, while many immune defences are aimed at preventing biofilms. And certain bacteria, like Vibrio fischeri, typically invade only certain species and tissues.

The signals that trigger biofilm formation may also regulate how a bacterium's relationship with its host switches from harmless to pathogenic, Mandel suggests. It might be that similar bacterial genes control both infection and symbiosis. That means learning how squid bacteria work might reveal new targets for fighting human infections.