Unhealthy change: diversity in a bacterial colony can prolong infections.

Researchers have long known that diversification strengthens large groups, from the stocks in a winning portfolio to the wildflowers in a field. New findings suggest that the same idea applies to bacterial communities and may explain why some infections are notoriously difficult to treat.

Many species of bacteria that form lump-shaped colonies can also live as biofilms, large clusters of organisms encased in a self-produced slime. Biofilms frequently capitalize on vulnerable sites in people, such as eyes irritated by contact lenses or open wounds, to cause chronic infections that are tough to eradicate.

While studying biofilms in the lungs of cystic fibrosis patients, Pradeep Singh and his colleagues at the University of Iowa in Iowa City noticed a strange phenomenon. When they used a small number of identical Pseudomonas aeruginosa bacteria to start a biofilm culture, the microbes quickly diversified into several types that looked and behaved differently.

Individuals isolated from the new biofilms had different nutritional needs, swimming styles, and defensive-chemical programs. Previous studies had noted that P. aeruginosa creates several different colony types.

Singh's team speculated that diversity within biofilms could give the bacteria an advantage in maintaining chronic infections. To test their hypothesis, the researchers exposed different groups of the bacteria to the disinfectant hydrogen peroxide. While biofilm communities composed of multiple P. aeruginosa types weathered the chemical onslaught, colonies containing only one bacterial type quickly perished.

The researchers traced the source of the species' diversity to a gene called recA. Biofilms composed of mutant bacteria missing the recA gene showed far less diversity and died rapidly, much as individual colonies did, when exposed to hydrogen peroxide.

Singh and his colleagues say that these results, which will be published in the Nov. 23 Proceedings of the National Academy of Sciences, parallel previous findings about diversity in other biological communities. When plant or animal groups are composed of several subpopulations, differences between individual members of a community can help the entire population survive environmental changes. Ecologists frequently refer to this idea as the "insurance hypothesis."

Although scientists are just beginning to consider bacterial groups as communities defined by cooperative interactions among individuals (see p. 330), Singh's team suggests that the insurance hypothesis may apply to the bacteria in biofilms.

"Basically, everyone knew that there was diversity [within P. aeruginosa bacteria] and that diversity is a good thing because it might help survival. But nobody did the experiment to show that a diverse population survives better and a less diverse population doesn't,' says Bill Costerton, a microbiologist at Montana State University in Bozeman and an author of a commentary accompanying Singh's report. Other researchers "did the experiments in their heads and assumed the results,' Costerton says.

Singh suggests that once researchers are convinced that a biofilm's survival hinges on diversity, they may develop new ideas for fighting chronic infections. "It maybe that we should start thinking of chronic infections from the ecological perspective," he says. "One would have to account for the capabilities of a diverse group instead of a large number of identical clones".