After growth in a biofilm, bacteria adopt varied phenotypes.


Infectious bacteria often grow in their hosts as biofilms. Blaise Boles, Matthew Thoendel, and Pradeep Singh (University of Iowa, Iowa City, IA) now report that growth in biofilms causes Pseudomonas aeruginosa bacteria to diversify. The resultant diverse populations are probably more able to cope with changing environmental conditions, providing a bacterial correlate of the resistance of diverse forest ecosystems.

Earlier workers recognized that these bacteria, when isolated from their human hosts, show a remarkable degree of diversification. Singh's group noted a similar diversity of colony morphology after a brief in vitro passage through biofilm conditions. After ruling out contamination, the team observed that diversity extended to bacterial nutrient requirements, swimming abilities, and production levels of a protective pigment. Biofilms grown from small flat colonies were better at dispersing their progeny and biofilms grown from tall, wrinkly colonies could better resist antimicrobial treatments.

The diversification was dependent on RecA activity. RecA may be required if biofilm conditions induce a stress response and thus DNA damage; error-prone repair could then be a source of mutation. Alternatively, the super-dense, encased conditions of a biofilm may induce a specialized RecA-dependent recombination as a programmed response to provide greater diversity.

“It's intriguing to speculate that group living initiates a program to produce diversification,” says Singh. “However, at the moment we don't have any evidence for that.” He must first confirm that growth in a biofilm results in higher rates of mutation rather than just increased selection pressure. If confirmed, the programming idea could intensify the debate over one view of evolution—where mutation rates are generally fixed and selection is the only driving force.

In the human host, an ecologically diverse community of bacteria may do better, because in any new environment there will be at least some subpopulation that thrives. Singh will test this idea by infecting animals with bacteria lacking RecA. If RecA bacteria adapt less readily, then attacking the diversification process may be one way to target persistent infections.


Boles, B.R., et al.
Proc. Natl. Acad. Sci. USA.