Sticky films of microbes form as the result of competition, not cooperation, between bacterial strains.

Scientists had thought that biofilms form cooperatively, with multiple strains of bacteria growing to protect one another. But when Kevin Foster of the University of Oxford, UK, and his colleagues mixed two different strains of the opportunistic pathogen Pseudomonas aeruginosa, they found that the strains competed with each other, with one producing antibiotics called pyocins to kill its opponent. This mixture produced greater amounts of biofilm than did the individual strains, and introducing other antibiotics into the mix further stimulated biofilm formation.

The results suggest that the clinical use of antibiotics could be causing bacteria to make more biofilms, making the microbes harder to stamp out.

PLoS Biol. 13, e1002191 (2015)