A protein secreted by human immune cells prompts an infectious bacterium to mutate, protecting it from drugs and immune attack.

Pseudomonas aeruginosa is a leading cause of fatal lung infections in people with cystic fibrosis (CF). A team led by Daniel Wozniak of Ohio State University in Columbus found that LL-37, a human protein produced by immune cells called neutrophils, triggered the bacteria to make a sticky, sugary coating that is often seen in chronic infections of P. aeruginosa.

At low levels, LL-37 entered bacterial cells and bound to their DNA. By disrupting DNA replication, the protein induced mutations that boosted production of the sugar coating and resistance to the antibiotic rifampin. The mutations were similar to those seen in P. aeruginosa isolated from people with CF.

PLoS Pathog. 10, e1004083 (2014)