Sub-lethal concentrations of antibiotics increase mutation frequency in the cystic fibrosis pathogen Pseudomonas aeruginosa.

Approximately 80% of adult patients with cystic fibrosis (CF) become chronically infected with Pseudomonas aeruginosa and consequently require antibiotic therapy at intervals throughout their lives. Achieving lethal concentrations of antibiotics in the lung remains a challenge. Recent evidence from Escherichia coli and Staphylococcus aureus suggests that the generation of hydroxyl radicals by sublethal concentrations of antibiotics may induce mutagenesis and confer bacteria with resistance to a wide range of antimicrobials. As Ps. aeruginosa can persist for many years following colonization of the airways and during this time it is repeatedly exposed to bactericidal antibiotics, we tested whether its exposure to sublethal levels increases mutation frequency. We demonstrate that sublethal levels of three classes of bactericidal antibiotics commonly used against Ps. aeruginosa infections, β-lactams, aminoglycosides and quinolones lead to an increase in mutation frequency, varying between c. threefold increase with aminoglycosides and a c. 14-fold increase in mutation frequency with β-lactam antibiotics. These findings could be clinically significant because exposure to sublethal concentrations of antibiotics during chronic infection leading to increased mutation frequency may facilitate adaptive radiation of pathogenic bacteria in the heterogeneous environment of the CF lung.