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, beta-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 beta-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. Significance and Impact of the Study A wide spectrum of antibiotics is used against infections of the lungs of cystic fibrosis (CF) sufferers, who are subjected to antibiotic therapy at regular intervals throughout their lives. However, high antibiotic concentrations are difficult to achieve in vivo, and bacteria that are repeatedly exposed to sublethal levels develop resistance. We demonstrate that sublethal levels of clinically important bactericidal antibiotics lead to an increase in mutation frequency in the major CF pathogen Pseudomonas aeruginosa. These findings could be clinically significant because sublethal concentrations of antibiotics present during chronic infection may act as mutagens and facilitate adaptation of bacteria to the CF lung.