Stationary biofilm growth normalizes mutation frequencies and mutant prevention concentrations in Pseudomonas aeruginosa from cystic fibrosis patients

Bacterial biofilms play an important role in the persistent colonization of the respiratory tract in cystic fibrosis (CF) patients. The trade-offs among planktonic or sessile modes of growth, mutation frequency, antibiotic susceptibility and mutant prevention concentrations (MPCs) were studied in a well-defined collection of 42 CF Pseudomonas aeruginosa isolates. MICs of ciprofloxacin, tobramycin, imipenem and ceftazidime increased in the biofilm mode of growth, but not the MPCs of the same drugs. The mutation frequency median was significantly higher in planktonic conditions (1.1×10^-8) than in biofilm (9.9×10^-9) (p0.015). Isolates categorized as hypomutable increased their mutation frequency from 3.6×10^-9 in the planktonic mode to 6×10^-8 in biofilm, whereas normomutators (from 9.4×10^-8 to 5.3×10^-8) and hypermutators (from 1.6×10^-6 to 7.7×10^-7) decreased their mutation frequencies in biofilm. High and low mutation frequencies in planktonic growth converge into the normomutable category in the biofilm mode of growth of CF P. aeruginosa, leading to stabilization of MPCs. This result suggests that once the biofilm mode of growth has been established, the propensity of CF P. aeruginosa populations to evolve towards resistance is not necessarily increased.