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

Abstract

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)) (p 0.015). 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.

Biofilm dispersal cells of a cystic fibrosisPseudomonas aeruginosaisolate exhibit variability in functional traits likely to contribute to persistent infection

“…Interestingly, Garcia‐Castillo et al . () have reported that biofilm populations of CF isolates showed a lower mutation frequency compared to planktonic cultures. In contrast, Conibear et al .…”

Evolutionary dynamics of bacteria in a human host environment

“…Excluding the 1973 isolates from the calculation, we find the mutation rate to be 6.9 × 10 −11 SNPs/bp per generation. It is important to note that these estimates show no indication of increased rates of mutation, as has been suggested for biofilm-associated cells and CF infection-associated cells due to oxidative stress or other phenomena (28)(29)(30)(31)(32).…”

Emergence and spread of antibiotic resistance following exposure to antibiotics

“…Furthermore, horizontal gene exchange is enhanced in biofilms. Whereas the mutation frequency is usually stable over time in bacteria, an increased value of this parameter has been demonstrated in bacteria growing in biofilms (Molin & Tolker‐Nielsen, 2003; Driffield et al , 2008; García‐Castillo et al , 2011). So‐called hypermutators (=mutators) have a spontaneous mutation rate that is 10–1000 times higher than that of a wild‐type strain.…”