The evolution of antibiotic resistance associates with a mutation in an efflux pump. The evolution of antibiotic resistance associates with a mutation in an efflux pump. (A) Susceptibilities to piperacillin plus tazobactam determined at 37°C after 24 h of incubation by measuring the diameter of cell growth inhibition. Isolates from clades C2, C3, and C4 have the emrA mutation. Clade C1 isolates (including also isolate BM1) lacking the mutation differed significantly from those with the mutation (***, P < 0.001) by Tukey’s HSD multiple-comparison test for unequal group sample sizes. (B) Temporal variations in levels of resistance to piperacillin plus tazobactam of all B. multivorans colonies recovered from the CF patient under study. Arrows indicate the date that the patient was subjected to an antibiotic therapy. Inês N. Silva et al. mSystems 2016; doi:10.1128/mSystems.00029-16