The mutation Proton pump modulator C1397A in gyrB was a G·CT·A transversion characteristic for mutY and mutM mutants of the GO system leading to an amino acid substitution. Alteration of gyrB at position 1397 has previously been reported in a fluoroquinolone-resistant clinical strain of P. aeruginosa (Oh et al., 2003). Mutations in both gyrB and nfxB clarify the high-level resistance to ciprofloxacin (> 256 mg L−1) in this isolate. As ciprofloxacin can

stimulate the bacterial production of ROS (Morero & Argarana, 2009; Kohanski et al., 2010), and as PAOMY-Mgm mutator is defective in the repair of DNA oxidative lesions, we decided to investigate the relative fitness of the PAOMY-Mgm mutator compared with PAO1 in the presence of 0.1 mg L−1 ciprofloxacin (MIC ciprofloxacin = 0.19 mg L−1 for PAO1 and 0.19 mg L−1

and resistant subpopulation (+++) for PAOMY-Mgm). Prior to the experiment, we ensured that the PAO1 and PAOMY-Mgm mutant have statistically the same growth rate in LB (doubling time ± SD: 26.5 ± 0.6 and 25.7 ± 0.7 min, respectively) and that the concentration of 0.1 mg L−1 ciprofloxacin, which is just below the strains MIC had statistically similar inhibitory effect on the growth rates of the two strains (doubling time ± SD: 66.6 ± 3.2 and 64.3 ± 3 min, respectively) (Philipsen et al., 2008). PKC412 in vitro In the absence of selection pressure in the environment, the two bacterial populations co-existed and appeared equally fitted during the 5-day period of the experiment (Fig. 1a), whereas in the presence of 0.1 mg L−1 ciprofloxacin, the PAOMY-Mgm Olopatadine overtook the PAO1 population at day 3 (Fig. 1b). This was not seen for the single mutants inactivated in mutY or mutM

(Fig. S1 C–F). This suggests occurrence of a tolerant bacterial population more fitted to grow in the presence of ciprofloxacin in the PAOMY-Mgm population. To investigate the cause of the better fitness of the PAOMY-Mgm population compared with PAO1, we searched for ciprofloxacin resistant mutants in the mutator population. The MIC levels of ciprofloxacin were increased only by twofold and of chloramphenicol by eightfold in the adapted isolates compared with control isolates (not exposed to ciprofloxacin) (Table 3). This phenotype was associated with moderate increases in the expression levels of some of the genes encoding efflux pumps. The expression levels of mexD were increased 7- to 15-fold and of mexB twofold to fourfold compared with control, untreated isolates (Table 3). No differences in the expression levels of mexE and mexF were found (data not shown).