Prevention and correction of oxidative DNA lesions in Pseudomonas aeruginosa is 
ensured by the DNA oxidative repair system (GO). Single inactivation of mutT, 
mutY and mutM involved in GO led to elevated mutation rates (MRs) that 
correlated to increased development of resistance to antibiotics. In this study, 
we constructed a double mutant in mutY and mutM (PAOMY-Mgm) and characterized 
the phenotype and the gene expression profile using microarray and RT-PCR. 
PAOMY-Mgm presented 28-fold increases in MR compared with wild-type reference 
strain PAO1. In comparison, the PAOMYgm (mutY) single mutant showed only a 
fivefold increase, whereas the single mutant PAOMMgm (mutM) showed a 
nonsignificant increase in MR compared with PAO1 and the single mutants. 
Mutations in the regulator nfxB leading to hyperexpression of MexCD-OprJ efflux 
pump were found as the mechanism of resistance to ciprofloxacin in the double 
mutant. A better fitness of the mutator compared with PAO1 was found in growth 
competition experiments in the presence of ciprofloxacin at concentrations just 
below minimal inhibitory concentration. Up-regulation of the antimutator gene 
pfpI, that has been shown to provide protection to oxidative stress, was found 
in PAOMY-Mgm compared with PAO1. In conclusion, we showed that MutY and MutM are 
cooperating in the GO of P. aeruginosa, and that oxidative DNA lesions might 
represent an oxidative stress for the bacteria.