Effect of ciprofloxacin concentration on the frequency and nature of resistant mutants selected from Pseudomonas aeruginosa mutS and mutT hypermutators

MORERO NATALIA R.; MONTI MARIELA R. ; ARGARAÑA CARLOS E.

Congreso; XLV Reunión Anual de SAIB; 2010

Resistance of P. aeruginosa to the antibiotic ciprofloxacin (CIP) is mainly achieved through mutations in GyrA and ParC subunits of DNA topoisomerases, and the expression of the multidrug efflux pump MexCD-OprJ by mutations in its transcriptional repressor NfxB. We previously showed that for wild type and hypermutator strains of P. aeruginosa PAO1 (deficient in the Mismatch or 8-oxoguanine Repair Systems), nfxB mutations are dominant in cells selected at low drug concentration (SAIB 2009). In this work we show that the apparent mutation frequency of nfxB in wild type and hypermutator strains increases to very high values at subinhibitory concentration of CIP, and report the mutational spectra obtained. In order to identify nfxB mutants by luminescence and analyze with better precision this phenomenon, we worked with PAO1 strains carrying a chromosomal copy of MexCD-OprJ promoter fused to the lux operon. This allowed us to estimate that in the presence of subinhibitory concentration of CIP, nfxB was mutated in approximately 1/300 colonies in the hypermutator strains and 1/1200 colonies in the wild type strain. The absence of mutants in the inoculum and the presence of sectorized luminescent colonies containing mutated and non mutated cells, led us to conclude that the mutation process at very low CIP concentration occurs after the exposure of the bacterial cells to this drug