The mechanism of ciprofloxacin resistance in dihydrogen peroxide-induced mutants of Salmonella enterica subsp. enterica serovar typhimurium consists mainly in mutations in gyrA gene and less in mutations affecting ciprofloxacin uptake.

The effect of H(2)O(2) on the induction of ciprofloxacin (CFL) resistant mutants of Salmonella enterica subsp. enterica serovar Typhimurium was evaluated and determinants of CFL resistance in the mutants were analyzed. Factors associated with CFL resistance in H(2)O(2)-induced mutants included (i) mutations in gyrA gene, predominantly (63 %) Asp(87)-->Asn and less (37 %) Ser(83)-->Phe substitutions, (ii) mutations in the regulatory genes of MarRAB or SoxRS or in the individual structural genes of these operons. Such mutations are induced by H(2)O(2) in a much lower extent. Reduced OmpF expression simultaneously with enhanced efflux was detected only in one mutant strain and 20 % of mutant strains had increased CFL efflux from the cells.