Phenotypic changes in ciprofloxacin-resistant Staphylococcus aureus

The work described here continues our studies of the effects of subinhibitory concentrations of antibiotics on SOS and DNA repair gene expression in Staphylococcus aureus. Mitomycin C and the new-generation fluoroquinolone moxifloxacin induced expression of SOS response genes (lexA, recA, sosA, and umuC) in a ciprofloxacin-resistant CiprI strain of S. aureus. To examine phenotypic changes in Cipr strains mutated in CIP targets (GrlA and/or GyrA), we used Biolog Phenotype MicroArrays™. Two other Cipr strains mutated in the norA promoter region were used to study the effects of subinhibitory concentrations of DNA-damaging antibiotics on norA expression. We show that mitomycin C and moxifloxacin induced overexpression of the norA gene in Cipr strains. Finally, we confirm that subinhibitory concentrations of CIP increase mutation rates in S. aureus.