GyrA sequence analysis of Staphylococcus aureus and methicillin-resistant S. aureus strains selected, in vitro, for high-level ciprofloxacin resistance.

Four methicillin-susceptible Staphylococcus aureus and eight methicillin-resistant S. aureus (MRSA) isolates, all of which were ciprofloxacin susceptible (MIC < 2.0 micrograms/ml) were manipulated, in vitro, to achieve high-level ciprofloxacin resistance by means of up to 14 passages onto media containing increasing concentrations of ciprofloxacin. Resistance to ciprofloxacin at a concentration of at least 512 micrograms/ml was achieved in all 12 isolates tested. This resistance was continually detected during weekly passage on antibiotic-free media for 12 weeks. The parent and daughter cells from four strains had their gyrA sequenced from amino acid (aa) codons 70-100, the region of previous mutations in high level quinolone-resistant S. aureus. Mutations at aa codon 84 were seen in three of four strains, but appeared at varying levels of ciprofloxacin resistance. High-level resistance of S. aureus and MRSA to ciprofloxacin can be developed in vitro using multiple exposures to incremental concentrations of the drug. It is apparently due to multiple mechanisms and, once established, remains stable over time.