GYRA SEQUENCE-ANALYSIS OF STAPHYLOCOCCUS-AUREUS AND METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS STRAINS SELECTED, IN-VITRO, FOR HIGH-LEVEL CIPROFLOXACIN RESISTANCE

Four methicillin-susceptible Staphylococcus aureus and eight methicill in-resistant S. aureus (MRSA) isolates, all of which were ciprofloxaci n susceptible (MIC < 2.0 mug/ml) were manipulated, in vitro, to achiev e high-level ciprofloxacin resistance by means of up to 14 passages on to media containing increasing concentrations of ciprofloxacin. Resist ance to ciprofloxacin at a concentration of at least 512 mug/ml was ac hieved in all 12 isolates tested, This resistance was continually dete cted during weekly passage on antibiotic-free media for 12 weeks. The parent and daughter cells from four strains had their gyrA sequenced f rom amino acid (aa) codons 70-100, the region of previous mutations in high level quinolone-resistant S. aureus. Mutations at aa codon 84 we re seen in three of four strains, but appeared at varying levels of ci profloxacin resistance. High-level resistance of S, aureus and MRSA to ciprofloxacin can be developed in vitro using multiple exposures to i ncremental concentrations of the drug. It is apparently due to multipl e mechanisms and, once established, remains stable over time.