Uptake of 3H-norfloxacin in methicillin-resistant Staphylococcus aureus

To examine the possibility of a proton-motive efflux pump for quinolones in highly quinolone-resistant clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA), we studied 3H-norfloxacin uptake in two quinolone-resistant and two quinolone-sensitive strains of MRSA whose gyrA region surrounding amino acid codons 84 and 85 had been sequenced. Two strains were related (one sensitive and one resistant) in that both were recovered from a single patient, one before (sensitive) and one after (resistant) ciprofloxacin therapy. Drug uptake was assessed in four separate experiments running triplicate bacterial suspensions with radiolabeled drug added at time = 0. Sampling was performed in 10 min increments up to 50 min by a vacuum filtration method. The ionic uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCPH), was added at 40 min to test inhibition of a pump mechanism. The results demonstrated no statistically significant differences in uptake between the sensitive and resistant groups, and the uptake patterns were similar. CCCPH also induced an equivalent surge, or enhanced uptake among these strains, rendering an energy-dependent efflux pump an unlikely contributor to the high levels of resistance seen in our strains. Our findings support parallel studies done on these isolates that implicate mutational changes at amino acid codon 84 and/or codon 85 in the gyrA gene as an explanation for high-level quinolone resistance (MIC to ciprofloxacin greater than or equal to 16 mg/L) in MRSA.