Selection of high-level oxacillin resistance in heteroresistant Staphylococcus aureus by fluoroquinolone exposure

To study the effect of fluoroquinolone exposure on the expression of mec(A)-encoded oxacillin resistance, population analysis profiling was performed on four strains of fluoroquinolone-susceptible, mec(A)-positive, heteroresistant Staphylococcus aureus. Growth in the presence of 0.5 x MIC of a fluoroquinolone resulted in >10-fold increase in the proportion of the population that grew on agar containing oxacillin 128 mg/L. Ciprofloxacin exhibited a greater effect than moxifloxacin, levofloxacin and gatifloxacin (average 3400-, 220-, 170- and 49-fold increase in oxacillin-resistant colonies versus the control, respectively). The increase was directly proportional to the fluoroquinolone concentration and could be detected as early as 8 h after exposure to the fluoroquinolone. At 8 h, the absolute number of colonies that grew on oxacillin 128 mg/L was similar whether or not the isolate was exposed to the fluoroquinolone, but the total cfu on non-selective media decreased. The resultant oxacillin-resistant colonies also showed a 1.5- to 3-fold increase in fluoroquinolone MIC. No oxacillin resistance was observed on two similarly treated fluoroquinolone-susceptible, mec(A)-negative strains. It appears that fluoroquinolones influence oxacillin resistance by selective inhibition or killing of the more susceptible subpopulations in heteroresistant S. aureus. The surviving populations are more resistant to both oxacillin and fluoroquinolone. The mechanisms of resistance to the two agents may be unrelated but tend to be associated. This could explain in part the observed increases in fluoroquinolone-resistant MRSA.