Dichotomous selection of high-level oxacillin resistance in Staphylococcus aureus by fluoroquinolones

The objective of this study was to determine whether exposure of Staphylococcus aureus to early (ciprofloxacin or levofloxacin) and a recent fluoroquinolone (moxifloxacin) has differential potential as a mutator or selector for meticillin resistance. The potential of fluoroquinolones to act as mutators or selectors was studied in 24 strains each of healthcare-associated meticillin-susceptible S. aureus (MSSA) and meticillin-resistant S. aureus (MRSA) as well as 6 strains of community-acquired MRSA. Mutator or selector potential was studied first by exposing isolates to 0.5x the fluoroquinolone minimal inhibitory concentration (MIC) and screening for either single-step fluoroquinolone resistance or high-level oxacillin resistance; second, by exposing the heteroresistant MRSA P8 parent strain as well as fluoroquinolone-resistant subpopulations derived from strain P8 to constant fluoroquinolone concentrations ranging from 0.015 mg/L to 128 mg/L; and third, by exposing the heteroresistant MRSA population of strain P8 to fluctuating concentrations of ciprofloxacin, levofloxacin and moxifloxacin simulating oral doses of 500 mg twice a day, 500 mg once daily (qd) and 400mg qd, respectively, compared with amoxicillin/clavulanic acid 500 mg three times a day. Total viable counts and subpopulations resistant to 2x, 4x and 8x the fluoroquinolone MICs and to 32, 64 and 128 mg/L oxacillin [high-level oxacillin (hl-OXA)-resistant] were quantitated. None of the fluoroquinolones acted as a mutator; ciprofloxacin and levofloxacin selected for hl-OXA resistance, whereas moxifloxacin selected towards hl-OXA resistance by one order of magnitude less frequently. The P8 parent and fluoroquinolone-resistant subpopulations were eliminated by ciprofloxacin or levofloxacin concentrations >10-fold higher than the MICs, whereas moxifloxacin eliminated all subpopulations by concentrations 2-3-fold the MIC. Finally, exposure of P8 to fluctuating amoxicillin/clavulanic acid, ciprofloxacin and levofloxacin concentrations, respectively, caused a rapid selection of fluoroquinolone and hl-OXA resistance. Moxifloxacin reduced total viable counts rapidly, thus preventing the emergence of resistant subpopulations. In conclusion, fluoroquinolones do not act as mutators towards hl-OXA resistance. However, ciprofloxacin and levofloxacin are potent selectors of hl-OXA resistance, whereas moxifloxacin is a poor selector. In contrast to ciprofloxacin and levofloxacin, moxifloxacin exerts a high bactericidal activity against staphylococci, thus minimising the probability for selection of resistance. Thus, fluoroquinolones exert a dichotomous MRSA-selective potential in heteroresistant MRSA.