Intracellular activity of ciprofloxacin and moxifloxacin, a new 8-methoxyquinolone, against methicillin-resistant Staphylococcus aureus.

The intraphagocytic effects of ciprofloxacin and BAY 12-8039 (moxifloxacin), a new 8-methoxyquinolone, on Staphylococcus aureus showing various susceptibility patterns were examined and the results were compared with the activities of the drugs against extracellular bacteria. MICs of moxifloxacin and ciprofloxacin against methicillin-resistant and -susceptible staphylococci were 0.063 mg/L and 0.5 mg/L, respectively. Moxifloxacin was also active against methicillin-resistant S. aureus (MIC = 2 mg/L) that were resistant to ciprofloxacin (MIC = 32 mg/L). At concentrations of 0.1 x or 1 x MIC, both drugs showed only weak bacteriostatic effects against ingested and non-ingested staphylococci. At 10 x MIC, ciprofloxacin and moxifloxacin were bactericidal against both intracellular and extracellular S. aureus susceptible to oxacillin and against MRSA susceptible to ciprofloxacin. However, ciprofloxacin was bactericidal only at very high concentrations ranging from 5 mg/L to 320 mg/L. Against ciprofloxacin-resistant MRSA moxifloxacin was still active. The results suggest that moxifloxacin is active against intracellular staphylococci.