The emergence of resistance to ciprofloxacin during treatment of experimental Staphylococcus aureus endocarditis.

The efficacy of ciprofloxacin was compared with that of vancomycin in the rabbit model of methicillin-susceptible Staphylococcus aureus endocarditis. Animals were treated with ciprofloxacin, 25 mg/kg iv every 8 h or vancomycin, 17.5 mg/kg iv every 6 h, for 3, 6, or 9 days. Both drugs were found to be equally effective in the therapy of this infection, but the degree of reduction in bacterial counts was less than expected on the basis of previous studies. Additionally, resistance to ciprofloxacin in the test strain of S. aureus was seen to emerge in 12.5% of animals that received the drug. This raises concern about the use of ciprofloxacin as a single agent in the therapy of humans with serious systemic S. aureus infections.