Susceptibility of ciprofloxacin-resistant Staphylococci and Enterococci to clinafloxacin

Clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and Enterococcus faecalis obtained from the Ann Arbor Veterans Affairs Medical Center within the last decade were tested for susceptibility to ciprofloxacin and clinafloxacin. For MRSA isolates, the minimum inhibitory concentrations (MICs) of ciprofloxacin were several fold higher than those noted with clinafloxacin. Prior to the introduction of the fluoroquinolones (1984–1985), all MRSA isolates were susceptible to ciprofloxacin and clinafloxacin. By 1993, virtually all MRSA isolates were resistant to ciprofloxacin and a 50-fold increase in the MIC 50 and MIC 90 for clinafloxacin was seen. In 1985–1986, most enterococcal isolates were susceptible to ciprofloxacin and clinafloxacin. By 1993, one-third of all enterococci were resistant to both ciprofloxacin and clinafloxacin. Fluoroquinolone resistance developed more quickly in enterococci that demonstrated high-level gentamicin resistance. Thus, cross-resistance between clinafloxacin and ciprofloxacin was seen; however, the lower MICs of clinafloxacin for MRSA may allow the use of this drug for some MRSA infections.