In vitro quinolone-gentamicin antagonism demonstrated by time-kill and combination experiments with clinical isolates ofEnterococcus

Bacteriostatic and bactericidal activites of amoxicillin, pefloxacin, ofloxacin, ciprofloxacin, sparfloxacin, vancomycin, alone or associated with gentamicin, were evaluated over 22 months using time-kill and combination experiments, on 20 clinicalEnterococcus isolates (6E. faecium and 14E. faecalis) selected in a French district hospital. Sparfloxacin or ciprofloxacin were more frequently bacteriostatic or bactericidal than pefloxacin or ofloxacin but, unlike amoxicillin or vancomycin, the association with gentamicin was not synergistic in most cases and antagonisms were observed in some cases. These results suggest that the use of sparfloxacin or ciprofloxacin could be considered in some cases of infections withEnterococcus that are resistant to penicillins and glycopeptides and/or highly resistant to aminoglycosides. Despite Perri's report to the contrary, the effect of quinolones and gentamicin in combination use was not only synergistic but antagonistic, and we are alerting others to the possible antagonism involved.