The synergic effects of quinolones and oral cephem antibiotics on Serratia marcescens

The in-vitro effects of new quinolones and oral cephem antibiotics in various combinations on 26 strains of clinically isolated Serratia marcescens were assessed. The in-vitro activities of ofloxacin and ciprofloxacin in combination with either cefpodoxime or cefcamate were evaluated by the chequerboard titration method. These combinations showed synergistic and additive effects against most of the S. marcescens tested but no antagonism was observed. Time-kill experiments performed with two representative isolates of S. marcescens showed synergic effects with respect to the kill rates, when concentrations of quinolones and beta-lactam antibiotics, exhibiting bacteriostatic effects were combined. Exposure of S. marcescens to ciprofloxacin before the addition of cefpodoxime had no influence on the combination effect. When cefpodoxime was added first, however, the bactericidal effect was reduced. The induction of beta-lactamase by cefmetazole was enhanced by the combination of ciprofloxacin and cefmetazole. These results suggest that the synergic effects observed between new quinolones and cephem antibiotics are due to the stimulation of incorporation of cephems by combination with quinolones.