In vitro influence of metronidazole on the activities of ciprofloxacin against clinically important bacterial isolates

A continued in vitro investigation into drug-drug interaction is necessary to avert dangerous drug combinations and achieve effective therapy at lower drug concentrations. In this study, the interactions of ciprofloxacin and metronidazole and the effectiveness of the antibacterial combinations against selected bacterial strains were investigated by agar diffusion, macrobroth dilution and the checkerboard assay methods. The results showed that the combined antibiotics had better antibacterial effects than those produced by the ciprofloxacin while no inhibition zone was produced by metronidazole at the concentrations used. At the highest concentration of the combination, the inhibition zones ranged between 28±1.00 mm and 37±1.00 mm while Plesiomonas shigelloidis was highly resistant. While the minimum inhibitory concentrations (MICs) of ciprofloxacin ranged between 0.05 and 3.13 µg/ml and those of metronidazole were between 15.63 and 62.5 µg/ml, the MICs for the antibacterial combinations were between 0.02/0.12 and 3.13/1.56 (Cip/Met) µg/ml against all the test microorganisms. The percentage reduction in the MICs showed that while the MICs of ciprofloxacin were reduced by between 0 and 96.87%, those of metronidazole were reduced by between 0 and 99.80 % when the two antibiotics were combined. The interaction of ciprofloxacin and metronidazole was more of synergy (55.56 %) than additive/indifference (44.4%). The FICI indicating synergistic interaction ranged between 0.035 and 0.375, the FICI indicating additive/indifference interactions ranged between 0.532 and 1.25. No antagonistic interaction was recorded. The lack of antagonism between these antibacterial agents suggested that their combination would be an improved therapy over the use of each antibiotic individually and may be useful in attaining better therapeutic effects in infections where polymicrobial and resistant bacterial strains may be involved.