The aim of the current study was to investigate the combination effect of enrofloxacin and trimethoprim by their inhibitory and bactericidal activities against five bacterial species (E. coli, P. hemolytica, S. aureus, S. cholerasuis and a field isolate S. typhimurium. Minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), fractional inhibitory concentration (FIC) and time killing rate were performed using these isolates. Both antibiotics has shown similar MIC ranging from equal to 3 fold dilutions difference for each of the bacteria tested except for E.coli where enrofloxacin has shown better activity with more than ten fold dilutions less than trimethoprim. The fractional inhibitory concentration index from the results of checkerboard for enrofloxacin and trimethoprim showed a synergistic effect for P. hemolytica and S. typhimurium (field isolate), while no difference was observed for the remaining tested bacteria. In the combination of the two antibiotics with different ratios, compared to the MICs of the two antibiotics tested alone, the concentration of the two antibiotics in the combination has shown a 2-8 fold reduction against all bacteria tested. Furthermore, as the concentrations of enrofloxacin increase and trimethoprim decrease the minimum inhibitory concentrations for E. coli, P. hemolytica and S. aureus has shown a decrease. The other two bacteria didn’t show any change. Although all the combined ratios had similar MIC and MBC values compared to MIC and MBC tested alone, the concentration of each antibiotic in the combined ratios was lower by more than ten-fold compared to the MIC and MBC alone for both antibiotics. The time kill rate study for the antibiotics alone or in combination against E.coli and S. aureus had revealed higher inhibitions of bacterial growth with a difference of 2-4 log cfu/ml bacteria by the combination antibiotics after 12 hrs of incubation than tested alone. In summary, combination therapy with these two antibiotics may serve additive to synergistic effect and broad spectrum activity against the tested bacteria.

Key words: Combination, Enrofloxacin, Synergism, Trimethoprim