This study evaluated the in vitro activity of florfenicol (F) in combination with amoxicillin (AM) or cefuroxime (CRM) against pathogenic bacteria of animal origin, including E. coli, S. aureus, S. cholerasuis and P. mirabilis. The MIC of AM ranged from 16 to 256 μg/ml. The MBC of AM (64 μg/ml) was four-fold higher than its MIC value (16 μg/ml) for E. coli, and similar to the MIC for the other three species. The MIC of F ranged from 8 to 16 μg/ml. The MBC values of F for E. coli, S. aureus, and S. cholerasuis were eight-fold higher than the respective MIC values, and 32-fold higher than the MIC of P. mirabilis. The MIC of CRM ranged from 8 to 128 μg/ml. The MBC of CRM was the highest (³ 256 μg/ml), except for E. coli. The F/AM combination resulted in synergism (FIC index £ 0.5) for E. coli, S. aureus, and P. mirabilis and in-difference (FIC index >1) for S. cholerasuis. For F/CRM combination, synergism (E. coli and S. cholerasuis) and in-difference (S. aureus and P. mirabilis) were observed. Killing rate study showed a 1.5 - > 3 log 10 cfu/ml reduction of E. coli with F/AM compared to AM or F alone. The highest activity of the combinations was observed when F comprised at least 50% of the combination. Further studies using many bacterial isolates and various proportion of each drug would reveal the potential of a combination product containing F and AM/CRM for use in veterinary practice.

Key words: Animals; Antibacterial agents; Combination therapy; Synergism