EFFECT OF EFFLUX PUMP INHIBITORS ON ANTIMICROBIAL ACTIVITY OF FLORFENICOL AGAINST RESISTANT STRAINS OF PORCINE ACTINOBACILLUS PLEUROPNEUMONIAE AND PASTEURELLA MULTOCIDA

The aim of this study is to investigate whether combinational use of efflux pump inhibitors (EPIs) could improve florfenicol (FF) antimicrobial activity. Five EPIs including Carbonyl Cyanide Chlorophenylhydrazone (CCCP), omeprazole, Phenylalanine-arginine [Formula: see text]-naphthylamide (PA[Formula: see text]N), reserpine and verapamil were individually combined with FF and the minimum inhibitory concentration (MIC) against porcine Actinobacillus pleuropneumoniae and Pasteurella multocida were evaluated by the broth microdilution assay. The results indicated that CCCP demonstrated substantial improvement on the antimicrobial activity (FF MIC reduction [Formula: see text] folds) while PAßN showed minimal effect at high concentrations (80–120[Formula: see text][Formula: see text]g/mL). The MICs of FF were further examined with CCCP at 5[Formula: see text][Formula: see text]g/mL against resistant A. pleuropneumoniae and at 2[Formula: see text][Formula: see text]g/mL against resistant P. multocida. With this combination, a total of 75% (6/8) of A. pleuropneumoniae and 100% (8/8) of P. multocida resistant isolates showed significantly reduced ([Formula: see text] folds) FF MICs. In addition, 100% of the 16 resistant bacterial strains carried the floR gene that regulates the phenicol-specific efflux pump (FloR pump). The time kill experiments were used to verify the effect of CCCP demonstrating a bactericidal effect in resistant A. pleuropneumoniae and synergistic effect in resistant P. multocida. In contrast, the FF MICs were not significantly affected in the FF susceptible strains of these two bacteria. These findings suggested that despite small sample sizes, consistent beneficial effects of CCCP were evident such that the combinational use of selective EPI with FF might be an alternative antibacterial strategy against FF resistant A. pleuropneumoniae and P. multocida. The anti-efflux mechanism was indicated, but more researches to further decipher the mechanisms of CCCP’s effects are warranted.