DefinePK

Abstract

Objective: Oxytetracycline is an approved antibiotic and most frequently used drug in commercial aquaculture all over the world for treating bacterial diseases in fish. The intensive use of oxytetracycline has resulted development of bacterial resistance to oxytetracycline. It is reported that phytochemicals like quercetin, gallic acid, cinnamic acid and p-anisic acid have antibacterial properties. An attempt was made to observe the type of interaction upon incorporation of phytochemicals in combination with conventional antimicrobials like oxytetracycline with a view to decrease the antimicrobial load in aquaculture for minimizing resistance in bacterial pathogens in case combination of phytochemicals and oxytetracycline do possess synergy or additive effect. Combinations of oxytetracycline with polyphenolic compounds (quercetin, gallic acid, p-anisic acid, and cinnamic acid) against common gram negative bacterial pathogens of fish, such as Aeromonas hydrophila, Aeromonas salmonicida, and Edwardsiella tarda were tested. Methods: Antibacterial activity of oxytetracycline, quercetin, gallic acid, p-anisic acid and cinnamic acid was determined in vitro against selected bacterial pathogens individually followed by combination of oxytetracycline with polyphenols using serial microplate dilution method measuring minimum inhibitory concentrations (MIC). Fractional inhibitory concentration (FIC) indices were calculated. Results: Oxytetracycline and other polyphenolic compounds exhibited antibacterial action against the selected fish pathogens with mean MIC ranging from 0.5 to 2.5 mg ml-1. Based on FIC indices, synergistic interaction was observed for combination of oxytetracycline with quercetin or cinnamic acid against Aeromonas salmonicida, and with gallic acid against Edwardsiella tarda. However, combinations of oxytetracycline with gallic acid, p-anisic acid and cinnamic acid against Aeromonas hydrophila; Combinations of oxytetracycline with gallic and p-anisic acid against Aeromonas salmonicida and the combinations of oxytetracycline with p-anisic acid and cinnamic acid against Edwardsiella tarda revealed additive antimicrobial interaction Conclusion: Positive antibacterial interaction was evident between oxytetracycline and selected polyphenols in vitro against Aeromonas hydrophila, Aeromonas salmonicida, and Edwardsiella tarda.