INTERACTION EFFECTS OF LITSEA CUBEBA ESSENTIAL OIL AND ANTIBIOTICS ON ANTIBACTERIAL ACTIVITY AGAINST PATHOGENIC BACTERIA IN AQUACULTURE

Abstract

In this study, the interaction effects of Litsea cubeba essential oil (EO) and two antibiotics (ABs) on the antibacterial activity against pathogenic bacteria in aquaculture were investigated. Two reference strains (Escherichia coli ATCC 25922 (E. coli ATCC 25922) and Vibrio parahaemolyticus ATCC 17802 (V. parahaemolyticus ATCC 17802)) and six locally isolated aquatic pathogenic strains (E. coli 9C48, E. coli 11C123, Vibrio 2S4, Vibrio 2N38, V. parahaemolyticus ND201 and V. parahaemolyticus TB81) were used. The inhibitory effects of individual antimicrobial agent (L. cubeba EO, nalidixic acid, and oxytetracycline) were tested against eight strains by using broth microdilution assay in 96-well microplates. A higher inhibitory effect of L. cubeba EO was observed against isolated aquatic pathogenic (MIC = 1.15 - 2.30 mg/mL) than that in the reference strains (MIC = 5.53 mg/mL). The combination effects of L. cubeba EO and ABs often used in the treatment of bacteria effects in aquaculture (nalidixic acid and oxytetracycline) were evaluated by the checkerboard method. Fractional Inhibitory Concentration (FIC) values were determined to characterize the interaction among combinations. Out of 16 AB-EO combinations tested, 11 of them showed a synergistic effect (FIC ≤ 0.5), 3 of them showed an additional effect (0.5 < FIC ≤ 1.0) and 2 of them showed indifferent effect (1 < FIC ≤ 4), no antagonistic effect was observed. The antimicrobial synergy of EO and AB could enhance efficacy, reduce toxicity, decrease adverse side effects, and lower the dose of ABs used in aquaculture.