Abstract:
A total of 50 Aeromonas hydrophila strains isolated from farmed Nile tilapia (Oreochromis nilotica) in Thailand were used in this study. All strains were examined for susceptibilities to 16 antimicrobials, characterized for class 1 integrons and determined for the AheABC efflux system. Thirteen strains resistant to enrofloxacin and/or ciprofloxacin were examined for the mutations in the Quinolone Resistance-Determining Regions (QRDRs) of gyrA and parC. Up to 94% of the isolates were considered multidrug-resistant. Most isolates were resistant to streptomycin (92%), trimethoprim (88%) and amoxicillin (86%) and common resistance pattern was AMO-CAR-TMP-STR (20%). Class 1 integrons were detected in 23 isolates. Fourteen isolates harbored inserted gene cassettes i.e. aadA2, dfrA1-orfC, dfrA12-aadA2 and incomplete aadA2. Three strains could horizontally transfer class 1 integrons to the E. coli recipient. All strains were screened for the expression of the AheABC efflux system. Transcriptions of the aheB gene were detected in 39 isolates. Ten isolates were randomly selected for determination of the transcription level of aheB gene. The aheB transcription level in all isolates was higher than that of A. hydrophila ATCC 35654. The expression level was not correlated with multidrug resistance phenotype. All the strains tested were found to contain mutations in both gyrA and parC gene. The substitutions in gyrA and parC gene were Ser-83-Ile and Ser-80-Ile, respectively. The results of this study show that the multidrug-resistant A. hydrophila are widely distributed in farmed Nile tilapia in Thailand. Class 1 integrons and the AheABC efflux pump play an important role in multidrug-resistance and dissemination of antibiotic resistance. The data indicate the requirement of proper and prudent use of antimicrobial agents in aquaculture.
