Molecular characterization of integron-gene cassettes in multi-drug resistant Aeromonas hydrophila from fish

The aim of this study was to explore the distribution and composition of integron-gene cassettes in multi-drug resistant(MDR) Aeromonas hydrophila, and to understand the relationship between integrons and MDR, so that we can accurately evaluate the status of integron-gene cassette systems in the MDR mechanisms of A. hydrophila. To determine the distribution and molecular characterization of integron-gene cassettes in 28multi-drug resistant A. hydrophila isolates from fish, the K-B disk diffusion method was employed to determine the antimicrobial susceptibility of the isolates to 18 different antibiotics. PCR amplification was then performed to detect class I, II, and III integrons and their gene cassettes in A. hydrophila. The results indicate that A. hydrophila isolated from fish exhibited multi-drug resistance, with an antibiotic resistance rate of more than 60% to-lactams,macrolides, chloramphenicols, and tetracycline antibiotics, while it was more sensitive to fluoroquinolones. The drug resistant spectrum also differed greatly among the isolates. The PCR amplification results showed that out of28 isolates, 15(53.57%) harbored int1 I and 6(21.43%) harbored int1 II, however, no int1 III was detected in the isolates. Combined with the antimicrobial susceptibility results, the drug resistant rate of the integron positive strains was higher than that of the integron negative ones. In addition, class I integron positive strains exhibited a higher multi-drug resistant rate than those harboring class II integrons, which indicates that the integron-gene cassette system plays an important role in the occurrence of multi-drug resistance in A. hydrophila. The main cassettes were those encoding aminoglycoside adenyltransferase A(aadA), dihydrofolatereductase A(dfrA), and chloramphenicol acetyltransferase B(catB). The most prevalent gene cassette array was aadA2+dfrA12. In addition, there was an obvious difference in MDR among individuals containing class I integrons, hinting that no direct relationship exists between the MDR phenotype and gene cassettes in the A. hydrophila isolates. These findings suggest that the transfer of antibiotic resistance genes can occur through integron-gene cassettes among A. hydrophila.