High incidence of multiple intI1 genomic gene cassettes in Aeromonas strains

Abstract

Antimicrobial resistance profiles of Aeromonas have been used as an effective indicator for monitoring resistances in aquatic environments. Integron gene cassettes are a major reservoir of antibiotic resistance genes in bacteria. To date, little has been reported on the antimicrobial resistance or integrons of Aeromonas in bullfrog aquaculture. In this study, 27 isolates belonging to four Aeromonas species (A. hydrophila, A. veronii, A. caviae, and A. jandaei) were isolated from farmed diseased American bullfrogs (Rana catesbeiana). all of them were multidrug-resistant, exhibiting resistance to at least seven drugs from five antibiotic classes. Class 1 integrons (intI1s), containing nine types of gene cassette arrays encoding resistance to trimethoprim (dfrA1, dfrA12, dfrA15, dfrA17), streptomycin/spectinomycin (aadA2), fluoroquinolones/aminoglycosides (aac(6′)-Ib-cr), rifampin (arr3), proline-rich antimicrobial peptides (ptrB), and multiple antimicrobial agents involved in biofilm development (dgs), were detected in 26 (26/27, 96.3%) of the isolates. Seven patterns were identified, and six of them contained two or three gene cassettes, in 27 isolates. This work highlights unusually high frequency (21/27, 77.8%) of multiple intI1 gene cassette arrays and three novel intI1 gene cassettes or arrays (dgc, ptrB, and aac(6′)-Ib-cr-arr3) in the isolates of Aeromonas spp. from American bullfrogs.