Identification and characterization of class 1 integron-mediated antibiotics resistance among Shiga toxin-producing Escherichia coli isolates

A total of 46 isolates of Shiga toxin-producing Escherichia coli (STEC), including 23 O157:H7 and 23 non-O157 STEC strains, were analyzed for antimicrobial susceptibilities and the presence of class 1 integrons. All STEC strains were resistant to sulfamethoxazole trimethoprim, and the rate of resistance to ampicillin, and streptomycin was 30.4% and 28.3% respectively. Five isolates exhibited resistance to two or more antimicrobial classes. Multiple resistance to streptomycin, sulfamethoxazole, and tetracycline was most often observed. Class 1 integrons were identified among 11 STEC isolates, including sero-types O157:H7. All the amplified integron fragments were 1 kb in size. DNA sequence analysis revealed that 9 strains exhibited aadA1 and 2 strains exhibited aadA2 gene cassettes as well as encoding resistance to streptomycin and spectinomycin. Integrons identified among the O157:H7 and non-O157 isolates also possessed a similar aadA gene cassette. Furthermore, integrons identified among O157 and non-O157 strains were transferable via conjugation to another strain of E. coli O157:H7 and several strains of other species. Our results demonstrate that Class 1 integrons exist in the Shiga toxin-producing E. coli and determine their resistance to aminoglycoside antibiotics.