Molecular characterisation of class 1 integrons in Salmonella enterica serovar Choleraesuis isolates from southern Taiwan

Integron-mediated multidrug resistance in Salmonellaenterica serovar Choleraesuis poses a major concern in human salmonellosis in Taiwan. In this study, 71 strains of S. Choleraesuis harbouring a class 1 integron from humans and animals were characterised for detection of integrons, gene cassettes and the spvC virulence gene using molecular genetic techniques. All 71 strains tested were negative for class 2 and 3 integrons. Fifty-eight (81.7%) of the isolates harboured a class 1 integron with different gene cassettes, including dfr12-orfF-aadA2 in 50 strains, dfrA1-UN in 7 strains and aadA1 in 1 strain. Among the 71 isolates, 1 isolate of S. Choleraesuis H40 harboured two class 1 integrons, the first integron with the dfr12-orfF-aadA2 gene cassette and the second integron with a novel single gene cassette aadA22, a new variant of the aadA gene family. Moreover, 13 of the 71 isolates were negatively amplified by 5′-CS and 3′-CS specific primer pairs and represented a novel gene cassette array of sat-psp-aadA2-cmlA1-aadA1-like-qacH-tnp-sul3 in class 1 integrons. In mating experiments, the transferability of integron-mediated drug resistance in S. Choleraesuis isolates was demonstrated. The class 1 integron-borne gene cassette dfr12-orfF-aadA2 was characterised as becoming predominant in multidrug-resistant S. Choleraesuis isolates. Furthermore, both the dfrA1-UN gene cassette and the sul3-associated novel gene cassette were also identified in the S. Choleraesuis isolates. To date, this is the first report describing the dissemination of a sul3-associated integron in S. Choleraesuis isolates in Taiwan.