JAC Advance Access originally published online on June 16, 2006
Journal of Antimicrobial Chemotherapy 2006 58(2):305-309; doi:10.1093/jac/dkl248
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Occurrence of integrons and antimicrobial resistance genes among Salmonella enterica from Brazil
1 Oswaldo Cruz Institute Avenida Brasil 4365, 21045-900 Rio de Janeiro, Brazil 2 WHO Collaborating Centre for Antimicrobial Resistance in Foodborne Pathogens, Danish Institute for Food and Veterinary Research Bülowsvej 27, DK-1790 Copenhagen V, Denmark
Received 27 February 2006; returned 26 March 2006; revised 23 May 2006; accepted 24 May 2006
*Corresponding author. Tel: +45-72-34-60-00; Fax: +45-72-34-60-01; E-mail: faa{at}dfvf.dk
Objectives: To determine the occurrence of antimicrobial resistance genes and role of integrons among 135 antimicrobial-resistant Salmonella enterica from Brazil.
Methods: The presence of antimicrobial resistance genes, class 1 and 2 integrons and gene cassettes was analysed by PCR and sequencing. The genetic location of class 1 integrons was determined in 25 isolates by hybridization and plasmid transfer experiments.
Results: Fifty-five of the isolates were positive for class 1 integrons. Integron-positive isolates represented 17 different serovars and were mainly from human (n = 28) and animal (n = 13) sources. The gene cassette arrangements could be determined in 51 of the positive isolates, which harboured one [dfrA22, aadA1 or orf3 (putative trimethoprim resistance)], two [aadA1-dfrA1, aac(6')-Ib-orf1 (unknown function) or aacA4-aadA1], three [dfrA15b-cmlA4-aadA2, orf2 (unknown function)-dfrA5-orfD] or four [orf4-aacA4-blaOXA-30 (interrupted by an IS1 element)-aadA1] cassettes in their variable region. Only one isolate harboured a class 2 integron with the gene cassette array dfrA1-sat-aadA1. Several integron unrelated resistance genes were also detected in the isolates. Sulphonamide resistance was primarily mediated by sul2 and sul3, tetracycline resistance by tet(B) and tet(A), chloramphenicol resistance by catA1, streptomycin resistance by strA and ampicillin resistance by blaTEM. blaCTX and blaCMY-2 were found in cephalosporin-resistant isolates. Mating and hybridization experiments demonstrated that a high-molecular-weight plasmid mediated the gene transfer of integrons and additional resistance determinants.
Conclusions: The present study revealed that integron-mediated resistance genes contributed to the multiresistance phenotype observed in the isolates, but most resistance genes were located outside the integron structure, as independent genes. However, they might be located on the same conjugative plasmid.
Keywords: gene cassettes , Salmonella , resistance determinants
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