Chloramphenicol and kanamycin resistance among porcine Escherichia coli in Ontario

Objectives: The purpose of this study was to compare the distribution of chloramphenicol and kanamycin resistance genes across three populations of porcine Escherichia coli. Methods: PCR was used to assess the distribution of the major chloramphenicol and kanamycin resistance genes catA1, cmlA and floR, and aphA1, aphA2 and aadB in enterotoxigenic E. coli (ETEC), non-ETEC isolates from cases of diarrhoea and commensal E. coli from healthy pigs. Associations between these genes and resistance genes for other antimicrobials or virulence genes were assessed. Results: The chloramphenicol and kanamycin resistance genes were distributed differently among the three E. coli populations. While aphA1, aphA2 and aadB were evenly distributed among resistant ETEC, non-ETEC and commensals, the catA1 gene was significantly more frequent in ETEC than in non-ETEC and commensals. Transformation experiments confirmed statistical associations by demonstrating that elf, estB, astA, aadA and sul1 were located with catA1 on a large ETEC plasmid. Plasmids carrying cmlA also carried sul3 and aadA. Other plasmids carrying floR and aadB also carried tet(A), sul2, strA/strB, bla(CMY-) and occasionally aac(3)IV. Conclusions: The clustering of genes observed is a likely cause for chloramphenicol resistance persistence. Similar to tetracycline, chloramphenicol resistance genes are physically linked to virulence genes. This is not the case for kanamycin resistance determinants, which were linked to other resistance genes only.