Molecular characterisation and chromosomal mapping of antimicrobial resistance gene cassettes in Irish Salmonella enterica serotype Typhimurium: comparison with Campylobacter spp.

Two hundred and twenty-six randomly collected Irish Salmonella enterica ser otype Typhimurium were analysed for antimicrobial agent-encoding resistance genes by polymerase chain reaction (PCR). Most of these isolates were phag e typed as DT104 and were resistant (R) to five (or more) antibiotics inclu ding ampicillin, chloramphenicol, streptomycin, sulphonamide and tetracycli ne giving the characteristic R-type ACSSuT. Antibiotic resistance is often disseminated on R-plasmids and transposons. Recently a third mechanism was described involving a novel group of naturally occurring mobile genetic ele ment, integrons. Gene cassettes form part of the latter structure containin g one or more open reading frames (ORF) which encode antimicrobial resistan ce genes. Several gene cassettes were characterised by automated DNA sequen cing. These data were then used to generate DNA probes with which to map th e corresponding resistance genes to the S. Typhimurium genome. Macrorestric tion with Xba1 identified a conserved 10 kbp multiresistance gene cluster o r 'resistance island' in DT104 isolates of R-type ACSSuT. Apparently larger clusters were detected in non-DT104s additionally resistant to other antim icrobials. Integron-like structures were also identified in Campylobacter s pp. and these could potentially facilitate natural transformation in these organisms. Molecular characterisation of these chromosomal regions in S. Ty phimurium and Campylobacter spp. is a necessary step in limiting the spread of these pathogens.