Molecular characterization and occurrence of extended-spectrum beta-lactamase resistance genes among Salmonella enterica serovar Corvallis from Thailand, Bulgaria and Denmark

Fifty nine Salmonella Corvallis isolates from humans and food products in Bulgaria, Denmark, and Thailand were examined for antimicrobial susceptibility and characterized by pulsed-field gel electrophoresis (PFGE). Cephalosporin-resistant isolates were examined for the presence of genes encoding beta-lactamases by PCR and sequencing.

Ten different PFGE types were observed. One type (30 isolates) was recovered in all three countries; three types were found only in Bulgaria, two only in Denmark, two only in Thailand, and two both in Denmark and Thailand. Ten isolates were susceptible to all antimicrobial agents tested, whereas 41 were resistant to three or more antimicrobials.

Most resistance was observed among the isolates from Bulgaria. Of the 25 isolates from Bulgaria, 20 displayed resistance to ampicillin and the cephalosporins ceftiofur and cephalothin. All 20 isolates tested negative for bla(CMY-1), bla(CMY-2), and bla(ACC), but positive for bla(SHV), of which five were sequenced to bla(SHV-2).

Plasmid profiling and hybridization revealed that the bla(SHV) gene was located on plasmids of approximately 70 kb. Five plasmid profiles were found among these 20 isolates. The plasmid profiling confirmed the PFGE-type and was able to further subdivide the strains. Seventeen of these 20 isolates contained also bla(TEM), of which nine representatives were sequenced to bla(TEM-1B), or bla(TEM-1H).

One isolate contained bla(CTX-M-15), bla(SHV-2), and bla(TEM-1H), with the bla(CTX-M-15), and bla(TEM-1H) genes located on a 63-kb transferable plasmid. This study showed a high frequency of resistance among S. Corvallis isolated from humans and food products in Bulgaria, with a lower frequency in Thailand and Denmark.

The clonal relatedness among the isolates from three countries could indicate a recent spread of this serovar.