blaCTX-M genes in clinical Salmonella isolates recovered from humans in England and Wales from 1992 to 2003

Cefotaximases (CTX-M) are a rapidly growing class A beta-lactamase family that has been found among a wide range of clinical bacteria. One hundred and six isolates were selected from 278,308 Salmonella isolates based on resistance to ampicillin and cephalosporins and subjected to further characterization. Fourteen isolates were bla(CTX-M) PCR positive, and cefotaxime MICs for these isolates were >= 16 mg/liter. Furthermore, sequence analysis revealed the presence of type CTX-M9, -15, or -17 to -18. All 14 isolates presented different PFGE restriction profiles, although six Salmonella enterica serotype Virchow isolates formed a tight cluster. The bla(CTX-M) genetic determinants were present in transferable plasmids of similar to 63, 105, and > 148 kb. Plasmid restriction analysis showed that both horizontal transfer of similar plasmids among different clones and transfer of genes between different plasmids were likely mechanisms involved in the spread of blaCTX-M genes. We have found that CTX-M enzymes have emerged in community-acquired infections both linked to foreign travel and domestically acquired. This is the first report of a CTX-M enzyme in Salmonella in the United Kingdom. Also, it represents the first report of a bla(CTX-M) gene in Salmonella enterica serotype Stanley and a bla(CTX-M-15) gene in Salmonella enterica serotypes Anatum, Enteritidis, and Typhimurium.