Molecular analysis of ceftriaxone -resistant Salmonella enterica

The prevalence of expanded-spectrum cephalosporin-resistant Salmonella has increased steadily in the United States since 1998, and this resistance has been found in multiple serotypes. This study found that the blaCMY-2-encoding plasmids within 17 expanded-spectrum cephalosporin-resistant S. Typhimurium are associated with three genetic backgrounds. However, octamer based genome scanning and pulsed-field gel electrophoresis suggested that bla CMY-2-containing plasmids were found in a clonal population of S. Newport. Three plasmid backbones (A/C, B, and D) were found to encode blaCMY-2, and sequence of a type B plasmid indicated it is similar to pColIb-P9. The type D plasmid was similar to pP, isolated from S. Enteritidis, while the type C plasmid sequence is unique, having not been previously described. Each plasmid type shared an identical sequence in the blaCMY-2 region, consisting of two genes downstream of blaCMY-2, blc and sugE, similar to genes located immediately downstream of ampC in the C. freundii chromosome. The A/C and D plasmids had a portion of the C. freundii ecnR gene immediately downstream of sugE. In all plasmid types, blaCMY-2 was preceded by an insertion element, IS Ecp1, located in one of two AmpR binding sites upstream of blaCMY-2. Transcription of bla CMY-2, driven by the −10 and −35 promoters located within ISEcp1 was confirmed by demonstrating that cloned blaCMY-2 including ISEcp1 could hydrolyze cephalothin and had decreased susceptibility to cefoxitin. The bla CMY-2 clones with only the ampC promoter upstream of blaCMY-2 (no IS) were susceptible to cefoxitin and didn\u27t hydrolyze cephalothin. Although a similar insertion element has been shown to be capable of open-ended transposition, transfer of blaCMY-2 was not observed in this study. Previously published data has suggested that Salmonella isolates expressing AmpC β-lactamases have decreased growth rate invasiveness. Data presented here suggests that S. Typhimurium SL 1344 containing various β-lactamases may initially have a decreased growth rate and a decreased ability to invade HEp-2 cells. However, these deleterious effects can be rapidly compensated, possibly through compensatory mutations, which allow the organism to retain its wild-type growth rate and invasiveness