Submitted to: American Society for Microbiology
Publication Type: Abstract Only
Publication Acceptance Date: 6/29/2006
Publication Date: 9/9/2006
Citation: Poole, T.L., Ramlachan, N., Edrington, T.S., Anderson, R.C., Nisbet, D.J. 2006. High prevalence of incompatibility group IncA/C among multi-drug resistant Salmonella enterica serotype Newport isolated from dairy cattle [abstract]. American Society for Microbiology. p. 87.
Technical Abstract: Multi-drug resistant (MDR) Salmonella enterica serotype Newport exhibiting increased morbidity and mortality among man and animals has emerged in North America and Canada. Risk factors for infection by MDR S. Newport have included exposure to dairy farms, consumption of undercooked ground beef or unpasteurized milk and cheese. One hundred fifteen S. Newport strains isolated from dairy cattle in three states were phenotypically and genotypically characterized to elucidate genetic relationships among the isolates. Susceptibility testing and breakpoint determination was done according to CLSI (formerly NCCLS) protocols. Ninety-eight (85%) of these isolates were resistant to multiple antimicrobials (3 to 11). Eighty-four percent of the MDR S. Newport strains (those resistant to 7 or more antimicrobials) were resistant to expanded spectrum cephalosporins. Genotypic analysis included multiplex PCR amplification of 19 incompatibility group broad host-range plasmids. Broad host-range plasmids have the capacity to replicate in many different bacterial genera; however, no two plasmids of the same incompatibility group can replicate in the same bacterial cell. This characteristic has been used to classify broad host-range plasmids into incompatibility groups (Inc). Acquisition of resistance genes on these plasmids has been cumulative resulting, in some cases, to resistance against nearly all antimicrobial classes. Although characterization of resistance traits carried by broad host-range plasmids has been extensive, there has been little characterization of plasmid incompatibility groups among food borne pathogens. Ninety-two (94%) of the MDR S. Newport were positive for IncA/C. Two isolates were positive for IncA/C and IncN, and two were positive for IncA/C and IncI; other than these three, no other incompatibility groups were detected. Six of the MDR S. Newport isolates did not amplify any of the incompatibility groups tested; however, IncA/C was detected in 1 of the 17 pansusceptible isolates. This is the first known report describing a large reservoir of IncA/C in any bacterial pathogen. The acquisition of IncA/C may have contributed to the emergence of MDR S. Newport in the last ten years. Additional characterization of IncA/C from MDR S. Newport is underway to determine if virulence and resistance genes; particularly, to expanded spectrum cephalosporins are located on the plasmid.