Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2006
Publication Date: 8/1/2006
Citation: Bhaduri, S. 2006. Enrichment, isolation, and virulence of freeze-stressed plasmid-bearing virulent strains of yersinia enterocolitica on pork. Journal of Food Protection. 69(8):1983-1985. Interpretive Summary: Yersinia enterocolitica is a bacterium that is a common cause of food-borne illness in humans. Swine are the only known reservoir for pathogenic Y. enterocolitica, and this bacterium is a serious concern for the pork production and processing industry in the United States. While Y. enterocolitica are prevalent in swine, relatively little is known concerning the ability of this organism to survive on pork products when stored under refrigeration and freezing conditions. Therefore, the effects of freezing at -20oC on the enrichment, isolation, and detection of pathogenic Y. enterocolitica on pork chops were examined. The results showed that freezing inhibited the recovery of pathogenic Y. enterocolitica on pork chops when present at a low level of 0.5 CFU/cm2. Furthermore, Y. enterocolitica subjected to freezing on pork chops retained their pathogenic traits. Hence, pathogenic Y. enterocolitica may cause disease if frozen pork products harboring this pathogen are not sufficiently cooked. Thus, freezing cannot replace sanitary production and handling of foods. This research furthers the understanding of the factors affecting the survival of pathogenic Y. enterocolitica exposed to cold stress.
Technical Abstract: The influence of freeze-stress at -20oC on the enrichment, isolation, detection, presence of virulence plasmid, and expression of virulence of plasmid-bearing Yersinia enterocolitica (YEP+) inoculated on pork chops medallions was assessed. Pork chop medallions (10 cm2) artificially contaminated with 10, 1, and 0.5 CFU/cm2 of YEP+ strains (serotype O:3) were placed in sterile petri dishes at -20 degrees C for 24 h. The medallions were swabbed when frozen, after thawing at room temperature (RT) for 90 minutes and after thawing at 4 degrees C for 18 h. Swabs were enriched and YEP+ were detected and isolated using the Congo red binding and low calcium response assays. The YEP+ were isolated under all conditions on pork inoculated with 10 CFU/cm2 and at a level of 1 CFU/cm2 when thawed at RT and at 4 degrees C but not from frozen pork. The YEP+ were not isolated from pork inoculated with 0.5 CFU/cm2 and then frozen, whereas YEP+ were recovered when inoculated at this level from pork not subjected to freezing. Virulence of the strains isolated from frozen pork chops was confirmed by PCR and the expression of plasmid-associated phenotypes. These results indicate that YEP+ subjected to freezing on pork are potentially capable of causing food-borne illness and that freezing is not a substitute for safe handling and proper cooking of pork.