Submitted to: Meeting Abstract
Publication Type: Abstract only
Publication Acceptance Date: 3/11/2005
Publication Date: 5/1/2005
Citation: Bhaduri, S. 2005. Effect of freeze-stress on enrichment, isolation, and virulence of plasmid-bearing virulent Yersinia enterocolitica in pork chops. International Association for Food Protection Abstract. Paper No. P5-39. Interpretive Summary:
Technical Abstract: Relatively little is known concerning the ability of plasmid-bearing virulent Yersinia enterocolitica (YEP+) to adapt to freeze stress conditions in raw pork products. Furthermore, the ability to isolate/detect low levels of YEP+ in pork that is frozen prior to testing may be hindered. Therefore, the influence of freeze-stress at -20ºC on the enrichment, isolation, detection, presence of virulence plasmid, and expression of virulence of YEP+ inoculated on pork chops (PC) was assessed. The PC artificially contaminated with 10, 1, and 0.5 CFU/cm2 of YEP+ (strain GER O:3) were placed in sterile petri dishes at -20ºC for 24 h. The PC were swabbed when frozen, and after thawing at room temperature (RT) and 4ºC. Swabs were enriched in modified trypticase soy broth containing yeast extract and bile salts at 12ºC for 24 h, and then again incubated for 24 h after the addition of 4 ug/ml of Irgasan. The YEP+ were isolated by Congo red (CR) binding on CR brain heart infusion agarose (CR-BHO), and the presence of the virulence plasmid was assessed by the appearance of red pinpoint colonies showing low calcium response (Lcr) and CR binding. The YEP+ were isolated under all conditions on PC inoculated with 10 CFU/cm2 and at a level of 1 CFU/cm2 when thawed at RT and at 4ºC but not from frozen PC. The YEP+ were not isolated from PC inoculated with 0.5 CFU/cm2 and then frozen, whereas YEP+ were recovered at this level from PC not subjected to freezing. Results of a multiplex PCR targeting the chromosomal ail gene and a plasmid-associated virF gene further confirmed that YEP+ isolates from CR-BHO possessed the virulence plasmid. Moreover, multiple in vitro virulence assays including dye binding, Lcr, autoagglutination, and hydrophobicity indicate that these freeze-stressed isolates are potentially capable of causing food borne illness.