Recent comparison testing for foodborne disease pathogen (FBD) screening will reassure beef producers; not only can the Applied Biosystems RapidFinder STEC Detection Workflow help businesses release product faster, but it also matches the accuracy of the United States Department of Agriculture (USDA ) reference method. Screening experimental inoculations in raw beef showed that the RapidFinder real-time polymerase chain reaction (RT-PCR) workflow produced results consistent with USDA Microbiology Laboratory Guidebook (MLG) testing.
Shiga toxin–producing Escherichia coli (STEC) are highly virulent FBD pathogens that cause severe hemorrhagic diarrhea and hemolytic uremia syndrome. In vulnerable populations, including the elderly and children, the disease can be fatal. Seven STEC bacterial strains are known to cause most cases of this disease, with E. coli O157:H7 causing the majority. Of the remaining cases, non-O157 strains, serogroups O26, O45, O103, O111, O121 and O145, cause more than 80% of STEC-induced disease, according to FoodNet surveillance. Since 1994, following a severe outbreak originating from a fast food burger chain, the USDA Food Safety and Inspection Service (FSIS) declared the top seven STECs as adulterants in beef, a major source of the bacteria in the food chain.
STECs can affect many different foods, including fresh produce, dry goods and dairy, but beef is a major source because it is vulnerable to fecal or gut content contamination during animal slaughter. For strain O145, the infective dose is as low as 10 to 100 organisms.
STEC bacteria produce Shiga toxin, and this is associated with O-antigen genes stx1, stx2 and eae. These, along with several O157-specific markers, form the basis of many screening tests. The RapidFinder STEC Detection Workflow targets these virulence factors to screen and then positively identify the seven serotypes. This two-step approach is similar to the BAX PCR method employed in the MLG reference assay.
Fratamico, Bagi and Abdul-Weeker (2017) compared assay performance for RapidFinder STEC testing against the USDA reference method. They experimentally inoculated raw ground beef with the STEC strains.1 The researchers also used other common FBD bacteria such as Salmonella, Listeria and Shigella species as controls. They then tested the samples using the two protocols for RT-PCR screening.
Both approaches use enrichment prior to PCR amplification, and both screen for presence of STEC bacteria; if a positive result is obtained, the product needs to go for further testing by colony isolation and characterization to identify the strain. In each approach, a positive result in each of the two steps is needed to confirm STEC contamination.
Both the MLG BAX and RapidFinder methods detected experimental STEC contamination in the raw beef samples. Neither method resulted in false positive identifications for meat contaminated with the non-STEC pathogens. However, the shorter enrichment time required for the RapidFinder RT-PCR detection meant that results were available around three hours before those from the MLG BAX method.
As food producers know, not only is accuracy essential in protecting brand integrity, but faster results from PCR testing help prolong product shelf life for consumer satisfaction. This, in addition to the kit’s recent AOAC accreditation—an important quality validation in food industry testing—and its validation against the USDA reference method, positions the RapidFinder STEC Detection Workflow as a valuable tool on the food safety lab bench.
Learn more about STEC detection with RT-PCR.
1. Fratamico, P.M., Bagi, L.K. and Abdul-Wakeel, A. (2017) ‘‘Detection and isolation of the “top seven’’ Shiga toxin–producing Escherichia coli in ground beef: Comparison of RapidFinder kits to the U.S. Department of Agriculture Microbiology Laboratory Guidebook Method,” Journal of Food Protection, 80 (pp. 829–36). doi:10.4315/0362-028X.JFP-16-296
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