Abstract
Shiga toxin-producing Escherichia coli (STEC) and Salmonella are food-borne pathogens commonly associated with beef, and reliable methods are needed to determine their prevalence in beef and to ensure food safety. Retail ground beef was tested for the presence of E. coli O157:H7, STEC serogroups O26, O45, O103, O111, O121, and O145, and Salmonella using the DuPont™ BAX® system method. Ground beef (325 g) samples were enriched in 1.5 L of TSB with 2 mg/L novobiocin at 42°C for 18 h, and then evaluated using the BAX® System real-time PCR assays for E. coli O157:H7 and STEC suite, and the BAX® System standard PCR assays for E. coli O157:H7 MP and Salmonella. Samples positive for STEC target genes by the BAX® System assays were subjected to immunomagnetic separation (IMS) and plating onto modified Rainbow Agar O157. Enrichments that were PCR positive for Salmonella were inoculated into RV broth, incubated for 18 h at 42°C, and then plated onto XLT-4 agar. Presumptive positive STEC and Salmonella colonies were confirmed using the BAX® System assays. Results of the BAX® System STEC assays showed 20/308 (6.5%) of samples positive for both the Shiga toxin (stx) and intimin (eae) genes; 4 (1.3%) for stx, eae, and O26; 1 (0.3%) for stx, eae, and O45; 3 (1%) for stx, eae, and O103; and 1 (0.3%) for stx, eae, and O145. There were also 3 samples positive for stx, eae, and more than one STEC serogroup. Three (1.0%) of the samples were positive using the BAX® System real-time E. coli O157:H7 assay, and 28 (9.1%) were positive using the BAX® System Salmonella assay. STEC O103 and E. coli O157:H7 were isolated from 2/6 and 2/3 PCR positive samples, respectively. Salmonella isolates were recovered and confirmed from 27 of the 28 Salmonella PCR positive samples, and a portion of the isolates were serotyped and antibiotic resistance profiles determined. Results demonstrate that the BAX® System assays are effective for detecting STEC and Salmonella in beef.
Highlights
The USDA Food Safety and Inspection Service (FSIS) classified Escherichia coli O157:H7 as an adulterant in raw ground beef and began a verification testing program for this pathogen in 1994 in response to a large outbreak associated with undercooked ground beef
It has become evident that non-O157 Shiga toxin-producing E. coli (STEC), STEC serogroups O26, O45, O103, O111, O121, and O145 cause illnesses similar to those caused by E. coli O157:H7 (Gould et al, 2013)
E. coli O157:H7 and other STEC serogroups, as well as Salmonella can be found as contaminants in beef products, and use of a common enrichment/pre-enrichment medium that is suitable for growth of all of these pathogens simplifies screening since the pathogens can be detected from the same sample
Summary
The USDA Food Safety and Inspection Service (FSIS) classified Escherichia coli O157:H7 as an adulterant in raw ground beef and began a verification testing program for this pathogen in 1994 in response to a large outbreak associated with undercooked ground beef. Cattle are a major reservoir for STEC in the U.S, and outbreaks due to non-O157 STEC, including serogroups O26 and O111 have been associated with beef or contact with cattle (Paton et al, 1996; Ethelberg et al, 2009; Kaspar et al, 2010). The FSIS declared the top six non-O157 STEC as adulterants in beef trim, and FSIS verification testing for these pathogens began in June 2012 in domestic and imported beef manufacturing trimmings (Anonymous, 2011).
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