A Novel selective medium for simultaneous enrichment of Shiga Toxin-producing Escherichia coli and Salmonella in ground beef

Authors: EGGERS, JOSEPH - University Of Minnesota; FEIRTAG, JOELLEN - University Of Minnesota; OLSTEIN, ALAN - Paradigm Diagnostics, Inc; Bosilevac, Joseph - Mick

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2018
Publication Date: 7/6/2018
Citation: Eggers, J., Feirtag, J.M., Olstein, A.D., Bosilevac, J.M. 2018. A Novel selective medium for simultaneous enrichment of Shiga Toxin-producing Escherichia coli and Salmonella in ground beef. Journal of Food Protection. 81(8):1252-1257. https://doi.org/10.4315/0362-028X.JFP-17-520.
DOI: https://doi.org/10.4315/0362-028X.JFP-17-520

Interpretive Summary: Ground beef may be contaminated by disease causing bacteria like Salmonella or Shiga toxin-producing E. coli (STEC). Therefore, ground beef is tested by producers and regulators for these bacteria. Because these foodborne pathogens, if present, are at very low levels, the first step of testing involves enriching the ground beef in a liquid media to grow the numbers of bacteria to levels that they can be detected. The most commonly used liquid media is modified tryptic soy broth (mTSB). mTSB however is non-selective and allows many bacteria to grow and makes identifying Salmonella and/or STEC difficult. This work compared a new Salmonella and STEC selective media (SSS broth) to mTSB. Results showed that SSS broth controls background bacteria growth and allowed mostly Salmonella and STEC to grow out for detection. Thus, SSS broth provides an alternative growth media that makes it easier to detect multiple foodborne pathogens.

Technical Abstract: Microbiological analysis of ground beef for contamination by both Salmonella and Shiga toxin–producing Escherichia coli (STEC) is performed by the U.S. Department of Agriculture, Food Safety Inspection Service (FSIS), as part of its Performance Standards Verification Testing program. FSIS has established a zero tolerance for STEC serotype O157:H7 and serogroups O26, O45, O103, O111, O121, and O145 because they are regarded as adulterants. The detection and isolation of these specific serogroups presents a technical challenge necessitating time-consuming and costly laboratory procedures that often exceed the technical capabilities of many small internal and reference laboratories. We describe here a method using a novel STEC and Salmonella selective (SSS) broth that allows for simultaneous selective enrichment of STEC and Salmonella sp., providing isolation and detection from the same broth. The method only involves direct plating from beef enrichments to detect suspect isolates that can be easily confirmed by using immunoassays or PCR, rendering the isolation simpler and less costly than the current described methods. In a side-by-side comparison with modified tryptic soy broth (mTSB), the use of SSS broth resulted in primarily isolating STEC and Salmonella sp., while substantially suppressing the growth of other gram-negative Enterobacteriacae by 90%. Significantly more (X2 < 3.84) samples containing E. coli O157:H7 and STEC O26, O111, O121, and O145 and a nondifferent (X2 > 3.84) number of samples containing STEC O103 and O45 were identified when enriching in SSS broth. Coenrichment using six different Salmonella serovars showed numerically greater but not significant (X2 < 3.84) positive samples by using SSS broth compared with mTSB for a majority of serotypes.