In October, a multi-state outbreak of enterohemorrhagic E. coli (EHEC), or E. coli 0157:H7 was reported by the CDC related to McDonald’s quarter pound burgers. Initial testing indicated that the likely culprits were the beef patties or the slivered onions served on burgers. Ultimately, investigators narrowed down the source of the E. coli to yellow onions produced by California-based Taylor Farms, which has issued a recall of affected produce.
Although McDonald’s and Taylor farms reacted quickly to pull products that were likely to be contaminated with E. coli O157:H7, the Federal Drug Administration (FDA) report indicates that 90 people are known to have been infected, with 27 in hospital and one death. The onions in question were shipped widely, to about 12 states, and infected persons lived in 13 different states across the Midwest and Mountain regions.
The natural reservoir for Shiga-toxin producing E. coli (STEC) is cattle, for whom an infection is largely asymptomatic.1 Enterohemorrhagic E. coli (EHEC) is part of the broader category of STEC, but is one of the most dangerous that is commonly found in cattle.2 Fresh vegetables can be contaminated during many different stages of growth, harvest, packaging, preparation, and distribution, and research into contamination sources is still ongoing. However, a likely culprit is soil or water contamination from domestic or wild animals. And once the vegetables are contaminated, pathogenic bacteria can adhere strongly to the outside, or even colonize the inside, of a fruits or leaves. In fact, STEC adheres to some vegetable matter (especially tomato skins, spinach leaves, and alfalfa sprouts) more strongly than non-pathogenic E. coli.3
Laboratories play an important role in the surveillance of STEC outbreaks in the community and they should be prepared to detect and report any isolate of this organism.
Stool specimens submitted for bacterial enteric culture should be screened for Shiga toxin producing E. coli O157 (STEC O157). Traditionally, laboratories would us Sorbitol MacConkey (SMAC) plates as a screening medium. Most isolates fail to ferment sorbitol and will be recognized as colourless colonies on SMAC. Non O157 STEC are also responsible for outbreaks and may not be isolated unless other media or methods for toxin testing are used.4
Non-culture methods are aimed at the detection of the toxins or toxin genes and they have the advantage of detecting any of the STEC serotypes.
There are few immunoassays commercially available for the detection of Shiga toxin. Most assays recommend the use of enrichment broth cultures rather than direct testing of stool specimens because of the low amount of free toxin in stools. Some assays can differentiate between Stx1 and Stx2.5
Multiplex assays are now implemented in the clinical labs that are able to detect multiple enteric pathogens (including STEC) directly form the patient’s sample making the screening for this organism more readily available.
CMPT supports laboratories detecting these organisms by offering PT programs that allow for the detection of STEC in simulated stools by culture, immunoassays or multiplex technology (Shiga Toxin and Gastrointestinal Panel)
1 https://pmc.ncbi.nlm.nih.gov/articles/PMC7355788/
2 https://pmc.ncbi.nlm.nih.gov/articles/PMC6899298/
3 https://doi.org/10.1111/j.1462-2920.2010.02297.x
