Tracking and preventing Shiga toxin-producing E. coli (STEC) outbreaks

E. coli sounds like bad news to many people. In most cases it’s a perfectly normal inhabitant of our intestinal tracts. In healthy humans, it can reach up to 109CFU per gram in fecal matter, according to Human Health.  Most types of E. coli are non-pathogenic, meaning they do not cause disease. However, some types of E. coli are pathogenic , or disease-producing, and can cause illness in humans, including diarrhea, abdominal pain, fever and sometimes vomiting.

Shiga toxin-producing E. coli (STEC) are also known as “verocytotoxin-producing E. coli” (VTEC). It is a type of pathogenic E. coli that produces a potent toxin called Shiga toxin (Stx), also known as verotoxin or verocytotoxin. STEC can produce shiga toxin type 1 (Stx1), type 2 (Stx2), or both, encoded by stx1 and stx2 genes, respectively.

STEC is a broader group of E. coli that produce the Shiga toxin and are further classified based on other virulence factors that they produce. For example, STEC containing virulence genes for stx1 and/or stx2 and eae (intimin gene involved in attaching and effacing phenotype) are designated enterohemorrhagic E. coli (EHEC) making EHEC is a subset of STEC.  STEC and EHEC are often used interchangeably.

However, there are certain pathotypes of E. coli that only have stx1 and/or stx2 and not eae that have been implicated in outbreaks (Table 1). To further complicate the tracking of outbreaks, more than 400 STEC serotypes have been identified and only a subset of these have been correlated to illness in humans (Table 1).

The expert group assembled by WHO/FAO used extensive scientific review and concluded that the serotype of the STEC strain should not be considered a virulence criterion as strains with the same serotype may not carry the same virulence genes and pose the same risk and serotype is not reliable for risk assessment. Hence, screening of the known virulence genes is critical for risk assessment.

Table 1. Common STEC serotypes involved in global disease outbreaks

O Serogroup H antigen(s) eae Seropathotype Hemorrhagic
colitis (HC)
Hemolytic uremic syndrome (HUS)
O26 NM, H11 + B + +
O45 NM + B +
O91b NM, H21 C + +
O103 NM, H2 + B + +
O111 NM, H8 + B + +
O113 b H21 C + +
O121 H19 + B + +
O145 NM, H25, H28 + B + +
O157 NMa, H7 + A + +

aSorbitol fermenting

bSTEC without eae

STEC encompasses a diverse pathotype of E. coli that can cause severe disease which can result in hemorrhagic colitis (bloody diarrhea), and a type of kidney failure called hemolytic uremic syndrome (HUS) in humans. In North America, the most commonly identified STEC (EHEC) is E. coli O157:H7.

It has been classified as an adulterant in beef since 1994 by the U.S. Department of Agriculture (USDA). As there are other STEC (EHEC) serogroups that have been implicated in disease, USDA has declared 6 more STEC (EHEC) serogroups, i.e., O26, O45, O103, O111, O121, and O145 (Big 6 non-O157 STECs) to be adulterants. Some 265,000 STEC infections occur each year in the United States. Around 36 percent of these are probably caused by E. coli O157:H7.

Based on a WHO/FAO report, regulators monitor meat (mainly beef), dairy, produce, nuts, seeds and seed sprouts. Human outbreaks of STEC-related disease have been linked to consumption of contaminated food or water, direct contact with infected animals or environments contaminated by their feces, or by direct human-to-human contact with infected persons (i.e. secondary infections). Some foods such as unpasteurized (raw) milk, unpasteurized apple cider, and soft cheeses made from raw milk are considered to carry a high risk of infection with E. coli O157 or other STEC. Health officials often recommend that people avoid such foods completely.

STEC contamination of meat and milk can happen during harvest and processing, as cattle and other ruminant animals such as goats, sheep, deer, and elk are natural reservoirs of STEC. Water contamination, run-off from animal production units, or dust and insects from animal production facilities can spread STEC from these animals to food such as fresh produce.

STEC poses an important human health risk.  Doses as low as 10 CFU can lead to infection and cause severe illness to people infected. Because of these risks, the food industry and regulators are putting substantial resources and efforts into eliminating STEC from the food supply.

Regulatory agencies such as FDA are looking more into health risk caused by the broader STEC group (stx positive and eae negative) and not just EHEC. The recently revised FDA BAM method now uses gene screen for non-O157 STEC and O157:H7 and further characterization of non-O157 STEC as E. coli by colony isolation and confirmation of presence of stx gene. The confirmed isolates are further characterized for serotype and adherence factors using whole genome sequence data.

ISO 13136 uses methods to screen for STEC genes and further characterization of isolates with stx only and stx and eae (EHEC).  However, USDA FSIS is testing for STEC (EHEC) using methods to screen for both gene targets (stx1/2 and eae) in products under its regulatory scope and further characterization of positive samples (positive for both stx and eae) for the top 7 serotypes (O157, O26, O45, O103, O111, O121 and O145). The serotype screens are complicated and best left to well-trained labs.

The gene screens are easy to implement and given the simple workflow of 3M™ Molecular Detection Assay 2 family, these assays can be implemented in any laboratory conducting pathogen testing. 3M Food Safety has developed two STEC kits, the 3M™ Molecular Detection Assay 2 – STEC Gene Screen (stx) and the 3M™ Molecular Detection Assay 2 – STEC Gene Screen (stx and eae). The food processor can choose either of the kits based on the product they are testing and regulatory requirements.

You can read more about the evaluation of 3M Molecular Detection Assay 2 – STEC Gene Screen for the detection of STEC and Salmonella in beef and poultry matrices. To learn more, ask an expert. 

3M™ Molecular Detection Assay 2 – STEC Gene Screen (stx and eae)

The 3M Molecular Detection Assay 2 – STEC Gene Screen (stx and eae) is used with the 3M™ Molecular Detection System for the rapid and specific detection of shiga toxin gene (stx1 and/or stx2) and intimin gene (eae) from Shiga toxin-producing E. coli  The screening kit contains two separate reagent tubes, one to detect virulence genes stx1 and/or stx2 and the other to detect eae gene.. The 3M™ Molecular Detection System Software reports results separately for  stx and eae genes and uses results from both reagent tubes to call the sample positive or negative for STEC (EHEC). For a presumptive positive for STEC (EHEC), both gene targets (stx1 and/or stx2 and eae) must be positive. The stx assay does not differentiate between stx1 and stx2, but detects presence of either or both of these genes in the sample.

3M™ Molecular Detection Assay 2 – STEC Gene Screen (stx)

The 3M™ Molecular Detection Assay 2 – STEC Gene Screen (stx) is used with the 3M™ Molecular Detection System for the rapid and specific detection of shiga toxin genes (stx1 and/or stx2) from shiga toxin-producing E. coli  in enriched foods and food process environmental samples. The term STEC refers to E. coli pathotypes capable of producing shiga toxin type 1 (Stx1), type 2 (Stx2), or both, encoded by stx1 and stx2 genes, respectively. The kit contains reagents only for detection of shiga toxin genes and no other virulence factors from STEC. In addition, the assay does not differentiate between stx1 and stx2, but detects presence of either or both of these genes in the sample.

More about the 3M™ Molecular Detection System

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