Food Safety Testing – E. coli
Escherichia coli is a Gram-negative, coliform bacteria that is one of the most common species found in human digestive systems.¹ Named for the scientist who first identified it in 1886, Theodor Escherich, most E. coli are not pathogenic in humans. In fact, harmless E. coli strains are vital to normal intestinal flora. However, there are certain types of E. coli bacteria that are known to cause severe food poisoning upon consumption of contaminated food.¹
One of these, Shiga toxin-producing E. coli (STEC), produce a potent toxin called Shiga toxin (Stx)*. There are subtypes of Shiga toxin; STEC strains can produce Shiga toxin type 1 (Stx1, encoded by the stx1 gene), Shiga toxin type 2 (Stx2, encoded by the stx2 gene), or both. STEC are further classified based on other virulence factors that they produce. For example, STEC that also contain the eae gene (intimin gene involved in attaching and effacing phenotype) are designated enterohemorrhagic E. coli (EHEC) making EHEC a subset of STEC.
The most common STEC serotype, O157:H7, causes more than 75% of E. coli infections worldwide, as it is more common than other STEC serotypes and has a much lower infective dose as low as 10-100 cells.³ In addition to E. coli O157, other STEC serogroups have been implicated in disease outbreaks.³ These are generally referred to as non-O157 STEC, and include serogroups O26, O45, O103, O111, O121, O145 (termed the big 6 non-O157 STECs by the USDA), O91, O104, and O113.
When STEC organisms are ingested, they can cause symptoms including severe cramping, diarrhea, and hemorrhagic colitis.¹ An estimated 5-10% of patients diagnosed with STEC infections also develop a serious complication called hemolytic uremic syndrome which can cause acute renal failure, permanent kidney damage and even death. This complication is most often associated with stx2.¹,³
STEC live in the intestines of animals, and the most common source of infection in humans is from cattle. Infection in humans requires fecal matter from an animal with STEC to be ingested, which can come from contaminated food, improperly treated or undercooked food, or direct contact with infected people or animals. Produce can easily become contaminated as well if it is grown near livestock or the water used to grow the crops becomes contaminated with feces.¹
STEC is responsible for the vast majority of foodborne outbreaks and recalls related to E. coli around the world, compromising foods ranging from ground meats to vegetables to non-pasteurized juices and milks to frozen cookie dough. In the US, STEC strains lead to more than 250,000 estimated annual illnesses, with approximately one-third of those resulting from O157:H7.
Due to its ease of infection and high mortality rate, STEC is one of the most feared foodborne pathogens by producers and consumers alike. E. coli recalls can be incredibly damaging from a brand reputation perspective. Food producers across many different categories have put an increased focus on detection efforts for pathogenic E. coli as high-profile outbreaks have occurred in recent years. The deadliest E. coli outbreak known – sprouts grown in Germany that were released to the public containing the O104:H4 strain – sickened an estimated 3842 persons and resulted in 53 fatalities.⁴
Because most E. coli is not pathogenic in humans, testing for and detecting STEC in a timely, efficient manner has been a unique challenge for the food industry. The 3M™ Molecular Detection Assay 2 - E. coli O157 (including H7) was introduced in 2016 as a rapid method for detecting the strain most commonly associated with E. coli O157 outbreaks with simplicity, sensitivity and efficiency. Many manufacturers also wish to screen for non-O157 STECs, including eae-negative subsets like the aforementioned O104:H4 outbreak in Europe.
In the summer of 2019, 3M announced two additional E. coli tests, the 3M™ Molecular Detection Assay 2 - STEC Gene Screen (stx and eae) and the 3M™ Molecular Detection Assay 2 - STEC Gene Screen (stx). Instead of testing for a pathogen or a serotype, the new assays can rapidly detect the genes for stx1 and stx2, and/or eae, the intimin gene that allows the bacteria to attach to intestinal cells. The product introduction gives processors the ability to choose a rapid test that’s ideal for their unique products and processes.
*Shiga toxin is also known as verotoxin or verocytotoxin, and STEC strains can also be called “verocytotoxin-producing E. coli (VTEC)
¹ Centers for Disease Control and Prevention, E. coli; www.cdc.gov/ecoli/index.html
² Bacteriological Analytical Manual Chapter 4A Diarrheagenic Escherichia coli. https://www.fda.gov/food/laboratory-methods-food/bam-diarrheagenic-escherichia-coli.
³ U.S. Food & Drug Administration, Bad Bug Book (Second Edition); www.fda.gov/files/food/published/Bad-Bug-Book-2nd-Edition-%28PDF%29.pdf (PDF, 5 MB)
⁴ Improving Food Safety Through a One Health Approach: Workshop Summary. https://www.ncbi.nlm.nih.gov/books/NBK114499/.