What is E. coli and how does it cause food poisoning?
Escherichia coli (E. coli) is a highly studied, common species of bacteria that belongs to the family Enterobacteriaceae, so named because many of its members live in the intestines of humans and warm-blooded animals.
A gram-negative, facultatively anaerobic rod, Escherichia coli was named after Theodor Escherich, a German-Austrian pediatrician. Escherich isolated a variety of bacteria from infant fecal samples by using his own anaerobic culture methods and Hans Christian Gram’s new staining technique. After discovering E. coli in 1884, Escherich originally named the common colon bacillus Bacterium coli commune. Castellani and Chalmers proposed the name E. coli in 1919, but it was not officially recognized until 1958.
E. coli is a highly diverse species; there are numerous types and strains with different characteristics. Most types of E. coli occur as normal inhabitants of the intestines of animals and do not cause disease.
E. coli strains are grouped together into serogroups and serotypes based on differences in antigens in their cell walls (somatic, or “O” antigens) and flagella (“H” antigens). Non-motile strains may carry the added designation “NM.” The O antigen number corresponds to the serogroup and the O:H antigen number combination corresponds to the serotype. Specific O antigens and H antigens are each given numbers, assigned chronologically as new antigens are discovered. Hundreds of serotypes of E. coli have been identified. The most prominent and notorious serotype in the United States is E. coli O157:H7.
Escherichia coli – Diarrheagenic
According to the group of virulence determinants acquired, specific combinations were formed determining the currently known E. coli pathotypes (strains), which are collectively known as “diarrheagenic E. coli” (DEC). These are the types of E. coli that can cause foodborne disease (a.k.a. food poisoning) when food becomes contaminated by feces of animal or human reservoirs of E. coli (sometimes via water or soil). For example, foods of animal origin (e.g., beef, raw milk) can be contaminated by bacteria from animals’ hides during production, and produce (e.g., leafy greens, sprout seeds) can be contaminated by animal waste in fields via irrigation water. Importantly, diarrheagenic E. coli can not only be acquired from food, but also from consumption of contaminated water, contact with infected people, and for some, contact with infected animals or animal environments. Other pathotypes of E. coli have the ability to cause diseases such as urinary tract infections (uropathogenic E. coli, or UPEC), blood stream infections (extraintestinal pathogenic E. coli, or ExPEC), and meningitis (MAEC) if they gain access to these normally-sterile sites within the body.
Six E. coli pathotypes are associated with diarrhea:
- Enterotoxigenic E. coli (ETEC)
- Shiga toxin-producing E. coli (STEC) (a.k.a. Enterohemorrhagic E. coli, or EHEC)
- Enteropathogenic E. coli (EPEC) (a.k.a. attaching and effacing E. coli, or AEEC)
- Enteroaggregative E. coli (EAEC)
- Enteroinvasive E. coli (EIEC)
- And, possibly, diffusely adherent E. coli (DAEC)
Of these E. coli pathotypes, Shiga toxin-producing E. coli, or STEC for short, are the most well-known cause of illness in the United States, and they cause the most severe illness. STEC are also called verotoxigenic E. coli (VTEC), but for simplicity hereinafter will be referred to as STEC. Not all STEC can infect humans, and only a subset of these are responsible for human disease. The term enterohemorrhagic E. coli (EHEC) is commonly used to specify STEC strains capable of causing human illness. The STEC/EHEC serotype O157:H7 is a human pathogen found to be responsible for bloody diarrhea outbreaks and hemolytic uremic syndrome (HUS) worldwide.
The Shiga Toxin
The Shiga toxin is produced by Shigella dysenteriae and EHEC. It is named after the Japanese microbiologist Kioshi Shiga, who identified the etiologic agent of epidemic dysentery in Japan in 1898. Shiga called this organism Bacillus dysenteriae, but the name was later changed to Shigella dysenteriae to reflect its discoverer. The term Shiga toxin-producing Escherichia coli (STEC) refers to an E. coli strain that acquired the capacity to produce a Shiga toxin, through transfer of gene by means of a Shiga-toxin (Stx) phage.
There are two main types of Stx: Stx1 and Stx2. STEC strains that possess either type of Stx can cause bloody diarrhea and result in hospitalization, but strains that contain Stx2 (either alone or in combination with Stx1) are generally the ones that can cause very severe disease, including HUS. Almost all E. coli O157:H7 strains recovered from ill people have Stx2 (and sometimes Stx1 as well). Stx2 has a number of variants (e.g., Stx2a, Stx2c, Stx2d), and strains with some of those variants (e.g., Stx2a) are more likely to cause hemolytic uremic syndrome (HUS).
STEC cause an estimated 265,000 illnesses annually in the United States. The other pathotypes of diarrheagenic E. coli mentioned above also cause foodborne illness in the United States. ETEC in particular has caused periodic outbreaks, typically related to imported fresh produce, but is better known as a leading cause of traveler’s diarrhea. In general, diarrheagenic E. coli pathotypes other than STEC are less well known, less well studied, and cause less severe disease in the United States than STEC.