Shigatoxigenic Escherichia coli (STEC) and verotoxigenic E. coli (VTEC) are strains of the bacterium Escherichia coli that produce Shiga toxin (or verotoxin). Only a minority of the strains cause illness in humans. The ones that do are collectively known as enterohemorrhagic E. coli (EHEC) and are major causes of foodborne illness. When infecting the large intestine of humans, they often cause gastroenteritis, enterocolitis, and bloody diarrhea (hence the name "enterohemorrhagic") and sometimes cause a severe complication called hemolytic-uremic syndrome (HUS). Cattle are an important natural reservoir for EHEC because the colonised adult ruminants are asymptomatic. This is because they lack vascular expression of the target receptor for Shiga toxins. The group and its subgroups are known by various names. They are distinguished from other strains of intestinal pathogenic E. coli including enterotoxigenic E. coli (ETEC), enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC), and diffusely adherent E. coli (DAEC).
Background, biology Shiga toxin–producing Escherichia coli are zoonotic pathogens, in that they can be found in the gastrointestinal tract of cattle and sheep, and can infect humans. They are globally-occurring bacteria. The best known of these strains is O157:H7, but non-O157 strains cause an estimated 36,000 illnesses, 1,000 hospitalizations and 30 deaths in the United States yearly. Food safety specialists recognize "Big Six" strains: O26; O45; O103; O111; O121; and O145. A 2011 outbreak in Germany was caused by another STEC, O104:H4. This strain has both enteroaggregative and enterohemorrhagic properties. Both the O145 and O104 strains can cause hemolytic-uremic syndrome (HUS); the former strain shown to account for 2% to 51% of known HUS cases; an estimated 56% of such cases are caused by O145 and 14% by other EHEC strains.
Clinical presentation The clinical presentation in humans ranges from a mild and uncomplicated diarrhea to a hemorrhagic colitis with severe abdominal pain. Serotype O157:H7 may trigger an infectious dose with 100 bacterial cells or fewer; other strain such as 104:H4 has also caused an outbreak in Germany 2011. Infections are most common in warmer months and in children under five years of age and are usually acquired from uncooked beef and unpasteurized milk and juice. Initially a non-bloody diarrhea develops in patients after the bacterium attaches to the epithelium of the terminal ileum, cecum, and colon. The subsequent production of toxins mediates the bloody diarrhea. In children, a complication can be hemolytic uremic syndrome which then causes cytotoxins to attack the cells in the gut, so that bacteria can leak out into the blood and cause endothelial injury in locations such as the kidney by binding to globotriaosylceramide (Gb3). EHECs that induce bloody diarrhea lead to HUS in 10% of cases. The clinical manifestations of postdiarrheal HUS include acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia. The verocytotoxin (shiga-like toxin) can directly damage renal and endothelial cells. Thrombocytopenia occurs as platelets are consumed by clotting. Hemolytic anemia results from intravascular fibrin deposition, increased fragility of red blood cells, and fragmentation. Antibiotics are of questionable value and have not shown to be of clear clinical benefit. Antibiotics that interfere with DNA synthesis, such as fluoroquinolones, have been shown to induce the Stx-bearing bacteriophage and cause increased production of toxins. Attempts to block toxin production with antibacterials which target the ribosomal protein synthesis are conceptually more attractive. Plasma exchange offers a controversial but possibly helpful treatment. The use of antimotility agents (medications that suppress diarrhea by slowing bowel transit) in children under 10 years of age or in elderly patients should be avoided, as they increase the risk of HUS with EHEC infections.
Names Names of the group and its subgroups include the following. There is some polysemy involved. Invariable synonymity is indicated by having the same color. Beyond that there is also some wider but variable synonymity. The first two (purple) in their narrowest sense are generally treated as hypernyms of the others (red and blue), although in less precise usage the red and blue have often been treated as synonyms of the purple. At least one reference holds "EHEC" to be mutually exclusive of "VTEC" and "STEC", but this does not match common usage, as many more publications lump all of the latter in with the former. The current microbiology-based view on "Shiga-like toxin" (SLT) or "verotoxin" is that they should all be referred to as (versions of) Shiga toxin, as the difference is negligible. Following this view, all "VTEC" (blue) should be called "STEC" (red). Historically, a different name was sometimes used because the toxins are not exactly the same as the one found in Shigella dysenteriae, down to every last amino acid residue, although by this logic every "STEC" would be a "VTEC". The line can also be drawn to use "STEC" for Stx1-producing strains and "VTEC" for Stx2-producing strains, since Stx1 is closer to the Shiga toxin. Practically, the choice of words and categories is not as important as the understanding of clinical relevance.
Infectivity and virulence The infectivity or the virulence of an EHEC strain depends on several factors, including the presence of fucose in the medium, the sensing of this sugar and the activation of EHEC pathogenicity island.
Attaching and effacing To successfully colonize the gut of its host, EHEC relies on attaching itself to epithelial cells in the large intestine. A type III secretion system (T3SS) consisting of intimin and its translocated intimin receptor (Tir), is expressed on the cell membrane, allowing EHEC to intimately attach to host cells. T3SS secretes Tir into the host cell membrane and induces the formation of pedestals, resulting in attachment and effacing lesions on epithelial cells. Expression of T3SS associated genes is regulated by LEE and is activated through the EvgSA two component system in the presence of nicotinamide.
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