Shigatoxigenic and verotoxigenic Escherichia coli

Shigatoxigenic and verotoxigenic *E. coli*
Shigatoxigenic and verotoxigenic E. coli
Specialty	Infectious disease

Shigatoxigenic Escherichia coli (STEC) and verotoxigenic E. coli (VTEC) are strains of the bacterium Escherichia coli that produce Shiga toxin (or verotoxin).^[a] Only a minority of the strains cause illness in humans.^[2]^{[failed verification]} 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).^[3]^[4] 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.^[5] 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).^[6]

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.^[7]

The best known of these strains is O157:H7, but non-O157 strains cause an estimated 36,000^{[citation needed]} illnesses, 1,000 hospitalizations and 30 deaths in the United States yearly.^[8] Food safety specialists recognize "Big Six" strains: O26; O45; O103; O111; O121; and O145.^[8] 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.^{[citation needed]}

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 uses 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).^{[citation needed]} 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.^[6]

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.^[9] 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.^[6]

Names

Names of the group and its subgroups include the following.^[10] 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",^[3] 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).^[1]^[11]^: 2–3 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.

Name	Short form
enterohemorrhagic E. coli	EHEC
hemolytic uremic syndrome–associated enterohemorrhagic E. coli	HUSEC
shiga toxin–producing E. coli	STEC
shigatoxigenic E. coli	STEC
shiga-like toxin–producing E. coli	SLTEC
verotoxin-producing E. coli	VTEC
verotoxigenic E. coli	VTEC
verocytotoxin-producing E. coli	VTEC
verocytotoxigenic E. coli	VTEC

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.^{[citation needed]}

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.^[12]

Shiga toxins

Shiga toxins are a major virulence factor of EHEC. The toxins interact with intestinal epithelium and can cause systematic complications in humans like HUS and cerebral dysfunction if they enter the circulation.^[13] In EHEC, Shiga toxins are encoded by lysogenic bacteriophages. The toxins bind to cell-surface glycolipid receptor Gb3, which causes the cell to take the toxin in via endocytosis. The Shiga toxins target ribosomal RNA, which inhibits protein synthesis and causes apoptosis.^[14] The reason EHEC are symptomless in cattle is because the cattle do not have vascular expression of Gb3 unlike humans. Thus, the Shiga toxins cannot pass through the intestinal epithelium into circulation.^[5]

Regulation of the pathogenicity island

Scheme of the signalling cascade in EHEC with effect of the sensing of some signalling molecules in the virulence of eneterohaemorragic E. coli

EHEC becomes pathogenic through the expression of the locus of enterocyte effacement (LEE) encoded on its pathogenicity island. However, when EHEC is not in a host this expression is a waste of energy and resources, so it is only activated if some molecules are sensed in the environment. ^{[citation needed]}

When QseC or QseE bind with one of their interacting signalling molecule, they autophosphorylate and transfer its phosphate to the response regulator. QseC senses adrenaline, noradrenaline, and an Endonuclease I-SceIII, encoded by a mobile group I intron within the mitochondrial COX1 gene (AI3); whereas QseE senses adrenaline, noradrenaline, SO4 and PO4. These signals are a clear indication to the bacteria that they are no longer free in the environment, but in the gut.^{[citation needed]}

As a result, QseC phosphorylates QseB (which activates flagella), KpdE (which activates the LEE) and QseF. QseE phosphorylates QseF. The products QseBC and QseEF repress the expression of FusK and FusR. FusK and FusR are the two components of a system to repress the transcription of the LEE genes. FusK is a sensor kinase which is able to sense many sugars among which fucose. When fucose is present in the medium FusK phosphorylates FusR which represses LEE expression. ^{[citation needed]}

Thus when EHEC enters the gut there is a competition between the signals coming from QseC and QseF, and the signal coming from FusK. The first two would like to activate virulence, but Fusk stops it because the mucous layer, which is a source of fucose, isolates enterocytes from bacteria making the synthesis of the virulence factors useless. However, when fucose concentration decreases because bacterial cells find an unprotected area of the epithelium, then the expression of LEE genes will not be repressed by FusR, and KpdE will strongly activate them. In summary, the combined effect of the QseC/QseF and FusKR provide a fine-tuning system of LEE expression which saves energy and allow the mechanisms of virulence to be expressed only when the chances of success are higher.^{[citation needed]} There are rare STEC without LEE, see PMID 19239748.

Notes

^ Current classifications consider the two identical, and only use the "Shiga toxin" name. See § Names.^[1]

References

^ ^a ^b Scheutz F, Teel LD, Beutin L, Piérard D, Buvens G, Karch H, Mellmann A, Caprioli A, Tozzoli R, Morabito S, Strockbine NA, Melton-Celsa AR, Sanchez M, Persson S, O'Brien AD (September 2012). "Multicenter evaluation of a sequence-based protocol for subtyping Shiga toxins and standardizing Stx nomenclature". Journal of Clinical Microbiology. 50 (9): 2951–63. doi:10.1128/JCM.00860-12. PMC 3421821. PMID 22760050.
^ Croxen MA, Law RJ, Scholz R, Keeney KM, Wlodarska M, Finlay BB (2013). "Recent advances in understanding enteric pathogenic Escherichia coli". Clinical Microbiology Reviews. 26 (4): 822–80. doi:10.1128/CMR.00022-13. PMC 3811233. PMID 24092857.
^ ^a ^b Mainil, J (1999), "Shiga/verocytotoxins and Shiga/verotoxigenic Escherichia coli in animals", Vet Res, 30 (2–3): 235–57, PMID 10367357.
^ Phillips, A; Navabpour, S; Hicks, S; Dougan, G; Wallis, T; Frankel, G (2000). "Enterohaemorrhagic Escherichia coli O157:H7 target Peyer's patches in humans and cause attaching/effacing lesions in both human and bovine intestine". Gut. 47 (3): 377–381. doi:10.1136/gut.47.3.377. PMC 1728033. PMID 10940275.
^ ^a ^b Pruimboom-Brees, I; Morgan, T; Ackermann, M; Nystrom, E; Samuel, J; Cornick, N; Moon, H (2000). "Cattle lack vascular receptors for Escherichia coli O157:H7 Shiga toxins". Proceedings of the National Academy of Sciences. 97 (19): 10325–10329. Bibcode:2000PNAS...9710325P. doi:10.1073/pnas.190329997. ISSN 0027-8424. PMC 27023. PMID 10973498.
^ ^a ^b ^c Bae, Woo Kyun; Lee, Youn Kyoung; Cho, Min Seok; Ma, Seong Kwon; Kim, Soo Wan; Kim, Nam Ho; Choi, Ki Chul (2006-06-30). "A Case of Hemolytic Uremic Syndrome Caused by Escherichia coli O104:H4". Yonsei Med J. 47 (3): 437–439. doi:10.3349/ymj.2006.47.3.437. PMC 2688167. PMID 16807997. Two sentences were taken from this source verbatim.
^ Franz, Eelco; Rotariu, Ovidiu; Lopes, Bruno S; MacRae, Marion; Bono, James L; Laing, Chad; Gannon, Victor; Söderlund, Robert; van Hoek, Angela H A M; Friesema, Ingrid; French, Nigel P; George, Tessy; Biggs, Patrick J; Jaros, Patricia; Rivas, Marta (2018-10-29). "Phylogeographic Analysis Reveals Multiple International transmission Events Have Driven the Global Emergence of Escherichia coli O157:H7". Clinical Infectious Diseases. 69 (3): 428–437. doi:10.1093/cid/ciy919. hdl:2164/13183. ISSN 1058-4838.
^ ^a ^b Mallove, Zach (26 April 2010). "Lawyer Battles FSIS on Non-O157 E. coli". Food Safety News. Retrieved 2 June 2011.
^ Zhang, X; McDaniel, AD; Wolf, LE; Keusch, GT; Waldor, MK; Acheson, DW (2000). "Quinolone antibiotics induce Shiga toxin-encoding bacteriophages, toxin production, and death in mice". The Journal of Infectious Diseases. 181 (2): 664–70. doi:10.1086/315239. PMID 10669353.
^ Karch, Helge; Tarr, Phillip I.; Bielaszewska, Martina (2005). "Enterohaemorrhagic Escherichia coli in human medicine". International Journal of Medical Microbiology. 295 (6–7): 405–18. doi:10.1016/j.ijmm.2005.06.009. PMID 16238016.
^ Silva, Christopher J.; Brandon, David L.; Skinner, Craig B.; He, Xiaohua; et al. (2017), "Structure of Shiga Toxins and Other AB5 Toxins", Shiga toxins: A Review of Structure, Mechanism, and Detection, Springer, pp. 21–45, doi:10.1007/978-3-319-50580-0_3, ISBN 978-3319505800.
^ Yang, Wen; Sun, Hongmin; Yan, Jun; Kang, Chenbo; Wu, Junli; Yang, Bin (2023-06-27). "Enterohemorrhagic Escherichia coli senses microbiota-derived nicotinamide to increase its virulence and colonization in the large intestine". Cell Reports. 42 (6). doi:10.1016/j.celrep.2023.112638. ISSN 2211-1247. PMID 37294635.
^ Detzner, J; Pohlentz, G; Müthing, J (2020). "Valid Presumption of Shiga Toxin-Mediated Damage of Developing Erythrocytes in EHEC-Associated Hemolytic Uremic Syndrome". Toxins. 12 (6): 373. doi:10.3390/toxins12060373. ISSN 2072-6651. PMC 7354503. PMID 32512916.
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