- Shiga toxin
Shiga toxins are a family of related toxins with two major groups, Stx1 and Stx2, whose genes are considered to be part of the
genomeof lambdoid prophages.cite journal |author=Friedman D, Court D |title=Bacteriophage lambda: alive and well and still doing its thing |journal=Curr Opin Microbiol |volume=4 |issue=2 |pages=201–7 |year=2001 |pmid=11282477 |doi=10.1016/S1369-5274(00)00189-2] The toxins are named for Kiyoshi Shiga, who first described the bacterial origin of dysenterycaused by " Shigella dysenteriae". The most common sources for Shiga toxin are the bacteria "S. dysenteriae" and the "Shigatoxigenic group" of " Escherichia coli" (STEC), which includes serotypeand other enterohemorrhagic "E. coli".cite journal |author=Beutin L |title=Emerging enterohaemorrhagic Escherichia coli, causes and effects of the rise of a human pathogen |journal=J Vet Med B Infect Dis Vet Public Health |volume=53 |issue=7 |pages=299–305 |year=2006 |pmid=16930272]
There are many terms that
microbiologistsuse to describe Shiga toxin and differentiate between different forms of it. Many of these terms are used interchangeably.
* 1) Shiga toxin (Stx) - true Shiga toxin is produced by Shigella dysenteriae.
* 2) Shiga-like toxin 1 and 2 (SLT-1 and 2 or Stx-1 and 2) - the Shiga toxins produced by some E. coli strains. Stx-1 differs from Stx by only 1 amino acid. Stx-2 shares 56%
sequence homologywith Stx-1.
* 3) Cytotoxins - an archaic denotation for Stx, used in a broad sense.
* 4) Verocytotoxins - a seldom used denotation for Stx, from the hypersensitivity of
Vero cellsto Stx.
Shiga toxins act to inhibit
protein synthesiswithin target cells by a mechanism similar to that of ricintoxin produced by " Ricinus communis".cite journal |author=Sandvig K, van Deurs B |title=Entry of ricin and Shiga toxin into cells: molecular mechanisms and medical perspectives |journal=EMBO J |volume=19 |issue=22 |pages=5943–50 |year=2000 |pmid=11080141 |doi=10.1093/emboj/19.22.5943] After entering a cell, the protein functions as an N- glycosidase, cleaving several nucleobases from the RNAthat comprises the ribosome, thereby halting protein synthesis.cite journal |author=Donohue-Rolfe A, Acheson D, Keusch G |title=Shiga toxin: purification, structure, and function |journal=Rev Infect Dis |volume=13 Suppl 4 |issue= |pages=S293–7 |year=1991 |pmid=2047652]
The toxin has two subunits—designated A and B—and is one of the AB5 toxins. The B subunit is a pentamer that binds to specific
glycolipids on the host cell, specifically globotriaosylceramide (Gb3). Following this, the A subunit is internalised and cleaved into two parts. The A1 component then binds to the ribosome, disrupting protein synthesis. Stx-2 has been found to be approximately 400 times more toxic (as quantified by LD50 in mice) than Stx-1.
Gb3 is, for unknown reasons, present in greater amounts in renal epithelial tissues, to which the renal toxicity of Shiga toxin may be attributed.
The toxin requires highly specific receptors on the cells' surface in order to attach and enter the cell; species such as
cattle, swine, and deerwhich do not carry these receptors may harbor toxigenic bacteria without any ill effect, shedding them in their feces, from where they may be spread to humans.cite journal |author=Asakura H, Makino S, Kobori H, Watarai M, Shirahata T, Ikeda T, Takeshi K |title=Phylogenetic diversity and similarity of active sites of Shiga toxin (stx) in Shiga toxin-producing Escherichia coli (STEC) isolates from humans and animals |journal=Epidemiol Infect |volume=127 |issue=1 |pages=27–36 |year=2001 |pmid=11561972 |doi=10.1017/S0950268801005635]
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