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Chapter 16 : Shiga Toxins (Stxs): Multifaceted Pathogenicity Determinants
Category: Bacterial Pathogenesis
The Shiga toxins (Stxs) (also known as Vero toxins and formerly called Shiga-like toxins) constitute a family of toxins that features Stx from Shigella dysenteriae type 1 as the prototype. Among bacterial toxins, the Stxs are particularly potent poisons that are estimated to be as toxic per unit of weight as tetanus toxin and second only to botulinum toxin in potency. The major difference among the Stxs is reflected in the division of the Stx family into two groups, Stx/Stx1 and Stx2, based on the capacity of antitoxin to neutralize the cytotoxic activity of each member of the homologous but not the heterologous group. The Stx-producing Escherichia coli (STEC) bacteria grow within the colon and elaborate Stx1 and/or Stx2. The toxin crosses the intestinal barrier and enters the bloodstream. Once in the bloodstream, the toxin may bind to polymorphonuclear cells and circulate. The major target organ of the Stxs is the kidney, although sites within the central nervous system may also be affected. The interaction of Stx and bacterial lipopolysaccharide with cells of the immune system and locally within the kidney may lead to increased cytokine and chemokine expression. Some epidemiological evidence suggests that STEC strains that produce Stx2 cause more severe disease than STEC strains that make Stx1 or both Stx1 and Stx2.
Crystal structures of Stx and Stx2. The A1 subunits (gray) rise above the B pentamers at the bottom of each image. The A2 peptides are depicted in black, as are the active sites in the A1 peptides. Note how the N-terminal end of the A2 peptide hovers near the active site of the toxin.The program DeepView/Swiss-PDB Viewer was used to model X-ray crystallo-graphic coordinates of the Stx and Stx2 holotoxins. The Protein Data Bank accession numbers for the Stx and Stx2 crystal structures shown are 1R4Q and 1R4P, respectively.
Steps in the pathogenesis of STEC. After the Stx-expressing bacteria are ingested, they colonize the host large intestine (through intimin for EHEC bacteria, and possibly aided by Stx expression), the Stxs enter the bloodstream, and the toxin, most likely in combination with lipopolysaccharide (LPS), causes increased levels of chemokines and cytokines. The toxin directly damages glomerular endothelial cells in the kidney, and that damage in the presence of the chemokines and cytokines creates a prothrombotic environment that leads to the HUS.
Model of the mechanism by which Stx2 may aid STEC colonization. Stx2 produced by adherent EHEC is internalized, travels in a retrograde manner through the cell, and halts protein synthesis. Through a yet-to-be-defined mechanism, nucleolin localization at the cell surface is increased, thus providing a greater number of nucleolin receptors available for initial localized adherence. The net result is enhanced microcolony formation and intestinal colonization as demonstrated in the mouse. TGN, trans-Golgi network; ER, endoplasmic reticulum.
Similarities and differences among the Stxs
Comparison of the levels of amino acid similarity of the A and B subunits among the Stxs a