SHIGA TOXINS (StxS): MULTIFACETED PATHOGENICITY DETERMINANTS

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.