What events occur following exposure to Shigella toxin?

Updated: Apr 03, 2018
  • Author: Jaya Sureshbabu, MBBS, MRCPCH(UK), MRCPI(Paeds), MRCPS(Glasg), DCH(Glasg); Chief Editor: Russell W Steele, MD  more...
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Answer

In summary, events that occur on exposure to Shigella toxin are as follows:

  • The B subunit of holotoxin binds to the Gb3 receptor on the cell surface of brush-border cells of the intestines.

  • The receptor-holotoxin complex is endocytosed.

  • The complex moves to Golgi apparatus and then to the endoplasmic reticulum.

The A1 subunit is released and it targets 28S RNA of the ribosome, inhibiting protein synthesis. Stxs may play a role in the progression of mucosal lesions after colonic cells are invaded, or they may induce vascular damage in the colonic mucosa. Stx adheres to small-intestine receptors and blocks the absorption of electrolytes, glucose, and amino acids from intestinal lumen. The B subunit of Stx binds the host's cell glycolipid in the large intestine and in other cells, such as renal glomerular and tubular epithelia. The A1 domain internalized by means of receptor-mediated endocytosis and causes irreversible inactivation of the 60S ribosomal subunit, inhibiting protein synthesis and causing cell death, microvascular damage to the intestine, apoptosis in renal tubular epithelial cells, and hemorrhage (as blood and mucus in the stool).

Chromosomal genes control lipopolysaccharide (LPS) antigens in cell walls. LPS plays an important role in resistance to nonspecific host defense encountered during tissue invasion. These genes help in invasion, multiplication, and resistance to phagocytosis by tissue macrophages. LPS enhances the cytotoxicity of Stx on human vascular endothelial cells. Shigella chromosomes share most of their genes with E coli K12 strain MG1655, and the diversity of putative virulence genes acquired by means of bacteriophage-mediated lateral gene transfer is extensive. As a result of convergent evolution involving the gain and loss of functions, Shigella species have become highly specific human pathogens with variable epidemiologic and pathologic features.

A 3-kb plasmid that harbors information for the production of bacteriocin by S flexneri strains has been described. The production of this bacteriocin may be related to dysenteric diarrhea these bacterial strains produce.


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