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Research Journal of Microbiology
Year: 2007  |  Volume: 2  |  Issue: 3  |  Page No.: 209 - 215

Escherichia coli STb Enterotoxin Toxicity and Internalization Investigations: A Mini-Review

J. Daniel Dubreuil, Stephanie Penel and Carina Goncalves    

Abstract: STb toxin is produced by strains of enterotoxigenic Escherichia coli isolated from various animals including man. The molecule comprises 48 amino acids (M.W. of 5200 Da and a pI of 9.6) with 4 cysteines involved in the formation of 2 disulfide bridges. Studies on STb toxin have resulted in the elucidation of the role of sulfatide as the receptor. This molecule was confirmed as a functional receptor as alteration or blocking strategies reduced or abrogated the toxicity as determined in the rat ligated loop assay. A study using a chemical cross-linker indicated that hexamers and heptamers of STb were formed. Oligomer formation could be observed with intact toxin but was abrogated in presence of β-mercaptoethanol. A structure-function study of STb using point mutations on residues, as determined by nuclear magnetic resonance, known to point to the solvent indicated that, electrostatic and hydrophobic interactions are important for enterotoxicity and binding to sulfatide, respectively. Mutants unable to form oligomer also showed an important reduction in toxicity. An electrophysiological study using planar lipid bilayers technology indicated that ions channels were formed when STb was added. Recently, we conducted experiments on pig jejunal brush border membrane vesicles and observed permeabilization of this vesicular model. With electron microscopy techniques, we could observe internalization of STb toxin after administration into rat intestinal loops. Using a cellular model (NIH-3T3 fibroblasts) confocal microscopy and cell markers, we observed endocytosis of STb. Contrarily to native toxin, reduced and alkylated STb toxin was not internalized. Using the NIH-3T3 fibroblasts cell model, STb endocytosis was shown to be mediated by two endocytic pathways, one clathrin-dependent and the other caveolae-dependent. These data together indicate that STb, after recognition of its receptor, is internalized into susceptible cells.

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