Enterotoxins, colonization factors and serotypes of enterotoxigenic Escherichia coli from humans and animals.

Enterotoxigenic Escherichia coli (ETEC) strains may synthesize both thermolabile (LT-I and LT-II) and thermostable (STa and STb) enterotoxins. Whereas thermolabile enterotoxins are high molecular weight proteins (85,000 d-90,000 d) composed by a single enzymatic A subunit combined with five B subunits which enable toxin for the receptor recognition, thermostable enterotoxins are small peptide chains with molecular weight between 1,900 d and 5,000 d. In addition to the synthesis of enterotoxins, the ability of ETEC strains to cause diarrhoea is also conditioned by the possession of colonization factors which enable bacteria adhere-to and colonize the luminal surface of small bowel. Colonization factors in ETEC strains were located in rigid fimbriae and flexible fibrils constituted by protein subunits ranging in size from 14,500 d to 31,000 d and usually responsible for mannose-resistant haemagglutination with determined erythrocyte species. Both enterotoxins and colonization factors are controlled by plasmids. There exist plasmids which may code separately enterotoxins and colonization factors, and besides there also exist recombinant plasmids coding together these two virulence factors. Human ETEC strains may synthesize LT-I and/or STa enterotoxins, they may possess the colonization factors named CFA/I, CFA/II, CFA/III or CFA/IV, and they belong mainly to serogroups O6, O8, O15, O20, O25, O27, O63, O77, O78, O114, O115, O126, O128, O139, O148, O153, O159 and O167. ETEC strains from porcine origin synthesize LT-I, STa and/or STb, they possess the colonization factors K88, P987, K99 or F41, and they usually belong to serogroups O8, O9, O20, O45, O64, O101, O115, O138, O141, O147, O149 and O157. Bovine and ovine ETEC strains are usually STa producers harbouring on the bacterial surface K99 or F41 colonization factors and they belong to serogroups O8, O9 and O101. Nevertheless, some particular bovine ETEC strains synthesizing LT-II have been described. Thus, a high specificity level between ETEC strains causing diarrhoea in humans and domestic animals can be observed. This is mainly due to the specific recognition between bacterial colonization factors and the epithelial receptors during host-parasite interaction.