[Enterotoxigenic, verotoxigenic, and necrotoxigenic Escherichia coli in food and clinical samples. Role of animals as reservoirs of strains pathogenic for humans].

Toxigenic Escherichia coli of human and animal origin have been classified into three categories: enterotoxigenic E. coli (ETEC), verotoxigenic E. coli (VTEC), and necrotoxigenic E. coli (NTEC), ETEC are a major cause of infant diarrhoea in less-developed countries and frequently cause colibacillosis in domestic animals. Human ETEC strains may synthesize LT-I and/or STa enterotoxins and they may possess the colonization factors CFA/I to CFA/IV; porcine strains synthesize LT-I, STa and/or STb, and possess the colonization antigens K88, P987, K99 or F41; and bovine strains are usually STa producers harbouring on the bacterial surface K99 or F41 colonization factors. There is a high host-specificity, because of that ETEC from animals are not pathogen for humans. VTEC strains may produce three mainly types of verotoxins (VT1, VT2, VT2vp1) that are functionally and structurally related to the shiga toxin. The VTEC of human and bovine origins produce VT1, VT2 or both, whereas VT2vp1 is elaborated by E. coli that cause edema disease in swine. The VTEC strains belonging mainly to serotypes O157:H7 or H-, O26:H11 and O111:H-, are now considered to be the major cause of two human syndromes of hitherto unknown cause: hemorrhagic colitis and hemolytic uremic syndrome. Most outbreaks of VTEC infection occurred in USA, Canada and United Kingdom during the last ten years and have been linked to consumption of undercooked ground beef, and, to a lesser extent, to the drinking of unpasteurized milk. Thus, the principal reservoir of VTEC is the intestinal tract of cattle. By contrast, it is presumed that human beings are the major reservoir of ETEC, and that contaminated water is a principal vehicle for transmission of ETEC infections. NTEC strains are able to elaborate two types of cytotoxic necrotizing factors (CNF1 and CNF2). Strains of human origin usually produce CNF1, whereas bovine NTEC generally synthesize CNF2. NTEC strains are not responsible for food-associated outbreaks of gastroenteritis, but CNF1 and CNF2 are very good markers of the source of food contamination.