Pathogenic Escherichia coli found in food

Abstract

The bacteria constituting the species Escherichia coli are commonly found in the intestinal flora of man and animals, and were until late 1950s recognized as non-pathogenic normal cohabitants. However, certain strains might induce disease, and E. coli should therefore be regarded as a potential pathogenic organism. The pathogenic strains can cause distinct disease syndrome as different diarrheal diseases, wound infectionsm meningitis, septicemia, artherosclerosis, hemolytic uremic syndrome and immunological diseases such as reactive and rheumatoid arthritis. Several different groups of diarrhea-inducing strains are known. The enterotoxigenic E. coli (ETEC) strains produce one or more of toxins from the heat-labile and the heat-stable enterotoxin families. These strains possess specific adhesion fimbria for intestinal attachment and colonization. Some enteropathogenic E. coli strains (EPEC) produce one or more of the cytotoxins, but adhere also to intestinal cells interfering with the electrolyte transport system. The group of strains possessing invasive properties are designated enteroinvasive E. coli (EIEC). Recently, the enterohemorrhagic E. coli (EHEC) strains have been identified and shown to produce one or more of the cytotoxins (vero-cytotoxins, shiga-like toxins). Food originating from warm-blooded animals may be contaminated with E. coli, but contamination from human sources are more common for food involved in outbreak of disease. In general, strains causing disease in animals do possess other colonization factors than those found on human pathogenic strains. EIEC strains are, like Shigella, only known to induce disease in man. However, both healthy and sick cattle are suspected to be a major reservoir for EHEC strains, and several outbreaks have been associated with consumtion of meat or meat products. Cheeses have been the source of outbreaks of both ETEC and EIEC in Europe and the USA, while water seems to be a major source for the different diarrheic E. coli strains affecting children and tourists in the 3rd world. Strains causing non-enteric disease are less known as being transmitted to humans with food as a vector, but the importance of some of these diseases, should implicate further research on what role food plays in spreading these organisms. The recipient of the potential pathogenic E. coli through food, the humans, are also of different risk of contracting diseases. The factors of most importance seems to be the immunological and nutritional status of the host, and recently E. coli strains previously regarded as non-pathogenic, have been implicated in cases of disease in persons with AIDS. Because the species E. coli consists of both pathogenic and non-pathogenic strains, and due to the fact that the latter constitute a part of normal intestinal flora, differentiation between pathogenic and non-pathogenic strains becomes very important. Traditional cultivation from food samples, using selective enrichment broths, has been shown to give growth preference to strains of environmental origin compared to strains of human origin. Isolated strains have a tendency to loose extrachromosomal genes encoding important pathogenicity factors during such selective enrichment. It is of the greatest importance to be able to identify the pathogenicity markers such as toxins and adhesins, and differentiate these strains from non-pathogenic strains originating from animals or the environment. This is a difficult task, but genetic probing and PCR technologies can be useful tools in making this type of diagnosis efficient, and provide information of epidemiological importance, specially with respect to routes for spreading pathogenic E. coli strains with food as a vector.