Escherichia coli strains of nonenteropathogenic serogroups carrying eae but lacking the enteropathogenic E. coli adherence factor plasmid and Shiga toxin DNA probe sequences were isolated from patients (children, adults, and AIDS patients) with and without diarrhea in Brazil. Although diverse in phenotype and genotype, some strains are potentially diarrheagenic.

Escherichia coli strains of serotype O51:H40 were studied with regard to the presence of several virulence properties and their genetic diversity and enteropathogenicity in rabbit ileal loops. This serotype encompasses potential enteropathogenic strains mostly classified as being atypical enteropathogenic E. coli (EPEC) strains, which are genetically closer to enterohemorrhagic E. coli than to typical EPEC strains.Enterohemorrhagic Escherichia coli (EHEC) and enteropathogenic E. coli (EPEC) comprise two of the six diarrheagenic E. coli pathotypes (16). While EHEC secretes Shiga toxin(s), EPEC adheres to HeLa/HEp-2 cells, forming compact clusters that distinguish the localized pattern of adherence (LA) (16). The LA is mediated by the bundle-forming pilus encoded by the EPEC adherence factor plasmid (pEAF) (16).EHEC and EPEC share the ability to promote attachingeffacing (AE) lesions in intestinal cells (11) characterized by intimate bacterial adherence, localized microvillus effacement, and accumulation of polymerized actin and other cytoskeleton elements, resulting in pedestal-like structures underneath adherent bacteria (11). The AE lesion-associated genes reside in the locus of enterocyte effacement (LEE) (20), a pathogenicity island encoding a type III secretion system, regulators, chaperones, and effectors that interfere with diverse cell signaling processes (revised in reference 11). The LEE also encodes the outer membrane adhesive protein intimin (15) and its translocated receptor, Tir (18).EPEC is currently subgrouped into typical EPEC (tEPEC) and atypical EPEC (aEPEC), where pEAF is present only in the former group (17, 34). Most tEPEC and aEPEC strains belong to defined (traditional) EPEC serogroups (34). Nevertheless, we previously identified and characterized several E. coli strains of non-EPEC serogroups isolated in three cities in Brazil carrying the intimin-encoding gene (eae) and lacking Shiga toxin-encoding genes (12). That study emphasized the large diversity of such strains but allowed no identification of the virulence potential of individual serotypes. Moreover, as both diarrheic and nondiarrheic patients may carry aEPEC strains (34,36), it is important to evaluate the virulence potential of selected strains to identify those strains that are truly enteropathogenic.In this study, we examined all 10 strains of serotype O51: H40, the most frequently found serotype in our previous study (12), and an additional uncharacterized O51:H40 strain (7) for novel E. coli virulence properties, their enteropathogenic potentials in vivo, genetic diversity, and genetic relatedness to tEPEC and EHEC. These results were compiled with those of our previous study (12) to attain an overview of the virulence potential of serotype O51:H40. Table 1 presents the origins and the clinical and microbiological data of the patients carrying the 11 O51:H40 strains studied (7,12,13).