Virulence properties of atypical EPEC strains.

Virulence properties of 31 atypical enteropathogenic Escherichia coli (EPEC) strains isolated from cases of diarrhoea were examined. All except two strains adhered to HEp-2 cells in a localised adherence-like (LAL) pattern. With the exception of two strains, all were fluorescent actin staining (FAS) positive. Gentamicin HEp-2 invasion assay studies showed that all strains were invasive. Transmission electron microscopy of infected HEp-2 cells showed the characteristic attaching and effacing lesion and invasion of the cultured cells. Of the nine strains that hybridised with a DNA probe for alpha-haemolysin, five were haemolytic within 3 h of incubation, while the remaining strains were haemolytic only after incubation for 24 h. Three strains produced enterohaemolysin on blood agar. None of the 31 strains of E. coli induced fluid accumulation in the rabbit intestinal loop assay or displayed cytotoxic effects in HeLa and Vero cells. All the strains belonging to serotypes O26:H11, O26:H- and 0119:H2 expressed intimin beta, whereas all the strains from serotype O55:H7 expressed intimin gamma. The strains belonging to serogroup O111 expressed a non-typable intimin. The participation of intimin in LAL was supported by adhesion inhibition experiments in which antibodies to intimin significantly reduced the level of LAL.