59 PATTERNS OF HEp-2 CELL ADHERENCE OF DIARRHOEAGENIC ESCHERICHIA COLI

Abstract

In 1979 Cravioto et al. showed that most enteropathogenic E. coli (EPEC) strains adhered to cultured HEp-2 cells whereas non-EPEC strains rarely adhered. Two patterns of adherence were described. Most EPEC adhered in localised microcolonies (LA) although a few displayed a diffuse pattern of adherence (DA). A new pattern of adhesion in which bacteria assume a characteristic ‘aggregative’ pattern (AA) both on the HEp-2 cell surface and between cells has recently been described in diarrhoeagenic E. coli of non-classical EPEC serotypes. To study mechanisms of adhesion we examined by electron microscopy the attachment to HEp-2 cells of strains exhibiting LA, DA and AA. LA was characterised by bacteria intimately attached to cuplike projections of the HEp-2 cell surface with associated disruption of the apical cell cytoskeleton and loss of cell surface microvilli; attachment closely resembling EPEC adhesion to intestinal mucosa. Strains exhibiting DA and AA did not show intimate attachment nor did they cause any alteration of cell surface architecture. A distinct halo surrounding bacteria and separating them from the HEp-2 cell surface was characteristic of AA strains. Specific staining showed DA to be capsule-mediated whereas AA was correlated with the production of specific rod-like fimbriae. These ultrastructural observations show that LA, DA and AA reflect different mechanisms of bacterial adhesion to HEp-2 cells and support preliminary evidence that AA is representative of a new class of diarrhoeagenic E. coli. Based on adherence properties, it would also appear that DA strains of classical EPEC serotypes may not be ‘true’ EPEC.