**4.1 Localized adherence**

Typical EPEC strains adhere to HeLa, HEp-2, and other cell lines and to organ cultures in vitro in a distinctive pattern of three-dimensional microcolonies so-called localized adherence (LA) pattern within 3 h of infection (**Figure 1A**) [8, 24]. A similar adherence pattern has been seen in tissue biopsies of EPEC-infected humans [37]. The LA phenotype is mediated by a type IV fimbriae bundle-forming pilus (BFP) associated with the EAF plasmid, which mediates bacterium-to-bacterium adherence, resulting in formation of compact microcolonies [38].

Atypical EPEC strains may display a variant LA pattern designated LA-like (LAL) pattern, which is characterized by the presence of loose compact microcolonies or clusters of bacteria in few cells observed in tests using prolonged incubation periods (6 h) (**Figure 1B**) [39, 40]. Interestingly, the LAL pattern is determined in prolonged assays (6 h) of bacteria-cell interaction [39]. LAL is the most common pattern seen among EPEC strains; however, some strains display alternate adherence phenotypes such as diffuse adherence (DA) and aggregative adherence (AA) [19].
