**4.3 Invasiveness**

*The Universe of Escherichia coli*

**4.1 Localized adherence**

**4. EPEC virulence factors and genetics**

**4.2 Attaching and effacing (A/E) lesion**

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-bacte-

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].

The hallmark of EPEC infection is the ability of the organism to attach intimately to epithelial cells and efface microvilli (**Figure 1C**). This effect was first described by Staley et al. [41], although the term attaching and effacing (A/E)

*EPEC adhesion to epithelial cells: (A) localized adherence pattern (LA) of typical EPEC on HeLa cells; (B) localized adherence-like (LAL) pattern of atypical EPEC on HeLa cells; (C) attaching and effacing (A/E) of enterocytes by EPEC; and (D) small bowel biopsy of infant infected with typical EPEC O111ab:H2 [36].*

rium adherence, resulting in formation of compact microcolonies [38].

**54**

**Figure 1.**

Intracellular typical EPEC have been observed both in tissue culture and in small intestinal biopsies from an EPEC-infected infant (**Figure 1D**) [36]. Fletcher et al. [42] and Scaletsky et al. [43] have reported that EPEC O111:NM strains contain plasmid sequences that confer invasiveness upon *E. coli* K12 strains. However, despite their invasive potential in vitro, most EPEC are considered as noninvasive pathogens [44].

### **4.4 Biofilm formation**

Typical EPEC have the ability to form biofilms on abiotic surfaces under static conditions, or on a flow through continuous culture system, and a model of EPEC biofilm formation has been proposed [45]. Biofilm formation requires adhesive structures as type 1 pili, antigen 43, BFP, and the EspA filament (see below) as participants in bacterial aggregation during biofilm formation on abiotic surfaces [45]. Atypical EPEC strains have also been shown to adhere to abiotic surfaces (polystyrene and glass) [46, 47]. The non-fimbrial adhesin curli and the T1P were shown to mediate binding to these surfaces in some atypical EPEC at different temperatures [48, 49].
