, 2005). Some STEC serotypes harbor a large pathogenicity island, termed the locus of enterocyte effacement (LEE), required for the formation of attaching and effacing (A/E) lesions (McDaniel & Kaper, 1997). The LEE carries the eae gene encoding the adhesin intimin, responsible for the intimate adherence of the bacteria to the enterocyte and linked to cytoskeleton rearrangements. However, the presence of the LEE does not seem to be essential for full virulence, as a wide number of LEE-negative STEC strains have been associated with sporadic cases and small outbreaks of HC and HUS (reviewed in Bettelheim, 2007). Of these LEE-negative organisms, O113:H21 is one of the most

commonly isolated STEC serotypes in many regions. Clinical isolates of LEE-negative STEC typically express Stx2 and also harbor a c. 90-kb plasmid encoding several virulence factors, including find more EHEC hemolysin (Newton et al., 2009). Some studies have elucidated different mechanisms by which these strains interact with the host intestinal

mucosa and induce disease: for example it has been demonstrated that STEC O113:H21 can invade tissue-cultured cells (Luck et al., 2006). Besides the knowledge that some STEC strains do not carry the LEE, very little is known about other strategies used by these pathogens to adhere to and colonize the host intestine. Analysis of the genome sequence of E. coli O157:H7 showed that several O157-specific islands contain putative fimbrial biosynthesis operons, including two regions encoding the long click here polar fimbriae (Lpf). The

Lpf loci are related at the genetic and protein level to Lpf of Salmonella enterica Thalidomide serovar Typhimurium and the latter have been shown to facilitate attachment of the bacteria to intestinal Peyer’s patches (Bäumler et al., 1996). In E. coli O157:H7, expression of the lpf operon 1 (lpf1) in an E. coli K-12 strain was linked to increased adherence to tissue-cultured cells and has been associated with the appearance of long fimbriae (Torres et al., 2002, 2004). The lpf2 operon has also been associated with adherence to epithelial cells and its expression in some pathogenic E. coli strains is believed to be important for the development of severe diarrhea (Doughty et al., 2002; Osek et al., 2003). Escherichia coli O157:H7 strains harboring mutations in one or both of the lpf loci (named lpf1 and lpf2 operons) have diminished intestinal colonization abilities and persistence in several animal models of infection (Jordan et al., 2004; Newton et al., 2004; Torres et al., 2007). Further, these lpf mutant strains also displayed an altered human intestinal tissue tropism (Fitzhenry et al., 2006). Cumulative evidence indicates that homologues of the lpf genes are found in other pathogenic E. coli, Salmonella, Shigella and even in some commensal E.