Whereas substantial knowledge has been acquired on the genetic structure and diversity of GEIs in E. coli, relatively few data exist on comparative genomics of E. coli plasmids and bacteriophages. The complete genome sequences of four E. coli strains indicate that these strains differ considerably in number of proph-age-related sequences [30, 32-34]. It is well known that genes coding for different adhesins, resistance genes, and the heat-labile and the heat-stable enterotoxins of ETEC strains are localized on plasmids [64-67]. F17 fimbrial adhesin determinants have been discovered together with genes coding for the cytotoxic necrotiz-ing factor 2 on virulence plasmids of necrotoxigenic E. coli strains . Many
EPEC strains contain large EPEC adherence factor (EAF) plasmids which carry the bfp locus coding for bundle forming pilus and the per genes whose gene products are important for the regulation of LEE-encoded gene expression . The EAST1 toxin is frequently encoded on EAF plasmids . The presence of the so-called pO157 plasmid is characteristic of the majority of EHEC strains. This plas-mid carries the ehx genes coding for an enterohemolysin as well as for a catalase-peroxidase (katP) and potential adherence factors . Several other plasmids ranging in size from 2 kbp to 87 kbp have been described in O157:H7 strains, too. Their composition and importance for virulence of these strains is unclear. Most of the EAEC strains carry large plasmids which share a high degree of homology . The aggregative adherence fimbriae 1-encoding determinant has been detected on a 60-MDa plasmid which may also harbor the EAST1 toxin gene . EIEC strains share a large pInv plasmid (140 MDa) with S.flexneri. This plasmid carries the invasion-related genes which encode a type III secretion apparatus (mxi, spa) as well as secreted proteins (ipa) involved in the invasion phenotype and a toxin designated Shigella enterotoxin 2 . Other genes providing intestinal E. coli strains with advantageous properties, e.g., resistance to antibiotics, expression of colicins and siderophores, are plasmid-encoded as well. Colicin-encoding plasmids can also harbor siderophore- or other virulence-associated determinants [73-76]. The fact that gene blocks located on GEIs can also be present on plasmids indicates that a similar heterogeneity and diversity may exist among closely related plasmids as among functionally related islands in E. coli.
The different types of shigatoxins (Stx), the major virulence factor of enterohe-morrhagic E.coli (EHEC) strains, are usually encoded on temperate bacterio-phages . Another virulence-associated gene cluster frequently encoded on bac-teriophages in E.coli encodes the cytolethal distending toxin. Whereas the cdtIII determinant of ExPEC strain 1404 is plasmid-encoded , similar genes in EHEC strain 493/89 may have been acquired by bacteriophage transduction .
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