In recent years, knowledge about virulence traits and evolution of ExPEC has accumulated. ExPEC strains cause frequent infections in man and animals. The corresponding isolates are distinct from normal commensal and intestinal pathogenic E. coli isolates in that they typically derive from different phylogenetic groups and lack distinctive virulence factors characteristic of extraintestinal types of infection. Instead, they exhibit considerable genome diversity and possess a broad range of virulence factors including toxins, siderophores, adhesins, lipopo-lysaccharides, polysaccharide capsules, proteases, and invasins which are frequently encoded on islands and other mobile DNA elements . Suppression subtractive hybridization analysis of avian ExPEC isolates further underlined the marked genome plasticity among human and avian ExPEC isolates. Furthermore, it turned out that even among strains causing the same disease, different alternative virulence factors can be involved in a "mix and match" combinatorial fashion at each step of the infection [81, 82]. It has been shown that a considerable fraction of genetic information of ExPEC, which has so far been considered as virulence-associated, is also present in many commensal, probiotic E.coli isolates [61, 83]. Thus, many of these features can be considered rather as contributing to fitness (e.g., iron uptake systems, bacteriocins, proteases, fimbriae, and other adhesins), thereby generally increasing adaptability, competitiveness, and the ability to colonize the human body efficiently, than as typical virulence factors directly involved in infection. Whether a commensal E. coli will develop into a pathogen depends not only on the acquisition of fitness-conferring genetic information enabling successful colonization of the host, but also requires the presence of functional genes directly contributing to pathogenesis. This highlights the thin line between "virulence" and "fitness" or "colonization" factors and questions the definition of several "ExPEC virulence factors."
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