Mobile Genetic Elements and Evolution of Pathogenic E coli

Mobilizable plasmids and bacteriophages represent key elements to enable bacteria to exchange genetic information by conjugation and transduction. During recent years, genomic research revealed that pathogenicity islands (PAIs) have also played a major role for the transformation of avirulent into virulent bacteria. Only limited knowledge exists about the origin of PAIs, but it has been speculated that they may derive from integrating plasmids or phages that have lost the genes required for replication and self-transfer in exchange for a more stable association and inheritance with the host chromosome [11]. Evidence to support this model was in the findings that PAIs often encode phage-like integration systems, as has already been discussed in detail. Additional analysis of PAI-specific DNA sequences revealed the presence of regions with similarity to phage- and/or plas-mid-related genes or sequences [42, 43], which is another indication that PAIs may have evolved from other mobile genetic elements. A relationship between PAIs and plasmids is also supported by the fact that island-encoded properties have been detected on plasmids in related species or a different isolate of the same species. Examples are aerobactin-encoding genes that are often part of plasmids that additionally encode colicin ColV. As the aer genes of Shigella island SHI-2 are associated with regions of sequence homology to ColV-specific genes, parts of SHI-2 may have evolved from an integrating plasmid [27, 44]. Alternatively, some PAIs may have been acquired by mechanisms related to conjugation. An increasing number of elements (termed "conjugative transposons," "constins," or "conjugative genomic islands") have been discovered in gram-negative bacteria including E. coli that are normally integrated in the chromosome, but can excise in a precise manner to be subsequently transferred to recipient cells by conjugation [45]. In gram-negative bacteria, some of these elements exhibit several features reminiscent of PAIs, including a site-specific recombinase and lack of autonomous replication. In contrast to PAIs, they carry genes required for mat-ing-pair formation and conjugative DNA metabolism which are related to plas-mid-encoded conjugation systems [46].

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