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Paris and Lens but none in strain Philadelphia 1. Strain Paris encodes a 132-kbp plasmid and strain Lens a 60-kbp plasmid. These plasmids may play a role either in adaptation to the environment or in virulence, as both encode putative virulence factors, mobile genetic elements, and antibiotic resistance genes. In addition, both plasmids encode a paralogue of CsrA - a protein known to act as a repressor of transmission traits of L. pneumophila and as an activator of replication (Molofsky and Swanson 2003), suggesting that these plasmids may code proteins implicated in virulence. Furthermore, some of the genes present on the plasmid identified in strain Paris have previously been identified on a plasmid of the same size in an L. longbeachae isolate that was shown to be implicated in virulence of this species (Doyle and Heuzenroeder 2002). The role of specific plasmids in virulence of Legionella has not been clearly demonstrated, but a correlation between virulence in a mouse model and the presence of plasmids (Bezanson et al. 1994) and a higher persistence in the environment of strains containing plasmids (Brown et al. 1982) has been reported. Transfer of plasmids from a L. pneumophila donor to another L. pneumophila isolate or another Legionella species may be mediated by the dot/icm type IV secretion system (Vogel et al. 1998). The identification, characterization, and analysis of the functional role of Legionella plasmids might add to the understanding of versatility, adaptation, and virulence mechanisms of this pathogen.

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