[Different species of Legionella plasmids and virulence].

Plasmids with a molecular weight of 2.5 to 80 MD were shown to be present in a significant portion of different-type Legionella strains including high-virulence isolates L. pneumophila, serogroup 1, and L. bozemanii detected both in Russia and abroad in different sources. Plasmid-free derivative were obtained from the L. pneumophila strains each carrying only one different-size plasmid DNA. The variants had the same virulence as the original cultures for chicken embryos and guinea pigs, when the latter were infected in the bile sac or through intraperitoneal routes, respectively; their virulence was also similar to that of strains resistant to the normal serum of guinea pigs. Hence, the infection models used by us failed to show any action of plasmids on the virulence of Legionella.