Virulence Conversion of Legionella pneumophila by Conjugal Transfer of Chromosomal DNA

In this study, we examined whether virulence conversion occurs in Legionella pneumophila by conjugal transfer of chromosomal DNA. A virulent strain, K6, which has the genes for Km^r and LacZ⁺ transposed in the chromosome of strain Philadelphia-1, which belongs to serogroup 1, was used as one parent, and an avirulent strain, Chicago-2S, which is a spontaneous streptomycin-resistant derivative of strain Chicago-2 belonging to serogroup 6, was used as the other parent. Experiments in which K6 (approximately 2.6 × 10⁹ CFU) and Chicago-2S (approximately 8. 9 × 10⁹ CFU) were mated typically yielded 10³ Km^r Sm^r LacZ⁺ transconjugants. Thirty-two (about 2.8%) of 1,152 transconjugants belonging to serogroup 6 acquired the ability to grow intracellularly in Acanthamoeba castellanii and guinea pig macrophages. When guinea pigs were infected with sublethal doses of Legionella aerosols generated from one of these transconjugants (HM1011), they developed a severe pneumonia similar to that caused by donor strain K6. These results show that avirulent strain Chicago-2S changed into virulent strain HM1011 through conjugation with virulent strain K6. Furthermore, we showed that Legionella chromosomal virulence genes (icm-dot locus) were horizontally transferred by the conjugation system. The chromosomal conjugation system may play a role(s) in the evolution of L. pneumophila.