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Journal of Bacteriology, November 2003, p. 6712-6718, Vol. 185, No. 22
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.22.6712-6718.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Virulence Conversion of Legionella pneumophila by Conjugal Transfer of Chromosomal DNA

Hiroshi Miyamoto,^1* Shin-ichi Yoshida,² Hatsumi Taniguchi,¹ and Howard A. Shuman³

Department of Microbiology, University of Occupational and Environmental Health, Yahatanishi-ku, Kitakyushu 807-8555,¹ Department of Bacteriology, Graduate School of Medical Sciences, Kyushu University, Higashi-ku, Fukuoka 812-8582, Japan,² Department of Microbiology, College of Physicians and Surgeons, Columbia University, New York, New York 10032³

Received 22 January 2003/ Accepted 19 August 2003

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 x 10⁹ CFU) and Chicago-2S (approximately 8.9 x 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.

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* Corresponding author. Mailing address: Department of Microbiology, University of Occupational and Environmental Health, 1-1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807-8555, Japan. Phone: 81-93-691-7242. Fax: 81-93-602-4799. E-mail: miyamoto{at}med.uoeh-u.ac.jp.

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Journal of Bacteriology, November 2003, p. 6712-6718, Vol. 185, No. 22
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.22.6712-6718.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

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