Conjugal Transfer of a Virulence Plasmid in the Opportunistic Intracellular Actinomycete Rhodococcus equi

• Published in: Journal of Bacteriology Vol. 194; no. 24; pp. 6790 - 6801
• Main Authors: Tripathi, V. N, Harding, W. C, Willingham-Lane, J. M, Hondalus, M. K
• Format: Journal Article
• Language: English
• Published: Washington, DC American Society for Microbiology 01.12.2012
• Subjects: Actinomycetales Infections - microbiology; Actinomycetales Infections - pathology; Actinomycetales Infections - veterinary; Animals; Bacterial infections; Bacterial Proteins - genetics; Bacteriology; Biological and medical sciences; Conjugation, Genetic; deoxyribonucleases; Deoxyribonucleases - metabolism; DNA, Bacterial - genetics; foals; Fundamental and applied biological sciences. Psychology; Gene Transfer, Horizontal; genes; Genetics; Gram-positive bacteria; Horse Diseases - microbiology; Horses; humans; macrophages; Macrophages - microbiology; mice; Microbiology; Miscellaneous; Mutation; Plasmids; Plasmids - genetics; pneumonia; Rhodococcus equi; Rhodococcus equi - genetics; Rhodococcus equi - pathogenicity; Sequence Analysis, DNA; soil bacteria; Virology; virulence; Virulence; Bacteria; Actinomycetes; Conjugation; Intracellular; Rhodococcus equi; Veterinary; Genetic transfer
• ISSN: 0021-9193; 1098-5530; 1067-8832
• DOI: 10.1128/JB.01210-12

Rhodococcus equi is a facultative intracellular, Gram-positive, soilborne actinomycete which can cause severe pyogranulomatous pneumonia with abscessation in young horses (foals) and in immunocompromised people, such as persons with AIDS. All strains of R. equi isolated from foals and approximately a third isolated from humans contain a large, ∼81-kb plasmid which is essential for the intramacrophage growth of the organism and for virulence in foals and murine in vivo model systems. We found that the entire virulence plasmid could be transferred from plasmid-containing strains of R. equi (donor) to plasmid-free R. equi strains (recipient) at a high frequency and that plasmid transmission reestablished the capacity for intracellular growth in macrophages. Plasmid transfer required living cells and cell-to-cell contact and was unaffected by the presence of DNase, factors pointing to conjugation as the major means of genetic transfer. Deletion of a putative relaxase-encoding gene, traA, located in the proposed conjugative region of the plasmid, abolished plasmid transfer. Reversion of the traA mutation restored plasmid transmissibility. Finally, plasmid transmission to other Rhodococcus species and some additional related organisms was demonstrated. This is the first study showing a virulence plasmid transfer in R. equi, and it establishes a mechanism by which the virulence plasmid can move among bacteria in the soil.