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Conjugal transfer of a virulence plasmid in the opportunistic intracellular actinomycete Rhodococcus equi

V N Tripathi1, W C Harding, J M Willingham-Lane

  • 1Department of Infectious Diseases, College of Veterinary Medicine, University of Georgia, Athens, Georgia, USA.

Journal of Bacteriology
|October 9, 2012
PubMed

Insights

The virulence plasmid of Rhodococcus equi, crucial for causing pneumonia in foals and immunocompromised individuals, can be transferred between bacteria. This plasmid transfer, likely via conjugation, reestablishes virulence in recipient strains.

Area of Science:

  • Microbiology
  • Bacterial Genetics
  • Infectious Diseases

Background:

  • Rhodococcus equi is a soilborne bacterium causing severe pneumonia in foals and immunocompromised individuals.
  • A large virulence plasmid (~81-kb) is essential for R. equi's intramacrophage growth and virulence.

Purpose of the Study:

  • To investigate the transferability of the R. equi virulence plasmid.
  • To determine the mechanism of plasmid transfer and its impact on virulence.

Main Methods:

  • Conjugation experiments were performed between plasmid-containing (donor) and plasmid-free (recipient) R. equi strains.
  • Mutagenesis of the putative relaxase-encoding gene (traA) was conducted to assess its role in plasmid transfer.
  • Plasmid transmission was tested in other Rhodococcus species and related organisms.

Main Results:

  • The virulence plasmid was transferred at high frequency between R. equi strains via cell-to-cell contact, suggesting conjugation.
  • Deletion of the traA gene abolished plasmid transfer, while reversion restored it.
  • Plasmid transmission reestablished the capacity for intracellular growth in recipient strains.

Conclusions:

  • This study demonstrates the first instance of virulence plasmid transfer in R. equi.
  • Conjugation is the primary mechanism for virulence plasmid dissemination in R. equi.
  • The findings establish a mechanism for the spread of virulence factors among soilborne bacteria.

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