Survival strategy of intracellular Rhodococcus equi : the role of virulence plasmid.

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At least three virulence levels of Rhodococcus equi have been identified : virulence, intermediate virulence, and avirulence. Virulent R. equi is characterized by the presence of virulence-associated 15-to 17-kDa antigens (VapA), virulence plasmid DNA of 85 to 90 kb, and suppurative pneumonia in foals (mouse LD_<50>=10^6). R. equi strains of intermediate virulence are identified by a virulence associated 20-kDa antigen (VapB) and virulence plasmid DNA of 79 to 100 kb and are found in the submaxillary lymph nodes of pigs (mouse LD_<50>=10^7). To analyze the difference of virulence levels in R. equi, the virulence plasmid of the R. equi strain A3 of intermediate virulence (pREA3) was sequenced and compared with the DNA sequence of the virulence plasmid (p33701). pREA3 was 79,288 by and the plasmid encoded 71 open reading frames (ORFs). Of the 71 ORFs, 52 were found to be almost identical to those in the virulence plasmid (p33701). pREA3 also has a pathogenicity island containing 5 new virulence-associated protein (vap) genes [vapI, vapJ (2 copies), vapK, and vapL] located upstream of vapB. There was extensive homology in the region except the pathogenicity island between p33701 and pREA3. These results suggest that the difference in the structure and function of the pathogenicity islands between p33701 and pREA3 might be responsible for the difference in virulence levels of R. equi.