Abstract
Thirteen strains of Rhodococcus equi from 12 independent clinical cases were analyzed for the presence of virulence plasmid DNA. Of the clinical isolates except strain JRA-11, 10 contained an 85-kb virulence plasmid and the remaining 2 contained a 90-kb virulence plasmid. The twelve isolates expressed 15- to 17-kDa antigens and were virulent in mice. Strain JRA-11 contained no plasmid and was avirulent in mice. These strains were examined by Southern analysis with pREAT 701 plasmid probes, and their plasmid DNAs were hybridized with the probes. These results revealed that natural infections in foals could be principally caused by virulent R. equi, but not by avirulent organisms.
