Association with HeLa cells by Rhodococcus equi with and without the virulence plasmid

HeLa cell association was examined in eight Rhodococcus equi isolates containing 80-85 kb plasmids and their plasmid negative derivatives, and in two other plasmid negative R. equi strains. Seven of the eight plasmid positive strains possessed an 85 kb plasmid and produced the virulence-associated protein (VapA) detected by monoclonal antibody staining in immunoblots; one of the eight had an 80 kb plasmid but did not produce VapA. Curing of the plasmids by repetitive subcultures at 42 degrees C in broth was confirmed by colony and DNA dot blot hybridization with a 7.5 kb BamHI-HindIII plasmid fragment probe, by attempted isolation of plasmid DNA, and by demonstration of lack of VapA. Most strains associated well with HeLa cells and no relationship was found with plasmid status and possession of VapA. Association with HeLa cells was significantly greater for strains with a dry colony type than for those with a mucoid colony, a result which correlated with hydrophobicity of the colonies. HeLa cell association does not correlate with the presence of the virulence plasmid in R. equi.