Rhodococcus equi plasmids: isolation and partial characterization.

Fifty-four strains of Rhodococcus equi from different clinical sources (mainly horses and pigs) were examined for their plasmid content by two screening methods. Plasmids were detected in 49 of 54 strains. A plasmid of approximately 80 kb was isolated from 21 of 22 isolates from horses and 20 of 28 isolates from pigs, and a 105-kb plasmid was isolated from 7 of 28 isolates from pigs. The 80-kb plasmid was significantly associated with strains of equine rather than porcine origin, and the 105-kb plasmid was significantly associated with strains of porcine origin. The type strain, ATCC 6939, consistently failed to yield a plasmid. Restriction enzyme analysis of purified plasmid DNA confirmed the relatedness of the 80-kb plasmids isolated from strains of equine and porcine origin. More differences between the restriction patterns of plasmids from strains isolated from horses and from pigs than among strains from either species were observed. Restriction enzyme analysis also showed relatedness of the 105-kb plasmid to the 80-kb plasmid. Three strains shown by others to be virulent in horses or mice possessed the 80-kb plasmid, whereas three other strains not virulent for horses or mice lacked the plasmid, although one had the 105-kb plasmid. There was a significant but not perfect association between the presence of the 80-kb plasmid and production of a diffuse 17.5-kDa thermoregulated, virulence-associated protein. Further study is needed to determine whether this plasmid is associated with virulence in R. equi.