IDENTIFICATION OF VIRULENT RHODOCOCCUS-EQUI BY AMPLIFICATION OF GENE CODING FOR 15-KILODALTON TO 17-KILODALTON ANTIGENS

During a survey of the prevalence of virulent Rhodococcus equi at hors e-breeding farms by plasmid and protein profiles, cryptic plasmids of various sizes were found in 66 (3.8%) of 1,725 isolates from feces of horses and 129 (5.9%) of 2,200 isolates from soil, Twenty-two isolates , which contained cryptic plasmids of different sizes, were found by p lasmid profiles, and their protein profiles and mouse pathogenicities were examined. Of the 22 isolates, 7 were virulent R, equi, contained both virulence and cryptic plasmids, and expressed 15- to 17-kDa antig ens, The remaining 15 isolates were avirulent and did not express the antigens: 6 strains contained cryptic plasmids of two different sizes and 9 strains contained cryptic plasmids of various sizes. A PCR assay was developed for the rapid identification of virulence plasmids of R . equi, Oligonucleotide primers, derived from the sequence of a gene c oding for the 15- to 17-kDa virulence-associated antigens of R, equi, amplified a 564-bp product from all the tested isolates harboring a vi rulence plasmid, This PCR product hybridized with virulence plasmid DN A in the Southern hybridization assay. Virulence plasmid-cured derivat ives and all of the tested isolates harboring cryptic plasmids only we re negative. The PCR is a rapid, sensitive, and specific test for the identification of virulent R. equi from environmental isolates compare d with standard techniques, such as plasmid and protein profiles and t he mouse pathogenicity test, and is considered to be a useful tool for epidemiological studies.