Rhodococcus equi (R.equi) has been determined to have a major adhesion factor encoded by a rpl pathogenicity island which enables host colonisation, wherein the rpl pathogenicity islandis absent from non-pathogenic Rhodococcus species. Further, the proteins (Rpl) encoded by the rpl pathogenicity islandhave been determined to be major immunodominant antigens. There is provided a novel diagnostic marker and vaccine candidate for R. equi in horses and other susceptible species.
