Virulent Rhodococcus equi showing 15- to 17-kDa antigens, which is virulent in mice, was found to harbor an 85-kbp plasmid, and the 15- to 17-kDa antigens were found to be associated with possession of the 85-kbp plasmid of R. equi (S. Takai, T. Sekizaki, T. Ozawa, T. Sugawara, Y. Watanabe, and S. Tsubaki, Infect. Immun. 59:4056-4060, 1991). The expression of these antigens was temperature regulated: when cells were grown at a low temperature (25 to 32 degrees C), they did not express them, whereas they expressed them in large amounts when the cells were grown at a higher temperature (34 to 41 degrees C). The antigens were expressed on the cell surface, as evidenced by their susceptibility to proteolysis by a trypsin and by the biotin-avidin protein-blotting technique.
