A Western blot (immunoblot) assay is presented for the diagnosis of
nocardiosis with a specific immunodominant 54-kilodalton (kDa)
antigen purified from a culture filtrate of Nocardia asteroides by immunoaffinity chromatography. The chromatography column was prepared with
immunoglobulin G obtained from sera from patients with
lepromatous leprosy. Unbound solutes consisted of specific, partially purified N. asteroides
antigens, primarily a 54-kDa band, accompanied by two others of 31 and 62 kDa. The Western blot technique was applied to detecting the immunologic response to nocardiae. Immunodetection was performed according to the
biotin-
avidin system, which greatly improved the detection of
antibodies even in immunosuppressed hosts. Each of 16 serum samples from immunosuppressed or immunocompetent patients infected with N. asteroides reacted with the 54-kDa band, and two reacted with the 31- and 62-kDa bands. Each of the serum specimens obtained from patients with
mycetoma caused by Nocardia brasiliensis or Rhodococcus rhodochrous reacted with the 54-kDa band. There was no reaction to either the 54- or the 31-kDa
antigen with all serum samples obtained from patients with
tuberculosis, except one, with all serum samples obtained from patients with
leprosy, or with all sera obtained from healthy controls. The 54-kDa
protein is a candidate to be used as a probe to study the humoral immunologic response to nocardiae.