Typing Haemophilus ducreyi by indirect immunofluorescence assay

Rabbits were inoculated with untreated whole cells of 16 strains of Haemophilus ducreyi. Homologous titres in an indirect immunofluorescence test ranged from 1/320 to 1/1280. No cross reactions, or very few, were observed between antisera to these strains and 22 other (possible taxonomically related) species, strains, and genera. A large diversity in types of antigen was observed among the strains of H ducreyi. Antiserum from two antigens (35000 and 3138), however, reacted with all 16 H ducreyi antigens tested. At least nine of the strains of H ducreyi examined could be identified as determining the type of antigen.