Capsule Formation by Members of the Brucella Group

Twenty-seven strains of Brucella have been examined microscopically, culturally, and serologically for evidence of capsule formation. Five of these strains produced demonstrable capsules, either directly or after mouse passage, and 5 others showed serological and cultural characteristics similar to the encapsulated strains but did not produce demonstrable capsules. With few exceptions, the organisms showing characteristics of encapsulated organisms had been isolated from cases of Hodgkin's disease and those which showed characteristics of nonencapsulated organisms had been isolated from cases of brucellosis. The encapsulated strains were not agglutinable in ordinary antiserum, but were highly agglutinable in serums prepared against homologous organisms, and vice versa. The encapsulated strains served as poor antigens, in vivo. Encapsulated organisms can be denuded by cultivation in beef extract broth. These stripped variants are culturally indistinguishable from the encapsulated and nonencapsulated forms. They act as better antigens, in vivo, than encapsulated organisms and produce agglutinins for the encapsulated forms to high titer. Opsonic studies showed that injection of encapsulated organisms into rabbits was followed by low opsonocytophagic index increase, while the injection of nonencapsulated organisms produced a large increase in this index for homologous organisms. Since acceptance of this material for publication, an article by I. F. Huddle- son concerning the formation of capsules by Brucella has appeared in Proc. Exper. Biol. & Med., 43: 773, 1940.