Characterization of the group antigen of Chlamydia trachomatis.

A lipid complement-fixing group antigen is shared by all chlamydiae including trachoma organisms. A water-soluble polysaccharide antigen was obtained by alkali saponification from partially purified water-insoluble lipid antigen preparations made from three trachoma strains. The extracted antigen failed to fix complement with antibody but was capable, as a hapten, of inhibiting complement fixation by prior reaction with the antibody. This antigen diffused readily in agarose, producing a reaction of identity with several trachoma strains. The antigen could also be absorbed onto untreated rabbit red blood cells for passive hemagglutination. It is concluded that polysaccharide antigen is responsible for the group reactivity of these organisms and can be obtained from the lipid antigen by alkali saponification.