Fine structure of A and M antigens from Brucella biovars.

Brucella A and M epitopes were found on single O-polysaccharide chains of all biotype strains of this species. Lipopolysaccharides from the type and reference strains of five of the six Brucella species, B. abortus, B. melitensis, B. suis, B. canis, and B. neotomae, were extracted and purified. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in conjunction with silver staining and immunoblotting developed by monoclonal antibodies, showed bands characteristic of A, M, or mixed A and M antigens. The A antigen previously described as an exclusively alpha 1,2-linked homopolymer of 4,6-dideoxy-4-formamido-D-mannopyranose was shown by 1H and 13C nuclear magnetic resonance spectroscopy to possess a fine structure consistent with the low-frequency occurrence of alpha 1, 3-linked 4,6-dideoxy-4-formamido-D-mannopyranose residues. This feature was previously attributed only to the M antigen, which is also a homopolymer of the same sugar. B. melitensis biotype 3 and B. suis biotype 4 lipopolysaccharides showed characteristics of mixed A and M antigens. Immunoabsorption of these O polysaccharides on a column of immobilized A-antigen-specific monoclonal antibody enriched polymer chains with A-antigen characteristics but did not eliminate M epitopes. Composite A- and M-antigen characteristics resulted from O polysaccharides in which the frequency of alpha 1,3 linkages, and hence, M-antigen characteristics, varied. All biotypes assigned as A+ M- expressed one or two alpha 1,3-linked residues per polysaccharide O chain. M antigens (M+ A-) also possessed a unique M epitope as well as a tetrasaccharide determinant common to A-antigen structures. B. canis and B. abortus 45/20, both rough strains, expressed low-molecular-weight A antigen.