Immunochemical characterization of the outer membrane complex of Bacteroides fragilis subspecies fragilis.

The outer membrane of gram-negative bacteria is immunologically the most important structure of the cell because of its accessibility to host defense mechanisms. By electron micrographic studies, it has been shown that Bacteroides fragilis has a cell envelope typical of gram-negative bacteria. The outer memberane of this envelope was found to consist of protein, two polysaccharides bound to lipid in various degrees, and loosely bound lipid. The protein component of the outer membrane had a distinct pattern when studied by polyacrylamide gel electrophoresis. Similar peptide band pattern were seen in all strains of Bacteroides fragilis subspecies fragilis studied, but this pattern was distinct from the pattern found in the outer membranes of other subspecies. The carbohydrates and the protein components were both associated with subspecies-specific immunoprecipitins. We propose that this antigenic complex represents the outer membrane of B. fragilis in its native state and that the immunologically important antigens of this organism are associated with this complex.