Immuno-electron microscopic study of antigenic surface components of Actinomyces naeslundii in human dental plaque

Rabbit antiserum to formalin-killed Actinomyces naeslundii I was used to investigate the ultrastructural location and distribution of the antigenic sites of A. naeslundii. Antigenic sites were identified by an indirect technique using goat anti-rabbit IgG coupled to horseradish peroxidase, also used to identify individual cells of A. naeslundii in a mixed bacterial population and in freshly isolated dental plaque. The bound antibody and associated reaction product were visible in ultrathin sections as an electron-dense amorphous material (100-150 nm thick) in juxtaposition to the bacterial cell wall. The location of the immuno-reactants suggested that the antigens are distributed superficially and evenly over the entire bacterial cell surface. Bridge-like extensions of the immuno-reactants connected adjacent cells, suggesting that a limited amount of antigenic material might extend out from the cell wall to provide structural continuity with similar material on adjacent cells.