Phagocytosis of Actinomyces viscosus by murine macrophages: Susceptible and resistant strains

The phagocytosis of various strains of Actinomyces viscosus was studied using murine peritoneal macrophages and in vitro assay procedures. A. viscosus M100 was readily phagocytized when compared to controls using Staphylococcus epidermidis. The phagocytic uptake was enhanced by the addition of opsonic antibody and complement. The virulent (V) and avirulent (AV) strains of A. viscosus T14 were, however, refractory to phagocytosis. Indeed, the presence of specific antibody and complement had no enhancing effect on phagocytosis of these strains. Heat killed T14‐V and T14‐AV exhibited phagocytic indices similar to those of non‐opsonized S. epidermidis. The inhibitory effect of A. viscosus T14 is dependent upon bacterial viability and does not alter the viability of the cultured macrophages. Since both T14‐V and T14‐AV were capable of blocking phagocytosis, the response does not appear dependent on the ascribed virulence factors.