In this study, data on phagocytosis of and its survival inside macrophages are presented. Mouse peritoneal macrophages were incubated in the presence of one of five different strains of capsular type 2: a virulent wild-type strain (1591), a non-capsulated non-virulent mutant strain (M2), a poorly capsulated non-virulent mutant strain (M42), a non-virulent capsulated strain (1330), and the wild-type reference (virulent) strain 5735. Opsonized or non-opsonized bacteria were incubated with macrophages and samples were obtained after 1 and 3 h incubation. Phagocytosis as well as live and dead intracellular organisms were determined by acridine orange and crystal violet staining. After 1 h incubation, non-opsonized virulent and non-virulent capsulated bacteria were poorly phagocytosed (by less than 7% of the macrophages), whereas the non-capsulated non-virulent mutant strain was highly phagocytosed (by more than 68% of the macrophages). The M42 mutant strain was more phagocytosed than the capsulated strains but less than the non-capsulated M2 mutant strain (35%). In contrast, a higher percentage of live bacteria was observed inside macrophages for the capsulated strains (1591 and S735) than for the non- or poorly capsulated mutant strains (M2 and M42). Opsonization of bacteria with rabbit serum or heat-inactivated rabbit serum significantly increased phagocytosis. For every opsonized strain, after 3 h incubation, the percentage of live bacteria within macrophages was considerably lower than the corresponding non-opsonized strains. In conclusion, the capsule of type 2 appears to act as an important anti-phagocytic factor. However, virulent capsulated non-opsonized strains can be phagocytosed by mouse peritoneal macrophages within which they appear to survive for at least 3 h. Serum factors other than complement increase not only phagocytosis but also intracellular killing of of both capsulated and non-capsulated strains.


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