Phagocytic and serum killing of capsulate and non-capsulate Bacteroides fragilis

The relative susceptibilities of capsulate and non- capsulate variants of Bacteroides fragilis to serum and phagocytic killing were investigated. The capsule of B. fragilis did not confer resistance to serum killing. Phagocytic killing of non- capsulate B. fragilis occurred at bacterial concentrations of 1 X 10(6) and 1 X 10(7) cfu/ml. Capsulate B. fragilis organisms were also phagocytosed and killed at a concentration of 1 X 10(6) cfu/ml, but phagocytosis and killing were impaired at a concentration of 1 X 10(7) cfu/ml.