Albumin bound to the surface of M protein-positive streptococci increased their phagocytosis by human polymorphonuclear leukocytes in the absence of complement and bactericidal antibodies.

Using a phagocytosis assay (Leijh 1980, Infect. Immun. 30, 421), determination of chemiluminescence, and transmission electron microscopy, the influence of the binding of albumin to M protein-positive group A streptococci on their phagocytosis by human polymorphonuclear leukocytes (PMNL) was investigated. Coating of streptococci with albumin in the absence of other serum components enhanced phagocytosis but not killing of the bacteria. Complement had no additional effect on engulfment. Fibrinogen reduced the enhancing effect of albumin. Albumin had no additional effect on the high phagocytosis rate of an M-protein-negative strain. It could be demonstrated that human PMNL bound human serum albumin-gold conjugate. The results are discussed with regard to the role of plasma proteins in the phagocytosis of streptococci under in vivo-conditions.