The Inhibition of Surface Phagocytosis by the Capsular "slime Layer" of Pneumococcus Type Iii* by W. Barry

Surface phagocytosis has been shown to play an important rble in the mechanism of recovery in acute bacterial pneumonia (1-5). Operating in the absence of specific antibodies, this form of phagocytosis acts as an immediate defense reaction of the host and causes prompt destruction of fully encapsulated bacteria. Highly virulent strains of type I pneumococci, Friedl'ander's bacilli, group A beta hemolytic streptococci, and staphylococci have all been foundto be susceptible to surface phagocytosis. The phagocytability of type I I I pneumococcus by the surface mechanism has been subjected to special study for the following reasons. (1) This organism is one of the most virulent encountered in human disease. (2) I t possesses a particularly large capsule, the state of which determines its susceptibility to phagocytosis (6, 7). (3) Its capsule contains a polysaccharide with unique physicochemical properties (8). The present study indicates that type I I I pneumococcus is more resistant to surface phagocytosis than any organism thus far studied. Its exceptional resistance is related to the presence of an outer "slime layer" (9) which is prominent only on highly virulent strains of the organism. The slime layer, which can be visualized under the electron microscope, stains metachromatically with methylene blue and is demonstrable only on the surfaces of rapidly multi~ plying cells. When the slime layer is lost with the aging of the bacterial population, the organism is readily phagocyted by the surface mechanism.