Phagocytic and bactericidal activity of human neutrophils against two isolates of Group B streptococci Type Ic of differing pathogenicity.

The phagocytic and bactericidal activities of normal adult human neutrophils against 2 strains of Group B streptococci Type Ic of differing pathogenicity were examined. Both isolates were phagocytosed by the neutrophils in the presence of normal and homologous immune serum. However, the highly pathogenicity streptococci were killed less readily in the presence of immune serum than were the streptococci of low pathogenicity in the presence of immune or normal serum. This difference in killing ability was not due to a defect in phagocytosis by the neutrophils, but to a defect in bactericidal activity. The highly pathogenic streptococci were not killed in the presence of normal serum, but were readily phagocytosed by the neutrophils, in which they accumulated and eventually caused their destruction. The streptococci of low pathogenicity, however, were killed equally as well in the absence of specific antibody as in its presence. It is suggested that an in vitro assessment of neutrophil function against streptococci of differing pathogenicity for mice may provide a useful method by which the pathogenicity of streptococci for man can be compared.