Quantitative Studies on the Bactericidal Actions of Serum and Complement

Abstract

Summary: A rapid photometric assay method is presented for the titration of the bactericidal activity of antisera or of complement. Like the conventional plate count assay method it requires an initial reaction period, during which the organisms, antibody, and complement interact. In our method, the relative numbers of organisms surviving are then determined by their growth rate properties on subculture, the quantitation of the relative total growth being made with a photoelectric colorimeter. Linear plots are obtained (on normal-deviate or probit graph paper) between the percentage kill and the dosage (in logarithmic units). From this the 50% end point and the response slope may be determined. If the culture density is standardized, the standard deviation of the absolute titer is about 10%; without such control, only the titers relative to a reference serum may be determined with this precision. Several factors which influence the test are discussed.