The properdin system is responsible for the bactericidal action of normal human serum against a variety of microorganisms. The present work shows that the removal of properdin from serum also removes bactericidal activity. Addition of properdin to properdin-deficient serum restores bactericidal activity. A quantitative relationship exists between the final properdin concentration and bactericidal activity against sensitive organisms. The possibilities of a bactericidal assay for properdin are discussed.

It is demonstrated that, in addition to properdin, the four components of complement (present in RP_b) are necessary for the destruction of properdinsensitive bacteria. If any component is missing, bactericidal activity is lost; when the component is replaced, bactericidal activity is restored. Magnesium is also necessary for the bactericidal activity of the properdin system. Maximal bactericidal activity is obtained with magnesium concentrations similar to that of normal human serum (10^–3 to 10^–4 M). The bactericidal activity of the properdin system occurs only at temperatures above 15°.

Resistant strains have been encountered in species of bacteria sensitive to the properdin system. Resistance or sensitivity is a characteristic of the individual strain and not of the species.

The widespread occurrence of the properdin system in normal mammalian serum and the variety of bacteria destroyed by it suggest that the properdin system is a factor in natural resistance.