Influence of the alternate complement pathway in opsonization of several bacterial species.

In normal human serum chelated with magnesium (10 mM) and ethylene glycotetraacetic acid (10 mM) (MgEGTA), the classical pathway of complement activation is blocked; however, the alternate pathway of complement activation is intact. Diplococcus pneumoniae, Staphylococcus albus, Streptococcus viridans, Streptococcus faecalis, and Serratia marcescens were opsonized in normal human serum containing 10 mM MgEGTA. In contrast, Staphylococcus aureus and Pseudomonas aeruginosa were poorly opsonized in human serum chelated with 10 mM MgEDTA. Therefore certain bacterial species appear to require the classical pathway of complement activation to be opsonized and other bacterial species utilize the alternate pathway of complement activation.