Immunological specificity of heat-stable opsonins in immune and nonimmune sera and their interaction with non-encapsulated and encapsulated strains of Staphylococcus aureus.

The in vitro interactions between strains of Staphylococcus aureus and human polymorphonuclear leukocytes in the presence of immune and nonimmune sera were studied. Evidence indicated that phagocytosis of encapsulated strains occurred in the presence of specific homologous antiserum, whereas non-encapsulated strains were readily phagocytized by polymorphonuclear leukocytes in the presence of both normal and immune sera. Immunological analyses demonstrated that normal serum opsonins, which reacted with the non-encapsulated strains, were specifically directed against exposed mucopeptide moieties of the organisms. Sera rich in antimucopeptide antibodies were obtained from rabbits immunized with heterologous bacteria such as Escherichia coli and group A-variant streptococci and were shown to be effective in opsonizing the non-encapsulated strains of S. aureus. Fresh clinical isolates of S. aureus were noticeably more resistant to the opsonizing effects of the antimucopeptide antibodies. Results were presented which suggest that the surface structures of these clinical isolates are more diverse than laboratory-propagated strains and that these antiphagocytic surface antigens may be significant factors in masking the opsonizing effects of the mucopeptide opsonins which are present in most sera.