THE EFFECT OF SERUM OPSONINS ON THE PHAGOCYTOSIS OF STAPHYLOCOCCUS AUREUS AND ZYMOSAN PARTICLES, MEASURED BY FLOW CYTOMETRY

Human leukocyte phagocytosis of S. aureus and zymosan particles was measured by flow cytometry (FCM). Killing of bacteria was measured by a standard microbiological method. When pooled human serum was heated to 56 degrees C for 30 min, the percentage of phagocytosing polymorphonuclear neutrophilic leukocytes (PMNLs), the rate of phagocytosis and killing of S. aureus by the whole leukocyte population were reduced to about 50% of the control values. The results indicate that the impaired phagocytosis and killing were due to the lack of attachment of bacteria opsonized with heat-stabile serum opsonins, mainly IgG, to 50% of the PMNLs. A prey-predator model was used to compare phagocytosis of S. aureus in heated and control serum. The rate of phagocytosis by those PMNLs that were active in heated serum was the same as that of all the leukocytes in control serum, suggesting that heat-labile serum opsonins did not affect the rate of phagocytosis by these PMNLs. PMNLs and monocytes phagocytosed zymosan particles in control serum, but in heated serum only a fraction of the leukocytes, corresponding to the fraction of monocytes phagocytosed. Thus, all PMNLs seem to be capable of phagocytosis by heat-labile serum opsonins. The combined use of S.aureus and zymosan particles may be of advantage in rapid screening of serum opsonin activities and phagocyte function in infectious and haematological disorders.