Phagocytosis of Staphylococcus aureus by human leukocytes: quantitation by a flow cytometric and a microbiological method.

Phagocytosis of killed, fluorochrome stained or live Staphylococcus aureus by human leukocytes was measured by flow cytometry (FCM) or a microbiological method, respectively. The results were compared to those obtained by simulation using a prey-predator model. In the presence of an initial bacteria-to-phagocyte ratio of 4:1 to 160:1, the percentage of phagocytosing leukocytes was independent of the bacteria and phagocyte concentration. The number of phagocytosed or killed bacteria per phagocyte increased with increasing bacteria and decreasing phagocyte concentration. One per cent pooled human serum was sufficient for maximum phagocytosis to occur, but killing slightly increased in the presence of 10% pooled human serum. With medium or low initial bacteria-to-phagocyte ratios phagocytosis and killing closely corresponded to the results obtained by the prey-predator model. Maximally each phagocyte was associated with 80 bacteria (measured by FCM), about 45 being phagocytosed (internalized) and 40 killed. The model seems suitable for the simulation of phagocytosis and killing of S. aureus by human leukocytes.