Kinetics of phagocytosis and intracellular killing of Staphylococcus aureus and Escherichia coli by human monocytes.

The kinetic patterns of the phagocytosis and intracellular killing of Staphylococcus and Escherichia coli by monocytes were investigated separately to acquire more insight into the total process, i.e. from the ingestion to the death of the micro-organisms. Phagocytosis proved to be dependent on: (1) both the bacteria-to-monocyte ratio and the monocyte concentration; a concentration of at least 5 x 10(5) monocytes/ml proved necessary for the measurement of ingestion, whereas the rate of ingestion was found to be proportional to the number of extracellular bacteria until a maximum rate is reached, (2) the serum concentration in the incubation medium, which influenced both the rate of phagocytosis and the maximum number of bacteria taken up by one monocyte, and (3) the temperature, the highest rate of phagocytosis being reached at 37-41 degrees C. The intracellular killing proved to be dependent on: (1) the number of bacteria ingested; the rate of killing was proportional to the number of ingested bacteria until a maximum rate was reached; (2) the temperature, since a maximum rate of killing is only reached at 37-41 degrees C; at lower and higher temperatures the rate of killing is lower, in the latter case due to inactivation of extracellular stimuli. These separate data on the ingestion and killing processes made it possible to compute the theoretical numbers of extracellular, viable intracellular, and total intracellular bacteria for a model system consisting of 5 x 10(6) monocytes, 5 x 10(6) bacteria, and 10% serum. These calculated values are in agreement with the experimental data.