The Effect of Divalent Cation Depletion on Phagocytosis of Staphylococci **

In most experimental systems, phagocytosis of bacteria by polymorphonuclear leukocytes (PMN's) is interrupted by divalent cation chelation with Na2H2EDTA.1" This has led to the assumption that divalent cations are required for phagocytosis.' Phagocytic systems, in fact, measure the end result of a complex sequence of events which include interaction of the bacterium with plasma proteins of both antibody and non-antibody type (opsonization), attraction of the phagocytic cell (chemotaxis), presumed surface adherence of the bacterium to the PMN, and finally ingestion of the bacterium. The effects of divalent cation chelation on individual steps in this complex process can be studied in part by selecting experimental circumstances that make one particular step rate limiting. In the present studies the use of a very high bacteria-phagocyte ratio and bacteria opsonized in the presence of normal divalent cation concentration greatly favored the occurrence of phagocytosis. In this set of experimental circumstances it was possible to demonstrate that substantial ingestion of bacteria could subsequently take place in the virtual absence of divalent cations. These observations suggest that ingestion per se is not a divalent cation-dependent process.