Phagocytosis by the Cellular Slime Mold Polysphondylium Pallidum during Growth and Development

The phagocytic ability of amoebae of the cellular slime mold Polysphondylium pallidum, grown in shaken suspension, was examined. An established quantitative assay of the uptake of polystyrene (PS) beads was shown to be valid for this organism. The kinetics of phagocytosis were determined, and estimates of the concentration of PS beads necessary to achieve half-maximal phagocytic velocity (K(p)), as well as the maximal velocity itself (V(p) (max)), were made. Comparison with previously published data on Acanthamoeba and guinea pig leukocytes suggested that the P. pallidum amoebae had the lowest K(p), while the leukocytes had the highest V(p) (max). Beads approximately 1 microm in diameter appeared to be the optimal size for ingestion. Simultaneously with phagocytosis, comparable numbers of beads accumulated at the cell surface; this accumulation did not occur when phagocytosis was inhibited. Phagocytosis was depressed by protein in the medium, by increased osmolarity, and by inhibitors of aerobic metabolism. Starvation-initiated development, leading to encystment, was shown to affect the capacity of the cells to phagocytize, mainly by progressively decreasing the time span over which the cells ingested particles at a constant initial rate.