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Enhanced phagocytic response of macrophages to bacteria by physical impact caused by bacterial motility or centrifugation

The mechanism of enhanced phagocytic and chemiluminescent responses of macrophages caused by bacterial motility (T. Tomita, E. Blumenstock, and S. Kanegasaki, Infect. Immun. 32:1242, 1981) was studied. Both responses increased up to a certain level with an increased number of motile bacteria, such as Salmonella typhimurium, Escherichia coli, or Pseudomonas aeruginosa, added. In contrast, only a slight increase was observed with the motility (mot) mutants of these bacteria, even when 4,000 bacteria per single macrophage were added. If nonmotile bacteria were centrifuged together with a monolayer culture of macrophages, the number of bacteria ingested per macrophage increased dramatically. This phenomenon was not observed in the presence of cytochalasin B or at a low temperature, and about half of the associated bacteria were killed within 30 min of prolonged incubation, indicating that the bacteria were not simply embedded on the macrophage surface. An observed biphasic increase of ingestion with an increase in centrifugal force suggested the existence of a threshold velocity for efficient phagocytosis. The minimum centrifugal force required for maximal response was determined under the conditions in which equalized collision frequency between bacteria and macrophages was maintained when different centrifugal forces were employed. From the value obtained (5 x g), the required rate of movement was calculated as approximately 2.5 microns/s, supposing that the bacterium is spherical and has a 1-micron radius. This value is much lower than the velocity of movement of motile bacteria (20 to 50 microns/s). The results indicate that physical impact caused by bacterial motility is enough to induce a high response of macrophages.