The unsuitability of the uridine incorporation assay for the measurement of phagocytosis of Escherichia coli.

The uridine incorporation technique for assaying phagocytosis is based on the fact that polymorphonuclear leucocytes are impermeable to labelled uridine, and therefore ingested bacteria inside phagocytic vacuoles will be unable to take it up. Extracellular bacteria, including those adherent to the phagocytic cell surface, can do so however. Differences in uptake between bacteria alone and in the presence of phagocytic cells can be used to measure ingestion. The present paper describes the application of this technique to Escherichia coli O-86 as the test organism. It appears that with this test species, the method is unsuccessful, because exposure of the non-ingested bacteria to some soluble product of the triggered polymorphonuclear leucocytes causes a large increase in their uridine uptake rates, over that of the control bacteria. The nature of the product responsible is unknown. It is unconnected with change in the pH of the medium, is heat stable, and is only produced by polymorphonuclear leucocytes which are actively phagocytosing. It may be that a release of phagolysosome contents is responsible.