DNA release as a direct measure of microbial killing by phagocytes

A new assay for the precise measurement of microbial killing by leukocytes is presented. The method assumes that release of radioactively labeled DNA from the microbe is direct evidence of cell death. Human peripheral blood leukocytes incubated with [14C]thymidine-labeled Salmonella typhimurium released 32 to 59% of the radioactivity after 4 h and 63 to 75% after 18 h. Inactivated leukocytes released less than 5% of the radioactivity. None of the released radioactivity is retained within the leukocyte, and 60% remains precipitable with trichloroacetic acid. Leukocytes released substantial radioactivity from labeled Escherichia coli but only a slight amount from staphylococci. Mouse peritoneal macrophages were also shown to release radioactivity from Salmonella. The DNA release assay avoids the errors inherent in prior killing methods which measure viability by growth inhibition. It is rapid, reproducible, and highly specific.