DNA release as a direct measure of microbial killing. I. Serum bactericidal activity.

A new method for quantification of microbial killing is presented, based upon the assumption that release of DNA from an organism can be taken as direct evidence of cell death. The assay was applied to measurement of serum bactericidal activity. A "serum-sensitive" Escherichia coli, whose DNA was pre-labeled with [14C] thymidine, released 95 to 100% of its radioactivity upon exposure to human serum for 120 min. This was accompanied by a fall in viability with less than 0.1% of the bacteria surviving. Heating serum to 56 degrees C for 30 min completely abolished both DNA release and killing. Normal serum did not release DNA from a "serum-resistant". Salmonella typhimurium whereas exposure to ampicillin caused both significant killing and DNA release. This assay is highly specific, sensitive, and rapid.