Release of 51Cr-Endotoxin from Bacteria as an Assay of Serum Bactericidal Activity

In order to facilitate the study of normal serum on Gram-negative bacteria we have developed a bactericidal assay based on the release of 51Cr from labeled bacteria. Release of radioactivity was shown to be mediated by complement and paralleled the bactericidal action of sera measured by change in colony-forming units. Serum from the C6 deficient rabbits and serum that had been treated to inactivate complement did not release 51Cr. The rate of release of 51 Cr was decreased by absorption of serum with Bentonite but the maximum percentage released was not decreased. 51Cr was not released from serum resistant bacteria. The 51Cr appears to be associated with the lipopolysaccharide fraction of the bacteria. Since this structure is the target for the bactericidal reaction, the release of 51Cr is a direct measure of the effect of complement on the bacteria. Under most conditions this reaction leads to cell death so that the release of 51Cr can be used as an assay of bactericidal activity. It was shown that the assay was feasible with Escherichia coli, Salmonella and Hemophilus influenza B.