Turbidimetric method for quantifying serum inhibition of Limulus amoebocyte lysate

This study describes a method to quantify the inhibition of lipopolysaccharide (LPS) activity by serum with a turbidimetric Limulus amoebocyte lysate assay. Assays were performed in multiwell microplates, and turbidity was measured as the optical density at 380 nm with a microplate spectrophotometer. LPS potency was measured as the 50% maximal Limulus amoebocyte response (LR50) of LPS diluted with saline. By comparing LR50s in saline, LPSs from various species of bacteria were standardized against the U.S. Reference Standard Endotoxin, lot EC-5. The potency of Escherichia coli O113 and O18 and Serratia marcescens LPSs was found to be equal to that of the reference standard EC-5, whereas LPSs from two salmonella species were half as potent. The least potent LPSs tested, obtained from Klebsiella pneumoniae and E. coli rough mutant J5, were 5- and 10-fold less potent, respectively, than EC-5. As a measure of inhibition, the LR50 of LPS in serum was compared to the LR50 of LPS in saline. Serum inhibited the potency of LPS 103- to 6,400-fold compared with saline. A positive correlation was found between standardized potency in saline and serum inhibition of the various LPSs tested. Thus, LPSs from E. coli O113, O18, and EC-5 and S. marcescens, which exhibited the highest potency in saline, were inhibited the most by serum. Likewise, E. coli J5 and K. pneumoniae LPSs, which were the least potent tested, were the least inhibited. The degree of inhibition of all types of LPS tested increased with increasing serum concentration.