Direct serological assay for the heat-labile enterotoxin of Escherichia coli, using passive immune hemolysis

Sheep erythrocytes sensitized with the heat-labile enterotoxin (LT) of Escherichia coli exhibited passive immune hemolysis (PIH) when exposed to specific antitoxin and complement. Thus, PIH serves as the basis for an in vitro serological assay for LT that is sufficiently specific and sensitive to differentiate LT-positive and LT-negative E. coli isolates. The PIH assay for E. coli LT has been performed with the standard Microtiter system and also by a tube method employing the spectrophotometric determination of hemoglobin release. The spectrophotometric method enhances the sensitivity, accuracy, and objectivity of the PIH assay. The increased sensitivity of the spectrophotometric method also facilitates the identification of LT-positive cultures employing polymyxin "mini-extracts" of whole overnight (18 h) broth cultures of 2.0% Casamino Acid-0.6% yeast extract-salts medium rather than mini-extracts of cells derived from 3.5-h subcultures. Thus, large numbers of E. coli isolates can be individually tested for LT in less than 24 h after broth inoculation by a rapid in vitro assay which requires anti-LT serum as the only specific reagent.