Inhibition of immune hemolysis: serological assay for the heat-labile enterotoxin of Excherichia coli.

Sheep erythrocytes sensitized by incubation with the heat-labile enterotoxin (LT) of Escherichia coli are hemolyzed in the presence of anti-LT antiserum and complement. The Microtiter (Cooke Laboratory Products) technique was used to titrate anti-LT antibody in serum by this immune hemolysis reaction. Immune hemolysis was inhibited by preexposure of the anti-LT antiserum to soluble LT before addition of the LT-sensitized sheep cells. E. coli mini-extract preparations were obtained by the polymyxin release technique and assayed for LT by the lysis inhibition test (LIT) and by the adrenal cell assay. All 75 adrenal cell-positive E. coli isolates were positive in the LIT assay. Eight of 318 adrenal cell-negative isolates tested were positive in the LIT assay, possibly indicating the presence of biologically inactive toxin.