Comparison between enterotoxic activity and methanol solubility in heat-stable enterotoxins (STa and STb) from Escherichia coli of human, porcine and bovine origins.

We have investigated the enterotoxic activity of culture filtrates and their methanol extracted fractions from 10 ST (STa or STb) producing Escherichia coli strains from human, porcine and bovine origin, in the infant mouse test (IMT) as well as in the rabbit intestinal loop test (RILT). Unconcentrated culture filtrates and methanol-soluble fractions from the eight STa-producing strains were positive in the IMT while methanol-insoluble fractions obtained from these STa-positive strains, like methanol-soluble and -insoluble fractions from the two strains producing only STb, lacked activity in the IMT. Unconcentrated culture filtrates from all ST-producing strains were unable to cause fluid accumulation in the rabbit ligated intestinal loops after 6 h incubation. When this material, concentrated 5-fold, was tested again, the culture filtrates and methanol-soluble fractions from all STa-producing strains yielded strongly positive fluid accumulation in the RILT, whereas culture filtrates and their methanol extracted fractions from the strains producing only STb, like methanol-insoluble fractions from four STa-producing strains, caused slight fluid secretion in the rabbit intestinal loops.