Comparison of enterotoxic activities of heat-stable enterotoxins from class 1 and class 2 Escherichia coli of swine origin.

Pig small intestine develops age-dependent resistance to some (class 2 strains) enterotoxigenic Escherichia coli while remaining susceptible to others (class 1 strains). This study tested the hypothesis that class 1 and class 2 strains produce different subtypes of heat-stable enterotoxin (ST). The dose-response curves of small intestine to crude ST preparations from a class 1 and a class 2 strain were compared in several species. In infant mice, the class 1 ST preparation was less active than the class 2 ST preparation, whereas in rabbits the preparations were equally potent. However, in 1-, 7-, and 14-week-old pigs, the class 1 ST preparation was more active than the class 2 preparation. At low doses, both preparations caused reduced absorption in pigs of all three age groups, and at high doses the class 1 preparation caused secretion in all three age groups. In contrast, at high doses the class 2 preparation caused secretion in 1-week-old pigs but only reduced absorption in older pigs. when class 1 and class 2 ST preparations were fractionated by methanol extraction, in both cases the mouse-negative, pig-positive activity was associated with the methanol-insoluble fraction and mouse-positive, pig-positive activity was associated with the methanol-soluble fraction. The results are consistent with a hypothesis that class 1 and class 2 strains of enterotoxigenic E. coli produce different subtypes of ST and that the response of pig intestine to ST varies with both age and toxin subtype.