Enteropathogenic serotypes of
Escherichia coli
which have been incriminated by epidemiological evidence as responsible for epidemics of acute diarrhea in infants are often found to be nontoxigenic when tested by conventional systems such as Y1-adrenal, Chinese hamster ovary, and suckling mouse assays. Twelve such strains, representing four different enteropathogenic serotypes, were examined for their capacity to elaborate toxic materials which alter water transport. Ultrafiltration fractions prepared to contain either a high-molecular-weight, heatlabile or a low-molecular-weight, heat-stable form of toxin from each strain were perfused through rat jejuna in graded concentrations ranging from 100 μg to 0.1 ng/ml. Ten of the twelve enteropathogenic strains produced one or both toxin forms that induced water secretion at concentrations of 1 to 10 ng/ml. Values in this range are considered indicative of clinically significant enterotoxigenicity in this assay system, and toxins from well-documented toxigenic strains examined in this study were active at these same concentrations. Similar preparations from ten control strains from healthy persons were either inactive or evoked water secretion only at concentrations of 10 to 100 μg/ml. These observations suggest that enteropathogenic serotypes of
E. coli
isolated from epidemics of infantile diarrhea produce diarrhea by elaborating potent heat-labile and heat-stable toxin forms which alter water transport but which are inactive in conventional assay systems. The manner in which these toxins differ either quantitatively or qualitatively from those which stimulate the conventional test systems is unknown.
