Enterotoxigenic Escherichia coli (ETEC) strains that produce multiple
enterotoxins are important causes of severe dehydrating
diarrhea in human beings and animals, but the relative importance of these
enterotoxins in the pathogenesis is poorly understood. Gnotobiotic piglets were used to study the importance of heat-labile
enterotoxin (LT) in
infection with an ETEC strain that produces multiple
enterotoxins. LT(-) (DeltaeltAB) and complemented mutants of an F4(+) LT(+) STb(+) EAST1(+) ETEC strain were constructed, and the virulence of these strains was compared in gnotobiotic piglets expressing receptors for F4(+) fimbria. Sixty percent of the piglets inoculated with the LT(-) mutant developed severe dehydrating
diarrhea and
septicemia compared to 100% of those inoculated with the
nalidixic acid-resistant (Nal(r)) parent and 100% of those inoculated with the complemented mutant strain. Compared to piglets inoculated with the Nal(r) parent, the mean rate of
weight loss (percent per hour) of those inoculated with the LT(-) mutant was 67% lower (P < 0.05) and that of those inoculated with the complemented strain was 36% higher (P < 0.001). Similarly, piglets inoculated with the LT(-) mutant had significant reductions in the mean bacterial colony count (CFU per gram) in the ileum; bacterial colonization scores (square millimeters) in the jejunum and ileum; and clinical pathology parameters of
dehydration,
electrolyte imbalance, and
metabolic acidosis (P < 0.05). These results indicate the significance of LT to the development of severe dehydrating
diarrhea and postdiarrheal
septicemia in ETEC
infections of swine and demonstrate that EAST1, LT, and STb may be concurrently expressed by porcine ETEC strains.