Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2012
Publication Date: 3/16/2012
Citation: Casey, T., Connell, T.D., Holmes, R.K., Whipp, S.C. 2012. Evaluation of heat-labile enterotoxins type IIa and type IIb in the pathogenicity of enterotoxigenic Escherichia coli for neonatal pigs. Veterinary Microbiology. 3(18). Available: http://www.sciencedirect.com/science/article/pii/S037811351200185X. Interpretive Summary: Escherichia coli is the most frequently identified cause of diarrhea in newborn calves and pigs. These pathogens, called enterotoxigenic E. coli, adhere in the small intestine, grow and produce toxins that cause fluid secretion resulting in diarrhea. Several different E. coli toxins have been found that are believed to be important for causing diarrhea in animals. The E. coli toxins called heat-labile enterotoxins Type IIa and Type IIb (LT-IIa and LT-IIb) have not be extensively studied. The genes for LT-IIa and LT-IIb toxins were found infrequently in a collection of E. coli isolated from calves and pigs with diarrhea. However, purified LT-IIa and LT-IIb caused fluid secretion in the small intestine of calves, suggesting that these toxins might be important for causing diarrhea in animals. This was confirmed by demonstrating that expression of LT-IIa or LT-IIb in E. coli that can adhere in the intestinal tract is sufficient to cause severe diarrhea in newborn piglets. This information is important for accurate identification of pathogenic E. coli by diagnostic laboratories and is an important step in designing vaccines and other prevention methods for producers and veterinarians to prevent diarrhea in animals caused by E. coli.
Technical Abstract: Type II heat-labile enterotoxins (LT-II) have been reported in Escherichia coli isolates from humans, animals, food and water samples. The roles of the antigenically distinguishable LT-IIa and LT-IIb subtypes in pathogenesis and virulence of enterotoxigenic E. coli (ETEC) have not been previously reported. The prevalence of genes encoding for LT-II was determined by colony blot hybridization in a collection of 1,648 E. coli isolates from calves and pigs with diarrhea or other diseases and from healthy animals. Only five isolates hybridized with the LT-II probe and none of these isolates contained genes for other enterotoxins or adhesins associated with porcine or bovine ETEC. Ligated intestinal loops in calves, pigs, and rabbits were used to determine the potential of purified LT-IIa and LT-IIb to cause intestinal secretion. LT-IIa and LT-IIb caused significant secretion in the intestinal loops in calves but not in the intestinal loops of rabbits or pigs. In contrast, neonatal pigs inoculated with isogenic adherent E. coli containing the cloned genes for LT-I, LT-IIa or LT-IIb developed severe watery diarrhea with weight loss that was significantly greater than pigs inoculated with the adherent, non-toxigenic parental or vector only control strains. The results demonstrate that the incidence of LT-II appeared to be very low in porcine and bovine E. coli. However, a potential role for these enterotoxins in E. coli- mediated diarrhea in animals was confirmed because purified LT-IIa and LT-IIb caused fluid secretion in bovine intestinal loops and adherent isogenic strains containing cloned genes encoding for LT-IIa or LT-IIb caused severe diarrhea in neonatal pigs.