Significance of Heat-Stable and Heat-Labile Enterotoxins in Porcine Colibacillosis in an Additive Model for Pathogenicity Studies

Abstract

Although heat-stable (ST) and heat-labile (LT) enterotoxins produced by enterotoxigenic Escherichia coli (ETEC) have been documented as important factors associated with diarrheal diseases, investigations assessing the contributions of individual enterotoxins to the pathogenesis of E. coli infection have been limited. To address the individual roles of enterotoxins in the diarrheal؉ astA STb ؉ strain did not, although diarrhea developed in several piglets. The changes in the blood packed-cell volume and plasma total protein of gnotobiotic piglets inoculated with the LT-positive strains were significantly greater than those of pigs inoculated with the K88 astA/pBR322 strain (P ‫؍‬ 0.012, P ‫؍‬ 0.002). Immunochemistry image analysis also suggested that LT enhanced bacterial colonization in a gnotobiotic piglet model. This investigation suggested that LT is a major contributor to the virulence of K88؉ ETEC and that isogenic constructs are a useful tool for studying the pathogenesis of ETEC infection.Escherichia coli strains that colonize the small intestines, invade intestinal epithelial cells, and/or produce one or more toxins are important causes of diarrheal disease in both farm animals and humans. The virulence of enterotoxigenic E. coli (ETEC) is believed to be associated with the production of fimbrial adhesins and enterotoxins (1,19,35,36,51,54). Fimbrial adhesins mediate the attachment of bacteria to the surface of host epithelium cells and allow bacterial colonization. Fimbriae produced by different ETEC strains are quite diverse (21). In swine, ETEC strains that produce K88 (F4) or F18 are the most common currently associated with diarrheal diseases (19). These fimbriae apparently bind to glycoconjugates in the porcine enterocyte brush borders, and the absence of the respective glycoconjugate renders the animal resistant to bacterial colonization and consequent diarrheal diseases (14,15,20,48,49).Enterotoxins, including heat-stable enterotoxins (STa and STb) and heat-labile enterotoxin (LT) (23,25,39,45), have been found to disrupt intestinal fluid homeostasis and to cause hypersecretion of fluid and electrolytes through activation of adenylate cyclase (by LT) or guanylate cyclase (by STa) in small intestinal mucosal cells (26, 34). There are two major serogroups of LT found among E. coli strains: LT-I and LT-II. LT-I is associated with diarrheal diseases of both humans and animals, while LT-II is typically associated with diarrheal disease in animals. STs are small and monomeric molecules and may be associated with either human or animal disease (45, 55). STa and STb are the two classes of STs first recognized and differ from each other in both structure and enzyme activity (11,12). STa is produced by ETEC and other bacteria, while STb is found only associated with ETEC. A third ST, enteroaggregative E. coli (EAEC) EAST1, has been more recently identified. It is a plasmid-mediated enterotoxin, of low molecular weight and is frequently but not exclusively associated with EAEC isolated from children with p...

Escherichia coli STb toxin induces apoptosis in intestinal epithelial cell lines

“…The role of STb in swine diarrhea was demonstrated in a study by Berberov et al (2004) who revealed that 60% of piglets inoculated with an ETEC STb + EAST1 + LT -strain developed diarrhea (Berberov et al, 2004). This finding was confirmed by Zhang et al (2006) who observed fluid accumulation in piglets injected with F4 + STb + positive strains. Fluid accumulation decreased substantially in piglets injected with F4 + STb -negative strains.…”

“…Fluid accumulation decreased substantially in piglets injected with F4 + STb -negative strains. According to the authors, this proved that STb contributes to the virulence associated with ETEC bacteria (Zhang et al, 2006). Taken together, the findings obtained by Berberov et al (2004) (Berberov et al, 2004) and Zhang et al (2006) as well as the cell death of HRT-18 cells reported here shed new light on the contribution of STb to ETEC pathogenesis.…”

Functional Diversity of Heat-labile Toxins (LT) Produced by Enterotoxigenic Escherichia coli

“…Such a finding contrasts with the established role of pLT on the induction of secretory diarrhea in pigs and with recent evidence based on piglets inoculated with isogenic recombinant ETEC strains expressing either pLT or hLT (54,55). Because previous studies regarding pLT effects were mainly based on live ETEC cells or whole cell lysates and considering the prevailing notion that both toxins were supposedly identical in their biological activities, it is not surprising that studies aiming the biochemical and immunological characterization of hLT and pLT using equimolar amounts of purified toxins were not reported previously.…”

“…4). Displacement of the ␤1 strand in LTK4R places the Arg 7 residue into a position that favors its coordination through five interactions with four residues (Phe 52 , Val 53 , Arg 54 , and Ser 61 ) (Fig. 4C and supplemental Table S1), leading to a contraction of the loop encompassing residues 52-55 and reducing the NAD-binding crevice.…”

Early Events of Enterotoxigenic Escherichia coli Colonization on Gut Barrier Function: No Longer UndETECted?

“…Multiple combinations of LT, ST a , and ST b are produced by ETEC strains associated with diarrheal disease in pigs. Indeed, the most common ETEC strain isolated from diarrheic pigs produces LT and ST b , both associated with ETEC-related diarrhea [6]. Of the effects of bacterial enterotoxins on gut ion secretion, the pig jejunum is particularly sensitive to the secretory action of the heatstable enterotoxins [7].…”