Adsorption of Cholera and Heat-Labile Escherichia coli Enterotoxins by Various Adsorbents: an In Vitro Study

A variety of common inorganic adsorbents representing aluminas, zeolites, phyllosilicate clays, silica, and carbon were compared for their abilities to adsorb cholera toxin (CT) and heat-labile (LT) Escherichia coli enterotoxin. An appropriate assay system for the enterotoxins was developed using the Y-1 mouse-adrenal-tumor cell line, End points were determined by counting the number of rounded (cytotonic) cells at the relevant dilution. The adsorption varied between 177.0 × 10⁶ and 109.6 × 10² CYTU (cytotonic titer unit) for CT with charcoal and boehmite respectively, and between 60.7 × 10⁴ and 180.4 × 10¹ CYTU for LT with charcoal and boehmite respectively. Several of the other materials adsorbed CT and LT well, particularly attapulgite and sodium bentonite. The tightness of CT and LT binding to sodium bentonite and charcoal was determined by washing the adsorbent-enterotoxin pellets. Both toxins were strongly adsorbed, with dissociation of only 46.3 × 10° CYTU (<0.01 %) of the bound CT from sodium bentonite and 18.0× 10¹ CYTU (0.06%) of the bound LT from charcoal. The clay and charcoal pellets were assayed for their cytotonicity. Most of the activity of the adsorbed enterotoxins was lost: 93.1 and 89.6% for CT with sodium bentonite and charcoal, respectively, and 93.8 and 85.9% for LT with sodium bentonite and charcoal, respectively. The effect of dietary protein (casein) in enterotoxin adsorption by clay was also investigated. One percent casein (when adsorbed to sodium bentonite clay) completely blocked the adsorption of CT. When this protein-clay complex was treated with enzymes present in pancreatin, the digestive effect on the casein was sufficient to permit the adsorption of 137.6 × 10¹ CYTU of CT, although most of the blocking effect of casein remained. Further in vitro studies are needed to model the stomach, pancreatic, and intestinal digestive systems for determining if dietary proteins can block CT adsorption by clay in vivo. These results extend and support previously published data, obtained experimentally in rabbit and rat intestinal loops and from studies of children suffering spontaneous diarrhea, on the beneficial role of clays and other inorganic adsorbents in controlling enterotoxin activity.