We screened foods containing indigestible ingredients in the ability to adsorb Shiga toxin (Stx). When 5 mg of foods and dietary fibers such as dry vegetables and inulin were mixed and incubated with 0.5 mL of Stx solution (100 ng/mL) containing 0.5% bovine serum albumin, both Stx1 and Stx2 seemed to be adsorbed by only a fermented food, natto (a traditional Japanese food prepared from steamed soybeans by the biological action of Bacillus subtilis). We purified the Stx-adsorbing substance from natto by extraction with H O, acid treatment, Proteinase K treatment, and an ion exchange chromatography. The purified substance showed an average molecular mass of about 600 kDa. We identified it as poly-γ-glutamate (PGA) by amino acid analysis of its hydrolysate and peptide analysis after its treatment with Proteinase K. Purified PGA (MW: molecular weight = about 600 kDa) was considered to adsorb both Stx1 and Stx2 when we separated adsorbed and unadsorbed Stxs (MW = about 72 kDa) by an ultrafiltration method with a centrifugal filter unit (MWCO: molecular weight cut-off = 100 K). However, PGA with the ability to adsorb Stx was an insoluble form precipitated in the filter unit during centrifugation. PGA precipitated beyond the saturated density was also confirmed to well adsorb both Stx1 and Stx2 by an equilibrated dialysis method. To the best of our knowledge, this is the 1st report on food-adsorbing Stx.

We examined poly‐γ‐glutamate (PGA) from natto, a Japanese fermented food, in the ability to adsorb Shiga toxin 1 (Stx1) and Shiga toxin 2 (Stx2). The polymer was immobilized by direct coupling to EAH–SepharoseTM. The PGA–Sepharose (about 10 mg of ligand/mL of gel) adsorbed Stx2, but not Stx1: its dissociation constant (Kd) against Stx2 was calculated to be 14.0 μM. To analyze the binding site of PGA against Stx2, we similarly immobilized glutamate and glutarate. Glutamate– and glutarate–Sepharoses (each 7 μmol of ligand/mL of gel) similarly adsorbed Stx2, but not Stx1; Kd values against Stx2 were calculated to be 14.0 and 30.0 μM, respectively. The common structures of PGA–, glutamate–, and glutarate–Sepharoses were considered to be glutaryl groups. When we added the mixture of Stx2 and PGA–Sepharose to Caco‐2 cells (a human colon epithelial cell line), PGA–Sepharose was found to reduce the cytotoxicity of Stx2. Practical applications Poly‐γ‐glutamate (PGA) is a component of a traditional Japanese food, and is believed to be absolutely safe. Present study revealed that immobilized PGA adsorbed Stx2 produced by enterohemorrhagic Escherichia coli. To our knowledge, this is the first report on adsorbents specific to Stx2. Our results are probably useful for development of new functional foods with the ability to adsorb Stx2.

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