New Insights into the Detoxification of Chromium(VI) from Contaminated Soils by Human Intestinal Microbiota

Accidental ingestion of contaminated soil is one of the major routes of human exposure to heavy metals, with proven adverse effects on gastrointestinal health. However, the effects with transformation of heavy metals in soil in the gastrointestinal phase remain poorly understood. Here, we investigated the bioaccessibility and conversion of Cr(VI) in two contaminated soils with different properties during the intestinal phase. The high- and low-Cr-contaminated soils showed 42% and 64% bioaccessibility of Cr in the small intestinal phase, respectively, followed by substantial reduction and detoxification of Cr(VI) from small intestinal to colonic phase. In the colonic liquid phase, nearly all Cr was present as reduced Cr(III) for the low-Cr soil, whereas Cr(III) accounted for more than 50% of dissolved Cr for the high-Cr soil. Such transformation was primarily microbiota-driven, with key genera including Phascolarctobacterium, Enterobacter, Lachnoclostridium, and Parasutterella. Functional analysis suggested that the tricarboxylic acid cycle and riboflavin metabolism provided electron-donating capacity that promoted Cr(VI) reduction. In parallel, Fe(II) generation supported a secondary Fe(III)/Fe(II)-associated indirect contribution. The PLS-PM modeling further indicated that direct microbial reduction is the major Cr(VI) detoxification route compared with Fe mediated pathways. Our findings show that the intestinal microbiota could induce reduction and detoxification of soil Cr(VI), offering new insights into how heavy metals can be detoxified from contaminated soils after accidental ingestion.