Probiotic detoxification of heavy metals: functional assessment in simulated intestinal and ex vivo models

Overview

Pane and colleagues assessed three gut-derived probiotic lactobacilli strains — Lactiplantibacillus plantarum LP14, Lactobacillus crispatus LCR04, and Lactobacillus acidophilus LA12 — for their capacity to detoxify a mixture of inorganic cadmium, chromium(III), mercury(II), and lead(II) salts across three escalating model systems: a static in vitro co-culture / post-growth assay, the SHIME® dynamic gastrointestinal simulator (gastric, small-intestinal, and short-term proximal colonic phases), and a gut ex vivo system (GEVS) using murine small intestine. Strain- and metal-specific sequestration emerged: LP14 and LCR04 reduced heavy-metal bioavailability in the colonic SHIME compartment whereas LA12 showed minimal SHIME detoxification despite broad in vitro activity, and all three strains attenuated heavy-metal-induced epithelial barrier disruption and pro-inflammatory cytokine induction in the GEVS. The study contributes mechanism-of-action data on probiotic-mediated gut bioremediation of inorganic cadmium (Cd), Cr(III), inorganic Hg(II), and Pb(II); it does not measure contamination of any consumer product.

Key numbers

Heavy-metal stock concentrations used in the in vitro co-culture and post-growth tests (paper p. 5, §3.1 Results; salts: CdSO4, CrCl3, Hg(NO3)2, PbCl2):

• Cd(II): 1.1 mg/L final
• Cr(III): 17.6 mg/L final
• Hg(II): 8.8 mg/L final
• Pb(II): 14.9 mg/L final

Heavy-metal reduction (% w/w, mean ± SD, n = 3 biological replicates) by lactobacilli strains in the in vitro assay (Figure 1, paper p. 6):

Authors’ narrative (paper p. 6): Cd was reduced by all three strains in both co-culturing and post-growth conditions. chromium (Cr) reduction was highly strain-specific in co-culturing — substantial by LA12, virtually no reduction by LP14 or LCR04 — while in the post-growth test Cr reduction was comparable across all three strains. mercury (Hg) was not appreciably removed by LP14 in either test, whereas LA12 and LCR04 displayed high Hg removal in co-culture (less pronounced post-growth). lead (Pb) was “essentially not reduced by any of the strains in either test” (p. 6).

SHIME® bacterial growth across compartments (Figure 2, paper p. 7; ¹⁰log AFU/reactor and CFU/reactor, mean ± SD, n = 3 SHIME experiments):

• Inoculum bacterial counts in SHIME at start: 10.14 (LP14, LA12) or 10.15 (LCR04) ¹⁰log/reactor (SD < 0.1).
• Cell concentration remained stable (~10.1–10.3 ¹⁰log AFU/reactor) for all three strains through SI start, DUO end, JEJ end, and ILE end.
• After 24 h colonic incubation, LP14 and LCR04 displayed up to almost 2 ¹⁰log increase in AFU/reactor (LP14 reaching ~12.0, LCR04 reaching ~11.7); LA12 showed no significant growth in the colonic phase (~10.4).

SHIME® detoxification in the colonic compartment (Figure 3, paper p. 7; differential recovery of HMs by ICP-MS in cell pellets vs. cell-free supernatants after 24 h colonic incubation, n = 3 SHIME experiments):

• LA12 did not promote a significant variation in HM content in the supernatant or pellet relative to the SHIME blank, indicating no detectable colonic detoxification by this strain (text p. 6).
• Pb: predominantly accumulated by LCR04, with roughly 45 percentage-points higher recovery from the cell pellet relative to the control experiment (text p. 7).
• Cd and Cr: markedly accumulated by both LP14 and LCR04 (~20 to 40 percentage-points higher recovery from cell pellets vs. control) (text p. 7).
• Hg: the most recalcitrant metal — only a modest increase (~10 percentage-points) in metal associated with cell pellets was observed for LP14 and LCR04 relative to controls (text p. 7).
• Statistical significance levels for cell-pellet / supernatant differences vs. blank (per Figure 3 caption, reproducing the paper’s wording verbatim from p. 4 §2.4.5): * 0.01 < p ≤ 0.05; ** 0.01 < p ≤ 0.001; *** 0.001 < p ≤ 0.0001. The paper’s printed ** bound is non-monotonic with the * bound; reproduced as printed rather than corrected.
• End-of-ileum samples for all strains showed HM levels in the supernatant similar to the SHIME blank, indicating no HM detoxification under small-intestinal simulated conditions (text p. 6, “data not shown”).

Heavy-metal concentrations used in the GEVS ex vivo experiments (paper p. 8, §3.3): Cd 0.69 mg/L, Cr 31.2 mg/L, Hg 5.2 mg/L, Pb 6.9 mg/L (selected after preliminary range-finding indicated the in vitro / SHIME concentrations were too toxic for GEVS tissue viability).

GEVS endpoints (Figure 4, paper p. 9; mean ± SD, two independent biological experiments, two biological samples per condition each, technical duplicates; mRNA fold-change relative to control):

• FITC-Dextran (FD4) permeability: in the HM-mix (MIX) condition, all three lactobacilli panels showed a significant increase in permeability versus control (CTRL); pre-incubation with each strain’s cell-free supernatant (CFS) restored physiological permeability (LP14 panel: ** p ≤ 0.01 CTRL vs MIX, ** p ≤ 0.01 MIX vs MIX+LP14; LCR04 panel: *** p ≤ 0.001 CTRL vs MIX, *** p ≤ 0.001 MIX vs MIX+LCR04; LA12 panel: *** p ≤ 0.001 CTRL vs MIX, *** p ≤ 0.001 MIX vs MIX+LA12).
• Tight-junction gene expression in the MIX condition: claudin-2 (CLD-2), claudin-15 (CLD-15), and occludin (OCL) mRNA were dysregulated; pre-incubation with each strain’s CFS restored CLD-2 and CLD-15 levels (statistically significant across all three strains); OCL restoration was statistically significant only with LA12 (paper p. 8, §3.3).
• Pro-inflammatory cytokine mRNA (IFNγ, TNFα): MIX strongly elevated both (**** p ≤ 0.0001 vs CTRL across all three panels); pre-incubation with each strain’s CFS reduced both back toward control (significance levels: LP14 panel **** for IFNγ and ** for TNFα MIX vs MIX+LP14; LCR04 panel **** for IFNγ and * for TNFα; LA12 panel **** for IFNγ and **** for TNFα).
• Anti-inflammatory cytokine IL-10 mRNA: MIX down-regulated IL-10; pre-incubation with each strain restored or partially restored IL-10 (LP14 panel: **** p ≤ 0.0001 MIX vs MIX+LP14; LCR04 panel: ** p ≤ 0.01; LA12 panel: * p ≤ 0.1, per the Figure 4 caption’s atypical convention — see Verification notes).

Methods (brief)

Heavy metals were dosed as inorganic salts (CdSO4 Sigma 481,882; CrCl3 Sigma 230,723; Hg(NO3)2 Sigma 104,439; PbCl2 Sigma 268,690) dissolved in deionized water and sterilized by 0.22 µm filtration; stock solutions were matrix-matched to the calibration standards. Heavy-metal content was quantified by Inductively Coupled Plasma Mass Spectrometry on a Thermo Scientific iCAP RQ ICP-MS (RF power 1,550 W; collision gas flow 4–4.6 L/min) after closed-vessel digestion with concentrated nitric acid and H2O2 under reflux on a hot plate (20 min, to clear/transparent endpoint). The instrument measures total elemental concentration; the paper does not perform Cr(III)/Cr(VI) speciation or methylmercury/inorganic-Hg speciation. All experimental Cr was introduced as Cr(III) (from CrCl3); all experimental Hg was introduced as inorganic Hg(II) (from Hg(NO3)2). All HM experiments and quantifications were run in triplicate and results are expressed as reduction (%) by comparing levels before and after HM contact with bacterial cells, with initial concentrations in the stock solution as positive control. Bacterial viability was monitored by SYTO 24 / propidium iodide flow cytometry on a BD Accuri C6 Plus with FlowJo analysis (results in active fluorescent units, AFU). The SHIME® system used a dynamic upper-GI simulation (gastric pH 4.6 → 3.0 over 120 min via 0.5 M HCl dosing; small intestinal phase pH 5.5 → 6.5 → 7.5 over duodenal/jejunal/ileal sub-phases via 4.8 g/L NaHCO3 dosing; pancreatic juice with 15.9 g/L oxgall, 10.6 g/L pancreatin, 6.45 g/L trypsin, 8.1 mL chymotrypsin solution) followed by a short-term proximal-colon simulation (24 h, pH 6.5–5.8 maintained, anaerobic, 37 °C, 90 rpm) with sterile-filtered human fecal inoculum. The gut ex vivo system used resected small intestine from 13-day-old C57BL/6J mice cultivated under luminal flow (99 µL/h Iscove’s Modified Dulbecco’s Medium with 20% KnockOut serum replacement) in a silicone-based six-chamber rig under humidified O2 (95%) and CO2 (5%); tissues were stimulated for 5 h with Iscove’s medium alone (CTRL), the HM mix (MIX), or MIX with cell-free supernatants of each strain after 16 h MRS co-incubation with the HM mix at 1:10 (MIX + CFS LP14/LCR04/LA12). Endpoints: FITC-Dextran (FD4) permeability across the tissue, qPCR of CLD-2, CLD-15, OCL, IFNγ, TNFα, and IL-10 normalized to L34, and AlamarBlue tissue viability. Statistical analysis used Student’s t-test (in vitro and SHIME) or ANOVA (GEVS) via GraphPad Prism 7.

Limitations explicitly named by the authors (paper p. 11, §5): the SHIME® model lacks a complex competitive endogenous microbiota; the HM mixtures use defined inorganic-salt concentrations rather than dietary-bound or organic/methylated forms; the GEVS experiments do not dissect per-metal contributions or distinguish barrier reinforcement from sequestration; no in vivo data; absence of colonic ex vivo tissue limits compartment-level alignment with the SHIME finding that colonic conditions are where detoxification is most evident.

Implications

• Certification (HMTc): Contributes mechanism-of-action data for probiotic-mediated reduction of luminal Cd, Cr(III), inorganic Hg(II), and Pb(II) bioavailability under simulated GI conditions; does not generate occurrence data for any food matrix or consumer product, so it does not feed any product-category percentile pool. The Pb result is the most distinctive single finding (no in vitro reduction yet a ~45 percentage-points colonic-pellet accumulation by LCR04 under SHIME conditions), supporting the authors’ contention that live colonic metabolism — not cell-surface biosorption alone — drives Pb sequestration by L. crispatus LCR04 in this system.
• Microbiome: Direct metal–microbiome interaction paper. Adds to the body of evidence that specific lactobacilli species can immobilize ingested heavy metals in the colonic compartment via cell-pellet association and that pre-incubation with strain-derived metabolites (cell-free supernatants) protects intestinal epithelium from HM-induced tight-junction dysregulation and pro-inflammatory cytokine induction. Strain- and metal-specific behavior — LA12 active in simple in vitro assays but inactive in SHIME, LP14 and LCR04 reciprocally active in SHIME but not on Pb in vitro — is the load-bearing nuance for any downstream WikiBiome cross-link.
• App / Courses: Not directly applicable; this paper does not characterize a food, a supplement, or an exposure route. May surface in advanced course material on gut-microbiome-mediated detoxification mechanisms.

Related evidence

• Cadmium
• Chromium
• Mercury
• Lead

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