Overview
Shuting Hu and colleagues at Tianjin University constructed 24 engineered Escherichia coli BL21(DE3) strains that heterologously express the heavy-metal-binding proteins (HMBPs) MerR and ChrB from the chromium-resistant model bacterium Cupriavidus metallidurans CH34, then screened them for Hg²⁺ and Cr⁶⁺ adsorption from aqueous solution. Display mode (intracellular overexpression versus surface display via the Pseudomonas syringae ice-nucleation N-terminal anchor INPN), inter-domain linker chemistry (no linker, flexible Gly₄Ser, rigid proline-rich PAPAP, rigid helical AEAAAKEAAAKA, and a 96-bp intermediate repeat), and codon optimization (native versus codon-optimized merR_opt and chrB_opt) were varied combinatorially. The best mercury strain (M’-002: codon-optimized merR_opt surface-displayed via flexible linker on INPN) reached 658.66 µmol/g cell dry weight (CDW) at 300 µM Hg²⁺; the best chromium strain (B-008: intracellular chrB overexpression) reached 46.84 µmol/g CDW at 500 µM Cr⁶⁺. A mixed-bacterial system co-culturing M’-002 and B-008 adsorbed up to 1.14× and 4.09× more total ions than the single-strain controls in a four-metal Hg²⁺/Cr⁶⁺/Cd²⁺/As³⁺ cocktail. The paper enters the HMI corpus as upstream bioremediation-technology context for the metal-binding-peptide and HMBP literature already represented in the same source-retrieval peptide folder (Peptides Used for Heavy Metal Remediation: A Promising Approach, Biomining: A biological approach to recycling elemental components from end-of-life electronics, Phytochelatins: Sulfur-Containing Metal(loid)-Chelating Ligands in Plants, The Role of Metallothionein in Oxidative Stress, Metallothionein: A Comprehensive Review of Its Classification, Structure, Biological Functions, and Applications, Metallothionein: An overview). It does not measure heavy metals in any food, personal-care, or consumer matrix.
Key numbers
The paper reports adsorption capacities of engineered strains and the wild-type chassis under single-ion and mixed-ion conditions. No occurrence or exposure values for HMI heavy metals in any consumer matrix are reported; values below are the bioengineering parameters and AAS-quantified adsorption capacities the paper publishes.
| Parameter | Value | Notes |
|---|---|---|
| Engineered strains screened | 24 | Twelve overexpression (B-001 to B-005 ChrB; M-001 to M-005 MerR; with codon-optimized counterparts B’-001 to B’-005 ChrB_opt; M’-001 to M’-005 MerR_opt) plus four intracellular controls (M-006, M’-006, B-008, B’-008) |
| Chassis | E. coli BL21(DE3) | Plasmid pET28a; T7 promoter; kanamycin selection |
| Source organism for HMBPs | Cupriavidus metallidurans CH34 | merR GenBank CP050332.1; chrB GenBank CP000355.2; plasmid pMOL28 |
| Surface-display anchor | INPN | N-terminal domain of Pseudomonas syringae inaK ice-nucleation protein (Accession NC AF013159) |
| Linker chemistries tested | NL, FL (Gly₄Ser), RL (PAPAP, proline-rich), HL (AEAAAKEAAAKA, helical), 96BP repeat | FL gave the highest extracellular capacity for both Hg²⁺ and Cr⁶⁺ |
| Optimal IPTG induction | 1.0 mM | Peak adsorption capacity for all strains tested across 0.2 to 1.4 mM IPTG |
| Optimal induction time | 22 h | All four optimal strains (M’-002, M’-006, B’-002, B-008) |
| Optimal induction temperature | 26 °C for M’-002, M’-006, B’-002; 24 °C for B-008 | Range tested 16 to 28 °C |
| Wild-type BL21 Hg²⁺ baseline | ~270 to 280 µmol/g CDW at 500 µM Hg²⁺ | Fig. 3a curve only; paper text does not state a precise numerical baseline for BL21 |
| Wild-type BL21 Cr⁶⁺ baseline | ~11 µmol/g CDW at 500 µM Cr⁶⁺ | Fig. 3b; flat across concentrations |
| M’-002 maximal Hg²⁺ capacity | 658.66 µmol/g CDW at 300 µM Hg²⁺ | merR_opt surface-displayed via FL linker on INPN; “highest value reported to date” per Conclusions. Abstract reports 658.40 µmol/g; results-section figure reports 658.66 µmol/g (paper-internal rounding) |
| M’-006 maximal Hg²⁺ capacity | 602.34 µmol/g CDW at 300 µM Hg²⁺ | Intracellular merR_opt overexpression |
| B-008 maximal Cr⁶⁺ capacity | 46.84 µmol/g CDW at 500 µM Cr⁶⁺ | Intracellular chrB overexpression; abstract gives 46.84 µmol/g, p. 8 also reports 46.73 µmol/g (text inconsistency) |
| B’-002 maximal Cr⁶⁺ capacity | 28.46 µmol/g CDW at 300 µM Cr⁶⁺ | Codon-optimized chrB_opt surface-displayed via FL linker |
| Mixed-bacterial Hg²⁺ capacity (single-ion) | 449.04 µmol/g CDW at 500 µM Hg²⁺ | M’-002 + B-008 co-culture in single-ion solution (not four-metal cocktail); inoculation ratio 1:1 v/v at 1% to fresh LB; Fig. 3c |
| Mixed-bacterial Cr⁶⁺ capacity (single-ion) | 50.71 µmol/g CDW at 500 µM Cr⁶⁺ | Same M’-002 + B-008 co-culture in single-ion solution; Fig. 3d |
| Mixed-ion four-metal cocktail concentrations | 40, 100, 200 µM per ion (160, 400, 800 µM total) | Hg²⁺/Cr⁶⁺/Cd²⁺/As³⁺ co-administered; Fig. 4; the ×1.14 and ×4.09 mixed-vs-single fold-changes report at the 800 µM total cocktail per the abstract |
| Mixed/single fold-change | up to ×1.14 and ×4.09 | Total ions adsorbed by mixed bacteria vs single-strain control (abstract; Conclusions) |
| Pseudo-first-order rate constant K₁ | M’-002 0.1157 min⁻¹; M’-006 0.1394 min⁻¹; B’-002 0.1153 min⁻¹; B-008 0.0688 min⁻¹ | Table 3; pseudo-second-order model did not fit |
| q_e,cal (pseudo-first-order) | M’-002 336.6 mg/g; M’-006 177.0 mg/g; B’-002 4.455 mg/g; B-008 3.057 mg/g | Table 3 |
| q_e,exp | M’-002 111.88 mg/g; M’-006 97.96 mg/g; B’-002 1.46 mg/g; B-008 1.96 mg/g | Table 3; large discrepancy with q_e,cal flagged by the authors as model fit weakness |
| Drinking-water reference quoted in Discussion | Hg²⁺ 50 µg/L (0.249 µM); Cr⁶⁺ 6 µg/L (0.115 µM) | Quoted as “WHO Guidelines for Drinking-water Quality” values; numbers appear transposed vs WHO GDWQ 4th ed (Hg 6 µg/L; total Cr 50 µg/L provisional). Recorded as the paper states |
| Optimal strains do not meet WHO drinking-water targets | confirmed by authors | Even M’-006 fails the drinking-water removal-rate requirement at the highest concentrations tested |
Methods (brief)
Plasmid construction used pET28a as the expression vector. Native and codon-optimized merR (genome CP050332.1) and chrB (genome CP000355.2) were PCR-amplified from C. metallidurans CH34 plasmid pMOL28; codon-optimized merR_opt and chrB_opt were synthesized by GENEWIZ. EcoRI and HindIII restriction sites were appended to TF primers; HindIII and NheI flanked the linker; NheI and XhoI flanked TF primers. Linkers (FL Gly₄Ser, RL PAPAP, HL AEAAAKEAAAKA, 96-bp intermediate repeat) and the INPN anchor were inserted into five conservation plasmids (pE-NL, pE-FL, pE-RL, pE-HL, pKE-FL), ligated with T4 DNA ligase, and transformed into BL21(DE3). The resulting 24 strains include 10 surface-display constructs each for MerR and ChrB (5 linkers × 2 codon variants) plus 4 intracellular overexpression strains (M-006, M’-006, B-008, B’-008). Adsorption assays grew strains in LB broth (5 g/L yeast extract, 10 g/L NaCl, 10 g/L peptone, pH 7.2 to 7.4) to OD₆₀₀ 0.6 to 0.8 at 37 °C, cooled to 22 °C, induced with 0.2 mM IPTG (0.2 to 1.4 mM range in optimization), supplemented with target metal ions to 200 µM (50, 100, 300, 500, 1000, and 1500 µM range for concentration sweeps), and incubated for 12 h at 22 °C / 220 rpm. Cells were collected, washed three times with ddH₂O, oven-dried at 60 °C for 24 h, weighed, and microwave-digested for atomic absorption spectroscopy (AAS) quantitation of Hg²⁺ and Cr⁶⁺. Adsorption capacity (µmol/g CDW) was computed as total metal content divided by dry biomass. All experiments used three biological replicates. Pseudo-first-order and pseudo-second-order kinetic models were fitted; pseudo-first-order fit the data better but with substantial q_e,cal versus q_e,exp gaps that the authors attribute to model weakness.
For mixed-ion adsorption, the four-metal cocktail (Hg²⁺/Cr⁶⁺/Cd²⁺/As³⁺) was prepared at 40, 100, and 200 µM per ion (160, 400, and 800 µM total). For the mixed-bacterial system, the two optimal strains M’-002 and B-008 were separately activated, then co-cultured in fresh LB at 1:1 v/v at 1% inoculation ratio.
Implications
The paper is a primary bioremediation-engineering study: it constructs, optimizes, and benchmarks engineered E. coli strains for aqueous Hg²⁺ and Cr⁶⁺ removal. It does not measure mercury (Hg), chromium (Cr), lead (Pb), cadmium (Cd), arsenic (As), or any other HMI-tracked analyte in any food, supplement, personal-care, or consumer matrix. Its value to the HMI corpus is upstream context for the metal-binding-peptide and HMBP literature that the same source-retrieval peptide folder is populating: it shows what a state-of-the-art recombinant bacterial sorbent achieves under controlled laboratory conditions, against which broader review claims about peptide- and protein-based heavy-metal remediation can be calibrated. The paper itself notes that even the best-performing strain (M’-006) fails the WHO drinking-water removal-rate target at higher influent concentrations, which bounds practitioner expectations for engineered bacterial sorbents under real wastewater conditions.
Update history
The five most recent substantive edits to this page, classified major (evidence or structure moved), correction (a published value or statement was wrong and has been fixed), or minor (narrative rewritten without changing the underlying evidence). Each description is derived from what the edit did to this page; the linked commit is the authoritative record, routine regeneration passes are excluded, and the full version history lives in git.
| Commit | Date | Change | Description |
|---|---|---|---|
| b01ec52c31 | 2026-08-04 | major | 1 section added |
| 3171d069e3 | 2026-08-02 | major | 1 section added |
| 90a2f9a38e | 2026-07-30 | major | 1 section added and 1 removed; narrative text revised |
| eaa8eeeedf | 2026-07-27 | major | 1 section added and 1 removed |
| 50de6ab217 | 2026-07-27 | minor | Narrative text revised |