Enhanced Bioaccumulation of Heavy Metals by Bacterial Cells Displaying Synthetic Phytochelatins

Overview

Bae, Chen, Mulchandani, and Mehra (Department of Chemical and Environmental Engineering and Environmental Toxicology Program, University of California, Riverside) report the construction and characterisation of recombinant Escherichia coli JM105 strains that display synthetic phytochelatin analogs of defined chain length (EC8, EC11, EC20 with 8, 11, and 20 cysteines respectively) on the cell surface via a Lpp-OmpA fusion anchor, and demonstrate that whole cells displaying the longest analog (EC20) accumulate ≈15-fold more Cd²⁺ on a dry-weight basis than the wild-type control and ≈2-fold more than cells expressing EC20 periplasmically as an MBP fusion. Purified MBP-EC20 is shown to bind Cd²⁺ at a stoichiometry of ≈10 Cd²⁺ per peptide — about 40 % higher than the canonical 7 Cd²⁺ per peptide reported for mammalian metallothioneins. The paper reports no measurements in any food or environmental matrix; its value to the wiki is conceptual background for the Cadmium page and for the mitigation/remediation-evidence section on engineered-microbial biosorbents.

Why this matters

• It is one of the foundational primary studies for the surface-displayed-phytochelatin strategy of heavy-metal bioremediation, alongside the Sousa et al. (1996, 1998) LamB-MT fusion line of work that the authors benchmark against. The Lpp-OmpA-EC fusion system reported here goes on to seed a substantial downstream literature on engineered microbial biosorbents for cadmium and mercury remediation.
• It demonstrates a clean chain-length dependence: EC8 → ≈18, EC11 → ≈29, EC20 → ≈60 nmol Cd²⁺/mg dry cells, consistent with the predicted increase in metal-binding centres per peptide as cysteine content rises. The chain-length dependence is direct mechanistic support for the design principle that longer cysteine-rich peptides give higher whole-cell bioaccumulation capacity.
• It documents an unexpected localisation effect — surface-anchored EC20 outperforms periplasmically-expressed EC20 by a factor of ≈2 in whole-cell Cd²⁺ accumulation despite the periplasmic construct having higher per-cell expression — and attributes the gap to cysteine binding sites in the MBP-EC20 fusion being vacant or oxidised in the periplasmic context, with less than one Cd²⁺ associated per fusion protein in the unprocessed periplasmic preparation versus ≈10 Cd²⁺ per fusion after reduction with DTT.
• It provides a numerical anchor (10 Cd²⁺/EC20 vs 7 Cd²⁺/MT) for the comparative-stoichiometry literature on cysteine-rich heavy-metal-binding peptides, of relevance whenever the wiki’s mitigation page contrasts phytochelatin-based and metallothionein-based engineering routes.

Key numbers

• EC20 Cd²⁺ binding stoichiometry, purified MBP-EC20 fusion after DTT reduction: 9.9, 10.1, 9.8 Cd²⁺ per EC20 in Sephadex G-25 fractions 6, 7, 8 respectively (Fig. 4, p. 521). Reported in the Abstract as a stoichiometry of 10 Cd²⁺ per peptide.
• Mammalian metallothionein (MT) reference stoichiometry the authors compare against: 7 Cd²⁺ per MT (Hamer 1986; Stillman et al. 1992 — cited values, not measured here). Authors conclude EC20 has ≈40 % higher Cd²⁺ binding capacity than MTs on a per-peptide basis (Discussion, p. 522).
• Whole-cell Cd²⁺ accumulation, E. coli JM105 strains grown in MJS medium with 0.1 mM CdSO4, 1 mM IPTG induction, monitored to 16 h (Fig. 6, p. 522):
• pUC18 negative control: ≈4 nmol Cd²⁺/mg dry cell weight (Fig. 5; the exact bar value is not numerically tabulated and is read from the graph).
• pLO8 (Lpp-OmpA-EC8, 8 cysteines): ≈18 nmol Cd²⁺/mg dry cells.
• pLO11 (Lpp-OmpA-EC11, 11 cysteines): ≈29 nmol Cd²⁺/mg dry cells.
• pLO20 (Lpp-OmpA-EC20, 20 cysteines, surface-displayed): ≈60 nmol Cd²⁺/mg dry cells (called out in the Abstract as “a maximum of 60 nmoles of Cd²⁺/mg dry cells”).
• pM20 (MBP-EC20, periplasmic expression): ≈35 nmol Cd²⁺/mg dry cells (Fig. 5).
• Surface-displayed pLO20 / periplasmic pM20 ratio: ≈2-fold higher whole-cell Cd²⁺ accumulation for the surface-displayed construct (Abstract; Fig. 5).
• Periplasmic MBP-EC20 native binding (before DTT): less than 1 Cd²⁺ per MBP-EC20 fusion in the unprocessed periplasmic preparation (p. 521).
• CdSO4 challenge concentration in the bioaccumulation assays: 100 µM CdSO4 (Materials and Methods, p. 519); whole-cell experiments used 0.1 mM CdSO4 (Results, p. 522) — these are the same value reported in two unit conventions.
• Replicate counts: n = 5 independent experiments for the whole-cell Cd²⁺ accumulation data in Figs. 5 and 6 (figure legends, p. 521–522).
• Fusion-protein expression levels (SDS-PAGE autoradiograph, Fig. 2): Lpp-OmpA-EC8 18.5 kDa, Lpp-OmpA-EC11 19 kDa, Lpp-OmpA-EC20 21 kDa; MBP-EC20 47 kDa. MBP-EC20 expression is more than 10× higher than the surface Lpp-OmpA-EC fusions despite the surface display giving higher whole-cell Cd²⁺ accumulation (p. 520–521).
• Proteinase K accessibility experiment (Fig. 3): Lpp-OmpA-EC20 signal is “no longer detectable after 2 h” of proteinase K treatment, whereas MBP-EC20 signal shows “no observable decline… even after 21 h of incubation” — confirming surface vs periplasmic localisation respectively (p. 521).

The paper reports no lead (Pb), mercury (Hg), arsenic (As), nickel (Ni), aluminum (Al), chromium (Cr), tin (Sn), or other-metal measurements. Mercury is mentioned only as an analogous limitation case from Chen and Wilson (1997) in the Discussion (p. 522), not measured in this study.

Methods (brief)

The authors used E. coli JM105 (genotype recited in Materials and Methods, p. 519) as the recipient strain throughout. Synthetic genes encoding (Glu-Cys)nGly phytochelatin analogs of three chain lengths (EC8, EC11, EC20) were prepared from two complementary oligonucleotides synthesised by Research Genetics (Huntsville, AL); codon usage was deliberately varied between GAA/GAG (Glu) and TGT/TGC (Cys) to prevent unwanted hybridisation. Double-strand synthesis used the Klenow fragment (Promega, Madison, WI); the synthetic gene was cloned into the yeast–E. coli shuttle vector pVT102-U (Vernet et al. 1987) via BamHI and HindIII to generate pVT20, sequence-verified. The ec20 fragment was PCR-amplified from pVT20 and cloned downstream of a pre-existing 481-bp lpp-ompA cassette (Richins et al. 1997) in pUC18 to generate plasmid pLO20; analogous procedures produced pLO8 and pLO11. For periplasmic expression, the BamHI–HindIII ec20 fragment was cloned into pMAL-p2x (New England BioLabs) to generate pM20, fusing EC20 to the maltose-binding protein (MBP).

Cultures were grown in low-phosphate MJS medium (Sambrook et al. 1989) supplemented with 50 µg/mL ampicillin at 30 °C to OD600 ≈0.3 with 1 mM IPTG induction; 100 µM CdSO4 was added to allow EC expression in the presence of Cd²⁺. Radiolabelling for SDS-PAGE-autoradiograph confirmation of fusion-protein expression used [35S]-cysteine (1075 Ci/mmol, ICN) at a final concentration of 5 µCi/mL. Samples were separated on 12.5 % (w/v) SDS-PAGE per Laemmli (1970). Protease accessibility experiments used proteinase K (Sigma) at 10 µg/mL after resuspension in incubation buffer (15 % w/v sucrose, 15 mM Tris-HCl pH 7.8), with samples taken at 3 min, 10 min, 1 h, 2 h, 5 h, and 21 h and visualised by SDS-PAGE autoradiography.

For purified MBP-EC20 Cd²⁺ binding stoichiometry, MBP-EC20 was purified through an amylose affinity column (New England BioLabs); purity was confirmed by SDS-PAGE. Ten nmol of purified fusion protein was treated with 5 mM DTT in 50 mM Tris-Cl pH 7.4 for 2 h at 37 °C to reduce thiol groups, then 300 nmol Cd²⁺ was added for 1 h at 37 °C. The mixture was fractionated through a Sephadex G-25 column to separate the Cd²⁺-protein complex from free Cd²⁺ and DTT. Cd²⁺ and protein concentrations in each fraction were analysed by flame atomic absorption spectrophotometry (PerkinElmer AAS3100, Norwalk, CT) for cadmium and thiol assay per Grassetti and Murray (1967) for the protein component.

For whole-cell bioaccumulation, JM105 cultures expressing each plasmid construct were grown in MJS medium with 0.1 mM CdSO4 monitored to 16 h after IPTG induction. Cells were harvested, washed twice with double-distilled water, and disrupted overnight by treatment with concentrated nitric acid. The disrupted cells were diluted with double-distilled water and centrifuged at 4 °C for 10 min; Cd²⁺ in the soluble fraction was measured directly by atomic absorption spectrophotometry (PerkinElmer AAS3100).

Funding: UC Biotechnology Research and Education Program; US EPA contract grant R827227. No conflict-of-interest declaration is reported on the front page; the article appears under the standard Wiley copyright for Biotechnology and Bioengineering Vol. 70, No. 5, 5 December 2000, pp. 518–524.

Implications

• Microbiome: Not relevant. The work uses recombinant E. coli JM105 as a bench heterologous host; it does not engage the gut microbiome, the rhizosphere microbiome, or any natural microbial community. WikiBiome federation is not implicated.

Limitations

The work is bench-scale on whole cells grown in defined laboratory medium with a single Cd²⁺ challenge concentration (100 µM CdSO4 ≈ 11.2 mg/L cadmium (Cd) as element), well above the concentrations relevant to most environmental remediation contexts (industrial-effluent Cd is typically 0.1–10 mg/L; agricultural-irrigation Cd is typically 0.001–0.05 mg/L). The authors do not report a concentration-response curve, so the relevance of the bench stoichiometry to lower-Cd remediation streams is not characterised. The whole-cell uptake numbers (18 / 29 / 60 nmol Cd²⁺/mg dry cells across the EC8/EC11/EC20 series) are reported only as bar-graph readings in Fig. 5 and Fig. 6 with no explicit numerical table, so values in the Key numbers section are approximations within ≈±5 nmol/mg of the graphed bar tops. Cells were harvested after a single fixed induction-and-incubation interval (16 h post-induction); time-course data on Cd²⁺ uptake kinetics are not reported. The downstream-process question (recovery and recycling of the Cd-loaded biomass; cell stability under operating conditions) is acknowledged in the Discussion as “currently under investigation” but not addressed in this paper. The paper covers Cd²⁺ exclusively; the title’s plural “heavy metals” is not matched by experimental data on lead, mercury, or any other analyte, and the wiki’s metals source metadata is restricted to Cd accordingly. No replicate-level standard deviations are tabulated for the stoichiometry measurements (only n=3 fraction values are given for MBP-EC20). The 2000 publication date predates the current understanding of metallothionein and phytochelatin proteomics, so the comparative-stoichiometry framing against MTs (“40 % higher”) rests on the canonical 7 Cd²⁺/MT value of Hamer (1986) and Stillman et al. (1992); subsequent literature may have revised those reference values.

Related evidence

• Remediation evidence — drivers and interventions
• Phytochelatins and Metallothioneins: Roles in Heavy Metal Detoxification and Homeostasis
• Phytochelatins, the heavy-metal-binding peptides of plants, are synthesized from glutathione by a specific gamma-glutamylcysteine dipeptidyl transpeptidase (phytochelatin synthase)
• Phytochelatins and Cadmium Mitigation: Harnessing Genetic Avenues for Plant Functional Manipulation
• Phytochelatins: Sulfur-Containing Metal(loid)-Chelating Ligands in Plants

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.