CEI — Cytochrome-EET Index
An operational qPCR index that, per sample, estimates the relative capacity for cytochrome-driven extracellular electron transfer (EET) compared to total bacterial biomass.
What does CEI represent?
Core question: “How many EET/cytochrome markers do I detect relative to 16S (total bacteria)?” CEI is not a direct measurement of current or corrosion rate; it is a biological capacity indicator that must be interpreted together with electrochemistry (current/potential) and metal-loss data.
Formula
CEI (%) = 100 × Σ(EET/cytochrome gene copies) ÷ (bacterial 16S copies)- EET/cytochrome markers: e.g., mtrC, omcA (Shewanella), omcS, omcZ (Geobacter), hmc (Desulfovibrio).
- 16S: bacterial 16S rRNA gene as an anchor for total bacterial load.
- Weighted variant (optional):
CEI = 100 × Σ(wᵢ × markerᵢ) / 16Sif certain markers should carry more weight (e.g., omcZ).
How to compute gene copies
- Convert qPCR Cq to gene copies using the standard curve.
- Correct for extraction volumes, dilutions, and sample area/volume → report as copies/mL (water/sludge) or copies/cm² (biofilm/coupons).
- Average replicates; treat nondetects consistently (e.g., LOD/2).
- Optional: PMA-qPCR to distinguish intact cells from relic/free DNA.
Example calculation
- mtrC = 2.0×10⁴; omcA = 1.0×10⁴; omcS = 6.0×10³; omcZ = 4.0×10³; hmc = 8.0×10³ → Σ = 4.8×10⁴ copies/mL.
- 16S (bacterial) = 1.2×10⁷ copies/mL.
- CEI = 100 × (4.8×10⁴ / 1.2×10⁷) = 0.4%.
Interpretation (bands — calibrate per site)
- Low: < 0.5% → typically routine monitoring.
- Medium: 0.5–2% → targeted tests, review setpoints/inhibitors.
- High: > 2% → escalate mitigation and track impact (qPCR + ER/coupons).
Tip: trend CEI alongside Icath (µA/cm²) and ER/coupon data. If CEI and cathodic current rise together, an EET-driven MIC component is plausible. Rule-of-thumb for uniform steel loss: ~1 µA/cm² ≈ 0.011–0.012 mm/year (order of magnitude; pitting can exceed this).
Limitations (important)
- Gene presence ≠ activity: CEI does not directly report expression or electron flux; always combine with electrochemistry and metal-loss.
- Primer bias & 16S copy variation: can impact scale; therefore on-site trends matter more than absolute cut-offs.
- eDNA/biofilm matrix: PMA-qPCR helps to filter the free DNA fraction.
MICBUSTERS on-site qPCR: making CEI actionable
With on-site qPCR, MICBUSTERS turns CEI into operational decisions:
- Rapid, on location: results within hours instead of days; directly link CEI trends to process setpoints.
- Asset-specific marker panel: EET/cytochrome markers (mtrC/omcA, omcS/omcZ, hmc) alongside 16S and optional micH, tailored to your asset.
- Action-oriented reporting: CEI as a capacity indicator integrated with cathodic current and ER/coupon data, with clear thresholds and follow-up advice.
- Proof of impact: measurable effect of setpoints, flushing, and inhibitors within the same maintenance window.
