What is a cytochrome?

A cytochrome is a hemoprotein whose heme cycles between Fe(II)/Fe(III), enabling electron transfer. Classes (a, b, c) refer to heme type/binding; c-types are covalently bound via the CXXCH motif. In bacteria, membrane- and surface-exposed cytochromes underpin respiration and extracellular electron transfer (EET).

From single heme to ‘heme wires’: multi-heme c-types

Multi-heme cytochromes stack several hemes within one polypeptide. Efficient electron flow emerges from heme–heme coupling and a graded set of redox potentials. Canonical systems include:

• Shewanella (Mtr/Omc): MtrA (periplasm), MtrC/OmcA (outer membrane, decaheme), plus a β-barrel conduit to the surface.
• Geobacter (OmcS/OmcZ): outer-surface cytochromes forming nanowires; OmcZ filaments exhibit high conductivity and support large current densities.
• Desulfovibrio (SRB): periplasmic cytochrome c₃ (tetraheme) interfaces with hydrogenases and Hmc complexes toward sulfate reduction.

Cytochromes in EET: direct, mediated, hybrid

Multi-heme cytochromes enable direct transfer to/from solids; mediated routes via soluble shuttles (e.g., flavins) extend reach. Real biofilms often run a hybrid of both.

Why this matters for MIC

In MIC, the cathodic step frequently limits overall rates. Surface cytochromes can draw electrons from metals/conductive films more efficiently, raising cathodic current and corrosion.

• Current → loss (rule-of-thumb): 1 µA/cm² ≈ 0.011–0.012 mm/y uniform loss; +10 µA/cm² ≈ +0.12 mm/y (order-of-magnitude; pitting can exceed this).
• Consortia: EET-active, cytochrome-rich bacteria can coexist with methanogen-linked routes (e.g., micH), jointly aggravating corrosion.
• Films/minerals: FeS, magnetite and other conductive phases can bridge cytochromes to metal, shaping MIC kinetics.

MICBUSTERS on-site qPCR: making cytochrome-EET actionable

The practical question is: “Do we host a cytochrome-rich, EET-active community that elevates MIC risk?” Our on-site qPCR answers this within hours, via:

1) Marker panel (asset-specific)

• EET/cytochrome genes: outer-membrane/periplasmic markers such as mtrC/omcA (Shewanella), omcS/omcZ (Geobacter), hmc complex (Desulfovibrio).
• Community anchors: bacterial/archaeal 16S rRNA plus functional targets (e.g., mcrA for methanogens); optional micH for methanogen-linked corrosion.
• Assay design: degenerate primers as needed, matrix-validated efficiency/specificity (melt curves or probe assays).

2) Quantification & normalization

• Absolute qPCR: standard curves; report as gene copies per mL (fluids/sludge) or per cm² (biofilm/coupons).
• Relative indices: normalize EET markers to 16S to compute an Cytochrome-EET Index (CEI) (sum or weighted sum of mtrC/omcA/omcS/omcZ/hmc vs 16S).
• Live fraction (optional): PMA-qPCR to distinguish intact cells from relic DNA.

3) Reporting to decisions

• Trends & heatmaps: CEI, 16S load, functional markers across time and location.
• Electrochemistry linkage: Rising CEI concurrent with increased cathodic current supports an EET-driven MIC hypothesis.
• Action bands (heuristics; calibrate on site):
  • Low/stable CEI + low I_cath → routine monitoring.
  • Rising/high CEI + moderate I_cath → targeted tests (biofilm/cathode assays).
  • High CEI + high I_cath (>~13 µA/cm² ≈ >0.15 mm/y) → escalate mitigation; verify impact with qPCR + ER/coupons.

From genes to mm/y

qPCR quantifies capacity, not current. By co-trending CEI with cathodic current/potential and metal-loss (ER/coupons), site-specific models convert biology into predicted mm/y and document mitigation impact within a single maintenance window.

References (selected)

• Shi L. et al. (2016) Extracellular electron transfer mechanisms between microorganisms and minerals. Nat Rev Microbiol.
• Breuer M., Rosso K.M., Blumberger J. (2014) Electron flow in multiheme bacterial cytochromes. PNAS.
• Gu Y. et al. (2023) Structure & conductivity of Geobacter OmcZ nanowires. Nat Microbiol.
• Marsili E. et al. (2008) Flavin-mediated EET in Shewanella. PNAS.
• Valente F.M.A. et al. (2001) Tetraheme cytochrome c₃ in Desulfovibrio. J Bacteriol.
• Lahme S. et al. (2021) Severe corrosion linked to methanogens via micH. Appl Environ Microbiol.
• Knisz J. et al. (2023) MIC—more than microbes. FEMS Microbiol Rev.