Bulk RNA marker-gene PCR analysis

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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Viral infection marker gene expression (RT-qPCR)
Approach: RT-qPCR of virus-specific marker gene transcripts
Context: in situ, lab
Spatial scale: point sample
Temporal scale: hours to days
Units: qPCR Ct number (relative transcript abundance)
Community captured: bulk
Co-measurements: host cell abundance

Method Overview

Water samples are filtered onto membranes and RNA is extracted immediately after collection. RNA is reverse-transcribed to cDNA and quantitative PCR (RT-qPCR) is performed using primers targeting virus-specific marker genes — typically major capsid proteins (MCPs), RNA-dependent RNA polymerases (RdRp), or other genes unique to the virus of interest. The cycle threshold (Ct) value, compared to a standard curve of known copy number, gives absolute transcript abundance. High transcript levels of viral marker genes indicate active viral replication within host cells, and therefore active infection. Relative expression can be normalized to a host housekeeping gene to account for differences in total RNA loading.

Scale of measurement

Transcripts reflect expression at the moment of collection, integrating viral activity over the preceding minutes.

Data generated

Viral marker gene transcript concentrations (copies L^-1) or qPCR Ct values. Time-series measurements reveal when active viral infection peaks in the community.

Units & currency

Units are qPCR Ct number or transcript copies L^-1. The currency is transcript molecules (expression of specific viral marker gene).

Sample size

Typical samples are 1–2 L in volume.

Limitations

Viral transcript abundance is proportional to the level of active replication, not to viral load or host lysis rate directly. The assumption that viral gene expression always leads to cell lysis is violated in chronic or lysogenic infections. Absolute transcript abundance depends on extraction efficiency, primer efficiency, and the stability of the reference gene chosen for normalization.

Example Applications & Protocols

Classic examples

• Rodríguez et al. (2009) Application of PCR-based methods to assess the infectivity of enteric viruses in environmental samples

Common calculations/conversions

• Absolute copies L^-1 = (copies per reaction / template volume) × extraction volume / volume filtered.
• Relative expression: ΔΔCt normalised to host housekeeping gene.