RT-qPCR Quantification of SARS-CoV-2 Genomic and Subgenomic RNA from Infected Vero E6 Cell Supernatants Using a Standard Curve

Supernatant samples from infected Vero E6 cultures (e.g., MOI 0.01 and 0.1, harvested at 0, 12, 24, 48, 72 h) are extracted and assayed in technical triplicate. Each plate carries a 7-point 10-fold standard curve (1e7 to 1e1 copies/reaction) in triplicate, no-template controls (NTC), no-RT controls, and an extraction/inhibition control (spiked exogenous RNA). Biological replicates: n=3 independent infections. Genomic (N) and subgenomic (sgE) targets are run as separate reactions per sample.

Live SARS-CoV-2 culture and harvest are BSL-3 with respiratory protection and institutional approval; only chemically inactivated/lysed samples are removed to BSL-2 for extraction and qPCR. Verify inactivation efficacy before downstream handling. Wear appropriate PPE (gown, double gloves, eye/respiratory protection per BSL-3 SOP). Guanidinium-based lysis buffers must NOT contact bleach (toxic gas) - use a non-bleach decontaminant or segregate waste. Treat liquid waste and decontaminate surfaces per BSL-3 SOP; autoclave solids. Maintain RNase-free technique with dedicated pipettes and filter tips.

Standard curve (7-point IVT RNA) for absolute quantification and efficiency assessment. No-template control (NTC): water replaces template - must be undetermined or Cq > 40 to exclude contamination. No-RT control: confirms signal is RNA-, not DNA-, derived. Negative biological control: mock-infected Vero E6 supernatant must be negative. Positive control: a known-titer viral RNA sample. Extraction/inhibition control: exogenous spike-in RNA recovered within expected Cq confirms efficient extraction and absence of PCR inhibitors. UV-inactivated-virus control distinguishes residual input genomes (N positive) from active replication (sgE should be negative).

Standard curve linear over 1e1-1e7 copies with R2 > 0.99, slope ~ -3.1 to -3.6 (efficiency 90-110%). Productive infection yields rising N-gene copies, e.g., from ~1e4 copies/mL at 0 h to 1e8-1e10 copies/mL by 48-72 h. sgE becomes detectable by 12-24 h in active infection and remains undetectable in UV-inactivated controls. NTC and no-RT controls undetermined. Genome-to-PFU ratios of ~1e3-1e4 are typical for SARS-CoV-2.

• Vero E6 cells (ATCC CRL-1586)
• Viral RNA extraction kit (column- or magnetic-bead-based, e.g., QIAamp Viral RNA Mini)
• One-step RT-qPCR master mix (e.g., TaqPath 1-Step or equivalent, with RT enzyme)
• N-gene primers/probe (e.g., CDC N1: forward, reverse, FAM-BHQ probe)
• Subgenomic E primers/probe (leader-specific forward + E-gene reverse + FAM probe)
• In vitro-transcribed N-gene RNA standard, quantified by spectrophotometry and digital PCR
• Nuclease-free water, RNase-free tubes/tips with filter
• 96- or 384-well qPCR plates and optical seals
• Carrier RNA and exogenous spike-in control RNA (e.g., MS2 or synthetic)

To quantify SARS-CoV-2 RNA copy number in cell culture supernatants and lysates using a validated one-step reverse-transcription quantitative PCR (RT-qPCR) assay targeting the N gene (total/genomic RNA) and the leader-containing subgenomic E (sgE) RNA (active transcription), with absolute copy-number determination from an in vitro-transcribed RNA standard curve. This supports antiviral potency assays, growth-kinetics studies, and replication confirmation.

1. Harvest 140 uL supernatant; inactivate per institutional SOP (e.g., lysis buffer addition in the BSC) before removal from BSL-3.
2. Extract RNA per kit, eluting in 50 uL nuclease-free water; include a spiked exogenous control in one aliquot to monitor extraction/inhibition.
3. Prepare the standard curve: 10-fold serial dilutions of the IVT RNA standard from 1e7 to 1e1 copies/reaction in carrier-RNA-containing diluent.
4. Assemble 20 uL reactions: 5 uL template, master mix, 500 nM each primer, 125 nM probe. Set up N-gene and sgE reactions separately. Include NTC and no-RT controls.
5. Run RT-qPCR: 50 C 15 min (RT), 95 C 2 min, then 45 cycles of 95 C 3 s / 60 C 30 s, collecting FAM at 60 C.
6. Verify amplification curves and baseline/threshold; export Cq values.
7. For each sample, interpolate copies from the standard curve and apply the elution/volume dilution factor to report copies/mL.

Independent variables: time post-infection, input MOI (or antiviral concentration in drug studies). Dependent variables: N-gene copies/mL (total viral RNA) and sgE copies/mL (replication). Controlled variables: input cell number, extraction volume (140 uL) and elution volume (50 uL), reaction primer/probe concentrations, thermal cycling program, and instrument/threshold settings.

If supernatant RNA copy number reflects viral replication, then N-gene copies will rise over a productive infection time-course in a dose-dependent manner with input MOI, sgE RNA will be detectable only in actively infected (not UV-inactivated) cultures, and the standard curve will be linear (R2 > 0.99) across at least 6 log10 with amplification efficiency of 90-110%.

Generate the standard curve by linear regression of Cq vs log10(copies); compute efficiency = 10^(-1/slope) - 1. Interpolate sample copies, average technical triplicates (flag SD of Cq > 0.5 for re-run), and multiply by dilution factors to express copies/mL. Subgenomic RNA is reported as sgE copies/mL and as an sgE:N ratio. Use the qPCR instrument software (e.g., QuantStudio Design & Analysis) plus a spreadsheet or R for interpolation and dilution-factor handling. Apply the LOD/LOQ determined during validation to censor below-LOD values.

1. Standard curve efficiency <90% or >110%: primer/probe degradation or pipetting error - prepare fresh dilutions, re-quantify the IVT standard. 2. Signal in NTC: contamination - decontaminate workspace, use UNG/dUTP if available, separate pre/post-PCR areas. 3. High Cq variance across triplicates: pipetting or inhibitor carryover - increase template, dilute 1:10 to test inhibition, check the spike-in recovery. 4. sgE positive in UV-inactivated control: incomplete inactivation or genomic read-through - increase UV dose, verify sgE primer leader specificity. 5. Flat amplification despite expected positive: RT failure - confirm no-RT vs +RT difference, check master-mix RT enzyme and 50 C RT step.

Compare time points or treatments on log10(copies/mL) by one-way or two-way ANOVA (factors: time, treatment) with Tukey or Sidak multiple-comparison correction (alpha=0.05); n=3 biological replicates, technical triplicates averaged before statistics. Antiviral EC50 from copy-number dose-response is fit by 4-parameter logistic. Power: detecting a 1 log10 reduction with SD ~0.3 log10 at 80% power and alpha=0.05 needs n=3 per group; below-LOD values handled by Tobit/censored methods or imputed at 0.5xLOD.