The ongoing COVID-19 pandemic has drastically elevated the need for rapid, reliable, and affordable viral detection techniques. Current approaches often depend on methods or reagents that have faced supply chain shortages, at times resulting in severe backlogs. Furthermore, silica-based RNA purification can be time-consuming, expensive, and challenging to automate. In addition to RNA isolation, sample collection via nasopharyngeal (NP) swab can be challenging, especially for children or cohorts that need to be routinely tested. Collection of saliva is an easier and less invasive process than NP swabs, and SARS-CoV-2 viral particles are stable in saliva for several days without any transport medium 1.
For Research Use Only. Not For Use In Diagnostic Procedures.
Here we demonstrate sensitive detection of crosslink-inactivated SARS-CoV-2 viral particles in both saline and human saliva through lysis with a novel Viral RNA Extraction Buffer (VRE100). The workflow consists of a 5-minute room temperature incubation which circumvents the need for RNA purification and cleanup. Samples are immediately ready for direct RNA analysis using molecular detection techniques, including qRT-PCR and Loop-Mediated Isothermal Amplification (LAMP) assays.
Crosslink-inactivated SARS-CoV-2 particles (ZeptoMetrix® Corp.) were spiked into saline, saliva from healthy individuals, or transport medium at various concentrations. For lysis with Viral RNA Extraction Buffer (VRE100), saliva samples were centrifuged for 5 min at 15,000 RCF to remove particulates and supernatant was used for testing. 2 µl of VRE100 was added to 6 µl sample and incubated for 5 minutes at room temperature. For ProK + heat treatment, samples were spiked with Proteinase K to a final concentration of 2.4 mg/ml, vortexed for 1 min, heated at 95°C for 5 min, and briefly centrifuged to remove supernatant. 12 µl qRT-PCR Master Mix (QR0200) containing primers and probe directed to the N2 region was added directly to the RNA-extracted sample and analyzed on a BioRad CFX Connect™ instrument. qRT-PCR reaction conditions are shown below. For Research Use Only. Not For Use In Diagnostic Procedures.
Saline is frequently available as a storage medium for NP swabs, and saliva offers a promising alternative to sample collection. Crosslink-inactivated SARS-CoV-2 particles were spiked into either saline or saliva from four different individuals at various concentrations based on manufacturer reported values. Samples were then treated with Viral RNA Extraction Buffer (VRE100) and detected by qRT-PCR (Figure 1). To test compatibility with several commonly used transport media, crosslink-inactivated SARS-CoV-2 particles were spiked at 1X104 particles per mL into indicated medium, samples were either left untreated or treated with Viral RNA Extraction Buffer, and RNA was detected by qRT-PCR (Figure 2). For Research Use Only. Not For Use In Diagnostic Procedures.
Figure 1.Viral RNA Extraction Buffer (VRE100) sensitivity using qRT-PCR. A, Ct values obtained with decreasing amounts of viral particles in saline. B, Ct values obtained with decreasing amounts of viral particles spiked into human saliva. Each color represents saliva from a different healthy individual.
Figure 2.Comparison of virus detection in various transport media using Viral RNA Extraction Buffer (VRE100). Ct values are shown for treated or untreated samples in different media. Each condition was tested in duplicate.
To test comparability to a similar approach, crosslink-inactivated SARS-CoV-2 particles were spiked into saliva from four different individuals at various concentrations. Each sample was subjected either to lysis with VRE100, treatment with Proteinase K and heat, or left untreated, and all samples were amplified by qRT-PCR (Figure 3).
Figure 3.Viral RNA Extraction Buffer performance relative to similar techniques. A, Ct values from four different saliva samples. Each color represents saliva from a different individual containing spiked viral particles. B, Amplification curves from a representative saliva sample (each treatment analyzed in duplicate).
Colorimetric LAMP technology allows for a quick, simple assay readout that circumvents the need for qPCR instrumentation. Healthy human saliva was spiked with crosslink-inactivated SARS-CoV-2 (ZeptoMetrix® Corp.) at 5x104 viral particles per mL. 1.6 µl Viral RNA Extraction Buffer was added to 4.9 µl sample and allowed to incubate for 5 minutes at room temperature. Viral RNA was detected with a SARS-CoV-2 Rapid Colorimetric LAMP Assay Kit (NEB) as per manufacturer instructions, with heating for 40 minutes at 65°C, and results were determined by visual inspection (Figure 4). Synthetic SARS-CoV-2 RNA (1x105 copies, Twist Bioscience) was used as a positive assay control.
Figure 4.Viral RNA Extraction Buffer coupled with LAMP. Pink color indicated a negative result, while yellow indicated a positive result in which SARS-CoV-2 RNA was detected. Conditions with (+) or without (-) Viral RNA Extraction Buffer.
Viral RNA Extraction Buffer (VRE100) was effective in lysing inactivated SARS-CoV-2 particles at concentrations as low as 1x103 viral particles per mL in saline and 5x104 viral particles per mL in saliva. Compatibility with various commonly used viral transport media was demonstrated at 1x104 viral particles per mL. The workflow was compatible with analysis via both qRT-PCR and LAMP. Detection of SARS-CoV-2 in saliva with Viral RNA Extraction Buffer was more consistent and sensitive than treatment with Proteinase K and heat. The use of Viral RNA Extraction Buffer also saved time and cost compared to comparable methods requiring RNA cleanup. These results demonstrate that Viral RNA Extraction buffer can provide a quick and convenient workflow for the detection of viral particles in saline, saliva, or viral transport media.