What is the liquid in the virus transport medium? Why is it red?

The nucleic acid test, as the name suggests, is the collection of nucleic acids (by using nasal swabs or oropharyngeal swabs) and then amplification by PCR to detect the presence of a target sequence to determine whether a species is present. Since the outbreak of Covid-19, this technology has gradually been in the public eye.

Once the biological sample is collected, it is vital to ensure that the nucleic acid is intactly preserved. However, like bacteria, exfoliated cells, and other secretory components may have active ingredients that catalyze nucleic acid cleavage. In particular, the genetic material of Covid-19 is RNA, which is weaker than DNA and is easier to lyse.

Therefore, the viral transport medium(VTM) has only one mission – to keep the genetic material in the test sample intact. It can be divided into two transport media depending on whether it is inactivated.

Inactivated Transport Medium

The simplest method is to kill all viruses and cells and inhibit all potential RNA hydrolases. For example, surfactants/lysis salts are used to destroy the membrane structure of viruses/cells so that viral proteins lose physiological activity. The enzymes are inhibited with guanidine salts (e.g., guanidine isothiocyanate) to protect the nucleic acid, similar to the nucleic acid extraction lysate (TRIzol reagent). It is commonly used for routine biological testing to avoid the risk of aerosol infection by transmissible viruses.

Guanidine-free lysis inactivation type: A non-guanidine salt protein denaturing agent is added, which lyses the virus to inactivate it and release nucleic acids into the solution. There is no need of nucleic acid extraction, suitable for direct PCR amplification (but its solubility and denaturation capacity is relatively weak compared to guanidine salts)

Non-inactivated Transport Medium

Non-inactivating transport media are made of organic solvent and inert materials designed to maintain the viability of clinical specimens containing viruses. They suppress microbial contamination and protect samples during transport and processing. The main components of an inactivated viral transport medium are basal Hank’s balanced salt solution. Amphotericin B are added to inhibit competing bacteria and yeast. The HEPES buffer is used to maintain the proper pH level.

Personal protection should be prioritized because the non-inactivating VTM contains viable viruses/bacteria. There is a risk that the researcher will become infected. At the same time, costly cold-chain transportation is required for efficient preservation. It can be utilized for virus isolation, cultivation, and development of vaccines.

Summary

Although the two transport media serve similar purposes, there are still some fundamental distinctions. While non-inactivated VTM stabilizes cell/viral viability and prevents bacterial reproduction, inactivated, VTM protects nucleic acids from viruses and cell disease.

A pH indicator is typically added to the solution to visually observe the status of the solution (under different pH values, the colors alter).

 Phenol Red sodium salt (CAS:34487-61-1) is widely used as pH indicator. The solution is red in neutral, yellow in acidic, and purple in alkaline. The solution will turn yellow if the solution is tainted with bacteria and the pH declines. If you add vinegar to this fuchsia red solution, it should become yellow, so friends with conditions can test it.

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