How does viability PCR (vPCR) work?

Viability PCR (V-PCR) utilizes dyes such as PMA or PMAxx™. Both dyes are photoreactive and membrane-impermeable, and have a high affinity for DNA. They intercalate into double-stranded DNA (dsDNA), forming a covalent bond on exposure to intense visible light. This reaction inhibits PCR amplification of the modified DNA templates through two mechanisms: inhibition of template amplification by DNA polymerases and removal of the modified DNA during purification. 

Because these dyes cannot penetrate intact cell membranes, they selectively target dead cells with compromised membranes. When a viability dye is applied to a sample containing both live and dead bacteria, only the DNA of the dead bacteria with compromised membranes undergo modification.

In a quantitative PCR (qPCR) reaction, the DNA from dead cells will show delayed amplification and a higher cycle threshold (Ct) compared to the DNA from live cells.