Propidium monoazide is unreliable for quantitative live-dead molecular assays

Propidium monoazide (PMA) is a dye that distinguishes between live and dead cells in molecular assays like Polymerase Chain Reaction (PCR). It works by cross-linking to the DNA of cells that have compromised membranes or extracellular DNA upon photoactivation, making the DNA inaccessible for amplification. Currently, PMA is used to detect viable pathogens and alleviate systemic bias in the microbiome analysis of samples using 16S rRNA gene sequencing. In these applications, treated samples consist of different amounts of dead bacteria and a range of bacterial strains, variables that can affect the performance of PMA and lead to inconsistent findings across various research studies. To evaluate the effectiveness of PMA, we used a sensitive qPCR assay and post-treatment sample concentration to determine PMA activity accurately under varying sample conditions. We report that PMA is unreliable for viability assays when the concentration and composition of the bacterial mixture are unknown. PMA is only suitable for qualitatively assessing viability in samples containing a known number of dead microbes or extracellular DNA.