Propidium monoazide (PMA) is a photoreactive DNA-binding dye that preferentially binds to dsDNA.

[1] Visible light (high power halogen lamps or specific LED devices[2]) induces a photoreaction of the chemical that will lead to a covalent bond with PMA and the dsDNA.

[4] Theoretically, dead microorganisms lose their capability to maintain their membranes intact, which leaves the "naked" DNA in the cytosol ready to react with PMA.

[5] The main use of PMA is in Viability PCR but the same principle can be applied in flow cytometry or fluorescence microscopy.

However, the ability of PMA in differentiating viable and non-viable cells varies for different bacteria.