How does Propidium Monoazide (PMA) work?

PMA works by selectively penetrating nonviable microbial cells with compromised membranes and combining only with exposed DNA of nonviable bacteria. Once inside the cell, PMA intercalates into the double-stranded DNA and forms a covalent bond on exposure to intense light. 

When PMA is applied to a sample containing viable and nonviable bacteria, only the DNA of the nonviable bacteria with compromised membranes undergo modification. After PMA-treatment, only the DNA from viable bacteria is usable in qPCR, enabling researchers to obtain only the amplified DNA of living organisms.