Sodium chloride affects propidium monoazide action to distinguish viable cells.

Propidium monoazide (PMA) is a DNA-intercalating agent used to selectively detect DNA from viable cells by polymerase chain reaction (PCR). Here, we report that high concentrations (>5%) of sodium chloride (NaCl) prevents PMA from inhibiting DNA amplification from dead cells. Moreover, Halobacterium salinarum was unable to maintain cell integrity in solutions containing less than 15% NaCl, indicating that extreme halophilic microorganisms may not resist the concentration range in which PMA fully acts. We conclude that NaCl, but not pH, directly affects the efficiency of PMA treatment, limiting its use for cell viability assessment of halophiles and in hypersaline samples.