Enumeration of viable Escherichia coli by real-time PCR with propidium monoazide.

A photo-inducible DNA-binding dye, propidium monoazide (PMA), was used to distinguish viable and dead Escherichia coli cells. Microscopic observations using a combination of the dyes 4',6-diamidino-2-phenylindole and PMA indicated that PMA stained only dead cells, with membrane damage, red. Mixtures of viable and heat-treated E. coli cells were subjected to real-time polymerase chain reaction (PCR) with PMA treatment. Viable cell counts were linearly related to real-time PCR threshold cycle values for PMA-treated cells in the mixtures of viable and heat-treated cells, as long as the ratio of dead cells to viable cells was no greater than 10. In the wastewater treatment plants, total, viable and culturable E. coli were enumerated by real-time PCR, real-time PCR coupled with PMA treatment and the most probable number method using EC-MUG medium, respectively. The concentrations of viable E. coli in the wastewater treatment plants were much higher than those of culturable cells. In addition, viable cells were even more chlorine resistant than culturable ones.