Application of ethidium bromide monoazide for quantification of viable and dead cells of Salmonella enterica by real-time loop-mediated isothermal amplification

A rapid and efficient method for quantification and discrimination of Salmonella enterica ser. Enteritidis between viable and dead cells killed by heat was developed using ethidium bromide monoazide (EMA) in combination with a real-time loop amplified (Rti-LAMP) DNA assay. The use of 8.0 mu g/m1 or less of EMA did not inhibit DNA amplification in Rd-LAMP assays derived from viable cells. However, 8.0 mu g/mlof EMA notably inhibited DNA amplification and significantly increased the Tp values with dead cells. When the DNA from 2000 viable CFU was subjected to EMA-Rti-LAMP the resulting Tp value was 13 min. In contrast, the DNA from 2000 CFU completely heat destroyed CPU still yielded a Tp value, which was greatly increased to 33.1 min. When the DNA from viable plus heat killed CPU at a ratio of 7:1993 was subjected to EMA-Rti-LAMP, the resulting Tp value was 193 min, which was statistically identical (P <0.05) to the Tp value of 19.9 min. obtained with the DNA from only 7 viable CPU. These results indicate that even though 2000 dead cells yielded a Tp value of 33.1 min., low numbers of viable cells in the presence of much higher numbers of dead cells still yielded a linear plot for enumerating viable CPU from Tp values. In addition, propidium monoazide (PMA) was found to be ineffective in distinguishing between low numbers of viable and heat killed cells of S. enterica. (C) 2015 Published by Elsevier B.V.