Detection of viable Zygosaccharomyces bailii in fruit juices using ethidium monoazide bromide and real-time PCR

In this study, we use ethidium monoazide (EMA) a dye commonly used to differentiate viable and nonviable populations of bacteria in real-time PCR (QPCR) assays to eliminate the nonviable cells from the Z. bailii population. Thus we are able to determine the viable Z. bailii population using QPCR plus EMA. To do this we first, optimized the EMA exposure conditions; EMA concentration of 50 µg/ml with an incubation at 30 °C in the dark for 5 min. Followed by light exposure on ice, for 5 min using a 500 W halogen lamp at a distance of 12 cm. Using these optimized conditions, we determined that the assay could detect as few as 12.5 viable Z. bailii cells in the presence of 105 CFU/ml of heat killed-cells. The EMA assay was also more consistent at determining viable cell counts when compared to plating than fluorescent microscopy viable cell counts. Finally, we used the assay to determine the viable population in heat-treated (72 °C, 2 min), ethanol-treated and raspberry cranberry juice Z. bailii cultures. When examining Z. bailii cells treated with 70% ethanol the QPCR assay with EMA (1.22 × 102) showed a better correlation with plating (4.5 × 101 CFU/ml) compared to the QPCR assay without EMA (5.31 × 106 CFU/ml) and this was also seen in the other two injured populations. Thus we feel that we have designed an assay which will be useful for the detection of viable spoilage yeasts in various fruit juices.