Abstract:

Direct labeling of starter cells with the fluorescent dye in combination with flow cytometry for rapid assessment
of cell viability and survival status has practical significance for the scientific evaluation of the quality of various microbial
fermentation starters. Direct vat set yogurt starter consisting of either Lactobacillus bulgaricus or Streptococcus thermophilus
as well as their freshly harvested cells and heat-killed cells were evaluated for their viability using flow cytometry combined
with staining with two specific fluorescent dyes, 5(6)-cFDA and propidium iodide (PI). Double staining with 5(6)-cFDA
and PI was suitable for evaluating the cell viability of L. bulgaricus in direct vat set starter, which was low, only 4.7%, while
staining with 5(6)-cFDA alone could more effectively evaluate the viability of S. thermophilus in direct vat set culture, which
was almost 90%. Also, the same results were observed in confirmatory experiments. Finally, our results also showed that
PI did not give clear live/dead discrimination for S. thermophilus. Consequently, flow cytometry is a reliable tool to rapidly
evaluate the viability of direct vat set starter cultures in dairy industry.

Key words: fluorescent dye, flow cytometry, direct vat set starter, viability