Rapid and simultaneous quantification of viable Lactobacillus and Streptococcus thermophilus cells in yogurt by flow cytometry combined with novel oligonucleotide probes

Yogurt, one of the most consumed fermented dairy products, is produced with Lactobacillus and Streptococcus thermophilus as classical starter cultures. The enumeration of their viable bacterial cells is extremely important for yogurt production and supervision. This study developed 2 novel oligonucleotide probes (NOP), along with flow cytometry (FC) method for rapid and simultaneous quantification of the 2 probiotics. The NOP exhibited excellent specificity, as validated by 5 Streptococcus thermophilus strains, 11 Lactobacillus strains, and 32 other bacterial strains, and the FC method could accurately identify viable cells. This NOP-FC method achieved high recovery rates for Lactobacillus and Streptococcus thermophilus, showing good repeatability and strong linear relationship with the plate-based method. The entire method, including sample pretreatment, fluorescent staining, and FC analysis, could be completed within 2 h, outperforming all the current methods in time efficiency. In conclusion, this study innovatively established a NOP-FC method for rapid and simultaneous quantification of viable bacteria. The developed method is successfully applicable for the detection of yogurt during fermentation and the shelf life, providing a reliable foundation for fermentation monitoring and quality assessment of yogurt.