Quantitative and qualitative assessment of cells in several commercial probiotic products and evaluation of storage conditions on cell culturability and viability of a probiotic yeast

One of the essential criteria for determining the quality of products containing probiotics is the presence of a certain number of viable cells per gram of product. To transmit enough viable cells to the gastrointestinal tract, the population of probiotic microorganisms should be preserved during storage. In the present study, culturable cells of some commercial products were isolated and enumerated by the plate count and flow cytometric methods. The isolates were then identified by biochemical and molecular methods. The viability of yeast cells was then assessed at different temperatures during 30 days by the methods above. The results showed that Protexin restore® contained the same types and number of bacteria that were claimed by the manufacturer. In contrast, protexin balance® and Dailyeast® contained a lower number of bacterial cells and yeast cells, respectively. It is noteworthy that only 33% of yeast cells were culturable on YPG agar, while flow cytometric analysis showed that 87% of cells were viable and exhibited cell membrane integrity. Also, yeast cells in Dailyeast® product were stable at 4-35°C. In conclusion, using different culture media and physicochemical conditions for the enumeration of viable and culturable cells may lead to different results. Thus, a combination of different methods should be used to get good results.