New insights in enumeration methodologies of probiotic cells in finished products

The number of bacterial cells is currently recognized as the most important parameter for the efficacy and quality of finished probiotic or live biotherapeutic products (LBP). Cell enumeration is generally performed by culture-dependent methodologies like plate count (PC). These techniques are able to reveal the number of viable cells able to replicate and generate a colony. However, they are limited by their dependence on the combination of culture conditions (e.g. nutrients, temperature) selected for cell recovery. Additionally, they do not provide information on the heterogeneity of a bacterial culture, namely they do not detect the cells in a viable but not cultivable (VBNC) status. Flow-cytometry (FC) is a culture-independent methodology having the potential to enumerate selectively live and damaged or dead cells. FC relies on the use of specific probes for different cell targets (e.g. membrane, enzymes) to unveil information on the cell structure and physiological statuses within a bacterial population. In this context, we monitored three batches of freeze-dried Lactobacillus rhamnosus GG (ATCC 53103) during a 3 year of storage at different conditions of temperature and relative humidity, according to ICH guidelines, by means of PC and FC. The Arrhenius model was applied to assess the suitability of the model to predict the mortality of probiotic cells in finished products. The higher destruction rate (k) obtained by PC data compared to FC data suggests a faster reduction of cultivability compared to membrane integrity, probably representing a dynamic shift of the bacterial population into a VBNC/dormant status during storage time. Interestingly, this mechanistic approach works both for PC and FC methodologies increasing the chances to monitor biological phenomenon within a mathematical modelling. The combined use of PC and FC shed lights on the true bacterial potency within a closed system like a finished product and the complexity of its heterogeneity.