The first step to selecting interesting lactic acid bacteria for commercial use is testing their resistance to different physicochemical stresses. In this study, we evaluated the viability of Enterococcus faecium and Enterococcus durans, obtained from two traditional fermented cheeses, subjected to several stresses (thermal, osmotic, acidic, alkaline, oxidative, detergent, and alcoholic). The assessment of cell viability was conducted via flow cytometry (FCM) combined with nucleic-acid double staining (NADS) and was compared to the conventional plate count method (CFU). The findings from the two approaches indicated that Enterococcus faecium and Enterococcus durans demonstrated a substantial proportion of viable cells following exposure to osmotic, thermal, and acidic stress. The alkaline stress treatment does not diminish the proportion of viable cells. Both strains exhibited extensive sensitivity to SDS, oxidative stress, and experienced total cell death under alcoholic stress. We observed a satisfactory correlation between cell viability as measured by FCM and CFU under all stress conditions. These data demonstrate the existence of indigenous strains of Enterococcus spp. that exhibit notable stress resistance. FCM for viability enumeration is better than the conventional plate counting method due to its rapid results and precision, which offer an effective evaluation of live, dead, and permeabilised cells. This technique holds promise for physiological state research in dairy applications to evaluate the quality of fermented products and the viable cell count for probiotic manufacturing.
Aouimeur et al. (Tue,) studied this question.