Abstract:

Studies on the tolerability of traditional yogurt strains to gastric acid and bile salt are very important to evaluate probiotic strains in yogurt. The Streptococcus thermophilus and Lactobacillus bulgaricus in yogurt were evaluated for their  olerance to different pH and different concentrations of bile salt in mimic gastrointestinal environment using flow cytometry (FCM) combined with double-fluorescence dye staining by 5(6)-cFDA and PI. The cell viability was very small, and the percentage of labeled viable cells was only 41.8% at an exposure pH of 2.0 and became higher and higher as the exposure pH increased from 3.0 to 7.0 (3.0, 4.0 through 7.0), and the inhibitory effect on viable cells showed a progressive reduction. The percentage of labeled viable cells exposed to 1 g/100mL deconjugated bile salt (DBS) was 48.7%, and those of labeled viable cells exposed to 0.5 g/100mL, 0.25 g/100mL, 0.1 g/100mL and 0.05 g/100mL DBS displayed an increasing pattern. Flow cytometry and plate counts were comparatively used to determine cell viability. The above two species of strains exhibited strong tolerance at the conditions of pH 5.0－7.0 gastric acid and 0.05－0.5 g/100mL deconjugated bile salt. Flow cytometry count was superior to plate count.

Key words: yogurt strain, pH, deconjugated bile salt, gastric acid, living cell, percentage of labeled cells