Nutrient consumption patterns of Streptococcus thermophilus F7 under acid stress and their application in enhancing...

The nutrient consumption patterns of Streptococcus thermophilus F7 under acid stress were investigated in this study. The strain was incubated at 42°C for 16 h in a chemically defined medium with different pH values (6.5, 5.8, and 5.5), and consumption of a variety of nutrients including amino acids, vitamins, purine and pyrimidine bases, and ions was observed. The results showed that Leu was the most consumed amino acid at all pH levels, and the consumption values were 2.5, 3.4, and 4.5 μmol/cfu × 10^-5 at pH 6.5, 5.8, and 5.5, respectively. Consumption of biotin was the highest among vitamins, ranging from 48.3 nmol/cfu × 10^-5 at pH 6.5 to 126.7 nmol/cfu × 10^-5 at pH 5.5. At pH 6.5, adenine was the most consumed purine (78.3 nmol/cfu × 10^-5), whereas uracil became the most consumed base under more acid stress, with values of 122.5 and 122.2 nmol/cfu × 10^-5 at pH 5.8 and 5.5, respectively. Regarding ion consumption, Na⁺ was predominant at pH 6.5 (35.4 μmol/cfu × 10^-5) and 5.8 (41.0 μmol/cfu × 10^-5), whereas K⁺ was the most consumed ion when pH dropping to 5.5 (61.9 μmol/cfu × 10^-5). Certain highly demanded nutrients were individually added to the medium under the same incubation conditions, and the results revealed that at pH 5.8, the optical density at 600 nm of S. thermophilus F7 supplemented with 1.5 times or 2 times of isoleucine increased by 1.13 folds compared with the initial medium. Similarly, at pH 5.5, supplementation with 3 times of proline or adenine resulted in a 1.15-fold or 1.12-fold increase in optical density at 600 nm, respectively. This study showed significant differences in the consumption of various nutrients when culturing S. thermophilus F7, which had the highest requirement for amino acids, followed by ions, vitamins, and pyrimidine bases. Furthermore, the individual addition of amino acids and pyrimidine bases showed improvement in biomass production of the strain. The findings of the present study provide a basis on the nutrient requirements of S. thermophilus F7 and contribute to the design of media for high-cell-density cultivation.