Short communication: Lactose utilization of Streptococcus thermophilus and correlations with β-galactosidase and urease.

The ability to use lactose is critical for the application of Streptococcus thermophilus in fermented dairy products. Most studies have evaluated the use of lactose of S. thermophilus by measuring lactose utilization, but its correlation with β-galactosidase and urease has rarely been investigated. In this study, 10 strains of S. thermophilus isolated from fermented yak milk exhibited a diversity of β-galactosidase and urease activities, growth, and acid production in de Man, Rogosa, and Sharpe-lactose. Among the strains, 15G5 possessed the highest β-galactosidase activity and showed the highest cell growth, lactic acid production, and titratable acidity during fermentation. In contrast, 7G10, with the weakest β-galactosidase activity, produced the lowest lactic acid content and change in titratable acidity. Further investigation indicated that β-galactosidase activity of S. thermophilus showed significant positive correlations with the growth of cell densities, the production of lactic acid, and titratable acidity, and urease activity of S. thermophilus showed a significant correlation with the use of lactose and the production of lactic acid and acetaldehyde. These findings suggest that the differences of β-galactosidase and urease activities are essential for the performance in the lactose metabolism, growth, and acid production of S. thermophilus, providing new insights into strain selection and application.