Original Articles
Likun Cheng, Jing Wang, Shugua
Abstract
pH is a key culture parameter for growth of Streptococcus suis and formation of lactate, and an appropriate pH is important to increase the cell density of S. suis. Here, we investigated the effect of pH on S. suis fermentation, and the results indicated that the optimum pH range was 7.0-7.5 and pH controlled using a two-stage strategy [pH 7.0 (0-4 h) and pH 7.5 (4-10 h)] had a positive impact on improvement of cell density. Specifically, NaOH was used to adjust pH during the initial 4 h and mixture of NaOH and KOH (2:1, v/v) were used to control pH during 4-10 h, which resulted in the concentrations of Na+ and K+ below the threshold for inhibiting S. suis fermentation. Furthermore, a pH feedback feeding strategy was applied in S. suis fermentation, the concentration of residual glucose was maintained at approximately 0.10 g/L, and the accumulation of lactate decreased to 4.17 g/L, and the concentration of Na+ and K+ were 70.2 mmol/L and 30.1 mmol/L. which was accompanied by a high cell density (2.347) and viability (8.42x109 colony forming units/mL) because of the reduction of lactate accumulation and proper levels of pH, Na+ and K+.