New high-density fermentation method for producing high molecular weight polysialic acid based on the combination fermentation strategy.

Polysialic acid (PSA) is a long-chain linear amino polysaccharide with broad application prospects; however, its relatively low molecular weight limits its application range. This study aims to explore a new fermentation method of combining the three-phase pH control strategy, three-phase mixing speed control strategy, and exogenous substance to produce high molecular weight PSA. In brief, Escherichia coli K235 6E61 (CCTCC M208088) was used as a fermentation strain. 3 g·L-1 Na5P3O10 was added to the initial medium. At 0-12 h, the mixing speed was controlled to 250 r·min-1, and the pH was maintained at 7.2. At 12-20 h, the mixing speed was increased to 400 r·min-1, the pH was changed to 6.8, and 0.75% n-hexadecane was added at hour 16. After 20 h, the mixing speed was adjusted to 250 r·min-1; the pH was restored to 7.2. Air flow was regulated to 1.2 vvm throughout the experiment. The combination fermentation strategy greatly improved the molecular weight of PSA up to 498 kDa at 32 h, which is currently the maximum molecular weight of PSA produced through microbial fermentation. The yield of PSA reached 6.27 g·L-1 at the end of fermentation (36 h), which is also currently the highest yield of PSA produced by natural bacteria. Therefore, the proposed strategy could simultaneously increase the molecular weight and yield of PSA and is of great importance to the industrial production of high molecular weight PSA. Key points • A new fermentation process was explored to produce high molecular weight PSA. • The yield and molecular weight were improved by the combination fermentation strategy. • The maximum molecular weight and highest yield of PSA were obtained.