Continuous 2-Keto-l-gulonic acid fermentation by mixed culture of Ketogulonicigenium vulgare DSM 4025 and Bacillus megaterium or Xanthomonas maltophilia

The fermentation process of 2-keto-L-gulonic acid (2KGA) from L-sorbose was developed using a two-stage continuous fermentation system. The mixed culture of Ketogulonicigenium vulgare DSM 4025 and Bacillus megaterium DSM 4026 produced 90 g/L of 2KGA from 120 g/L of L-sorbose at the dilution rate of 0.01 h⁻¹ in a single-stage continuous fermentation process. But after the production period was beyond 150 h, the significant decrease of 2KGA productivity was observed. When the non-spore forming bacteria Xanthomonas maltophilia IFO 12692 was used instead of B. megaterium DSM 4026 as a partner strain for K. vulgare DSM 4025, the 2KGA productivity was significantly improved in a two-stage continuous culture mode, in which two fermentors of the same size and volume were connected in series. In this mode, with two sets of 3-L jar fermentors, the steady state could be continued to over 1,331.5 h at least, when the dilution rates were 0.0382 h⁻¹ and 0.0380 hour⁻¹, respectively, for the first and second fermentors. The overall productivity was calculated to be 2.15 g/L/h at 113.1 g/L and a molar conversion yield of 90.1%. In the up-scaling fermentation to 30-L jar fermentors, 118.5 g/L of 2KGA was produced when dilution rates in both stages were 0.0430 hour⁻¹, and the overall productivity was calculated to be 2.55 g/L/h.