The influence of growth rate-controlling feeding strategy on the surfactin production in Bacillus subtilis bioreactor processes.

Background: The production of surfactin, an extracellular accumulating lipopeptide produced by various Bacillus species, is a well-known representative of microbial biosurfactant. However, only limited information is available on the correlation between the growth rate of the production strain, such as B. subtilis BMV9, and surfactin production. To understand the correlation between biomass formation over time and surfactin production, the availability of glucose as carbon source was considered as main point. In fed-batch bioreactor processes, the B. subtilis BMV9 was used, a strain well-suited for high cell density fermentation. By adjusting the exponential feeding rates, the growth rate of the surfactin-producing strain, was controlled.

Results: Using different growth rates in the range of 0.075 and 0.4 h^-1, highest surfactin titres of 36 g/L were reached at 0.25 h^-1 with production yields Y_P/S of 0.21 g/g and Y_P/X of 0.7 g/g, while growth rates lower than 0.2 h^-1 resulted in insufficient and slowed biomass formation as well as surfactin production (Y_P/S of 0.11 g/g and Y_P/X of 0.47 g/g for 0.075 h^-1). In contrast, feeding rates higher than 0.25 h^-1 led to a stimulation of overflow metabolism, resulting in increased acetate formation of up to 3 g/L and an accumulation of glucose due to insufficient conversion, leading to production yields Y_P/S of 0.15 g/g and Y_P/X of 0.46 g/g for 0.4 h^-1.

Conclusions: Overall, the parameter of adjusting exponential feeding rates have an important impact on the B. subtilis productivity in terms of surfactin production in fed-batch bioreactor processes. A growth rate of 0.25 h^-1 allowed the highest surfactin production yield, while the total conversion of substrate to biomass remained constant at the different growth rates.