Impact of foaming on surfactin production by Bacillus subtilis: Implications on the development of integrated in situ foam fractionation removal systems

[Display omitted] •Small-scale batch foaming approach to assess the impact of foaming on cellular growth.•Foaming boosted surfactin titers, yields and productivities in B. subtilis.•Joint effect of foaming and glucose-limited conditions in surfactin production.•Stimulation of surfactin production via recycling foamed cells.•Fate of best surfactin producing B. subtilis cells within a foam recovery system. The impact of foaming on cell growth and surfactin production by Bacillus subtilis during a foam recovery process has been evaluated for the first time. First, a small-scale batch foaming approach was employed to assess whether fermentation performance is affected when B. subtilis cells are subjected to foaming. Results showed that foaming stimulated surfactin production, resulting in 1.13-fold higher surfactin producing abilities (17mg/gh) and higher biomass yields (∼0.27g/g) in comparison to fermentation performances achieved by using cells not subjected to foaming. The synergistic combination of glucose-limited conditions and foaming additionally triggered the surfactin yields on glucose (0.1g/g) and titers (0.41g/L), attaining a volumetric surfactin productivity of 17mg/Lh under batch cultivation. Second, continuous foam fractionation experiments enabled controlled segregation of cells by foaming. This revealed that foamed cells displayed 2-fold higher specific surfactin production rates (16mg/gh) than non-foamed cells (8mg/gh). Furthermore, cell growth, surfactin titers, and volumetric productivities were further enhanced by re-using spent B. subtilis cells from the column overflow. The present study demonstrated for the first time the concept of using foam fractionation for improving cellular growth while triggering surfactin production by recycling the spent B. subtilis cells resulted from a foam recovery process.