LAUSR

Abstract In this study, Mg–Al layered double hydride (LDH) was prepared and the impact of LDH addition on the biomass and surfactin yield for cultures of Bacillus subtilis CWS1 was examined. Various solid additives, including montmorillonite (MMT), activated carbon (AC) and LDH, were tested. Among these, LDH triggered the highest surfactin yield at 3789 mg/L. The optimal conditions for surfactin production were 2 g/L LDH supplied on the beginning of a 5-day cultivation, resulting in a 3.8-fold increase in yield. To reveal the mechanism of surfactin enhancement, observation was conducted via the optical microscope. The resulted images showed that when LDH was added, the cells became segmented, short and narrow compared to cells without any addition. It is apparent that LDH exhibits a slightly toxic effect, limiting cell growth. The surviving cells were forced to secrete more surfactin to protect themselves from the harsh environment. Since the cells could no longer grow, their only function was to secrete surfactin into the broth, leading to an extraordinarily high surfactin yield with a low biomass production. This suggests that an alternative means for efficient surfactin production is through the addition of LDH.