The receptor-binding domain of botulinum neurotoxin (HC fragment), is a promising botulism vaccine candidate. In the current study, fermentation strategies were evaluated to upscale HC fragment expression. A simple translation of the growth conditions from shake flasks to a batch fermentation process resulted in limited culture growth and protein expression (OD of 11 and volumetric protein yields of 123 mg/L). Conducting fed-batch fermentation with rich media and continuous nutrient supplementation significantly improved culture growth (OD of 40.3) and protein expression (1093 mg/L). A further increase in HC fragment yield was achieved by high cell density cultivation (HCDC). The bacterium was grown in a defined medium and with a combined bolus/continuous feed of nutrients to maintain desired oxygen levels and prevent acetate accumulation. The final OD of the process was 260, and the volumetric yield of the HC fragment was 2065 mg/L, which reflects improvement by an order of magnitude. Purified HC fragments, produced by HCDC, exhibited typical biochemical and protective characteristics in mice. Taken together, the advancements achieved in this study promote large-scale production of the HC fragment in E. coli for use in anti-botulism vaccines.
Keywords: Clostridium botulinum; fermentation; high cell density cultivation; recombinant protein expression; subunit vaccine.