Continuous processing can get products to market about 12 months faster than batch processing, according to a 2022 paper by the FDA. Understandably, the drive to transition to continuous bioprocessing is strong.
For fermentation products, the bulk of the transitional work has involved E. coli platforms. Little has been done to speed the transition for another industrial-scale expression platform, Bacillus licheniformis, which is known for its rapid growth, high productivity, and high-purity output.
Julian Kopp, PhD, a postdoctoral researcher at Technical University Vienna (TU Wien), and colleagues there and at White Biotechnology Research assessed the transferability of B. licheniformis from batch and fed-batch fermentation to continuous cultivation.
“The literature states fed-batches are applicable for transitioning to chemostat cultivation for other inductive systems,” Kopp, the paper’s corresponding author, tells GEN. “It was a surprise for us, therefore, to see that chemostat cultivations have to be screened individually from fed-batch cultivations, and that researchers have to establish hard data with continuous cultivations in order to determine suitable operating ranges.”
In a recent paper, they report significant differences in productivity between fed-batch and chemostat cultivations, despite similar growth rates. Kopp and colleagues found continuous systems showed “a clear dependence on generation time.” To ensure stable productivity for recombinant protease production, those cultivations needed dilution rates between 24% and 50% and specific substrate uptake rates above 30%. Fed-batch cultivations, conversely, needed low growth rates to enhance productivity.
Biopharmaceutical developers aiming to transfer B. licheniformis from fed-batch to continuous cultivation processes, they conclude, should not rely upon the data from fed-batch screenings.
A closer look at their data shows that chemostat cultivations with an intermediate dilatation rate of approximately 40% of the maximum uptake rate were optimal for product formulation. Productivity dropped significantly both below 25% and above 72%. Biomass formation remained consistent, although productivity, measured in terms of protein concentration, declined over time.
“Fed-batch cultivations showed a strong effect of uptake rate in regard to biomass formation and specific protease productivity,” the researchers note. That system, however, showed a 30% increase of space-time yield when compared to continuous cultivation. “We attributed this effect to the higher amount of energy being applied, whilst still maintaining derepressed conditions, in comparison to the constant-fed reference processes,” they report.
Throughout their experiments, even the worst of the fed-batch cultivations significantly outperformed continuous cultivations. For perspective, the least productive screening had a yield of 80, specific protease productivity of 119 (+/- 31), and an uptake rate of 48 (+/-1). Meanwhile, the most effective continuous cultivation screening had a space-time yield of 46 (+/- 11), a specific protease productivity of 69 (+/- 17), and an uptake rate of 50 (+/-4).
“Our results indicate that in a derepressed induction system, a narrow operational range that supports either biomass growth or induction is essential for effective continuous cultivation,” the researchers conclude.