Process analysis of the anaerobe Phocaeicola vulgatus in a shake flasks and fermenter reveals pH and product inhibition

Abstract

The anaerobic gut bacterium Phocaeicola vulgatus (formerly Bacteroides vulgatus) has a significant role in the human gut microbiome. It can produce bioactive compounds with antimicrobial properties and industrially relevant organic acids like succinate. However, there is a knowledge gap in understanding the metabolism of P. vulgatus, as cultivation of anaerobic gut bacteria is challenging and usually conducted with enriched microbiota cultures. We aim to close this gap by characterizing this anaerobe bacterium in different cultivation conditions and scales. In this work, axenic cultures were studied in a shake flask and 2 L fermenter scale to characterize the influence of initial pH, buffer concentration, osmolality, and product inhibition on growth and organic acid production by P. vulgatus. Both cultivation systems had online gas measurements for total gas and CO2 production. HPLC analysis generated closed carbon balances, accounting for all produced acids. Total gas transfer rates and CO2 transfer rates revealed that 65% of produced gas was attributed to H2, while just 35% was connected to CO2 production. A minimum buffer concentration of 50 mM MOPS and an initial pH of 7.3 were identified to mitigate pH inhibition in P. vulgatus cultivations with a defined minimal medium and glucose as substrate. The initial addition of lactate showed an inhibitory effect, starting at a concentration of 1 g L−1. On the contrary, initial acetate addition was beneficial for organic acid production. A comparison of a pH-buffered and a pH-controlled 2 L fermentation demonstrated a switch in acid production toward succinate under pH control. The study provides insight into improved cultivation conditions for the gut bacterium P. vulgatus and demonstrates a successful scale-up from the shake flask to the 2 L bioreactor. By applying pH control in the bioreactor, growth was increased, and the organic acid production was switched from lactate to succinate. Even though P. vulgatus could serve as a production organism for interesting bioactive compounds and organic acids, further characterization and improvement are necessary to improve titers.