Steering the metabolism of Bacillus subtilis under oxygen-limited conditions with anode assisted electro-fermentation

Abstract

Background: Bacillus subtilis is generally regarded as a ubiquitous facultative anaerobe. Oxygen is the major electron acceptor of B. subtilis, and when oxygen is absent, B. subtilis can donate electrons to nitrate or perform fermentation. An anode electrode can also be used by microorganisms as the electron sink in systems called anodic electro-fermentation. The facultative anaerobic character of B. subtilis makes it an excellent candidate to explore with different electron acceptors, including an anode. This study aimed to optimise industrial aerobic bioprocesses using alternative electron acceptors. In particular, the change of metabolism and end product spectrum of B. subtilis with different electron acceptors, including anode from the electro-fermentation system, was investigated. Results: B. subtilis was grown using three electron acceptors, i.e., oxygen, nitrate, and anode (poised at a potential of 0.70 V vs. standard hydrogen electrode). The results showed oxygen had a crucial role for cells to remain metabolically active. When nitrate or anode was applied as the sole electron acceptor anaerobically, immediate cell lysis and limited glucose consumption were observed. In anode assisted electro-fermentation with a limited aeration rate, acetoin, as the main end product showed the highest yield of 0.78 ± 0.04 molproduct/molglucose, 2-fold higher than without poised potential (0.39 ± 0.08 molproduct/molglucose). Conclusions: Oxygen controls B. subtilis biomass growth, alternative electron acceptors utilisation and metabolites formation. Limited oxygen/air supply enabled the bacteria to donate excess electrons to nitrate or anode, leading to steered metabolic pathways. The anode assisted electro-fermentation showed its potential to boost acetoin production for future industrial biotechnology applications.