Prophage induction and acetate availability are associated with distinct Lactiplantibacillus plantarum electrode responses.

UNLABELLED: Lactic acid bacteria can use an electron transport system to pass reducing equivalents to extracellular mediators. This redox flexibility decouples carbon and electron balances and presents the opportunity to use electrodes to engineer fermentations. We show that growth with an anode may be a two-edged sword for the fitness of the model lactic acid bacterium Lactiplantibacillus plantarum grown in a chemically defined medium supplemented with quinones, depending on substrate and cultivation regime. In batch culture on glucose, the anode slows growth despite yielding an over four-fold higher ratio of acetate to lactate than the open-circuit control, a fermentation profile associated with higher ATP yield. In batch culture on the more reduced substrate mannitol, the anode facilitates growth by enabling the dissipation of reducing equivalents. However, when acetate is also present as an alternate electron sink, the anode prolongs the lag phase while maintaining a growth rate similar to the open-circuit control, despite the expected ATP cost of acetate-to-ethanol reduction. Under semi-continuous cultivation on mannitol in the absence of acetate, anode polarization reduces growth yield relative to open-circuit conditions. Although the anode promotes more energetically favorable fermentation patterns during batch growth on both glucose and mannitol, these are likely counterbalanced by prophage induction to affect population growth, as confirmed by transcriptomic analysis, extracellular DNA measurement, and transmission electron microscopy. Together, these results indicate that extracellular electron transfer associated with an anode is not necessarily beneficial for fermenters and prompt further inquiry into the context-dependent nature of this style of metabolism.

IMPORTANCE: A hybrid metabolism has been described in widespread fermenters where energy conservation through substrate level phosphorylation is coupled with electron transfer to external electron acceptors via extracellular redox mediators. This mechanism shapes interactions in microbial systems and presents opportunities in biotechnology. The variable physiological effects observed here of using an electrode as an electron sink highlight both the potential and challenges of applying electrodes to regulate fermentations. We show that polarization of an anode can hinder Lactiplantibacillus plantarum growth and is associated with induction of prophages, increasing the rate of lysis and thereby possibly counteracting the benefits of metabolic flexibility and more energetic fermentation patterns. These results make a new connection between electron balancing and a mobile genetic element and suggest a dynamic, context-dependent role of these mediators as public goods.