Rewiring the respiratory pathway of Lactococcus lactis to enhance extracellular electron transfer

Scrutinizing a Lactococcus lactis mutant with enhanced capacity for extracellular electron transfer reveals a unique role for a novel type-II NADH dehydrogenase

Lactococcus lactis, a lactic acid bacterium used in food fermentations and commonly found in the human gut, is known to possess a fermentative metabolism. L. lactis, however, has been demonstrated to transfer metabolically generated electrons to external electron acceptors, a process termed extracellular electron transfer (EET). Here, we investigated an L. lactis mutant with an unusually high capacity for EET that was obtained in an adaptive laboratory evolution (ALE) experiment. First, we investigated how global gene expression had changed, and found that amino acid metabolism and nucleotide metabolism had been affected significantly. One of the most significantly upregulated genes encoded the NADH dehydrogenase NoxB. We found that this upregulation was due to a mutation in the promoter region of NoxB, which abolished carbon catabolite repression. A unique role of NoxB in EET could be attributed and it was directly verified, for the first time, that NoxB could support respiration in L. lactis. NoxB, was shown to be a novel type-II NADH dehydrogenase that is widely distributed among gut microorganisms. This work expands our understanding of EET in Gram-positive electroactive microorganisms and the special significance of a novel type-II NADH dehydrogenase in EET.IMPORTANCEElectroactive microorganisms with extracellular electron transfer (EET) ability play important roles in biotechnology and ecosystems. To date, there have been many investigations aiming at elucidating the mechanisms behind EET, and determining the relevance of EET for microorganisms in different niches. However, how EET can be enhanced and harnessed for biotechnological applications has been less explored. Here, we compare the transcriptomes of an EET-enhanced L. lactis mutant with its parent and elucidate the underlying reason for its superior performance. We find that one of the most significantly upregulated genes is the gene encoding the NADH dehydrogenase NoxB, and that upregulation is due to a mutation in the catabolite-responsive element that abolishes carbon catabolite repression. We demonstrate that NoxB has a special role in EET, and furthermore show that it supports respiration to oxygen, which has never been done previously. In addition, a search reveals that this novel NoxB-type NADH dehydrogenase is widely distributed among gut microorganisms.

Thiamine-Starved Lactococcus lactis for Producing Food-Grade Pyruvate

Lactococcus lactis is a safe lactic acid bacterium widely used in dairy fermentations. Normally, its main fermentation product is lactic acid; however, L. lactis can be persuaded into producing other compounds, e.g., through genetic engineering. Here, we have explored the possibility of rewiring the metabolism of L. lactis into producing pyruvate without using genetic tools. Depriving the thiamine-auxotrophic and lactate dehydrogenase-deficient L. lactis strain RD1M5 of thiamine efficiently shut down two enzymes at the pyruvate branch, the thiamine pyrophosphate (TPP) dependent pyruvate dehydrogenase (PDHc) and α-acetolactate synthase (ALS). After eliminating the remaining enzyme acting on pyruvate, the highly oxygen-sensitive pyruvate formate lyase (PFL), by simple aeration, the outcome was pyruvate production. Pyruvate could be generated by nongrowing cells and cells growing in a substrate low in thiamine, e.g., Florisil-treated milk. Pyruvate is a precursor for the butter aroma compound diacetyl. Using an α-acetolactate decarboxylase deficient L. lactis strain, pyruvate could be converted to α-acetolactate and diacetyl. Summing up, by starving L. lactis for thiamine, secretion of pyruvate could be attained. The food-grade pyruvate produced has many applications, e.g., as an antioxidant or be used to make butter aroma.

Lactiplantibacillus plantarum uses ecologically relevant, exogenous quinones for extracellular electron transfer

IMPORTANCE

While quinones are essential for respiratory microorganisms, their importance for microbes that rely on fermentation metabolism is not understood. This gap in knowledge hinders our understanding of anaerobic microbial habitats, such in mammalian digestive tracts and fermented foods. We show that Lactiplantibacillus plantarum, a model fermentative lactic acid bacteria species abundant in human, animal, and insect microbiomes and fermented foods, uses multiple exogenous, environmental quinones as electron shuttles for a hybrid metabolism involving EET. Interestingly, quinones both stimulate this metabolism as well as cause oxidative stress when extracellular electron acceptors are absent. We also found that quinone-producing, lactic acid bacteria species commonly enriched together with L. plantarum in food fermentations accelerate L. plantarum growth and medium acidification through a mainly quinone- and EET-dependent mechanism. Thus, our work provides evidence of quinone cross-feeding as a key ecological feature of anaerobic microbial habitats.

Superior anodic electro-fermentation by enhancing capacity for extracellular electron transfer

Anodic electro-fermentation (AEF), where an anode replaces the terminal electron acceptor, shows great promise. Recently a Lactococcus lactis strain blocked in NAD+ regeneration was demonstrated to use ferricyanide as an alternative electron acceptor to support fast growth, but the need for high concentrations of this non-regenerated electron acceptor limits practical applications. To address this, growth of this L. lactis strain, and an adaptively evolved (ALE) mutant with enhanced ferricyanide respiration capacity were investigated using an anode as electron acceptor in a bioelectrochemical system (BES) setup. Both strains grew well, however, the ALE mutant significantly faster. The ALE mutant almost exclusively generated 2,3-butanediol, whereas its parent strain mainly produced acetoin. The ALE mutant interacted efficiently with the anode, achieving a record high current density of 0.81 ± 0.05 mA/cm2. It is surprising that a Lactic Acid Bacterium, with fermentative metabolism, interacts so well with an anode, which demonstrates the potential of AEF.

Anodic Respiration of Vibrio natriegens in a Bioelectrochemical System

Vibrio natriegens promises to be a new standard biotechnological working organism since it grows extraordinarily fast, its productivity surpasses E. coli by far, and genomic tools are getting readily available. Recent studies provided insights into its extracellular electron transfer pathway, revealing it to be similar to other well-known electroactive organisms. Therefore, we aimed to show for the first time that V. natriegens donates electrons from its metabolism to an electrode by direct contact as well as via an artificial redox mediator. Our results demonstrate current densities up to 196 μA cm-2 using an artificial mediator. Via direct electron transfer, 6.6 μA cm-2 were achieved within the first 24 h of cultivation. In the mediated system, mainly formate, acetate, and succinate were produced from glucose. These findings favor V. natriegens over established electroactive organisms due to its superior electron-transfer capabilities combined with an outstanding metabolism.