Lactiplantibacillus plantarum uses ecologically relevant, exogenous quinones for extracellular electron transfer
Extracellular electron transfer (EET) is a metabolic process that frequently uses quinones to couple intracellular redox reactions with extracellular electron acceptors. The physiological relevance of this metabolism for microorganisms that are capable of EET, but unable to synthesize their own quinones, remains to be determined. To address this question, we investigated quinone utilization by Lactiplantibacillus plantarum, a microorganism required for food fermentations, performs EET, and is also a quinone auxotroph. L. plantarum selectively used 1,4-dihydroxy-2-naphthoic acid (DHNA), 2-amino-3-carboxy-1,4-naphthoquinone (ACNQ), 1,4-naphthoquinone, and menadione for EET reduction of insoluble iron (ferrihydrite). However, those quinones used for EET also inhibited L. plantarum growth in non-aerated conditions. Transcriptomic analysis showed that DHNA induced oxidative stress in L. plantarum and this was alleviated by the inclusion of an electron acceptor, soluble ferric ammonium citrate (FeAC), in the laboratory culture medium. The presence of DHNA and FeAC during growth also induced L. plantarum EET metabolism, although activity was still dependent on the presence of exogenous electron shuttles. To determine whether quinone-producing bacteria frequently found together with L. plantarum in food fermentations could be a source of those electron shuttles, L. plantarum EET was measured after incubation with Lactococcus lactis and Leuconostoc mesenteroides. Quinone-producing L. lactis, but not a quinone-deficient L. lactis {Delta}menC mutant, increased L. plantarum ferrihydrite reduction and medium acidification through an EET-dependent mechanism. L. plantarum EET was also stimulated by L. mesenteroides, and this resulted in greater environmental acidification and transient increases in L. plantarum growth. Overall, our findings revealed that L. plantarum overcomes the toxic effects of exogenous quinones to use those compounds, including those made by related bacteria, for EET-conferred, ecological advantages during the early stages of food fermentations.