Lactiplantibacillus plantarum
uses ecologically relevant, exogenous quinones for extracellular electron transfer
Authors: Stevens Eric T. , Van Beeck Wannes , Blackburn Benjamin , Tejedor-Sanz Sara , Rasmussen Alycia R. M. , Carter Mackenzie E. , Mevers Emily , Ajo-Franklin Caroline M. , Marco Maria L. 
Published: November 2023

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 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, that performs EET and is also a quinone auxotroph.
L. plantarum
selectively used 1,4-dihydroxy-2-naphthoic acid (DHNA) and more hydrophilic naphthoquinones for EET reduction of insoluble iron (ferrihydrite). However, quinones used for EET also inhibited
L. plantarum
growth in non-aerated conditions. Transcriptomic analysis showed that DHNA-induced oxidative stress in
L. plantarum,
but this was alleviated when the electron acceptor, ferric ammonium citrate (FeAC), was included in the growth medium. Although DHNA and FeAC induced
L. plantarum
EET, this metabolism was still dependent on direct access to environmental electron shuttles. To determine whether quinone-producing food fermentation bacteria could be sources 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
Δ
menC
mutant, increased
L. plantarum
ferrihydrite reduction and medium acidification through an EET-dependent mechanism.
L. plantarum
EET was also stimulated by
L. mesenteroides
, resulting in greater environmental acidification and transient increases in
L. plantarum
cell numbers. Our findings show that
L. plantarum
overcomes the toxic effects of exogenous quinones to use those compounds for EET-conferred, ecological advantages during the early stages of food fermentations.

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.