Bacterial extracellular electron transfer in plant and animal ecosystems

Extracellular electron transfer (EET) is a bioelectrochemical process performed by electrochemically active bacteria (EAB) found in host-associated environments, including plant and animal ecosystems and fermenting plant- and animal-derived foods. Through direct or mediated electron transfer pathways, certain bacteria use EET to enhance ecological fitness with host-impacting effects. In the plant rhizosphere, electron acceptors support the growth of EAB such as Geobacter, cable bacteria, and some clostridia that can result changing iron and heavy metal uptake by plants. In animal microbiomes, EET is associated with diet-derived iron in the intestines of soil-dwelling termites, earthworms, and beetle larvae. EET is also associated with the colonization and metabolism of some bacteria in human and animal microbiomes, such as Streptococcus mutans in the mouth, Enterococcus faecalis and Listeria monocytogenes in the intestine, and Pseudomonas aeruginosa in the lungs. During the fermentation of plant tissues and bovine milk, lactic acid bacteria like Lactiplantibacillus plantarum and Lactococcus lactis may use EET to increase their growth and food acidification, as well as decrease environmental oxidation-reduction potential. Thus, EET is likely an important metabolic pathway for host-associated bacteria and has implications for ecosystem function, health and disease, and biotechnological applications.