mGem: Facilitated fermentation—an underappreciated mode of energy conservation

Here, we introduce a new name into the bacterial energy conservation lexicon: facilitated fermentation. This name is necessary because the more familiar terms "respiration" and "fermentation" do not adequately describe how electron balancing is coupled to energy conservation for organisms that engage in this metabolism. Facilitated fermentation is when ATP is predominantly made via a substrate-level pathway that is redox-coupled to a terminal electron acceptor reduced outside of the cell. The coupling is often facilitated by an extracellular electron shuttle or outer membrane protein that shuttles electrons from the electron transport chain to the extracellular acceptor. Naming facilitated fermentation is timely because it has recently been demonstrated to support both growth and non-growth states in bacteria that are important in nature and disease. We hope that the introduction of this term will inspire future research to evaluate the extent of facilitated fermentation's prevalence and impact in the microbial world and beyond.