FIG 8 .

Proposed model for P. mirabilis energy metabolism during swarming. The rotation of flagellum and oxidative phosphorylation are dependent on the proton gradient that is generated by membrane respiration to oxidize NADH to NAD⁺. The majority of NADH is formed during the complete oxidation of pyruvate to CO₂ in the TCA cycle. The decreased interval between swarming and consolidation for aerobic TCA cycle mutants suggests that the capacity to reoxidize NAD⁺ and establish a proton gradient is due to inappropriate fermentation caused by loss of the complete TCA cycle and by the diminished capacity to metabolize fumarate. Extracytoplasmic substrate oxidation of dihydrogen and cytoplasmic reduction of glycerol could maintain the proton gradient and allow for reoxidation of NAD⁺ in the absence of electron transfer to oxygen. The ability to support motility without respiring oxygen may reflect the reduced energy demand caused by multicellular cooperation during swarming.