Differential effects of sodium ions on motility in the homoacetogenic bacteriaAcetobacterium woodii andSporomusa sphaeroides

Abstract

The strictly anaerobic homoacetogenic bacteriaAcetobacterium woodii andSporomusa sphaeroides differ with respect to their energy metabolism. Since growth as well as acetate and ATP formation ofA. woodii is strictly dependent on Na⁺, but that ofS. sphaeroides is not, the question arose whether these organisms also use different coupling ions for mechanical work, i.e. flagellar rotation. During growth on fructose in the presence of Na⁺ (50 mM), cells ofA. woodii were vigorously motile, as judged by light microscopy. At low Na⁺ concentrations (0.3 mM), the growth rate decreased by only 15%, but the cells were completely non-motile. Addition of Na⁺ to such cultures restored motility instantaneously. Motility, as determined in swarm agar tubes, was strictly dependent on Na⁺; Li⁺, but not K⁺ partly substituted for Na⁺. Of the amilorides tested, phenamil proved to be a specific inhibitor of the flagellar motor ofA. woodii. Growth and motility ofS. sphaeroides was neither dependent on Na⁺ nor inhibited by amiloride derivatives. These results indicate that flagellar rotation is driven by$\Delta \tilde \mu _{Na^ + }$ inA. woodii, but by$\Delta \tilde \mu _{H^ + }$ inS. sphaeroides.