Motility and chemotaxis in alkaliphilic Bacillus species

Alkaliphilic Bacillus species grow at pH values up to approximately 11. Motile alkaliphilic Bacillus use electrochemical gradients of Na+ (sodium-motive force) to power ion-coupled, flagella-mediated motility as opposed to the electrochemical gradients of H+ (proton-motive force) used by most neutralophilic bacteria. Membrane-embedded stators of bacterial flagella contain ion channels through which either H+ or Na+ flow to energize flagellar rotation. Stators of the major H-coupled type, MotAB, are distinguishable from Na+-coupled stators, PomAB of marine bacteria and MotPS of alkaliphilic Bacillus. Dual ion-coupling capacity is found in neutralophilic Bacillus strains with both MotAB and MotPS. There is also a MotAB variant that uses both coupling ions, switching as a function of pH. Chemotaxis of alkaliphilic Bacillus depends upon flagellar motility but also requires a distinct voltage-gated NaChBac-type channel. The two alkaliphile Na+ channels provide new vistas on the diverse adaptations of sensory responses in bacteria.