A genus of motile, nonsporeforming, aerobic bacteria (family Spirillaceae) containing gram-negative, rigid, helical, or helically curved cells that are 0.2-1.5 mcm in diameter. Motile cells contain fascicles of flagella at one or both poles. Some species can grow anaerobically with nitrate instead of oxygen as the terminal electron acceptor. These organisms are chemoorganotrophic, possessing a strictly respiratory metabolism. They do not ferment carbohydrates; a few species can oxidize a limited variety of carbohydrates. The habitat of these organisms is fresh water. The type species is Aquaspirillum serpens. [L. aqua, water, + spirillum, coil]