Motility studies and taxonomy of a rod-shaped bacterium with unusual flagellar fascicles, Aquaspirillum fasciculus sp. nov.

In 1971 Strength and Krieg reported the isolation of a floe-forming gram-negative freshwater rod which exhibited large bipolar flagellar fascicles. Despite the intense activity of the fascicles, the organism had appeared unable to swim. However, free-swimming has now been demonstrated within the highly viscous floes formed by the organisms in viscous solutions of gelatin, DNA and methyl cellulose. With each of these viscous agents there existed an optimum concentration for motility. Higher or lower concentrations led to a decrease in motility. In methyl cellulose, strain XI exhibited optimum motility at a viscosity of 200 cp, while strains X and XI were optimal at 10 cp.

Nitrogenase activity was demonstrated by the use of the semisolid medium of Döbereiner and Day. No nitrogenase activity occurred when liquid medium was used, and the organisms were obligately microaerophilic in nitrogen-free medium. When (NH₄)₂SO₄ was supplied, no nitrogenase activity occurred and the organism grew best aerobically.

The characteristics of strains X, XI and XII indicated that the organisms should be assigned to genus Aquaspirillum. This decision was based on the following considerations: large bipolar fascicles of flagella, a strictly respiratory metabolism, inability to attack carbohydrates, presence of intracellular poly-β-hydroxybutyrate granules, positive reactions for catalase, oxidase and phosphatase, lack of tolerance to 3% NaCl, occurrence of a "polar membrane" in thin sections, and a DNA base composition of 62 to 65% G+C. The strains have been placed in the species Aquaspirillum fasciculus sp. nov.