Morphology and physiology of Spirochaeta aurantia strains isolated from aquatic habitats.

1. Seven strains of Spirochaeta aurantia were isolated from pond and swamp water by means of a selective technique which utilized the ability of these organisms to move through bacterial filters and to diffuse through agar media. Although most of the isolations were accomplished when enrichment media low in carbohydrates were used, all seven strains were found to be exclusively saccharolytic. 2. The isolates could be divided into two groups on the basis of cell morphology: a loosely coiled group, and a tightly coiled group with markedly smaller wave length and wave apmlitude. Spirochetes of the latter group possessed a slightly lower GC content in their DNA. The isolates were facultative anaerobes, synthesized carotenoid pigments which conferred an orange color to aerobic colonies, and utilized a variety of carbohydrates--but not amino acids--as energy sources. Exogenous thiamine was required by six isolates tested, riboflavin by four, and biotin by one. The major products of glucose fermentation were acetate, ethanol, CO2 and H2. Growth of the isolates was inhibited by a variety of antibiotics. Determinations of GC contents of DNA showed that strains of S. aurantia are phylogenetically distant from spirochetes classified in the genera Treponema and Leptospira. 3. S. aurantia populations inoculated in the center of agar medium plates migrated in the form of growth rings toward the periphery of the plates. The rate of migration of glucose-utilizing rings was greatest at low glucose concentrations (e.g., 0.02 g/100 ml). It was concluded that migration of cells present in these rings was mainly due to a chemotactic response to glucose which served both as the attractant and the substrate. Chemotaxis of S. aurantia toward glucose may be used as a selective factor in isolating this bacterium from natural environments. 4. The subspecific epithet stricta is proposed to recognize, taxonomically, the tightly coiled strains of S. aurantia.