Group of bacteria found widely in nature in slow-running water.
Many species are attached to underwater surfaces. They are characterized by a threadlike, branching arrangement of cells enclosed in a sheath. The sheaths of some are variously encrusted with iron or manganese oxides, depending on the water. One of the best-known is a common species, Sphaerotilus natans ; in polluted water it has thin, colourless sheaths, and in unpolluted water containing iron it has yellow-brown, iron-encrusted sheaths that often grow into long slimy tassels.