group of microorganisms found widely in nature in slow-running water, many species of which are attached to submerged surfaces. They are characterized by a filamentous arrangement of cells enclosed in a sheath. The sheaths of Leptothrix, Crenothrix, and Clonothrix are variously encrusted or impregnated with iron or manganese oxides, depending upon the water.

One of the best known sheathed bacteria is Sphaerotilus natans, a common species, which in polluted water has thin and colourless sheaths and in unpolluted water, containing iron, has yellow-brown iron-encrusted sheaths that often grow into long slimy tassels.