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Sphaerotilus A genus of Gram-negative bacteria which are not assigned to any taxonomic family. Cells are rod-shaped, occurring mostly in chains which are enclosed by thin sheaths; single cells also occur, and these are motile by means of a tuft of flagella. Sphaerotilus species are aerobic and chemo-organotrophic. There is 1 species, S. natans, found in sewage treatment plants and in slow-moving freshwater streams polluted with sewage and other effluents. It occurs attached to plants, stones, etc., forming a major component of the slimy growth known as sewage fungus.