Thiothrix is a genus of filamentous sulfur-oxidizing bacteria, related to the genera Beggiatoa and Thioploca. They are usually Gram-negative (but can be Gram-variable) and rod-shaped (0.7–1.5 µm in width by 1.2–2.5 µm in length). They form ensheathed multicellular filaments that are attached at the base, and form gonidia at their free end. Thiothrix live primarily in flowing water containing a source of sulfide but are also present in activated-sludge waste water treatment systems. They are aerobic or microaerophilic in their oxygen requirements.