Sphaerotilus natans is an aquatic periphyton bacterial organism associated with polluted water. These tightly sheathed filamentous bacteria colonies are commonly but inaccurately known as "sewage fungus"[1]
Straight or smoothly curved filaments 1.5 μm in diameter and 100 to more than 500 μm in length are formed by rod-shaped cells with clear septa growing within a long, tubular sheath. An adhesive basal element at one end of the filament can aid attachment to solid surfaces.[2] The sheath offers some protection from predators, and the ability to anchor in flowing water allows access to a passing stream of food and nutrients.[3] Individual mature cells swarm out of the protective tube to colonize new sites.[4] Each motile mature cell has an intertwined bundle of flagella appearing as a single flagellum consisting of a long filament with a short hook and a basal body complex, but it is distinguishable by electron microscope as 10 to 30 strands with diameters of 12.5 to 16 nm each. S. natans stores reserves of poly- beta -hydroxybutyrate as internal bioplastic globules making up 30 to 40% of the dry weight of a colony.[3] Gram and Neisser staining reactions are negative.[5]
S. natans requires dissolved simple sugars or organic acids as a food supply, but needs less phosphorus than many competing organisms and can tolerate low oxygen concentrations.[5] Capability to deposit elemental sulfur intracellularly in the presence of hydrogen sulfide is believed to be a detoxifying mechanism. S. natans requires either cobalamin or methionine as a trace nutrient.[3] S. natans filaments can aid development of a periphyton biofilm trapping suspended particles and stabilizing colonies of other organisms including Klebsiella and Pseudomonas.[2]
S. natans is described as a key taxon in sewage fungus, a polymicrobial biofilm that proliferates in rivers with a high organic loading[6] [7] [8] such as from sewage discharges, industrial effluents or runoff from airport de-icing.[9] It is also implicated in active sludge bulking[10]
Sphaerotilus natans is often associated with a buoyant floc (or "bulking sludge") causing poor solids separation in activated sludge clarifiers of secondary sewage treatment.[4] Metal surfaces covered with S. natans may experience accelerated corrosion if the slime creates a barrier causing differential oxygen concentrations.[11] S. natans slimes may reduce quality of paper produced by paper mills that use recycled water.[2]