Prototheca is an aerobic, achlorophilic and unicellular organism with ovoid shape; it is often confused with yeasts. Prototheca cells are fairly common in a variety of enviroments such as sewage, freshwater, soil, rotting leaves, faeces, skin and various mammals (deer, cat, dog, cattle and humans). Prototheca species are widespread in housing areas, pens and pastures used by dairy cattle and can cause mastitis in dairy cattle. Protothecosis is an uncommon infection of humans and animals. Most infections are clinical and remain as chronic infections. Five species of Prototheca have been identified: P. moriformas, P. stagnora, P. ulmea, P. wickerhamii, P. zopfii; only the last two of these cause infection in humans and animals. Little is known about the ecology of Prototheca. Failure to isolate Prototheca spp. may be explained by the fact that they are readly over-grown by bacteria and fungi when culture is attemped from contaminated sources, as well as the fact that they superficially resemble yeasts. For this reason a specific culture media has been developed for Prototheca spp. (c) 2006 Lippincott Williams & Wilkins.