Prototheca wickerhamii is a ubiquitous alga that does not have chlorophyll. It is widely present in the environment and is a rare cause of infection in humans (protothecosis) and most commonly presents as nodules of the skin. Most cases reported have a suppressed immune system (from drugs or disease). Infection usually results by direct traumatic inoculation.[full citation needed]
Diagnosis can be made through culture of infected fluid in Sabouraud dextrose agar or by visualization of sporangia containing sporangiospores on tissue biopsy (using hematoxylin/eosin, GMS, or PAS histochemical stains). The organism incites a chronic granulomatous inflammation with infiltrate of histiocytes, lymphocytes, giant cells and occasional eosinophils. The organism has thick wall, internal septations, measures 3-11 µm in diameter. The sporangia have very small wedge-shaped endospores arranged radially and moulded (morula-like form).
Differential diagnosis: Protothecal sporangia may be confused with Coccidioides immitis, which are much larger