Prototheca is a genus of single-celled organisms classified as achlorophyllous algae, meaning they lack the chlorophyll required for photosynthesis. These organisms are common in nature, existing as free-living saprobes that feed on decaying organic matter. While most species are harmless, a few act as opportunistic pathogens in humans and animals. This can lead to protothecosis, a rare infectious disease ranging from localized skin infections to life-threatening systemic illness.
Biological Identity and Environmental Presence
The genus Prototheca is unique because it is an alga that has lost its photosynthetic ability, often leading to its misidentification as a fungus. These organisms are colorless, heterotrophic, and belong to the family Chlorellaceae, closely related to the green algae genus Chlorella. Prototheca species are spherical, yeast-like cells that reproduce asexually through the internal formation of sporangiospores, also known as endospores.
These microalgae are ubiquitous, obtaining nutrients from decaying matter in a saprophytic lifestyle. They thrive in moist, organic-rich settings, including fresh and salt water, soil, sewage, and the slime flux of trees. Animal waste, especially in dairy farming, also serves as a significant reservoir, with species like P. bovis causing mastitis in cattle.
Exposure to the organism is generally exogenous, occurring through direct contact with contaminated environmental sources. Infection typically results from the traumatic inoculation of the organism into the skin or subcutaneous tissue, such as through a wound or minor cut. The two species most commonly associated with human disease are Prototheca wickerhamii and Prototheca zopfii.
Manifestations of Protothecosis
Protothecosis is a rare human disease presenting in three distinct clinical forms that vary in site and severity. The most frequent presentation is cutaneous protothecosis, a localized skin infection often following minor traumatic injury. These lesions can appear as plaques, nodules, ulcers, or areas of thickened, inflamed skin, typically on the limbs.
A second localized form is olecranon bursitis, involving inflammation of the bursa located at the elbow. Patients usually present with a swollen, tender elbow bursa, sometimes with a history of repeated trauma. Both cutaneous and olecranon bursitis infections are commonly seen in individuals with otherwise healthy immune systems.
The third form, disseminated or systemic protothecosis, is the rarest but most severe, involving the spread of the organism to internal organs. This systemic infection is almost exclusively observed in severely immunocompromised patients, such as organ transplant recipients or those undergoing chemotherapy. Affected organs may include the gut, liver, spleen, and central nervous system, potentially leading to life-threatening sepsis. Immunosuppression transforms this localized opportunistic infection into a widespread, high-mortality disease.
Diagnosis and Treatment Protocols
Confirmation of protothecosis relies on laboratory identification of the causative organism from an infected tissue sample or fluid. Diagnostic material, such as a tissue biopsy or aspirated fluid, is typically sent for both culture and microscopic examination. The organism grows well on standard fungal culture media, often producing smooth, yeast-like, white to cream-colored colonies.
Microscopic analysis visualizes the characteristic morphology of Prototheca. The presence of sporangia containing internal endospores, sometimes described as a morula-like or “wagon-wheel” structure, suggests infection. Specialized stains, such as periodic acid-Schiff or Grocott’s methenamine silver, highlight these structures in the tissue.
Treatment strategies are dictated by the disease form and severity, often combining medical and surgical approaches. Localized infections, like cutaneous lesions and olecranon bursitis, are managed with surgical debridement to remove infected tissue. This is usually supplemented with oral antifungal agents, such as fluconazole or itraconazole, which are effective against the organism.
For systemic or disseminated protothecosis, aggressive, long-term intravenous therapy is required. Amphotericin B, a polyene antifungal medication, is considered the most active drug against Prototheca species and forms the mainstay of treatment for widespread disease. Due to the chronic nature of these infections, medical therapy is often prolonged, lasting months to ensure complete eradication.