Pneumocystis jirovecii Pneumonia (PCP) Classification
Pneumocystis jirovecii pneumonia (PCP) is a fungal infection, not a bacterial infection. 1
Organism Classification
Pneumocystis jirovecii is now definitively classified as a fungus based on DNA analysis and cell wall composition. 1 The organism was historically thought to be a protozoan due to certain biological characteristics it shares with protozoa, but molecular studies have conclusively placed it within the fungus kingdom. 1, 2, 3
Key Classification Points:
DNA sequencing and cell wall composition analysis demonstrate that P. jirovecii belongs to the fungus kingdom, despite sharing some biological features with protozoa. 1, 3
The organism cannot be cultured in routine microbiology laboratories, which is an important practical limitation distinguishing it from many other fungi. 1
Taxonomic nomenclature has evolved: the organism affecting humans is specifically named Pneumocystis jirovecii, while P. carinii refers only to the organism found in rats. 1
Clinical Implications of Fungal Classification
The fungal nature of P. jirovecii has important diagnostic and therapeutic implications:
Diagnostic testing includes fungal-specific markers such as β-D-glucan (BDG), which is a component of fungal cell walls and can aid in PCP diagnosis. 1, 4
Treatment approaches reflect its fungal nature, though the first-line agent trimethoprim-sulfamethoxazole has both antibacterial and antifungal properties. 2, 5
The organism is grouped with other pulmonary invasive fungal infections (pIFI) in clinical guidelines for immunocompromised patients, particularly those with hematological malignancies. 1
Common Pitfall to Avoid
Do not confuse the historical classification with current taxonomy. Older literature and some clinicians may still refer to "Pneumocystis carinii pneumonia" or describe it as having "protozoan-like" features, but the definitive classification based on modern molecular analysis is fungal. 1, 3 This matters for understanding transmission patterns, diagnostic approaches using fungal biomarkers, and grouping with other opportunistic fungal infections in immunocompromised hosts.