Pneumocystis jirovecii

Pneumocystis jirovecii is a yeast-like fungus that is the causative agent of pneumocystis pneumonia (PCP), primarily affecting individuals with weakened immune systems such as those with HIV/AIDS, cancer, organ transplants, or on immunosuppressive therapy[1][4][7][9]. Formerly known as Pneumocystis carinii in humans, it was reclassified based on host specificity; P. jirovecii infects humans while other species infect animals[1][3]. The organism exists mainly in two forms within the lungs: the trophic form and the cyst form. The trophic form attaches to alveolar epithelial cells and proliferates by binary fission; sexual reproduction leads to formation of cysts containing up to eight spores[2][8]. The cell membrane lacks ergosterol—making many antifungal agents ineffective—and instead contains beta-glucans which are useful for diagnosis but not directly targeted by most drugs[3]. Transmission occurs via airborne spread of cyst forms from person-to-person. Most healthy people are exposed during childhood and may carry the organism asymptomatically; disease develops when immunity wanes significantly[5][7]. Diagnosis relies on detection of fungal DNA or visualization in lung samples. Therapeutically, Pneumocystis jirovecii itself is not a molecular drug target like an enzyme or receptor, but rather an infectious pathogen treated with antimicrobial agents that inhibit its metabolic pathways—most notably folic acid synthesis using trimethoprim-sulfamethoxazole as first-line therapy[3]. Alternative drugs include pentamidine and atovaquone. Because this entry refers to an entire microorganism—not a specific molecular target—it should be flagged as incorrect for structured drug-target databases focused on receptors/enzymes/etc., though it remains highly relevant clinically due to its role in opportunistic infections among immunocompromised patients. "The causative organism of pneumocystis pneumonia (PCP) is the fungus called Pneumocystis jirovecii... Cell membrane lacks ergosterol; hence antifungal agents such as azoles and amphotericin B products are inactive against Pneumocystis... The fungus must synthesize its folic acid—a typical target for treatment." [3]

Other names

Pneumocystis carinii (former name)P. jiroveciiP. cariniiPCP organismPneumocystis fungus

Inhibition of folic acid synthesis pathway enzymes (e.g., TMP-SMX targets dihydropteroate synthase and dihydrofolate reductase)[3]