Microbiology · Yeasts and yeast-like fungi
Pneumocystis jirovecii !
- Fungus
- Exam priority: medium
An atypical fungus that cannot be cultured; it causes severe pneumonia in advanced HIV or other immunosuppression. Prevented with trimethoprim-sulfamethoxazole.
Updated 2026-11-01
Also called: Pneumocystis, formerly P. carinii
At a glance
- Shape
- Round or cup-shaped cysts, sometimes crushed
- Family
- Pneumocystidaceae (Taphrinomycotina)
- Type
- Atypical fungus (once thought a protozoan); cysts and trophic forms; not culturable
- Stain
- Silver (GMS): cyst wall; Giemsa: trophic forms; direct immunofluorescence
- Size
- Cysts of about 4–7 µm
Morphology
- Thick-walled cysts that stain with silver and often look wrinkled or cup-shaped
- Small trophic forms that stain with Giemsa
- Its membrane has cholesterol instead of ergosterol
- Does not grow in culture media
Transmission
- Airborne, from person to person
- Healthy adults can carry it without symptoms and pass it on
- Most children are infected in early years without falling ill
Reservoir: Humans (asymptomatic carriers)
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Attaches to type I pneumocytes and fills alveoli with a foamy exudate
- Without ergosterol it escapes azoles and amphotericin B
- Variable surface glycoproteins that evade the immune response
Diseases it causes
- Pneumocystis pneumonia: dry cough, fever and progressive breathlessness with hypoxemia Pneumocystis pneumonia (PCP) →
- An AIDS-defining illness, typical with CD4 below 200 cells/µL HIV infection and AIDS →
Clinical picture
- Desaturation on exertion with a nearly normal lung exam
- Bilateral ground-glass opacities on X-ray or CT
- Risk of pneumothorax
- In people without HIV (transplant, steroids) the course is faster and more severe
Diagnosis
- Bronchoalveolar lavage or induced sputum with silver (GMS) or direct immunofluorescence
- PCR on a respiratory sample
- Raised serum beta-D-glucan (supportive)
- Raised LDH and bilateral ground glass on CT
Treatment
Trimethoprim-sulfamethoxazole (folate synthesis inhibitors) is the treatment of choice; alternatives are pentamidine, atovaquone, and primaquine with clindamycin. Corticosteroids are added for moderate or severe hypoxemia. Classic antifungals do not work.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Trimethoprim-sulfamethoxazole prophylaxis in HIV with CD4 below 200 cells/µL, in transplant recipients and with prolonged corticosteroids
- Prophylaxis alternatives: dapsone, atovaquone or inhaled pentamidine
- Antiretroviral therapy to restore CD4
- Avoid sharing a room with other immunosuppressed patients
Test yourself
Think of the answer, then tap the question to see it.
At what CD4 count is Pneumocystis prophylaxis started in people with HIV?
When CD4 is below 200 cells/µL (also with oropharyngeal candidiasis).
Which drug class is used to treat and prevent Pneumocystis pneumonia?
Trimethoprim-sulfamethoxazole, a combination that blocks folate synthesis.
Why does Pneumocystis not respond to amphotericin B or azoles?
Its membrane has cholesterol instead of ergosterol, the target of those antifungals.
Which stain shows Pneumocystis cysts in bronchoalveolar lavage?
Gomori methenamine silver (GMS); direct immunofluorescence is also used.
Related
Related conditions
← Back to the group: Yeasts and yeast-like fungi
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Pneumocystis Pneumonia Basics (CDC, 2024); Pneumocystis jirovecii pneumonia: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).
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Representative model, for educational purposes only.