# Pneumocystis jirovecii

> Pneumocystis jirovecii for NEET-PG Microbiology, covering pneumonia in HIV, the lack of ergosterol, diagnosis, co-trimoxazole treatment and prophylaxis.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/pneumocystis-jirovecii
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Mandell Douglas and Bennett's Principles and Practice of Infectious Diseases; Harrison's Principles of Internal Medicine
- How to cite: "Pneumocystis jirovecii", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/pneumocystis-jirovecii

## Direct answer

Pneumocystis jirovecii is a fungus that causes pneumonia in profound immunosuppression, most often in HIV when the CD4 count falls below about 200 cells per microlitre. It lacks ergosterol in its membrane, so azoles and polyenes are ineffective. Treatment is high-dose co-trimoxazole, with corticosteroids added for hypoxic disease, and the same drug serves as prophylaxis.

## What you must remember

- **Classification:** once classed as a protozoan, now recognised as an ascomycete fungus on the basis of its genetic and ribosomal features.
- **No ergosterol:** the cell membrane contains cholesterol instead, so polyenes and azoles have no target.
- **Clinical picture:** subacute dry cough, exertional dyspnoea and fever, with bilateral interstitial or ground-glass shadows on chest imaging.
- **Diagnosis:** induced sputum or bronchoalveolar lavage examined by silver stain, direct fluorescence or PCR, with serum beta-D-glucan supporting the diagnosis.
- **Treatment:** high-dose co-trimoxazole for 21 days, with prednisolone added when room-air PaO2 is below 70 mmHg.
- **Prophylaxis:** co-trimoxazole started when the CD4 count is below 200 in HIV, and used in transplant and haematological patients as well.
- **Sulfonamide allergy:** alternatives such as atovaquone are used for milder disease, and primaquine with clindamycin for more severe disease.

## Pneumonia with a falling CD4 count

A man with newly diagnosed HIV and a CD4 count of 120 cells per microlitre has a month of dry cough and exertional breathlessness, with bilateral ground-glass shadows on chest radiograph.

1. **Think of the CD4 threshold:** exertional breathlessness with bilateral ground-glass shadows and a low CD4 count raises Pneumocystis strongly, even when the presentation looks subacute.
2. **Measure oxygenation and LDH:** a room-air PaO2 below 70 mmHg marks hypoxic disease that needs steroids, and a raised LDH supports the diagnosis.
3. **Sample the lung:** induced sputum or bronchoalveolar lavage is examined by silver stain or direct fluorescence, and PCR is more sensitive.
4. **Check beta-D-glucan:** a raised serum beta-D-glucan supports the diagnosis, although it is not specific for Pneumocystis.
5. **Start co-trimoxazole:** high-dose treatment is given for 21 days, with prednisolone added when hypoxia is present.
6. **Plan prophylaxis and antiretrovirals:** co-trimoxazole continues until CD4 recovers on antiretroviral therapy, and antiretrovirals are started early after Pneumocystis treatment begins.

Each step builds toward the lung sample and the oxygen status, so treatment rests on evidence rather than the chest film alone.

## Where the exam tries to catch you

Pneumocystis questions usually test the absence of ergosterol and the standard drug and duration.

- **Trap:** prescribing fluconazole or amphotericin B for Pneumocystis pneumonia — **Correct:** the organism lacks ergosterol, so these antifungals fail, and high-dose co-trimoxazole is the treatment.
- **Trap:** assuming a negative routine sputum culture excludes the disease — **Correct:** the organism cannot be grown on routine media, so silver stain, direct fluorescence or PCR is needed.
- **Trap:** stopping prophylaxis after the acute episode — **Correct:** prophylaxis continues until CD4 recovers on antiretroviral therapy, because the risk returns when immunity falls.

## Frequently asked questions

### Why do antifungal drugs fail against Pneumocystis jirovecii?
Pneumocystis jirovecii lacks ergosterol in its membrane and uses cholesterol instead, so polyenes and azoles have no target. Co-trimoxazole acts on folate synthesis and is the effective treatment. This is why exam questions link Pneumocystis pneumonia to co-trimoxazole rather than antifungals.

### At what CD4 count is Pneumocystis prophylaxis started in HIV?
Co-trimoxazole prophylaxis is started in HIV when the CD4 count falls below 200 cells per microlitre. It is also used in transplant and haematological patients at risk. Dapsone or atovaquone can replace co-trimoxazole when it is not tolerated.

### When are corticosteroids added in Pneumocystis pneumonia?
Corticosteroids are added to co-trimoxazole in moderate to severe Pneumocystis pneumonia with hypoxia, usually when room-air PaO2 is below 70 mmHg. They reduce inflammatory lung injury when started early. Steroids are given alongside antimicrobial therapy, never in place of it.

### Which tests detect Pneumocystis jirovecii in respiratory samples?
Silver stains such as Gomori methenamine silver, direct fluorescent antibody staining and PCR detect Pneumocystis jirovecii in induced sputum or bronchoalveolar lavage. Because the organism cannot be cultured on routine media, these methods are needed. A raised serum beta-D-glucan supports the diagnosis.