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[Pneumocystis jirovecii]

Pneumocystis jirovecii cysts are thick-walled, rounded and approximately 5-8 µm in size, although thin-walled cysts exist also.  Cysts contain up to eight intracystic bodies.  Precysts have also been described for this organism.  Precysts are spherical, 4-7 µm in diameter and do not contain intracystic bodies (but may contain one or more nuclei).  Cysts are found in the lungs and many other extrapulmonary specimens, especially in immunocompromised patients.

A	B

A:  Three cysts of P. jirovecii in a bronchoalveolar lavage (BAL) specimen, stained with Giemsa.  Note the presence of several smaller, isolated trophozoites.
B:  Cysts of P. jirovecii in lung tissue, stained with silver stain.  The walls of the cysts are stained black; the intracystic bodies are not visible with this stain.