Recognition of Pneumocystis carinii by gram stain in impression smears of lung tissue.

In 12 of 20 (60%) biopsy-proven cases of Pneumocystis carinii pneumonia, the diagnosis was first suggested by examination of routine Gram stains of impression smears made from infected lung tissue and later confirmed by methenamine-silver staining. The cysts appeared as 5- to 7-microns unstained spheres, each containing six to eight intracystic gram-negative bodies (sporozoites). Although the Gram stain does not appear to be as sensitive as more traditional staining techniques for the detection of P. carinii, clinical microbiologists should be aware of the morphology of this organism in gram-stained specimens because this relatively simple procedure gives quick results.