|Description:||At a magnification of 1000X, this photomicrograph reveals Pneumocystis jirovecii fungi, which were present in this Giemsa-stained impression smear of rat lung tissue. Formerly known as Pneumocystis carinii, and classified as a protozoa, investigative tests upon this organism’s nucleic acid and biochemical composition has since placed it in the Kingdom of Fungi.These fungi are found in the lungs of mammals where they reside without causing overt infection until the host's immune system becomes debilitated. Then, an oftentimes lethal pneumonia can result. Note the round cyst in the very middle of this image containing eight immature haploid neuclei, as well as numbers of freed trophozoites.|
Asexual phase: trophic forms replicate by mitosis. Sexual phase: haploid trophic forms conjugate and produce a zygote or sporocyte (early cyst). The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst). Spores exhibit different shapes (such as, spherical and elongated forms). It is postulated that elongation of the spores precedes release from the spore case. It is believed that the release occurs through a rent in the cell wall. After release, the empty spore case usually collapses, but retains some residual cytoplasm. A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist. The organism causes disease in immunosuppressed individuals, including those with AIDS or undergoing chemotherapeutic treatments.