Abstract

The patient reported had a fulminant, refractory diarrhea of unknown etiology producing about 81 of diarrheal fluid daily which required continuously large volumes of intravenous fluid therapy. No protozoon other than Blastocystis hominis was present. Bacterial counts were very low in the fecal material because of continuous antibiotic therapy. Blastocystis hominis was present in large numbers, averaging for a 5-day period, 8.3 X 10(6)/ml of diarrheal fluid. Treatment with metronidazole for 12 days had no effect on either the diarrhea or the numbers of B. hominis present. The patient died from aspiration pneumonia without a firm diagnosis of his underlying disease. Identification of an unusual form of B. hominis seen only in diarrheal fluid was confirmed through cultivation, specific fluorescent antibody staining, as well as by transmission electron microscopy and light microscopy using hematoxylin staining and Nomarski interference contrast. The most significant result of this study is the description of ultrastructure of the in vivo trophozoite form of B. hominis. The persistence of certain morphologic features of the in vivo and in vitro forms of B. hominis is described. In vivo, B. hominis has a larger nucleus than in vitro, more prominent, well structured nucleoli, and hundreds of mitochondria with numerous delicate saccular cristae in a clear electron-lucent matrix and complex internal structure. Many cytoplasmic vesicles lined with ribosomes are present. The in vivo structure differs from the granular culture form of B. hominis, which has a smaller nucleus (nuclei), no distinct nucleoli, mitochondria characterized by a uniform structureless electron dense matrix, and few cytoplasmic, ribosome-lined vesicles. A feature retained in both in vivo and in vitro B. hominis is a distinct crescentic band of nuclear chromatin.