Balantioides coli
Balantioides coli, trophozoites
The trophozoites of Balantioides coli are very large, measuring 40 to 200 micrometers in maximum dimension, and are covered by a circumferential layer of cilia. They have a large "kidney-bean" shaped macronucleus and a smaller (and less easily visualized) micronucleus. Shown here are stained (Carmine stain) and unstained examples of this large organism. My predecessor, Dr. John Thompson, used to call them the "aircraft carrier of the fecal flotilla".
Balantioides coli, trophozoites in tissue
Balantioides coli can cause invasive intestinal disease in a similar fashion to Entamoeba histolytica: the trophozoites invade through the mucosa and then laterally into the underlying submucosa, resulting in production of flask-shaped ulcers. The two parasites can be differentiated by identifying the trophozoites in tissue; the trophozoites of Balantidium coli are very large, measuring 40 to 200 micrometers in maximum dimension, and are covered by a circumferential layer of cilia which can be appreciated even in tissue sections. Depending on the plane of section, the large "kidney-bean" shaped macronucleus can also often be seen. In comparison, the trophozoites of E. histolytica are much smaller (10 to 60 micrometers), are not ciliated, and have a very different appearing nucleus.
Balantioides coli, cysts
Like the trophozoites, the cysts of Balantioides coli are larger than other protozoal cysts, measuring 50 to 70 micrometers in diameter. Both cysts and trophozoites have a large "kidney-bean" shaped macronucleus. It is often necessary to focus up and down in order to appreciate the entire shape of the macronucleus.