As an exception to the rule, B. coli is best diagnosed on a wet prep, where cysts and/or trophozoite forms can be seen. They are quite large and can even be seen using 10x and 40x objectives. They have a characteristic large, dark staining (with iodine) kidney bean-shaped macronucleus and a small micronucleus (which is not often seen).
The trophozoites, which are more commonly seen than the cysts, are oval- or pear-shaped on the wet prep. Their size ranges from 50-100 μm long x 40-50 μm wide. They can also be seen on trichrome stained smears but are sometimes missed as they stain very darkly and the internal features may be obscured, or they can be mistaken for helminth eggs.
Although molecular methods are being developed and used in research and veterinary labs, they are not typically available in US clinical labs.
31. DPDx. "Parasites - Balantidiasis - Figure D: B. colitrophozoite in a wet mount, 1000× magnification. Note the visible cilia on the cell surface. Image contributed by the Oregon Public Health Laboratory." CDC.gov, 6 Jun 2019, https://www.cdc.gov/dpdx/balantidiasis/index.html
