Balantidium coli is not normally pathogenic because it usually feeds simply by engulfing small particles in the intestinal contents. However, it may secrete proteolytic enzymes and cause a small ulcer in intestinal epithelium.
This photo shows a section of a small ulcer within the intestine. The cavity is filled by trophozoites, recognizable by the shape of the macronucleus in some. (Because these are sections, the appearance of the macronucleus, and even whether it is visible or not, will depend on the location of the cut in each particular trophozoite.)