SEM of the lumenal surface of the intestine of a mouse. The dehydration-condensed residue of the intestinal biofilm is seen to occupy much of the tissue surface, and to be composed of a rich mixture of bacterial and protozoan species. A large Giardia cell is seen to be attached to the surface (P) of the intestinal epithelium, while a detached cell of the same species shows its well-developed sucker structure, and the microvillar surface of opposite side shows the scars of previous protozoan attachment. Attachment plays a large role in the microbial ecology of the intestine, because the intestinal mucus exerts powerful shear forces that tend to remove loosely attached organisms.