Abstract
The large intestinal microbiota is characteristically viewed as being a homogeneous entity, yet the proximal colon and distal bowel differ markedly in relation to their nutritional availabilities and physicochemical attributes. Moreover, individual species and assemblages of microorganisms exist in a multiplicity of different microhabitats and metabolic niches in the large gut, on the mucosa and in the mucus layer, as well as in the gut lumen. Examination of intestinal material by scanning electron microscopy and fluorescent light microscopy shows that most of the bacteria are not freely dispersed, but occur in clumps, and in aggregates attached to plant cell structures and other solids. With respect to the numerically predominant species, bacteria attached to surfaces in the gut lumen appear to be phylogenetically similar but physiologically distinct from non-adherent populations. These adherent organisms are more directly involved in the breakdown of complex insoluble polymers than unattached bacteria, which provides a competitive advantage in the ecosystem. In healthy people, mucosal populations are more difficult to study than faecal bacteria due to difficulty in gaining access to the bowel, and has restricted studies on these communities. Consequently, little information is available concerning the composition, metabolism and health-related significance of bacteria growing at or near the mucosal surface.