THE HUMAN LARGE INTESTINAL ECOSYSTEM

The adult human colon typically contains over 200 g of digestive material (Banwell et al., 1981; Cummings et al., 1990; 1992), with the average daily output of faeces in Western countries being approximately 120 g (Cummings et al., 1992). A large proportion of this is microbial cell mass with bacteria comprising approximately 55% of faecal solids in persons living on Western-style diets (Stephen and Cummings, 1980). The large intestine is an open system in the sense that food residues from the small intestine enter at one end, and together with bacterial cell mass, are excreted at the other end. Because of this, the colon is often viewed as being a continuous culture system, although only the proximal bowel really exhibits characteristics of a continuous culture.

The large intestine is a complex microbial ecosystem in which bacteria exist in a multiplicity of microhabitats and metabolic niches. The microbiota comprises several hundred bacterial species, subspecies and biotypes. Microbial cell counts are generally in the region of 1011–1012 per gram of gut contents. Some organisms occur in higher numbers than others, but about 40 species make up approximately 99% of all readily culturable isolates (Finegold et al., 1983). Viable counting indicates that bacteria belonging to the genera Bacteroides, Bifidobacterium and Eubacterium, together with a variety of anaerobic Gram-positive rods and cocci predominate in the gut (Finegold et al., 1983), however, molecular methods of analysis indicate that other groups are also numerically important, including atopobium, faecalibacterium and clostridia belonging to the C. coccoides group (Harmsen et al., 2002).