ABSTRACT
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The human digestive tract is home to a huge and diverse population of microorganisms, largely bacteria. The most densely colonized region is the colon (Figure 15.1) with around 1012 bacteria per gram of contents. Indeed, colonic bacteria account for around 95% of the living cells in the human body. The colon is a highly reducing anaerobic environment and hence the most numerous bacterial groups present are strict anaerobes such as bacteroides, bifidobacteria, and clostridia (Figure 15.2), although over 500 species are known. It is estimated that these species only represent around 60% of the true diversity of the colonic microbiota. This complex ecosystem is sustained by nutrients arriving from the small intestine and is very metabolically active; it is believed that the colon represents the most metabolically active organ in the body. Products of bacterial metabolism range from fatty acids, largely as a result of the metabolism of saccharides to phenolic compounds derived from protein breakdown. The bacterial population also produces a wide range of enzymes that further act on nutrients and primary products of metabolism.
