In the large intestine or colon, bacteria can reach a density of 10 10–10 12 organisms per gram and comprise more than 1000 species, including obligate anaerobes, such as Bacteroides, bifidobacteria, fusobacteria, and peptostreptococci, as well as obligate and facultative aerobes, such as enterobacteria and lactobacilli. Higher bacterial densities of 10 8 organisms per gram can be found in the ileum, which is the distal portion of the small intestine. The stomach and proximal segments of the small intestine, including the duodenum and jejunum, have relatively low bacterial densities of approximately 10 3–10 5 organisms per gram of luminal contents, at least in mice. The gastrointestinal mucosa is a unique environment that becomes exposed to a massive and diverse microbial ecosystem shortly after birth ( Macpherson, 2006).
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