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The large intestine hosts the most densely populated microbial ecosystem in the human body. This complex community primarily consists of anaerobic bacteria, with Bacillota (formerly Firmicutes) and Bacteroidota (formerly Bacteroidetes) as the predominant groups. The distribution of these microbes varies along different sections of the large intestine, influenced by local environmental factors such as oxygen availability and nutrient composition.
The cecum, located at the beginning of the large intestine, contains facultative anaerobes—microorganisms that can grow in both the presence and absence of oxygen—such as Escherichia and Enterococcus. These bacteria are well-suited to the region’s fluctuating oxygen levels. Additionally, aerotolerant bacteria, such as Lactobacillus, are present, contributing to the initial stages of carbohydrate metabolism and helping maintain pH balance.
In the proximal colon, the microbial community is dominated by members of the phylum Bacteroidota and strict anaerobes, including Clostridium species. These bacteria play essential roles in fermenting dietary fibers into key metabolites, particularly short-chain fatty acids (SCFAs) such as acetate, propionate, and butyrate. These SCFAs serve as vital energy sources for colonocytes, support the integrity of the intestinal barrier, and exhibit antimicrobial and anti-inflammatory properties.
In the distal colon, where nutrient availability is lower, the microbiota shifts further toward specialized carbohydrate fermenters such as Ruminococcus and Faecalibacterium. These microbes excel at breaking down complex carbohydrates, producing SCFAs that nourish epithelial cells and help maintain the intestinal barrier. Additionally, SCFAs in this region help regulate immune responses and inhibit the growth of pathogenic bacteria. Importantly, these regional differences reflect general patterns rather than strict compartmentalization, and substantial overlap exists among microbial communities throughout the large intestine.
Beyond bacteria, the large intestine also hosts other microbial groups. Methanogenic archaea, particularly Methanobrevibacter smithii, play a key role by removing excess hydrogen generated during fermentation, converting it into methane to help continued microbial metabolism. Fungi, although present in smaller numbers, include species such as Candida. These organisms contribute to carbohydrate metabolism and interact with both bacterial communities and host cells, shaping the metabolic landscape of the large intestine.
