The human ecosystem
Matthew Child and George Macfarlane explain the role of gastrointestinal microbiota
Genetic analysis shows that our gastrointestinal tracts are home to more than 100 000 billion (1014) individual micro-organisms of perhaps 36 000 different species. And more than 90% of the cells in our bodies are non-human.1 These bacteria form a diverse and complex ecosystem with a total gene pool (microbiome) more than 100 times larger than the human genome—in effect we are hybrid “superorganisms.” The types and numbers of bacteria differ from the stomach to the distal colon, reflecting the changes in pH, concentration of oxygen, and availability of nutrients. Small numbers persist in the stomach (notably Helicobacter pylori, which causes ulcers) and the small intestine, but most of these organisms are found in the anaerobic environment of the large intestine (table).
Maintenance of this population of resident microbes, termed the gut microbiota, has several advantages. By competing for nutrients and epithelial receptors and producing bacteriocins (antibiotic peptides) and fermentation