Gut microbiota in health and disease

Gut microbiota in health and disease

The human gastrointestinal (GI) tract contains complex consortia of trillions of microorganisms (approximately 1 × 10¹³ to 1 × 10¹⁴, biomass > 1 kg), thousands of bacterial phylotypes, as well as hydrogen-consuming methanogenic archaea with a collective genome (also termed microbiome), the majority of which resides in the colon  As a whole, the microbiome represents more than 100 times the human genome. Thus, the human ‘metagenome’ consists of a mixture of genes embedded in the human genome (Homo sapiens) and in the genomes of our microbial partners. The microbiota and its microbiome provide us humans with additional gene products, we lack, and these serve many functions pertinent to the maintenance of our homeostasis. The human gut microbiota system may be regarded as a ‘microbial organ’ within the gut, which contributes to multiple host processes including the defense against pathogens at the gut level, immunity (mediated through a number of signal molecules and metabolites), the development of the intestinal microvilli, and the synthesis of several vitamins. Accumulating evidence indicates that the gut microbiota has a crucial role in conditions including obesity, diabetes, non-alcoholic fatty liver disease, inflammatory bowel disease (IBD) and even cancer. Gut microbial composition among healthy humans is influenced by host genotype, diet, age and sex, and it appears that organic disease and drugs can modulate microbiome composition and activities. At birth, the gut is sterile and is colonized immediately, ultimately developing into a stable community, although there are marked variations in microbial composition between individuals. Within an individual the composition is remarkably stable at different anatomical locations along the gut, but absolute number vary greatly, ranging from 10¹¹ cells g⁻¹ content in the ascending colon to 10^{7–8 in the distal ileum and 10−2–3 in the proximal ileum and jejunum.}