An Extraordinary Balance

Microbial Composition Along the GI Tract

The composition of the gut microbiome is not homogenous and differs by location along the GI tract.10,13 Differences in diversity and absolute counts of bacterial species are dictated by local biochemical factors, including pH and oxygen levels.5,10

In the stomach and upper small intestine, acidic pH and high oxygen levels limit bacterial abundance and type, favoring the growth of aerobes.10

In the colon, a more alkaline pH, slower motility, and lower oxygen levels enable greater diversity and abundance and promote the growth of anaerobes.10

Microbial Imbalance and Chronic Disease

Although composition varies widely, a healthy gut microbiome is generally described as abundant, diverse, and resistant to stress-related change.3,5 When altered, microbial composition can favor the development of chronic diseases, such as3,5,7,14,21,22:

  • Obesity
  • Inflammatory bowel disease
  • Irritable bowel syndrome
  • Type 2 diabetes mellitus
  • Autism
  • Psoriasis

The gut microbiome may also affect mental health via the gut-brain axis.14,21,23 Evidence suggests that signals from the CNS can alter the gut microbiota and, in return, these alterations can affect brain function.23 Changes in gut microbial composition have been associated with psychiatric disorders, such as major depressive disorder; however, studies cannot conclude whether dysbiosis was a cause or consequence of these disorders.14,15,21,23 In addition, this connection has been assessed primarily in animal models and has not been validated in humans.14,21,23

microbial composition in different areas of GI tract, including esophagus, stomach, duodenum. Jejunum, ileum, colon

Adapted, with permission, from: Gorkiewicz G, Moschen A. Gut microbiome: a new player in gastrointestinal disease. Virchows Arch. 2018;472(1):159-172.

GI = gastrointestinal
pH = power of hydrogen
CNS = central nervous system

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