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 as^{3,5,7,14,21,22}:

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.

CAUSES AND CONSEQUENCES OF GUT DYSBIOSIS

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

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