After the gut microbiota develops, it stabilizes during adulthood.  However, its composition changes in old age.  An increase in facultative anaerobes, a change in the proportioning of bacteroidetes and firmicutes, and a decrease in bifidobacteria have all been observed in the elderly.  These phenomena are associated with age-related immune senescence and inflammation.  

Michigan State University

The human gut is home to many types of bacteria that produce metabolites that are used to maintain homeostasis in various physiological processes, including:

- vitamin synthesis
- pathogen protection
- immunological development
- intestinal angiogenesis
- intestinal epithelium cell development

Every person has a unique and dynamic niche of microorganisms in their gut.  If the microbiota harbors harmful pathobionts or if keystone species that regulate the ecological community become threatened in some way, a condition known as microbial dysbiosis develops.  

Gut Microbiota in Homeostasis

Ahlawat, S., & Sharma, K. K. (2020). Immunological co-ordination between gut and lungs in SARS-CoV-2 infection. Virus Research, 198103.
https://www.sciencedirect.com/science/article/pii/S0168170220308649
doi: 10.1016/j.virusres.2020.198103

Immunological co-ordination between gut and lungs in SARS-CoV-2 infection

The inflammatory mediators associated with the cytokine storm in COVID-19 leads the increased permeability of the lungs.  This allows for SARS-CoV-2 and the inflammatory mediators to enter the bloodstream and migrate from the lungs to the intestine.  SARS-CoV-2 can decrease ACE2 expression and alter the gut microbiota which can alter normal immune responses.  Additionally, the inflammatory mediators can weaken the integrity of the gut, allowing for microbes and their metabolites to leak out.  This can cause microbial dysbriosis which may contribute to why diarrhea is a common symptom of COVID-19.  

Gut-Lung Axis of COVID-19

Just as some gastrointestinal (GI) infections are accompanied by respiratory manifestations, many respiratory infections are coupled with GI symptoms.  It is thought that the organs are linked to each other's homeostasis through an immunological connection governed by the microbes within the microbiotas.  Consequently, they play a key role in shaping both normal and pathological immune responses.  

Bi-directional Communication Network Between the Gut and Lung Microbiotas

Intestinal ACE2 controls the expression of anti-microbial peptides (AMPs) and regulates intestinal homeostasis through its role in dietary amino acid uptake.  A lack of tryptophan, specifically, is thought to lead to adverse effects as it and its metabolites influence microbial ecology, microbial metabolism, and the interaction between the microbiota and its host.  Animal studies have shown that ACE2 knock outs had altered gut microbiotas, decreased AMP expression, and impaired amino acid uptake.  

ACE2 Modulates Gut Microbiota Composition

Plant fibre compounds that are indigestible for humans, but serve as food for the good bacteria in our bodies and allow the beneficial bacteria to grow and flourish. 


Prebiotics 

The highest mortality rates from SARS-CoV-2 are found  in the elderly and in people with certain comorbidities, such as diabetes. These groups tend to have less diverse gut microbiomes. 

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