Changes to the respiratory and intestinal microbiota affects susceptibility to infections, especially in the elderly. So could having a healthy microbiota reduce the severity of symptoms caused by SARS-CoV-2?
The microbiota is also referred to as “the last discovered organ in the human body.” It’s the collection of microorganisms that reside in our bodies; from the skin, to internal organs.
Numerous studies in recent years have demonstrated the multitude of interactions between the microbiota and the human immune system. Because the coronavirus provokes a strong immune response, it was inevitable for researchers to investigate the role the microbiota play on the severity of the infection.
This is where the so-called “gut-lung axis” comes into play. This is a link between the two organs via the lymphatic and circulatory systems, which allows the transfer of immune system cells between the two organs. In particular, studies in animal models have demonstrated that a gut microbiota in dysbiosis (out of balance) increases the susceptibility to both influenza virus infection and lung inflammation.
Gastrointestinal Symptoms and SARS-CoV-2
In demonstrating the involvement of the intestine, there are the gastrointestinal symptoms that occur in about 40% of patients, coming in the form of diarrhea, nausea, vomiting, and abdominal pain and the presence of viral RNA in the stool. In fact, the ACE-2 receptor, which is also the gateway for virus entry into human cells, is abundantly present in both the lungs and intestinal cells.
What are the interactions between the microbiota and the coronavirus?
Researchers have formulated two hypotheses:
- A microbiota in dysbiosis (out of balance) may promote infection, due to a reduced effectiveness of the immune system. We know that dysbiosis occurs very frequently in the elderly, obese people and in people with chronic diseases associated with various states of inflammation (i.e. in that part of the population in which the symptoms of COVID-19 are greater and more severe).
- Coronavirus infection, even in subjects in which the microbiota is in eubiosis (i.e. in balance), leads to dysbiosis through the competition between commensal protective bacteria (i.e. good) and pathogenic bacteria favoring the latter (click here for details).
It’s currently unknown which of the hypotheses is prevalent. The only thing that is certain is that a balanced microbiota is essential to maintaining a healthy body.
How can a healthy microbiota reduce the effects of SARS-CoV-2 infection?
It’s been proven through several studies that protective commensal bacteria (i.e., the good ones, if we contrast them with pathogens), have a protective ability to benefit the organism in which they reside — and against viruses. One example is a bacterium, Bacillus Subtilis. This bacterium destroys the surface of several viruses, including the virus responsible for influenza A. In addition, Bacillus strains compete with pathogenic bacteria by promoting the growth of other good bacteria such as Lactobacillus.
In addition, the prevalence of the Bacteroidetes species, is generally associated with milder symptoms of COVID-19, while other species such as Firmicutes are more commonly found in harshly-affected patients and are also often discovered in these patients’ stool.
The microbiota is not only important for fighting pathogenic bacteria and viruses but it also has a nutritional effect on intestinal cells. One of the most important nutrients produced by the microbiota is butyrate, which has a positive effect on regulating mucus production and the proliferation of enterocytes (i.e. cells in the epithelium of the intestinal villi).
Even more important is its function as a modulator of the immune response. Moreover, a healthy microbiota is able to reduce the production of pro-inflammatory cytokines such as interleukin-6 (IL-6). Very high levels of IL-6 are found in Coronavirus-infected patients with severe symptoms, so it can be assumed that the reduction of IL-6 concentration by a healthy microbiota could reduce the more severe effects of SARS-CoV-2 infection (click here for details).
The importance of the microbiota being in balance
In conclusion, much remains to be clarified about the interaction between the microbiota and SARS-CoV-2. However, the studies carried out so far are in agreement that a balanced microbiota (i.e. in eubiosis), significantly contributes to the efficiency of the immune system and therefore to the fight against any infection and therefore limits its symptoms.
Moreover, considering the gastrointestinal system as a site of infection and, consequently, of possible transmission, it’s essential to develop effective treatments that not only take the respiratory system into account.
In fact, the Chinese National Commission on Health has suggested the use of probiotics for COVID-19 patients, in order to improve the composition of the intestinal flora and its metabolites (click here for further reading).
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