The authors of a recent paper ask what role gut bacteria might play in COVID-19. They outline strands of existing evidence and conclude that a link between the two is plausible, but that more research is necessary.

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Is there a relationship between COVID-19 and gut bacteria?

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Scientists have implicated gut bacteria in a number of conditions. From type 2 diabetes to depression, researchers have observed relationships between a wide range of disease states and the organisms that live in our gastrointestinal tract.

The recent paper outlines previous research that demonstrates links between lung health in general and gut bacteria.

On the surface, a link between the gut and COVID-19 might seem unlikely. However, there are a number of reasons to suspect such a relationship.

For instance, gastrointestinal symptoms are a relatively common feature of COVID-19. In one study, more than half of those with the disease reported digestive symptoms, including diarrhea and vomiting.

Another link between the new coronavirus and the gut involves ACE-2 receptors. These receptors are SARS-CoV-2’s entry point into cells; they are expressed in a few anatomical sites, including the lungs and the gastrointestinal tract.

Also, researchers have detected SARS-CoV-2 in the stool of people with COVID-19.

In the new paper, which appears in the journal Virus Research, the authors outline how our microbiome might influence either our risk of developing COVID-19 or the severity of the disease. Although there is no direct evidence, the researchers collate various lines of converging evidence.

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A link between the lungs and the gut seems somewhat unexpected. However, as the authors discuss in their paper, this is not a new idea. The so-called gut-lung axis describes the cross-talk that occurs between gut microbiota and the lungs.

This communication travels in both directions: Endotoxins and metabolites that bacteria produce in the gut can travel through the blood and influence the lungs. In a similar manner, inflammation in the lungs can affect bacteria residing in the gut.

“This raises [the] interesting possibility that [SARS-CoV-2] might also have an impact on the gut microbiota,” the authors explain. “In fact, several studies have demonstrated that respiratory infections are associated with a change in the composition of the gut microbiota.”

They also note that some studies have demonstrated links between acute respiratory distress syndrome, which occurs in severe cases of COVID-19, and gut microbiota. Further to this, they explain that in mice, “removal of certain gut bacteria by antibiotic[s] leads to increased susceptibility to influenza virus infection in [the] lungs.”

From decades of research, it has become increasingly clear that the microbiome plays a role in the immune system.

As the authors of the new paper explain, gut bacteria help “tune the immune cells for pro- and anti-inflammatory responses, thereby affecting [their] susceptibility to various diseases.”

They also write: “The composition of balanced gut microbiota is known to have a major influence on the effectiveness of lung immunity. Germ-free mice, devoid of their intestinal microbiota, have been shown to have impaired pathogen clearance capability in the lung.”

Other scientists have revealed that the repeated use of antibiotics is associated with an increased risk of lung and other cancers, once again shining a light on a potential link between the gut and lungs.

Although scientists have not proven a direct link between SARS-CoV-2 infection and gut bacteria, the study authors believe that the circumstantial evidence is enough to warrant further investigation.

Unsurprisingly, diet plays a significant role in shaping the composition of bacterial populations in the gut. For instance, one study demonstrated that individuals who follow a plant-based Mediterranean diet have very different populations of gut bacteria compared with individuals who consume a meat-based Western diet.

The authors of the new paper also mention a study that looked at gut bacteria, dietary fiber, and allergic reactions in the lungs. The scientists found that when microbes metabolize dietary fiber, levels of short-chain fatty acids in the blood increase. This has a protective effect against allergic inflammation in the lungs.

As the authors of the allergy study conclude, “Our results show that dietary fermentable fiber and [short-chain fatty acids] can shape the immunological environment in the lung and influence the severity of allergic inflammation.”

With these findings in mind, the authors of the latest paper write:

“[E]ffective nutritional strategy and specific functional foods aiming at the microbiota for specific population group may be the need of the hour.”

To add to the discussion, in a study from last month, scientists analyzed stool samples from 15 individuals with COVID-19. They took samples two to three times each week for the duration of the participants’ stay in the hospital.

The scientists compared these data with stool samples from 15 healthy individuals and six people with pneumonia but without COVID-19. They found that, across all time points, “Patients with COVID-19 had significant alterations in fecal microbiomes compared with controls.”

However, no studies so far have resolved the question of cause and effect; understanding whether gut bacteria influence COVID-19 risk or whether having COVID-19 alters gut bacteria will take more research. Regardless, the authors conclude that “[s]trategies to alter the intestinal microbiota might reduce disease severity.”

To date, there is no firm evidence to suggest that SARS-CoV-2 interacts with gut bacteria in a meaningful way, or that the microbiome influences the severity of COVID-19. However, taking all the information in the new paper together, it certainly seems to be a credible theory.

At this stage, we can only conclude that gut bacteria could play a part in an individual’s susceptibility to COVID-19 or the severity of their symptoms. More research is necessary and, with high levels of interest in both the microbiome and SARS-CoV-2, this is sure to follow.

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