Gut microbiome, immune system and COVID-19

By Despina Marselou

Dietitian specialized in autoimmune diseases and chronic inflammation

Despite the fact that SARS-CoV-2, the virus that causes COVID-19, is known to attack our respiratory system, causing cough, shortness of breath and pneumonia, there have been cases of patients with gastrointestinal (GI) problems, like diarrhoea, vomiting and abdominal pain.

This suggests that the presence of viral receptors in GI epithelial cells in combination with an imbalance in the gut microbiome and a not so robust immune system can lead to GI complications. Furthermore, the risk of serious infection by COVID-19 is greater amongst those who have high blood pressure, type 2 diabetes and obesity, conditions that are associated with changes in the composition of the gut microbiome. This inevitably raises questions on whether our gut microbiome plays a role in the severity of COVID-19.

The gut microbiome and the immune system

The symptoms of diarrhoea, vomiting and abdominal pain that occur in some patients may originate not only from antibiotics given to deal with secondary bacterial pneumonia, but also from the virus’ ability to infect the gut and reproduce in it. What’s more, the receptors used by SARS-CoV-2 to infect the host cells are found in the oral cavity, the oesophagus, the stomach and in other organs like the kidneys, the gallbladder, the cardiac muscle and even the cerebellum. There’s even a small series of cases that support the involvement of the GI environment in COVID-19, in which some patients had an imbalance in the microbiome composition, lacking in Lactobacillus and Bifidobacterium, which is certainly going to lead to further research.

The gut-lung axis isn’t something new and is already known to affect the progress of certain respiratory conditions. The gut-lung axis describes the crosstalk 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. The respiratory symptoms, the GI tropism of SARS-CoV-2 (which means that certain pathogens show preference to certain cells) and the alteration in the gut microbiome in some cases, make scientists believe that the GI tract is a possible target when it comes to the disease’s management and transmission.

The role of nutrition

It is well established that a healthy interaction between our immune system and the gut microbiota is crucial for the maintenance of our body’s homeostasis and health and in general, diversity of bacteria seems to be a good indicator of a “healthy gut” since lower bacterial diversity has been reproducibly observed in people with inflammatory bowel disease, type 1 and type 2 diabetes, obesity, and arterial stiffness. Several studies have demonstrated that individuals who follow a mostly plant-based diet have better diversity and richness of gut bacteria compared with individuals who consume a meat-based, Western diet. Additionally, the first major results of the American Gut Project, the largest published study to date of the human microbiome, showed that participants who ate more than 30 different plant types per week had gut microbiomes that were more diverse and had fewer antibiotic resistance genes in their gut than those who ate 10 or fewer types of plants per week.

Another study that looked at gut bacteria, dietary fibre, and allergic reactions in the lungs showed that when microbes metabolize dietary fibre, levels of short-chain fatty acids in the blood increase. This has a protective effect against allergic inflammation in the lungs, an indication that dietary fibre can shape the immunological environment in the lung and influence the severity of allergic inflammation.

How might probiotics help?

Two randomized-controlled studies have investigated the use of probiotics in patients on mechanical ventilation. The study by Morrow et al suggested that patients treated with Lactobacillus were significantly less likely to develop microbiologically confirmed Ventilator Associated Pneumonia (VAP) compared with patients treated with placebo, while the authors of the second trial concluded that therapy with the probiotic bacteria B. Subtilis and E. faecalis are an effective and safe means for preventing VAP.

Not long ago, the National Health Commission in China and the National Administration of Traditional Chinese Medicine recommended probiotics in cases of patients with severe symptoms of COVID-19 in order to help prevent a secondary bacterial infection. These are preliminary findings but may light the path for more research on probiotics and the prevention of pneumonia. Currently a research team in Belgium is exploring the potential of specific strains of lactobacilli in the nasopharynx and oropharynx to reduce viral activity via the barrier‐enhancing and anti-inflammatory effects, and reduce the risk of secondary bacterial infections in COVID‐19. In the United Kingdom, there is an ongoing Phase II randomized, double‐blind, placebo‐controlled trial to evaluate the efficacy and safety of oral live biotherapeutic product (in addition to standard supportive care for hospitalized COVID‐19 patients for 14 days).

The available data show that infection with SARS-CoV-2 moves beyond the lungs and can affect, in addition to the immune system, the GI system. We need more research before we can recommend probiotics to patients with COVID-19. In the meantime we should aim to follow the recommendation of a recent review on the impact of nutrition on COVID-19

“it is critical that individuals refrain from eating foods high in saturated fats and sugar and instead consume high amounts of fibre, whole grains, unsaturated fats, and antioxidants to support immune function”.