The gut microbiome and severity of SARS-CoV-2 infection: what is the link?

In the recent years, studies of the human microbiome have aroused great interest. Several evidences suggest a connection between the gut microbiome and the human immune response at the pulmonary level, which has been defined as the "gut-lung axis". The clinical symptoms of COVID-19 are varied and include gastrointestinal manifestations such as diarrhea, which has been linked to alterations in the gut microbiome; imbalance of the immune response; and delayed viral clearance. The aim of this narrative review was to address the role of the gut microbiome in the respiratory health and in particular, its association with the severity of COVID-19. The gut microbiome plays several important roles therefore; its balance is determinant for the human health, due to its relationship with several essential physiological processes, including maturation of both of the innate and the adaptive immune responses. Intestinal dysbiosis has an impact on the respiratory mucosa, and in turn on infection of the intestinal epithelial cells by SARS-CoV-2, which can induce intestinal inflammation and gastrointestinal symptoms. All these symptoms could contribute to an altered inflammatory immune response to SARS-CoV-2, favoring infection, dissemination and severity of the disease. Knowledge about the roles of the gut microbiome and its interactions in the context of SARS-CoV-2 infection could help to find biomarkers involved in COVID-19-related dysbiosis, as well as to determine the possible therapeutic targets for treatment of these patients.