The role of respiratory microbiota in the protection against viral diseases: respiratory commensal bacteria as next-generation probiotics for COVID-19.

On March 11, 2020, the World Health Organization declared a pandemic of coronavirus infectious disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and imposed the biggest public health challenge for our civilization, with unforeseen impacts in the subsequent years. Similar to other respiratory infections, COVID-19 is associated with significant changes in the composition of the upper respiratory tract microbiome. Studies have pointed to a significant reduction of diversity and richness of the respiratory microbiota in COVID-19 patients. Furthermore, it has been suggested that , and are associated with severe COVID-19 cases, while and are significantly more abundant in asymptomatic subjects or with mild disease. These results have stimulated the search for new microorganisms from the respiratory microbiota with probiotic properties that could alleviate symptoms and even help in the fight against COVID-19. To date, the potential positive effects of probiotics in the context of SARS-CoV-2 infection and COVID-19 pandemics have been extrapolated from studies carried out with other viral pathogens, such as influenza virus and respiratory syncytial virus. However, scientific evidence has started to emerge demonstrating the capacity of immunomodulatory bacteria to beneficially influence the resistance against SARS-CoV-2 infection. Here we review the scientific knowledge regarding the role of the respiratory microbiota in viral infections in general and in the infection caused by SARS-CoV-2 in particular. In addition, the scientific work that supports the use of immunomodulatory probiotic microorganisms as beneficial tools to reduce the severity of respiratory viral infections is also reviewed. In particular, our recent studies that evaluated the role of immunomodulatory strains in the context of SARS-CoV-2 infection are highlighted.