Microbiota plays a crucial role in the maturation, development and functions of both innate and adaptive immune system [156]. The gut microbiota has been shown to affect lung health through a vital crosstalk between gut microbiota and lungs, called the “gut-lung axis”. This axis communicates through a bi-directional pathway in which endotoxins, or microbial metabolites, may affect the lung through the blood and when inflammation occurs in the lung, this, in turn, can affect the gut microbiota. The immunological health of the gut, primarily mediated by the microbiota, influences lung health via the “gut-lung axis”. In addition, microbial communities inhabiting the mucosal surfaces of the respiratory tract also contribute towards host defense against viral respiratory infections (VRIs). Acute VRIs are associated with microbial dysbiosis in these communities, thus acting the optimal functioning of the immune system. Alterations in the microbiota during influenza virus infection contributes to the pathogenesis of secondary bacterial infections, thus increasing the severity of the clinical course in the absence of appropriate immune responses [157, 158]. It is also known that alterations of the immune functions associated with chronic inflammation and related metabolic dysfunctions lead to a compromise of innate and acquired immune functions in the host [159, 160]. Moreover, chronic inflammation and the use of antibiotics are known to accompany disorders in the gut microbiota, resulting in dysbiosis and aggravation of immune dysfunctions [161]. In addition, the prevalence of comorbid conditions (including chronic lung disease, diabetes, hypertension and cardiovascular diseases) and old age predispose to infection, the development of ARDS and pneumonia, factors already observed for other infections such as influenza [162]. This point raises an interesting possibility that the new SARS-Cov-2 may also have an impact on the gut microbiota. Indeed, several studies have shown that respiratory infections are associated with a change in the composition of the gut microbiota. Numerous experimental and clinical observations have suggested that the gut microbiota plays a key role in the pathogenesis of sepsis and ARDS [163]. Moreover, it is known that the signals derived from the intestinal microbiota tune the cells of the immune system for pro and anti-inflammatory responses which thus influence the susceptibility to various diseases (Fig. 2).
Fig. 2 Graphical representation of immune homeostasis disequilibrium during SARS-Coronavirus 2 infection. ARDS: acute respiratory distress syndrome; T lymphocyte; IL: interleukin; Th: helper T lymphocyte; TNF: tumour necrosis factor