**12. Microbiome-immunity crosstalk mechanisms and COVID-19**

The microbiome is composed of a great number of microorganisms which in the human body are found in the gut and also on the skin and other mucosal microenvironments. Studies have shown that microbiome is active participant in a host functions like immunity and other metabolism activities. Other functions which have a microbiome impact are nutrition end products and the circadian clock operation [28–30]. Studies have shown that lung microbiome can modify the risk and consequences of COVID-19 by activating the immune response. On the same line, other suggestions show that bacterial co-infections as well as the gut-lung cross talk may be important players in COVID-19 disease prognosis. Recent studies have established that COVID-19 patients with GI complications experience more respiratory distress when compared to the patients without GI complications [31, 32]. Interestingly, obesity, *diabetes mellitus*, cardiovascular diseases and old age-related disorders have been related to weak microbiome communities and the same category of patients have also been more vulnerable to COVID-19 and SARS CoV-2 attacks [33–36]. Even in gut microbiome, COVID-19 patients were found to be deficient to beneficial commensals like *Eubacterium ventriosum, Faecalibacterium prausnitzii,* and *Roseburia* these microbes highly correlate to disease severity [37–39].

### **13. Recommendations**

The recommendations from this crucial evaluation of the immune system role of the microbiome is that in future we should not over look natural mechanisms which have established an equilibrium with the immune system. This should also involve the educating of masses on how to cultivate the natural human microbiome defense systems as physical barriers. The training and boosting of the immune system both in children and adults respectively should regionally be evaluated. This is because increased traveling and climate change cannot be assumed to be of little consequences in pandemic control strategies in light of the microbiome roles.
