Figure 3.

Schematic overview on the role of tryptophan metabolism at the interface of the microbiota and the immune system in SARS-CoV-2 infection. The drawing shows how the nasal mucosa and microbiota (healthy or dysfunctional) determine the outcome of infection. The healthy nasal microbiota positively interacts with the nasal mucosa improving the local immune response and allows the control of virus penetration with limited systemic effect. Conversely, a dysfunctional microbiota has a negative impact on the nasal mucosa, reducing the capacity of the nose to prevent the virus from causing overt disease. The tryptophan co-metabolism between the host and the microbiota may condition the response to viral infection and shift the balance between the different scenarios. Details are described in the text.