Fig. 1.

Possible pre- or probiotic mechanisms in the fight against COVID-19. Through increasing levels of positive micro-organisms in the gut, this can impact on a range of pathways that could be of benefit in the fight against COVID-19: (1) Intervention with pre- and probiotics positively alters the bacterial community in the gut, often to the detriment of potentially pathogenic micro-organisms (through the production of metabolites and/or competitive exclusion. This can reduce the risk of gut-related illness. (2) The gut microbiota produce SCFA that have systemic effects in the body, including provision of a cellular energy source. (3) Increased SCFA levels are associated with improved tight intestinal junctions, possibly restricting the passage of endotoxin from the gut lumen; these effects may be in part mediated by cytokines. Increased levels of bifidobacteria have also been associated with improvement of tight junctions. It is worth noting that both ageing and obesity (which are COVID-19 risk factors) are associated with a poorer gut barrier function. (4) Free fatty acid receptors can be found on dendritic cells, as such SCFA stimulation of dendrites can impact on regulatory T-cells resulting in enhanced macrophage activity and increased anti-inflammatory cytokines. (5) Macrophages are associated with pro-inflammatory status; however, SCFA have been observed to down-regulate pro-inflammatory cytokine release. By supporting the immune function and reducing inflammatory status, the gut microbiota could be a tool to aid the body in defending from COVID-19.