The gut microbiome affects the functions of CNS macrophages. In vivo experiments using a germ-free (GF) mouse model revealed that the microbiome plays an important role in microglial development and maturation, in addition to influencing the functioning of the adult brain.
Hasavci, D. and Blank, T. (2022). Age-dependent effects of gut microbiota metabolites on brain resident macrophages. Frontiers in Cellular Neuroscience 16. doi:10.3389/fncel.2022.94452