The existing data highlight a recurrent microbiota signature for nano-Ag, TiO2, ZnO and SiO2, characterized by an alteration of the F/B ratio, a depletion of Lactobacillus strains (SCFA and AhR ligand producers) and an increase in the abundance of Proteobacteria, which may resemble the microbiome shift in IBD, CRC or obesity where gut dysbiosis play a key pathogenic role. This evidence concerns the gene AHR and obesity due to melanocortin 4 receptor deficiency.