This is accompanied by an overabundance of gram-negative, LPS-producing bacteria in the gut microbiome of people with PD [112, 114, 115], and elevated epithelial TLR4 levels in PD colonic biopsies coupled with increased cytokine expression and dysbiosis [60], indicating a link between microbial LPS, a leaky gut, TLR4 signalling and intestinal PD pathology. This evidence concerns the gene TLR4 and Parkinson disease.