It is still unclear how the gut microbiota may affect brain physiology, although a number of studies have now highlighted how an altered microbiota can lead to enhanced TLR4 signalling that results in changes in glial cell function,27,28 while others have noted how the microbiota can affect blood brain barrier permeability.29 Notably, a human PD-patient derived gut microbiota was also shown to exacerbate PD-like pathology in an ⍺-synuclein overexpressing mouse model of PD, and this was proposed to be mediated by the altered production of short-chain fatty acids (SCFAs).30 Here, TLR4 is linked to Parkinson disease.