TNF and Tremor: For example, a study of two Taiwanese PD populations, tremor and non-tremor dominant, found Bacteroides species to be more abundant in the non-tremor PD group, correlated with plasma levels of TNF-α and severity of motor symptoms defined by UPDRS part III, as well as an overall decreased abundance of Prevotella species in both groups of PD patients compared to non-PD controls (Lin et al., 2019).