Targeting the gut microbiota as part of the microbiota-gut-brain axis has been proposed as a potential interesting new therapeutic strategy for neurodegenerative disorders, such as Parkinson’s and Alzheimer’s disease. In this review, the original articles from April 2020 to March 2021 tackling this research topic, are discussed. Multiple research groups investigated the mechanisms involved in the altered microbiota-gut-brain axis in Parkinson’s and Alzheimer’s disease, pointing out correlations between the gut microbiome and specific (patho)physiological properties or gastrointestinal functions and the potential of gut microbial markers as novel diagnostic biomarkers. Furthermore, the interventions described to target the gut microbiota, included probiotics (e.g., VSL#3, Bifidobacterium strains), prebiotics (e.g., plant and yeast extracts), supplements (e.g., minerals), medication (e.g., acetylcholine esterase inhibitors, antibiotics) and alternative therapies (e.g., acupuncture). Positive effects on the different components of the microbiota-gut-brain axis were demonstrated, such as an improvement of gut microbiota dysbiosis (including a restoration of the Bacteroidetes to Firmicutes ratio, a decrease in Clostridiales, and an increase in Lactobacillus and short-chain fatty acids), intestinal barrier dysfunction, inflammation (central/systemic/intestinal), motor deficits, cognitive impairment, neurotoxicity, pathological lesions in the brain, and amyloid β (Aβ) deposition. These results show a major role for the modulation of the microbiota-gut-brain axis as a new therapeutic strategy for

both Parkinson’s and Alzheimer’s disease. Since the available data predominantly resulted from preclinical studies, future research should be focused on translational studies with a bench-to-bedside approach in order to eventually incorporate new treatments targeting the microbiota-gut-brain axis in the clinic.