The cause of Parkinson's disease (PD) is unknown and a reliable biomarker to identify PD patients as early as possible is urgently needed. Nerve cells near the nose and in the gut become first affected in PD and patients frequently suffer from loss of smell and constipation. The nose and gut harbor very high amounts of bacteria that influence our body functions in many ways, even in the brain. The investigators are examining a possible role of bacteria of the nose and gut in the pathogenesis of PD. This may lead to a better understanding of what PD causes and may open new possibilities for diagnosis and treatment.