The aim of this study is to evaluate whether the administration of Bacillus Subtilis influences gut and blood biomarkers relevant to the proposed mechanism(s) of action, as well as being acceptable as a regular supplement for people with Parkinson's disease.

Following informed consent, participants will undergo screening. If eligible, they will be invited to attend a baseline visit (week 0) to complete clinical assessments and provide a faecal and blood sample before starting their assigned intervention. Each participant will be randomised 1:1 to visually identical capsules containing either Bacillus Subtilis or placebo to be taken for 24 weeks. Telephone follow-up will be performed at week 8 to ensure compliance and answer any participant questions. Repeat clinical assessments and faecal/blood sample collection will take place at weeks 24 (treatment end date) and 36 (to assess longevity of biomarker changes after the intervention has stopped). Faecal samples will be collected at home within seven days prior to the clinic visits and brought to the appointment. Microbiota composition in faecal samples and blood biomarkers will be analysed in participant samples. Motor and non-motor symptom rating scales will also be administered.

Inclusion Criteria:

• Diagnosis of Parkinson's disease
• Able to provide written informed consent
• Over 50 years old
• Motor symptom duration over 12 months
• Stable dopaminergic medication regime for at least 3 months

Exclusion Criteria:

• Use of probiotic supplements and probiotic-fortified foods (such as probiotic drinks and yoghurts) within past 3 months
• Use of antibiotics within past 3 months
• Previous gastrointestinal surgery or chronic organic bowel disorder
• History of clinically significant motor fluctuations
• History of postural instability or falls
• Current smoker
• Known or suspected allergy to probiotics
• Regular use of antacids, proton pump inhibitors, laxatives or anti-diarrheal drugs
• Use of hypoglycaemic or diabetes drugs
• Participation in clinical trial of investigational medicinal product within past 3 months
• Co-morbidities or other factors that, in the opinion of the investigator, make the patient unlikely to comply with study protocol