Inclusion Criteria:
1. Confirmed diagnosis of Parkinson's disease based on UK Brain Bank criteria
2. Between the ages of 40-80 years
3. Hoehn and Yahr stage between 1-3 (mild to moderate PD) in the "ON" state
4. Mild to moderate depression (clinical diagnosis of mild to moderate depression in the MINI clinical interview and/or BDI-II score of 14-28, and/or IDS-C score of 12-36, and/or QIDS-SR score of 6-15 in the "ON" state)
5. Women of childbearing potential must agree to use a medically approved method of birth control (e.g., hormonal contraceptives, intrauterine devices, vasectomy/tubal litigation, barrier methods and double barrier method) and must have negative pregnancy test results at screening and baseline
6. Willingness to maintain current physical activity levels during study period
7. English proficiency
Exclusion Criteria:
1. Atypical parkinsonism
2. Active suicidality
3. Active psychosis
4. Cognitive score (MoCA) of \< 21 in the "ON" state
5. Severe depression (BDI-II score \> 28 and/or IDS-C score \> 36, and/or QIDS-SR score \> 15 in the "ON" state)
6. Probiotic, saccharomyces boulardii and/or antibiotic use in the past 3 months (yogurt, kefir, and other probiotic containing foods are allowed)
7. The use of natural health products that affect depression (e.g., St. John's Wort, passion flower, gaba, 5-htp, kava, bacopa, efa's)
8. Change in schedule of concurrent psychotherapy or brain stimulation for the treatment of mood or anxiety disorders in the last 4 weeks
9. Change in antidepressant or anxiolytic medication (including benzodiazepines) within the last 4 weeks
10. Change in Parkinson's medication within the last 2 weeks
11. Neurological disease other than PD, including Alzheimer's disease, multi-infarct dementia, Huntington's disease, normal pressure hydrocephalus, a brain tumor, progressive supranuclear palsy, seizure disorder, subdural hematoma, multiple sclerosis, or history of significant head trauma
12. A significant immune-compromised condition due to either a health condition or use of an immune suppressant (e.g., AIDS, lymphoma, chemotherapy treatment, patients undergoing long-term systemic corticosteroid or immunosuppressant treatment)
13. A known bleeding disorder
14. Current illness (e.g., a cold or flu-like symptoms) and infections (e.g., hepatitis, HIV, gastroenteritis, fungal, or parasitic infections)
15. Allergy to corn starch or corn
16. Concurrent treatment for Parkinson's disease with Duodopa
17. Change in Deep Brain Stimulation (DBS) stimulation parameters in the last 4 weeks
18. New onset of significant psychiatric symptoms following DBS procedure that are considered likely related
19. Women who are pregnant, breast feeding, or planning to become pregnant during the course of the trial
20. Unstable medical conditions or serious disease/conditions (e.g., cancer undergoing active treatment, poorly controlled diabetes)
21. Drug and/or substance abuse
22. The patient, in the opinion of the Investigator, is unable to adhere to the requirements of the study. This includes any patient who, in the judgment of the Investigator, is likely to be noncompliant during the study, or unable to cooperate because of a significant language barrier or cognitive impairment
