Effects of Probiotics on Symptoms of Depression

Inclusion Criteria: 1. Diagnosis of Major Depressive Disorder (MDD) according to DSM-5 as determined by MINI 2. Current depressive episode with a MADRS score of 20 3. Males and females between ages 18 and 65 4. Able to understand and comply with the requirements of the study 5. Provision of written informed consent Exclusion Criteria: 1. Current use of any antidepressant drug 2. Three or more previous episodes of depression 3. Failure to respond to another treatment in the current episode 4. Use of any antibiotic drug in the past 4 weeks (may be eligible to participate after a 1-month washout period). Should a participant require an antibiotic drug during the 16 week study period, they may continue with the study but must take the probiotic 4-6 hours before or after they take the antibiotic. 5. Use of any sleep medication in the past 4 weeks (may be eligible to participate after a 1-month washout period) 6. Milk, yeast, or soy allergy 7. History of alcohol or substance dependence in the past 6 months 8. Use of probiotic product in the past 2 weeks (may be eligible to participate after a 2-week washout period) and during the trial 9. Use of any type of laxative 10. Consumption of products fortified in probiotics 2 weeks before and during the trial 11. High risk of suicide (score 4 or more on item 10 of MADRS) 12. Psychotic symptoms determined by the MINI 13. Bipolar Disorder determined by the MINI 14. Women who are pregnant, breastfeeding, or planning to become pregnant during the course of the trial (determined by a pregnancy test performed at the screening visit and confirmation of the use of appropriate contraception) 15. Immunodeficiency (immuno-compromised and immuno-suppressed participants; e.g. AIDS, lymphoma, participants undergoing long-term corticosteroid treatment, chemotherapy and allograft participants) 16. Unstable or severe medical conditions (e.g. cancer, cardiovascular, renal, lung, diabetes, psychiatric illness, bleeding disorders, etc.) 17. The use of natural health products (NHPs; e.g. St. John's Wort, melatonin, passion flower, etc.) that affect depression or sleep is not allowed in the trial 18. Electroconvulsive therapy (ECT) in the year prior to participation in the study 19. Taking medication or other not-permitted treatment that cannot be safely discontinued