This study will be an 8-week open-label pilot study examining subjective and objective changes in mood, anxiety, cognition, and sleep before and after the introduction of a probiotic supplement containing Lactobacillus helveticus and Bifidobacterium longum in 10 treatment-naïve participants diagnosed with major depressive disorder (MDD). In order to examine the full composition of the micro biome, the investigators will also be collecting and analyzing fecal samples as well as blood samples to examine changes in plasma levels of inflammatory markers, 5-HT, and tryptophan in order to look at possible underlying mechanisms of any changes seen.

Inclusion Criteria:

1. Diagnosis of Major Depressive Disorder (MDD) by Mini International Neuropsychiatric Interview (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. Use of any antidepressant drug
2. Use of any antibiotic drug in the past 4 weeks (may be eligible to participate after a 1-month washout period)
3. Use of any sleep medication in the past 4 weeks (may be eligible to participate after a 1-month washout period)
4. Milk, yeast, or soy allergy
5. History of alcohol or substance abuse in the past 6 months
6. Daily use of probiotic product in the past 2 weeks (may be eligible to participate after a 2-week washout period)
7. Use of any type of laxative
8. Consumption of products fortified in probiotics
9. Severely suicidal
10. Experiencing psychosis or bipolar episode
11. History of epilepsy or uncontrolled seizures

Individual participant data will not be shared.