Inclusion Criteria:
* Female and male participants 15 to 24 years of age
* Fluency in English
* Access to smartphone, tablet, or computer, on which program "Zoom" can be run for remote visits
* Diagnosed with a primary DSM-5 clinical depressive disorder (Major Depressive Disorder, Persistent Depressive Disorder, or Other Specified Depressive Disorder). The DSM-5 clinical depressive disorder will be confirmed by the Schedule for Affective Disorders and Schizophrenia for School-Aged Children-Computerized Version (K-SADS-COMP)47 to participants \<18 years and the Structured Clinic Interview for DSM-5 (SCID-V) to participants ≥18 years.
* Participants must be under the care of a primary care clinician and/or mental health professional
* Currently on SSRI for at least 8 weeks
* BDI-II score greater than 13
Exclusion Criteria:
* Current diagnosis of other DSM-5 mental health disorders except for anxiety disorders and ADHD as long as depressive disorder is the primary diagnosis
* Current medically-treated with stimulant medication for ADHD unless subject is on stable dosage of stimulant medication and there are no plans to change the stimulant medication or dosage for at least 8 weeks.
* Current diagnosis of any Axis II personality disorder(s) as assessed by the K-SADS-COMP or SCID-V.
* Oral probiotic use 1 month prior to phone screening
* Oral or IV antibiotic use 1 month prior to phone screening
* Fermented foods (e.g., yogurt, kombucha) 1 month prior to phone screening
* Oral antioxidant or omega 3 supplements \<6 weeks prior to phone screening
* Current alcohol intake greater than 2 drinks per week
* Smoking cigarettes (\>5 during last 6 months) or tobacco (pipe or hookah #1 time during last month)
* Any plans to make significant change in diet and lifestyle
* Immunocompromised patients
* Allergies to milk, soy, or yeast
* Tanner Stage \<3
* Current active suicidal ideation that requires psychiatric hospitalization or suicidal ideation with plan or intent to attempt suicide as assessed by the Columbia-Suicide Severity Rating Scale (C-SSRS)
* Prior suicide attempt within past 6 months that required hospitalization and/or visit to the Emergency Department
* History of failure to clinically respond to 2 or more adequate trials of 2 different first-line SSRI antidepressant medications
* Any contraindication to MRI (e.g., braces)
* Pregnancy or any plans to become pregnant during the study is an exclusion criterion for entrance into the study. Women of reproductive capability will be asked about pregnancy or plans to become pregnant. Women of reproductive capability will be asked to employ at least one of the following allowable contraception methods until they complete their second MRI: birth control implant, birth control shot, birth control patch, birth control pill, condom, internal condom, birth control sponge, cervical cap, spermicide, fertility awareness (calendar method), outercourse and abstinence.
