Depression study probes link between antidepressants, brain steroids, and gut bugs

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

All participants must meet the following criteria: * Age 21-60 and able to give informed consent. * Not "needle phobic," by self-report. * English-speaking (to allow accurate use of behavioral rating scales and verbal cognitive tests). * Females of child-bearing capacity must be non-pregnant (confirmed by urine pregnancy test) and using effective non-hormonal birth control (e.g. abstinence, condoms, IUD). * Good general medical health; no significant uncontrolled illnesses that will invalidate the designated outcome measures. * Clinical labs (electrolytes, liver function test, CBC) with no clinically significant abnormalities that result in medical treatment that will invalidate the designated outcome measures. * Negative urine toxicology (drugs of abuse) screen. * Taking no medication or drugs likely to interfere with the study objectives (including statins or medications that affect hormones \[e.g. birth control pills or steroids\]). * Free of all psychotropic medications (including antidepressants) for at least 6 weeks (with the exception of prn short-acting benzodiazepines or sedative-hypnotics, \< 3 doses per week, and none for 5 drug half-lives before the study). * No vaccines for at least 4 weeks prior to baseline blood draw (including the flu shot). * Not currently anemic (Hct of 36-48 for females and 38-54 for males, or Hgb of 12.5-20) and has not donated blood for at least 8 weeks prior to baseline blood draw. * No neurological disorders and no history of concussion with a black-out that lasted \> 10 minutes. * Willing to provide stool sample Additional criteria for Depressed Participants: * Current DSM-5 diagnosis of Major Depressive Disorder, unipolar, with non-psychotic features. * Baseline 17-item Hamilton Depression Rating Scale (HDRS) rating of \>= 17, or Baseline 25-item HDRS rating of \>= 20. * Current depressive episode duration of \> 6 weeks. * No current, active suicidal intent; HDRS "suicidality" item rating \<= 2 OR by clinician determination. * No recent (\< 6 month) history of substance or alcohol use disorder(s), with the exception of tobacco use (DSM-5 criteria). * No current (in the last month) diagnosis of Post-Traumatic Stress Disorder (DSM-5 criteria). * No anticipated changes in psychotherapeutic interventions during the course of the study. Additional criteria for Normal Controls: • No history of DSM-5 Axis I diagnoses