Inclusion Criteria:

• Outpatients or inpatients with nonpsychotic major depressive disorder (MDD) or Bipolar I or II Disorder.
• A score of >16 on the QIDS
• Outpatients or inpatients for whom antidepressant treatment is deemed appropriate by the treating clinician
• Subjects who are between 18-65 years of age

Exclusion Criteria:

• Contraindications to citalopram treatment
• Axis I or II disorder other than depression that is the primary reason for seeking treatment intervention and/or psychiatric care
• Subjects diagnosed with Borderline Personality Disorder (BPD) as their primary diagnosis.
• For healthy controls, a first degree relative who has been diagnosed with an Axis I disorder
• Patients with schizophrenia, schizoaffective disorder, or bipolar I disorder
• Antidepressant treatment within 4 days of study (1 week if fluoxetine). Subjects currently on antidepressant medication with subtherapeutic results in terms of depression management after providing informed consent, will undergo a medication taper and discontinuation prior to initiation of citalopram treatment. The subject must be off of previous antidepressants for at least 4 days week prior to starting citalopram (1 week if fluoxetine). The subject will be closely monitored by the research study psychiatrist (with or without additional monitoring from primary clinical psychiatric providers). The medication taper is left up to the research study psychiatrist in consultation with patient's primary care or psychiatric provider. Study subjects who cannot be safely tapered from their medication or experience adverse effects during the taper will be excluded from the study
• Study subjects using their antidepressant medication for management of nicotine dependence, chronic pain, migraine prophylaxis, or other diagnoses will not be eligible for the study unless they remain on a stable dose of the medication for the 12 weeks of the study.
• Trazodone, melatonin, and diphenhydramine may be used as rescue medications for insomnia. Benzodiazepines may be used for treatment of anxiety, not to exceed 4 mg/24 hour of lorazepam
• Subjects who are currently on an antibiotic or an antibiotic within 2 weeks. (Topical antibiotics are OK)
• Daily use of aspirin, NSAID's or Warfarin (low dose of baby aspirin OK)
• Subjects unable to give informed consent are excluded
• Pregnant subjects will be excluded
• Subjects who are currently breastfeeding and who plan to continue breastfeeding will be excluded
• Postmenopausal women are not eligible for this study