Background: There are epidemiological and preclinical studies in vivo that support the biological plausibility of the association between periodontal diseases and major depression (DM), through the hypothesis of a leaky mouth by periodontitis as a source of neuroinflammation. Therefore, this association should be studied in depth in carefully designed cross-sectional studies in humans to specifically assess this relationship.

Objectives: Primary: determine if periodontitis can be associated with the development of DM. Secondary: (1) to estimate the prevalence of periodontal diseases (gingivitis and periodontitis) in patients with and without DM; (2) to determine whether oral, periodontal, and fecal (bacterial, viral, and fungal) metagenomic microbiomes, inflammatory mediators, and intestinal barrier integrity are associated with periodontal and mental health variables. Material and method: A cross-sectional analytical study with two groups is designed: * Control group (without DM): subjects without known mental health pathologies will be included, who present a PHQ-9 index of 5 or less. They will be recruited from the control group of a population-based study PsychoBioma TRIAD (C.P. PSQ-19-2 - C.I. 19/474-E). They will be matched by age, gender, and socioeconomic status. * Group of cases (MD patients): subjects with moderate DM will be selected, characterized by HPQ9 index values of 9 or higher. They will be selected among those patients who attend the Mental Health consultations associated with the San Carlos Clinical Hospital. The study will consist of three visits: * Visit in Mental Health Consultations: in this visit the subject will be evaluated to determine if he meets the eligibility criteria. You will be informed of the purpose of the study and you will be invited to participate and sign the informed consent. After that, a structured clinical interview for the DSM-IV (SCID) will be conducted and the subject will fill in a series of specific scales on a study-specific electronic device \[Beck Depression Inventory (BDI); UCLA Loneliness Scale, Center for Epidemiologic Studies Depression scale \[CES-D\]; Childhood Trauma Questionnaire short form (CTQ-SF); The World Health Organization Quality of Life questionnaire (WHOQOL); Hamilton scale (HAM-D17); Global Assessment of Functioning (GAF) Scale\]. * Dental School Visit: Subject will receive a comprehensive periodontal examination. A subgingival microbiological sample, a saliva sample and a blood sample will also be taken. The patient will be given a specific vial to collect stool samples. * At the participant's home: the stool samples will be deposited by the participants at home in the specific collection vial.