Inclusion Criteria:  General criteria:  Male and/or female; Age between 50-85 years; Having given their free, informed, written consent, signed by the participant and the investigator (at the latest on the day of inclusion and before any examination required by the research); Patient benefiting from a social security scheme  AD group:  Diagnosis of Alzheimer's disease according to the international ATN criteria of Albert et al, (1) who had a diagnostic lumbar puncture and CSF biomarker assay and brain imaging with analysis of hippocampal atrophy; topographic and pathophysiological markers must be positive to confirm the etiopathogenic diagnosis; Mild to moderate stage with MMSE score between 10 and 26 ; Symptomatic treatments specific to Alzheimer's disease (acetylcholinesterase inhibitors and memantine) and psychotropic treatments (anxiolytics, antidepressants, neuroleptics) are authorized but must be at fixed doses for more than 3 months ;  Control group:  - Patients seen in memory consultation for concern of having AD but for whom the workup is normal. These patients will have had a diagnostic lumbar puncture during the same memory consultation.  Exclusion Criteria:  Inclusion criteria:  General criteria:  Male and/or female; Age between 50-85 years; Having given their free, informed, written consent, signed by the participant and the investigator (at the latest on the day of inclusion and before any examination required by the research); Patient benefiting from a social security scheme  AD group:  Diagnosis of Alzheimer's disease according to the international ATN criteria of Albert et al, (1) who had a diagnostic lumbar puncture and CSF biomarker assay and brain imaging with analysis of hippocampal atrophy; topographic and pathophysiological markers must be positive to confirm the etiopathogenic diagnosis; Mild to moderate stage with MMSE score between 10 and 26 ; Symptomatic treatments specific to Alzheimer's disease (acetylcholinesterase inhibitors and memantine) and psychotropic treatments (anxiolytics, antidepressants, neuroleptics) are authorized but must be at fixed doses for more than 3 months ;  Control group:  - Patients seen in memory consultation for concern of having AD but for whom the workup is normal. These patients will have had a diagnostic lumbar puncture during the same memory consultation.  Exclusion criteria:  General criteria:  Absence of a family caregiver to complete the neuropsychological scales and questionnaires ; Patient living in a medical institution; Illiteracy or inability to complete the psycho-behavioral tests; Major physical or neurosensory problems likely to interfere with the tests; Contra-indication or refusal to perform the biological tests; Refusal to carry out neuropsychological tests. Refusal of stool collection Digestive neoplasia in progress or < 5 years old Digestive surgery with intestinal resection ; Presence of inflammatory bowel disease or other familial gastrointestinal pathology ; Chronic use of laxatives; Antibiotics taken within one month prior to inclusion; Short-term life-threatening conditions (active cancer, unstable heart failure, severe liver, kidney and respiratory failure); Chronic psychosis or psychotic episodes; Alcohol or drug addiction; Epilepsy and other non-degenerative diseases of the central nervous system Vitamin B12 deficiency and unsupplemented folic acid ; Untreated hypothyroidism; Patient deprived of liberty, by judicial or administrative decision; Major protected by law; Patient in a period of relative exclusion from another protocol; Refusal to participate in the protocol.  AD group :  Insufficient clinical and paraclinical information for the diagnosis of AD; Genetic form of AD (known genetic mutation);  Control group: No specific non-inclusion criteria