A Study of Probiotic Powder as an Add-on Treatment for Depression With Anhedonia

INTERVENTION: Placebo group:Escitalopram plus matching placebo powder;Probiotic group:Escitalopram plus Bifidobacterium longum subsp. longum probiotic powder; CONDITION: Major depressive disorder with anhedonia PRIMARY OUTCOME: Change in HAMD‐17 total score from baseline to week 8; SECONDARY OUTCOME: Change in TEPS total score from baseline;Clinical response rate at week 2;Clinical response rate at week 8;Clinical response rate at week 4;Change in related metabolite levels from baseline;Change in gut microbiota composition from baseline;Change in QIDS‐SR16 total score from baseline;Change in GSRS total score from baseline;Change in SHAPS total score from baseline;Change in GAD‐7 total score from baseline;Clinical remission rate at week 8; INCLUSION CRITERIA: 1.Diagnosed with major depressive disorder according to DSM‐5 criteria by the investigator and confirmed using the Mini‐International Neuropsychiatric Interview (MINI), either first episode or recurrent episode, without psychotic symptoms; 2.Aged 18–55 years, male or female; 3.If antidepressants were used before enrollment, a washout period of at least five half‐lives is required before enrollment; 4.HAMD‐17 score =17 at both screening and baseline; clinically significant anhedonia defined as a SHAPS score =3 at screening according to the original binary scoring method; 5.Able to understand and comply with study requirements and willing to sign written informed consent.