Study protocol: efficacy and safety of Limosilactobacillus fermentum WIS32 as an adjunctive treatment for major depressive disorder—a randomized, double-blind, placebo-controlled trial

Major depressive disorder (MDD) is one of the leading causes of disability worldwide. Although antidepressant medications are widely used as first-line treatments, a substantial proportion of patients experience insufficient therapeutic response or limited tolerability, highlighting the need for additional safe and effective adjunctive treatment strategies. In recent years, growing evidence has suggested that the microbiota–gut–brain axis may play an important modulatory role in the onset, maintenance, and treatment response of depression. This study aims to evaluate the efficacy and safety of Limosilactobacillus fermentum WIS32, a human breast milk–derived probiotic strain, as an adjunctive intervention to stable antidepressant therapy in patients with MDD, and to further explore its potential effects on selected biological markers. This is a single-center, randomized, double-blind, placebo-controlled clinical trial. A total of 86 patients with MDD diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), are required to complete the study. Allowing for an anticipated dropout rate, 108 participants will be recruited. All participants will continue stable antidepressant treatment and will be randomly assigned in a 1:1 ratio to receive either Limosilactobacillus fermentum WIS32 or a matched placebo for 8 weeks. The primary outcome is the change in the 17-item Hamilton Depression Rating Scale (HAMD-17) score from baseline to week 8. This manuscript describes protocol version 1.0, dated 8 April 2025. The trial was prospectively registered in the Chinese Clinical Trial Registry (ChiCTR2500100388) on 8 April 2025, a WHO ICTRP Primary Registry, which collects and publicly displays the WHO Trial Registration Data Set (TRDS). Participant recruitment began on 10 June 2025 and is ongoing at the time of submission. Recruitment is expected to be completed by December 2026.