A50 TRANSFER OF DEPRESSIVE-LIKE PHENOTYPE TO GNOTOBIOTIC MICE DEPENDS ON MICROBIAL FEATURES SPECIFIC TO INDIVIDUAL PATIENTS

Abstract Background Major depressive disorder (MDD) affects approximately 4.4% of the global population. Despite its high prevalence, little is known about the mechanisms underlying this disorder. Recent studies in both humans and rodents have suggested that the intestinal microbiota may play a role in depression. Altered microbiota composition has been found in a subset of MDD patients. Preclinical studies have suggested that fecal microbiota transplant using pooled MDD patient samples can induce depressive-like behaviour in rodents. We have previously shown that the use of different microbiota donors with irritable bowel syndrome results in the induction of different phenotypes in recipient mice. Thus, we have hypothesized that pooling microbiota samples abrogates features that are unique to individual donors. Aims (1) Investigate whether the transfer of individual MDD patient microbiota can induce depressive-like behaviour in germ-free (GF) mice (2) Identify features of individual MDD patient microbiota that are associated with the depressive-like phenotype Methods GF NIH Swiss mice of both sexes (min. n=10 per group, total n=110) were colonized with either fecal microbiota from a single donor, MDD patient (MDD1-4) or matched healthy control (HC1-4), or pooled fecal microbiota from MDD1-4 or HC1-4. Mouse behaviour was assessed, using the open field test, three chamber sociability assay, tail suspension test, and sucrose preference test. Stool samples were collected throughout the experiment for 16S rRNA gene sequencing. Results Mice colonized with microbiota from patient MDD1 exhibited depressive-like behaviour, as assessed by the sucrose preference test and sociability assay, when compared to mice colonized with HC1 microbiota. This was not true for mice colonized with individual microbiota from the other three patients (MDD2-4) or with pooled MDD microbiota. Comparative analysis of the 16S data revealed a significant difference in Faith’s Phylogenetic Diversity between MDD1 microbiota and pooled MDD microbiota. Four bacterial species were found to be significantly associated with the depressive-like phenotype in mice: Bacteroides acidifaciens, Bacteroides ovatus, unclassified species of Phascolarctobacterium (Veillonellacae family), and Eggerthella lenta. The relative abundances of these species did not differ significantly between the two pooled groups. Conclusions Microbiota from some, but not all, MDD patients can induce a depressive-like phenotype in GF mice. The ability to induce depressive-like behaviour in GF mice is lost when microbiota from multiple patients is pooled. Specific bacterial species may be responsible for the successful transfer of the depressive-like phenotype to mice. Funding Agencies NIH

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