Major depressive disorder (MDD) is a highly heterogeneous disorder, with some patients displaying immune activation and altered intestinal microbiota composition when compared to healthy controls. In recent years, the transfer of fecal microbiota pooled from several MDD patients has been used to model depression in recipient rodents. However, we have previously observed the induction of donor-specific phenotypes in mice receiving microbiota from individual irritable bowel syndrome and generalized anxiety disorder patients. Therefore, we assessed the efficacy of fecal microbiota transplant (FMT) using individual versus pooled MDD patient microbiota to induce depressive-like behaviour in recipient rodents. We observed that pooling microbiota from several patients abrogated microbial features unique to individual donors. Mice that received pooled microbiota displayed different behavioural and immune phenotypes when compared to mice that received individual patient microbiota. Two individual MDD microbiota donors, patients MDD1 and MDD5, altered the behaviour of recipient mice when compared to controls. We identified several microbial species that may underlie the anxiety- and depressive-like behaviours observed in MDD1 and MDD5 mice. Additionally, altered expression of neural and immune genes was observed along the gut-brain axis of mice colonized with MDD1 microbiota. As microglia activation may play a role in our model, we developed a protocol for the isolation and phenotyping of adult mouse microglia that will facilitate future research efforts. Overall, our results demonstrate the heterogeneity of the microbial underpinnings of MDD and support the use of individual patient microbiota in future FMT experiments. / Thesis / Master of Science (MSc)

Clostrioides difficile is a common cause of nosocomial (hospital-acquired) infections. Patients receiving antibiotic treatment experience dysbiosis of gut microbiota, and C. difficile, normally held in check by the various other organisms, takes this opportunity to propagate. Symptoms of infection generally include diarrhea, colitis, dehydration, and fever. Understanding that C. difficile generally only causes illness when it is the dominant bacterium (i.e. when growth is relatively unchecked by other microbes), it is appropriate to investigate potential competitive organisms that may be introduced after antibiotic courses or during active C. difficile infection to effectively displace it. Fecal samples from the University of Central Florida Lift fecal collection station were aseptically plated onto modified cycloserine cefoxitin fructose agar (CCFA). Visually remarkable colonies (certain colonies that looked unique in comparison to others) were restreaked on new plates of the same media to verify growth, then transferred to brain heart infusion-supplemented (BHIS) plates for propagation. Colonies were inoculated in glycerol stocks for storage, then grown in BHIS liquid media to prepare for identification. Genomic extraction was performed on each sample, and spectrophotometric quantification and gel electrophoresis were executed to confirm successful extraction. Genomic samples will be sent to an external laboratory for identification via polymerase chain reaction and Sanger sequencing. We hypothesize that at least one bacterial strain from the fecal collection station will potentially inhibit C. difficile infection. Should such an organism be identified, it follows that the efficacy of its application in conventional hospital settings may be examined. Current regulation of fecal microbiota transplants, an effective therapeutic practice, is cumbersome, and changing the classification of fecal transplants may improve timeliness and effectiveness of treatment.