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Research on commensal gut microbes, or microbes with which humans share a symbiotic relationship, has provided evidence of a microbe-brain interaction, and has highlighted the importance of the gut-brain axis. Many external factors that may affect the meticulous composition of the gut flora have demonstrated a subsequent downstream effect on mental health. Similarly, the state of an individual’s mental health has also demonstrated an effect on the composition of the gut flora. Negative health effects may also be neurological or psychological in nature. Individuals may encounter illnesses such as chronic fatigue syndrome, or they may suffer from psychological disorders such as anxiety and depression. The importance of this bidirectional communication emphasizes the destructive effects of gut dysbiosis, and raises the question of whether or not supplementation with specific strains of probiotics can offset any negative physical and mental symptoms. This thesis reviews the health effects of three prominent probiotic microbes that are endemic to the human gut. These are Lactobacillus rhamnosus, Bifidobacterium infantis, and Bifidobacterium breve. The effects of these three microbes are evaluated using both rodent models and human studies. Studies analyzing rodent models discovered that rodents raised in microbe-free environments and rodents raised with chronic induced stress, both demonstrated improved sociability and attenuation of depressive and anxiety-like behaviors following supplementation of either of these three microbes. In human models, individuals that had suffered from clinical anxiety or depression, or that experienced these psychological symptoms as a result of functional gastrointestinal diseases such as irritable bowel syndrome and functional dyspepsia, demonstrated significantly improved mental health outcomes following consistent supplementation of either Lactobacillus rhamnosus, Bifidobacterium infantis, or Bifidobacterium breve. While the effects of probiotics on the gut-brain axis in humans are not as robustly studied, and findings may not be translatable between animal models and human models, there are a plethora of studies conducted in human models that indicate an observable positive health effect that should be further analyzed. Further research on the mechanism of microbes on the gut-brain axis can illuminate the intimate connection between the gut and the brain. These findings can underline the importance of a healthy diet as what we ingest and what we bring into our gut, will inevitably have an effect on the composition of our gut microbiome. Hopefully, the implications of this thesis can illuminate and curb the many adverse downstream health effects of unhealthy eating, and other unfavorable lifestyle choices.