Microbes have been shown to have a relationship with stress and anxiety-related behaviors. Studies have shown exaggerated neuroendocrine responses to stress and anxiety-like behavior in germ-free mice. This observation was reversible when mice were colonized with bacteria, reducing anxiety-like behaviors. But, this phenomenon is thought to only be possible if mice are colonized in a critical time window during the early-life/adolescence. Additionally, there has been evidence of the microbiome modulating emotional behavior. Probiotics with Bifidobacterium and Lactobacillus have shown to have beneficial effects on anxiety and depression in a few different studies. Another mental state that microbes seem to influence is that of autism spectrum disorder. ASD-like behavior of mice was associated with alterations in the gut microbiome. A study also showed that germ-free mice showed reduced sociability and had social cognition deficits compared to conventionalized mice. These social deficits were reversed by the colonization of bacteria. Lastly, gut microbes have been shown to exert influence over leptin and ghrelin, appetite-regulating hormones. There have been shifts in science that indicate microbes do indeed affect the brain and a persons’ mental state but much of the research done has been on rodents. More research is required to truly prove this causational relationship in humans and understand the mechanisms at which microbes affect the brain.