Memory and the microbiome: Scientists are discovering more and more ways gut health relates to brain health

Among those involved in psychiatric and neurological studies, little thought has been given to a potential relationship between the central nervous system and microorganisms. This in spite of the decades’-worth of information, which includes a potential link between the gut microbiome and behaviors and diseases of the brain. But new research presented at the 2018 annual meeting of the American Association for the Advancement of Science (AAAS) may help change that.

Janet Jansson, director of biological science at the Pacific Northwest National Laboratory, and her colleagues discovered that transplanting Lactobacillus (a probiotic bacteria believed to be beneficial to overall health) into germ-free mice improved their memory function. Compared to near-identical mice who were bred without microbes, these animals showed signs that they had developed “a much better memory.” Additionally, the mice with enhanced memories also appeared to undergo positive metabolic changes that were brought on by Lactobacillus.

“We are finding strong evidence of associations between the brain and microbiome. Now we are trying to discover the biochemical and genetic factors responsible. We have some exciting preliminary findings, but they are not yet published,” Jansson told

Gut health and the mind

Jansson’s work supports an earlier literature review from 2014. The multi-institutional and international team behind it cited numerous studies wherein germ-free animals displayed intensified behaviors in response to external stimuli. The work that begat all others in this vein, a 2004 study from Japan, showed recolonizing the bellies of germ-free mice with a specific species of Bifidobacterium greatly tempered an “exaggerated hypothalamic-pituitary-adrenal axis response to restraint stress.” Various other investigations since then have found exposing germ-free mice to new and unpleasant environments caused them to exhibit behaviors indicative of anxiety and stress. However, introducing gut bacteria or probiotics into these animals reduced and even reversed the behaviors.

In fact, the researchers noted that studies with animal models have pointed to Lactobacillus and Bifidobacterium positively affecting behaviors relating to anxiety and depression. These included Bifidobacterium longum regulating anxiety-like behaviors in colitis model rodents, and the combination of Lactobacillus rhamnosus and Lactobacillus helveticus reverting stress-induced memory problems in mice infected with Citrobacter rodentium. (Related: Probiotics fight depression: Healthy gut bacteria found to reduce symptoms.)

As for the effects of gut microbiomes on human brains, there have been a handful of small-scale studies that suggest a connection but don’t fully define it. A 2009 study that focused on patients suffering from chronic fatigue syndrome noted that taking a Lactobacillus-containing probiotic could reduce anxiety but not depression. An earlier study from 2007 that used a similar probiotic found that this probiotic had no significant influence on the overall mood of the participants. However, there was a slight change in mood among study subjects in the lowest mood state tertile. Though these studies hint towards probiotics benefiting mood and behaviors in humans, more clinical trials with a larger study population would be necessary.

A more definitive link has been established between gut microbiota and autism spectrum disorder. “Alterations in the communication between the gut microbiome and the brain, including alterations in the composition and metabolic products of the gut microbiome, have been implicated in the complex pathophysiology of [autism spectrum disorder],” said the researchers, noting that germ-free mice have been observed showing lower sociability and deficits in their social cognition abilities. While it’s unlikely that poor gut health is attributable to all cases of autism spectrum disorder, it remains an intriguing possibility.

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