A study by De Palma, et al. used germ-free and specific pathogen-free mouse models to investigate the effects of early-life stress. Researchers reported that stress (maternal separation) altered the hypothalamic-pituitary-adrenal axis in mice. Colonizing adult germ-free mice with the same microbiota led to distinct microbial profiles in mice who had experienced early-life stress. After colonization, behaviour was altered in the stressed mice but not in control mice. This study showed that stress-induced changes in physiology resulted in dysbiosis; this dysbiotic gut microbiota was required for anxiety-like behaviour to manifest.
A mechanistic study by D’Mello, et al. examined a mouse model of liver inflammation (with resultant changes in brain function) and reported that a probiotic reduced sickness behaviour in the mice. This was associated with a decrease in circulating TNF-alpha levels and a reduction in microglial activation. The results suggest a new pathway through which the gut and the brain interact in systemic inflammatory diseases.
Meanwhile, another gut-brain study investigated the effects of probiotics in healthy humans. A randomized, placebo-controlled trial by Steenbergen, et al. reported a beneficial effect of a multispecies probiotic on cognitive reactivity to sad mood, in a 4-week intervention with non-depressed individuals. Probiotic intervention was associated with reduced rumination and aggressive thoughts.
De Palma G, Blennerhassett P, Lu J, Deng Y, Park AJ, Green W, Denou E, Silva MA, Santacruz A, Sanz Y, Surette MG, Verdu EF, Collins SM, Bercik P. Microbiota and host determinants of behavioural phenotype in maternally separated mice. Nat Commun. 2015 Jul 28;6:7735. doi: 10.1038/ncomms8735.
D’Mello C, Ronaghan N, Zaheer R, Dicay M, Le T, MacNaughton WK, Surrette MG, Swain MG. Probiotics Improve Inflammation-Associated Sickness Behavior by Altering Communication between the Peripheral Immune System and the Brain. J Neurosci. 2015 Jul 29;35(30):10821-30. doi: 10.1523/JNEUROSCI.0575-15.2015.
Steenbergen L, Sellaro R, van Hemert S, Bosch JA, Colzato LS. A randomized controlled trial to test the effect of multispecies probiotics on cognitive reactivity to sad mood. Brain Behav Immun. 2015 Aug;48:258-64. doi: 10.1016/j.bbi.2015.04.003. Epub 2015 Apr 7.