The gut microbiome refers to the collection of the microorganisms and their genomes in the gut habitat which are now regarded as a critical node in the brain-gut axis. One outcome of the recent intense focus on this 'virtual organ' using metagenomic approaches is the realisation that microorganisms in our gastrointestinal tract outnumbers the human cells in our bodies by a factor of 10 and contains 150 times as many genes as out genome. The complex role of the gut microbiota within the brain-gut axis is just beginning to be charted, in contrast to a well-developed understanding of the reciprocal communication between the enteric nervous system (often referred to as the second-brain) and the central nervous system.
Behaviour and the brain-gut microbiome:
Researchers are just beginning to understand the wide influence exerted by the gut microbiota on brain and behaviour. For instance, the influence of gut microbiota on cognitive function and working memory deficits. Recent research undertaken by the Department of Medicine, David Geffen School of Medicine, UCLA, investigated whether the consumption of a fermented milk product with probiotic (FMPP), for 4 weeks, by healthy women could impact cognitive performance. In this study, healthy women with no gastrointestinal or psychiatric symptoms were assigned to groups given FMPP, a non-fermented milk product or no intervention. The FMPP contained Bifidobacterium animalis subsp. Lactis, Streptococcus thermophiles, Lactobacillus bulgaricus, and Lactococcus lactis subsp. Lactis. Preliminary findings from the foregoing study have provided evidence that intake of a fermented milk product with a probiotic can alter brain activity in regions of relevance to cognitive performance. Interestingly, further research has indicated that individuals diagnosed with irritable bowel syndrome (IBS) experience cognitive alterations related to regions of the brain, including the hippocampus, resulting in working memory deficits. This is fascinating when we take into consideration the microbial disturbances that can be associated with IBS.
Depression and Microbiota:
A link between the microbiota and depressive-like behaviours is also apparent pre-clinically following the administration of probiotic strains such as L. rhamnosus, B infantis and a formulation of L.helveticus and B. longum. Interestingly, in healthy volunteers, the foregoing combination of probiotic strains alleviated psychological distress including an index of depression. This being said, clinically, the microbiota composition of depressed subjects has, to date, only been reported in one preliminary study which reported no clear compositional differences from their control subjects (those not diagnosed with depression). Nonetheless, in light of the influence probiotic strains can have on healthy volunteers, a study may benefit from a more detailed bioinformatics analysis of the results. For this reason, further studies are urgently required to adequately address and unravel the complex interactions that might exist between depression and human microbiota.
Stress and Microbiota:
Early-life stressors are associated with increased anxiety and depressive-like behaviours, as GI disorders that have a stress component. Given the influence of stress on the gut-brain axis, appropriate development of the HPA (hypothalamus, pituitary, adrenal) axis is essential to the balanced functioning gut-brain axis. Stress has long been known to influence the composition of the gut microbiota, and research has shown that stress in early life can alters gastrointestinal bacterial content. Hence, early life stress capable of activating the HPA axis can impact on the developing microbiota and vice versa, ultimately leading to an imbalance in the gut microbiota and an inappropriate stress response.
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A: Mark Hinchey Naturopathy, 601 Glebe Road Adamstown, 2289, New South Wales, Australia