The Forgotten Organ – The Human Microbiota

The symbiotic relationship between humans and the microbes living inside us is increasingly linked to health, disease and even behaviour Dr Eva Sirinathsinghji

The rapidly progressing study of the human microbiota is revealing that humans are not individual self-contained beings, but instead hugely complex super-organisms that blur the distinction between where ‘we’ end and ‘they’ begin.

The human microbiota consists of an estimated 100 trillion cells [1], at least 10 times the number of human cells, and new research is revealing how this symbiotic relationship determines human health and disease.  Colonising our outer surface and deeper layers of the skin, gastrointestinal tract, saliva, oral cavity and conjunctiva, the human microbiota affect many aspects of human physiology including immune cell development, digestion and extraction of nutrients, fat distribution, metabolism, angiogenesis, and even regulation of memory, mood, and well-being. Consequently, disturbances of the human microbiota are implicated in many disease states including chronic inflammation, bowel disease, metabolic disorders, cancers, depression and anxiety, autism and memory impairments. Further, many aspects of current lifestyle are thought to have resulted in changes in the human microbiota including improved sanitation, environmental toxins, food processing, diets, vaccinations, increased antibiotic use, decline in parasite infections,  caesarean sections, smaller family size, refrigeration, less crowded living conditions and sedentary habits.

Two huge collaborative efforts in this rapidly expanding field include the International Human Microbiome Consortium [2] and the US National Institute of Health’s Human Microbiome Project (HMP) [3], the latter a 5 year project concluding in 2013 aimed at characterising the microbiotic communities residing in different parts of the body. Modern deep sequencing and metabonomic technologies - the study of the genetic material taken directly from its natural environment - have been employed to identify species previously unknown because they could not be cultured in the laboratory. The enormous data-gathering by the HMP has already yielded 190 papers involving the development of a reference set of microbial genomes and computational tools for analyzing new data sets, the establishment of a resource repository, and the exploration of relationship between microbiota and disease.

The new projects build on much older wisdom of the benefits of probiotics to digestion and overall health. Most, if not all cultures seem to have examples of probiotic foods in their traditional cuisine, including foods such buttermilk, sauerkraut, kefir, fermented soya (miso), fermented millet, fermented sorghum, pulque (fermented probiotic beverage from Mexico), that were consumed to maintain good health. Indeed one of the common probiotics researched today is the fungus Saccharomyces boulardii, which grows on the outside of lychee skins. It was first isolated when scientists noted people consuming the skin to protect themselves from cholera. It is also used to treat Candida infections [4].

We shall be featuring a series of reports on the human microbiota and health, the first of which dealing with our mind, mood and behaviour. 

Article first published 20/11/13

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References

1. Ley RE, Peterson DA, Gordon JI. Ecological and evolutionary forces shaping microbial diversity in the human intestine. Cell 2006, 124, 837-848
2. The International Human Microbiome Consortium, www.human-microbiome.org. http://www.human-microbiome.org/, accessed 1^st November 2013
3. The Human Microbiome Project. http://commonfund.nih.gov/hmp/, accessed 1^st November 2013
4. Saccharomyces boulardii Wikipedia.org. http://en.wikipedia.org/wiki/Saccharomyces_boulardii