Are all our modern health issues linked to our microbiome?

June issue of Scientific American

The human microbiome is definitely the sensation of this Summer 2012. [Kind of a Carly Rae Jepsen for science!]

In June alone, our microbial inhabitants were featured on the cover of Scientific American (watch their beautiful infographics), Nature, Science and Microbe…

The Human Microbiome Project (HMP), a $170 million research consortium funded by the US National Institute of Health (NIH), just released two reports in Nature accompanied by fifteen publications in PLoS ONE. The consortium, strong of about 200 researchers, studied the diversity of microbes inhabiting the body (skin, mouth, nose, gut, urogenital tract) of 242 healthy people, using new sequencing technologies to catalog the microbes.

The bottom line? Microbial diversity is very high between healthy individuals. It is therefore impossible to define a typical ‘healthy’ microbiome. In a way, every individual develops his/her own personal set of microbes. The diversity, however, is not totally random and patterns are clearly present in various areas of the body.

June 8 issue of Sci

The amount of sequence data produced by HMP is almost overwhelming: 1000 times more than for the Human Genome Project! No doubt there is still a tremendous amount of information to mine in this dataset. And HMP is not the only project out there. Metagenomics of the Human Intestinal Tract (MetaHIT) is a European-funded project that released its results last year. Both programs are ending this year and are now looking for a follow-up, as Michael Balter clearly explains in a Science News article. Balter writes:

“One big question for the future is whether to stress gathering more data on healthy and unhealthy microbiomes or focus on establishing causal relationships between the microbiome and disease.”

I think this is the crux of the matter. Can we go beyond correlation of microbial diversity with illness, and show some causal effects? This is essential since many studies suggest a role of the microbiome in crucial aspects of our health.

Obesity, for instance, is repeatedly pointed at. Helicobacter pylori, the ulcer-causing bacterium, has now been shown to help regulate the production of ghrelin, a human hormone that controls food intake, and therefore reduce weight gain—this is the work of Martin Blaser at New York University. More and more children, Blaser warns, grow without H. pylori in their stomach. Is it enough to explain the epidemy of obesity? Probably not, but this may not be trivial at all. 

June issue of Microbe

Bacteria may not be the only microbes influencing obesity. The Adenovirus 36 (Adv36), which infects humans, has the power to modify its host physiology (Atkinson, 2012).  Indeed, obesity correlates with Adv36 infection in adults, and even more in children. As Atkinson rightly writes in his article:

“Having an infectious agent responsible for obesity forces all of us to reconsider this condition, which is widely held to be self-inflicted through inappropriate diet and activity levels.”

So, who are we to blame? Bacteria, viruses, our dietary habits? Most probably there is no single culprit, but the realization that our microbes matter so much is revolutionary.

Immunity is another issue of tremendous importance. Here as well, evidence is piling up to suggest that commensal microbes actively participate in the building of a healthy human immune system (Olszak, 2012). And just as for obesity, autoimmune disorders (for instance asthma) are on the rise in our modern society. Our beneficial microbiome is globally in decline, due to an increase in hygiene standards and the common use of antibiotics (two things, it has to be noted, that are intrinsically good and save millions of lives every year). 

June 14 issue of Nature

The ‘microbiota hypothesis’, proposed by Sarkis Mazmanian from Caltech, suggests that the lack of exposure to commensals is triggering the increase in allergic disease. This view is shared by Martin Blaser, who showed that H. pylori can also protect against asthma. Blaser writes in his abstract (Blaser, 2012):

“The phenomenon of disappearing ancient microbiota may be a general paradigm driving the diseases of modernity.”

So, back to the title question: are all our modern health issues linked to our microbiome? I suppose the answer is the classical scientific answer: Maybe, but it’s complicated and we don’t understand the whole story yet.

But definitely a strong light now shines on the human microbiome and its role in our health and well-being.

References: 

• Pennisi E. (2011). Girth and the gut (bacteria). Science Vol. 332, pp.32-33.
• Ackermann J. (2012). The ultimate social network. Scientific American Vol. 306 (6), pp.36-43.
• Atkinson R. L. (2012). Virus-induced obesity in humans. Microbe Vol. 7 (6), pp.263-267.
• Balter M. (2012). Taking stock of the human microbiome and disease. Science Vol. 336, pp.1246-7.
• Blaser M. J. (2012). Equilibria of humans and our indigenous microbiota affecting asthma. Proc Am Thorac Soc  Vol. 9 (2), pp. 69-71.
• Hunter P. (2012). The changing hypothesis of the gut. EMBO Reports Vol. 13(6), pp.498-500.
• Olszak T. et al. (2012). Microbial exposure during early life has persistent effects on naturalkiller T cell function. Science Vol. 336, pp.489-493.
• Pennisi E. (2012). Microbial survey of human body reveals extensive variation. Science Vol. 336, pp. 1369-1371.