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human microbiome bdebate

Synopsis B·Debate - Microbiome: from forgotten organ to a new frontier

Each human being plays host to some 100 trillion bacteria, of some 10,000 different types, weighing an average of 2 kilograms. And while in the past we dreamed of being free of microorganisms, today we know that aspiration was misguided.


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The bacteria we co-exist with are key to processes like digestion and educating the immune system. But their role goes much further, as variations or alterations in their composition have been associated with all sorts of diseases: obesity and diabetes, allergies, certain types of cancer. Recent studies suggest that they may also be linked to infection with the Human Immunodeficiency Virus (HIV) or even to behavioral disorders like autism. This is why the microbiome, the community of microorganisms that inhabit the human body —more than 90% of which are bacteria— is now known as the forgotten organ.

Some of the top international experts in the microbiome presented and commented on the latest work in this area at B·Debate, an initiative of Biocat and the “la Caixa” Foundation to promote scientific debate.

The main conclusions of this debate are:

  • In terms of the microbiome, richer is better: greater bacterial diversity tends to correspond with better overall health.
  • According to the hygiene hypothesis, increased asepsis in developed countries has reduced the diversity of beneficial microorganisms, contributing to the increase in allergies, obesity and diabetes.
  • Some bacteria have been linked to certain types of cancer, like colon or pancreatic. With HIV, the intestinal damage caused by the virus allows bacteria to pass into the blood and accelerates cell ageing.
  • The intestinal microbiome can even impact behavior and has been linked to cases of autism. Research is being done on the use of prebiotics, probiotics and even fecal transplants to treat this and other conditions.En termes de microbioma, és millor ser ric: una major diversitat bacteriana se sol correspondre amb una millor salut en general.


From a bacteria-free world to the ‘microbiome boom’

We’ve known that microorganisms exist for some time now, but until recently they were contaminants. The dream was to be bacteria free,” remembered  Francisco Guarner, head of the IBD Section at Vall d’Hebron University Hospital and one of the scientific leaders of this B·Debate. But when scientists managed to breed mice without germs, “they didn’t grow, had many allergies and died surprisingly easily”: microorganisms are essential to a normal life.

Since World War II, there has been a rise in the number of cases of chronic non-communicable conditions like cancer, autism and allergies. This hasn’t happened in developing countries. One possible explanation is referred to as the hygiene hypothesis: the use of antibiotics, increase in cesarean births, decrease in breastfeeding and, in general, increased asepsis in wealthy countries has limited our contact with certain microorganisms that are necessary to educate our defenses. This is why, according to Guarner, “the immune system makes more mistakes and changes to the traditional microbiome may be the origin of this increase in disease.”

But why the relatively sudden interest in the microbiome? Because of technology. “New DNA sequencing techniques have allowed us to look at and study our other genome, that of all those microorganisms,” said M. Luz Calle, director of the Bioinformatics and Medical Statistics Group at the University of Vic-Central University of Catalonia. This technology can not only identify the types of microorganisms but also the genes they contain.

In general, we have an average of 600,000 microbial genes in our gut alone and, in the words of Dusko Ehrlich, director emeritus of the French National Institute for Agricultural Research, “with regard to the microbiome as well, it’s better to be rich than poor.” The lower the diversity, the greater the tendency to accumulate fat, to develop diabetes and, in general, to be in poorer health.




A good part of the studies on the microbiome are done on feces, as the gut is home to more than 90% of all the microorganisms in our bodies. In 2011, one of these studies divided the population into three groups according to the composition of the bacteria in their feces.

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“Some types of cancer tend to be seen more in certain families but don’t appear to be genetic,” said Núria Malats, principal investigator in the Genetic and Molecular Epidemiology Group at the CNIO. The microbiome may be responsible for some of those cases, like colon cancer.

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A person whose microbiome is lacking in diversity can improve its wealth through their diet. A diet rich in fruit, vegetables and whole foods, many different sources of fiber, seems to be the healthiest option for us and our bacteria.

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