What does it mean to be human? It’s a deceptively simple question, but the answer is complex. The reasonis that most of our cells are not human at all. We depend on a vast army of microbes to stay alive: a friendly “microbiome” that protects us against germs, breaks down food to release energy, and produces vitamins.

Yogurt that might stop a heart attack

Your digestive system alone is home to roughly 100 trillion microbes – about 10 times the number of cells in the major organs. A team co-led by Prof Jeroen Raes of the Flemish Institute of Biotechnology has discovered that we all have one of three basic ecosystems of bugs in our guts – but strangely, the type for each person is unrelated to their race, native country or diet. They label these “enterotypes” the “bacteriodes”, “prevotella” and “ruminococcus”, to reflect the species of bug that dominate in each. People with a bacteriodes ecosystem, for example, have a bias towards bacteria that get most of their energy from carbohydrates and proteins.

This revelation has prompted much interest, because it could explain differences in our ability to digest food. A few years ago, Prof Jeffrey Gordon’s team at Washington University School of Medicine found that the intestines of obese people contain a slightly different repertoire of microbes when compared with slim people. In the Flemish study, researchers found a similar correlation between obesity and the abundance of bacteria that extract energy rapidly from food.

Prof Jeremy Nicholson, of Imperial College London, doubts that the latest find is of huge biological significance, since the three enterotypes probably have similar roles and capabilities. Yet he believes that one day, it might be possible to engineer enterotypes, which could be used (for example) to boost the number of calories extracted from poor diets by children in developing countries.

That is not to say that it will be easy. The human gut contains about 1,500 bacterial species, so tinkering with their ecology in a controlled way may be tricky. Although there are products that claim to manipulate bacteria, such as prebiotics, which fuel certain microbes, and probiotics (such as yogurts) that contain live bacteria, we still understand too little to do this reliably.

What does it mean to be human? It’s a deceptively simple question, but the answer is complex. The reason is that most of our cells are not human at all. We depend on a vast army of microbes to stay alive: a friendly “microbiome” that protects us against germs, breaks down food to release energy, and produces vitamins.

Your digestive system alone is home to roughly 100 trillion microbes – about 10 times the number of cells in the major organs. A team co-led by Prof Jeroen Raes of the Flemish Institute of Biotechnology has discovered that we all have one of three basic ecosystems of bugs in our guts – but strangely, the type for each person is unrelated to their race, native country or diet. They label these “enterotypes” the “bacteriodes”, “prevotella” and “ruminococcus”, to reflect the species of bug that dominate in each. People with a bacteriodes ecosystem, for example, have a bias towards bacteria that get most of their energy from carbohydrates and proteins.

This revelation has prompted much interest, because it could explain differences in our ability to digest food. A few years ago, Prof Jeffrey Gordon’s team at Washington University School of Medicine found that the intestines of obese people contain a slightly different repertoire of microbes when compared with slim people. In the Flemish study, researchers found a similar correlation between obesity and the abundance of bacteria that extract energy rapidly from food.

Prof Jeremy Nicholson, of Imperial College London, doubts that the latest find is of huge biological significance, since the three enterotypes probably have similar roles and capabilities. Yet he believes that one day, it might be possible to engineer enterotypes, which could be used (for example) to boost the number of calories extracted from poor diets by children in developing countries.

That is not to say that it will be easy. The human gut contains about 1,500 bacterial species, so tinkering with their ecology in a controlled way may be tricky. Although there are products that claim to manipulate bacteria, such as prebiotics, which fuel certain microbes, and probiotics (such as yogurts) that contain live bacteria, we still understand too little to do this reliably.

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