Without the "good" bacteria in our guts, we could not digest food. You might expect that we would all have the same set of bacteria to provide the chemical machinery that does the job. But this turns out to be only half true.

Knowing that gut bacteria are key to digestion and metabolism, Jeffrey Gordon of Washington University School of Medicine in St Louis, Missouri, and his colleagues went in search of a core group of bacterial species that aid digestion. They expected to find these species living in the guts of most healthy people.

When the researchers analysed faeces from 154 people this turned out not to be so. The subjects did, however, all possess the same core group of bacterial genes needed for digestion, albeit from different species (Nature, DOI: link).

It is this combination of genes, rather than any particular species, that is necessary for a healthy gut, says Gordon. "We've learned that you can have different collections of species, yet the gene functions represented in these collections are broadly shared."

The analysis also flagged up differences between the bacterial genes of obese and lean people. Obese individuals had a greater proportion of genes for digesting fat, protein and carbohydrates, which might make them better at extracting and storing energy from food. Gordon hopes that a better knowledge of these genes might suggest new ways of combating obesity.

Interestingly, participants who were related shared similar gut bacteria species as well as genes.