You are what you eat (Image: Science Photo Library)

Your genes influence everything from eye colour to risk of disease – and now it appears they shape the collection of bacteria that line your gut. Tweaking these bugs – rather than the genes themselves – may be a way to treat inherited diseases and perhaps even protect against obesity.

The bugs in your gut are vital to your health. An imbalance of gut bacteria has been linked to all kinds of problems, from obesity and diabetes to heart disease and dementia. It is thought that we collect our all-important microbiota from our surroundings – the first colonisation probably happens in the womb, before we receive a giant dose at birth. Throughout life, everything from the air you breathe and the home you live in, to your diet and drug intake can influence your microbiota. But what about your genes?

Five years ago, Tim Spector of Kings College London attended a presentation of the results of a study in twins that suggested a person’s genes don’t affect their microbiome. “I thought, ‘I don’t believe this’,” says Spector, who conducts genetic research in twins. “Virtually everything I study in twins has a genetic component.” So he teamed up with the study’s presenter, Ruth Ley at Cornell University in New York, to run a much larger study.


The pair, along with their colleagues in the UK, US and Israel, analysed the gut bacteria of 977 twins based in the UK by looking at the bacterial content of their faeces. Each of the volunteers donated a sample, which the team put through a process that pulls out fragments of DNA that can be used to identify the different types of bacteria present. The team then compared how similar gut bacteria were in identical twins (which have the same set of genes) and non-identical twins (who are assumed to share a very similar environment, but do not have exactly the same genes).

When the group compared the levels of similarity in gut bacteria generally, they found no difference between identical and non-identical twins, suggesting that genes don’t influence the microbiome. But when they looked at the heritability of different groups of bacteria separately, they found that some bacteria were more similar in identical than in non-identical twins. In other words, some components of the microbiome are heritable.

Intriguingly, the groups of bacteria that are heritable are ones that have previously been linked to disease, the team found. “It could be one of the mechanisms for genetic inheritance of disease – a route we haven’t thought of before,” says Spector.

“The big excitement is that the most heritable bacteria is Christensenella,” says Andy Clark at Cornell University, who co-authored the study. Volunteers with higher levels of Christensenella bacteria in their guts were more likely to have a lower body mass index than those with low levels.

“Everyone has a small amount of it, around 1 per cent, but some people have as much as 10 per cent,” says Spector. “We think the more you have, the more protected you are against obesity.”

To test this theory, the team fed the volunteers’ fecal samples to mice bred without a microbiome. The mice that received a sample low in Christensenella gained significantly more weight, at a significantly faster rate, than those fed samples rich in Christensenella.

“We have to be careful not to assume that all we need to do to reduce obesity is add some Christensenella,” to patients’ guts,” says Emma Allen-Vercoe at the University of Guelph in Ontario, Canada. “I would love for this to be true, but we don’t yet know enough about the downstream consequences of such a treatment on the overall functionality of the microbiota.” Allen-Vercoe gives the example of cane toads, which, since they were introduced to Australia in the 1930s, have wreaked havoc on native wildlife. “Ecosystems are very complicated beasts, and a change in one aspect of it can have unexpected changes in another,” she says.

Spector and Clark’s research is ongoing, and the team hope that, eventually, we will be able to reduce a person’s disease risk by tweaking their microbiome with prebiotics – ingredients that promote the growth of beneficial microbes – and probiotics, doses of living organisms themselves. “We can’t change our genes, but we can change our microbiome,” says Spector.

Journal reference: Cell, DOI: 10.1016/j.cell.2014.09.053