The microbiome is more than just our partners-in-digestion – they affect our health too. They have been linked to a variety of medical conditions, including allergies, immune diseases, and even obesity. For example, the balance of the two major groups – the Bacteroidetes and Firmicutes – could influence our body weight.

Fat mice and humans have a less diverse milieu of gut bacteria, with a greater proportion of Firmicutes to Bacteroidetes in their bowels. This ratio increases if we eat high-fat diets and falls if we eat low-fat diets. And if the gut bacteria from fat mice are transplanted into mice with no gut bacteria of their own, they can make the new hosts overeat and pile on the pounds. This research suggests that gut bacteria could be manipulating us for their own ends. Some species send out signals that make us hungrier, encourage us to eat more, and affect the way we store fat. And some of our immune genes help to moderate these signals.

As we learn more about our bacterial partners, we might eventually find ways of influencing them to improve our health. This is already happening. In 2008, Alexander Khoruts from the University of Minnesota managed to cure a woman with a “vicious gut infection” by giving her a transplant of her husband’s gut bacteria.