Obviously, the microbiota are not the whole story behind the obesity epidemic. We now need to understand how they interact with other things that affect our risk of becoming obese, not least of all, our own genes. And there is much we still don’t know about our life-long passengers, such as how they sense and respond to their host’s condition, how they are passed on, or how they are affected by our diet. By answering these questions, scientists could then assess whether actively shifting our bacterial balances could help to stem the worldwide increase in obesity levels