Take a stroll through the supermarket and you’ll find plenty of products stuffed with ‘friendly bacteria.’ Bacteria – the good guys! Chug them down and they’ll help to look after you.

We’re used to the idea that our guts are teeming with microbes and that they can be topped up with fancy yoghurts. We give the bacteria somewhere to live and, in return, they help us to digest our food. It’s a mutually beneficial, two-way street.

Only this cosy relationship isn’t quite what it seems.

Hidden among their ranks is a dark cabal of interlopers. To call them ‘unfriendly bacteria’ would be an understatement. These are microbial hell-raisers and as we get older, they slowly start to wreak havoc.

It turns out that our gut microbes aren’t always the good guys we think they are. A growing body of evidence suggests that our gut bacteria play a key role in many of the negative aspects of ageing, such as disease, frailty and the faltering of the immune system.