The gut is a dark and hidden world. We usually only know what’s happening within it when something goes badly wrong. But there are trillions of microbes in our guts; with their first-hand experience, they’re perfectly positioned to tell us about what’s going on inside ourselves. Pamela Silver from Harvard Medical School is now training them to do that. Her team is transforming gut bacteria into journalists—microscopic reporters who cover the bowel beat.

So far, they have loaded E.coli—a common gut microbe—with a genetic circuit that allows it to detect and remember the presence of an antibiotic. These engineered bacteria can colonise the guts of mice to no ill effect. If the mice get the antibiotic in their diet, the