We know that the 100 trillion microbes in the human body are important to our health. What’s harder to know is how to use them to make us healthy.

Normally, our resident microbes–the microbiome–carry out a number of important jobs for us, from fighting off pathogens to breaking down food for us. If they get disrupted, we  suffer the consequences. Sometimes antibiotics can upset the ecological balance in our bodies so severely, for example, that rare, dangerous species can take over.

For decades, doctors and scientists have searched for microbes that can promote our health by taking up residence in our bodies. They’ve had some modest success in treating people by giving them a single species at a time. (Don’t be