In October 2017, Graham Hatfull received an urgent email from across the pond. A microbiologist colleague of his named James Soothill was desperately looking for a way to help two patients at the Great Ormond Street Hospital in London. The pair of teenagers, a girl and a boy, had cystic fibrosis, a genetic condition where the lungs can’t clear mucus or disease-causing bacteria. And they had both recently received double lung transplants as a result. The surgeries had gone well. But shortly after, infections long simmering inside their young bodies erupted from their sutures. And as Soothill noted in his message, the bacterial strains now spreading across their skin and through their tissues were impervious to all the hospital’s antibiotics.

With no more drugs to try, they were put on palliative care plans. But maybe Hatfull had a hail mary in his freezers. Since the late 1990s, the University of Pittsburgh micro­biologist had been enlisting students to help him amass the world’s largest collection of bacteriophages—viruses that prey solely on bacteria—from around the world. Perhaps a phage or two among those 15,000 vials sitting at –80 degrees Celsius might overpower the bacterial assaults on the lives of the two British patients.

In the end, there were four. By January, Hatfull’s team had identified one phage that could attack the boy’s strain. But they were too late—he had succumbed to his infection earlier that month. The girl, though, has been receiving a cocktail of three phages from Hatfull’s lab since June—including two that were genetically modified to better attack her bacteria.

Though she's still recovering, her skin lesions have mostly disappeared, and her liver and lungs are back from the brink of organ failure. She’s also back to more normal teenage things, like posting silly cat photos to Facebook and baking cupcakes. The results of this drastic intervention, published today in the journal Nature Medicine, represent the first-ever use of engineered phages in a human patient. The success offers hope that the emerging field of synthetic biology might reboot the 100-year-old Soviet science of phage therapy to arm doctors with a potent new weapon against superbugs.

“At first we were just excited to have two more strains to test on our phages,” says Hatfull. But as his team’s search for viral predators with a taste for Mycobacterium abscessus began to turn up promising leads from deep within the phage library, it became an all-consuming quest for the young research associates in his lab. “Once they smelled blood in the water, they worked tirelessly to turn this thing from a hypothetical to something we could put in a box and ship to London.”

The University of Pittsburgh researchers dug up three phages that could successfully invade the female patient’s strain of M. abcessus: Muddy, ZoeJ, and BPs. (Because most of Hatfull’s phage library is collected and characterized by undergraduate research volunteers, the names can get pretty funny: ChickenNugget, TGIPhriday, and IAmGroot are among the recent additions.)

But Muddy, which was scraped off the underside of a rotting eggplant by a student in Durban, South Africa, in 2010, was the only phage that has what’s called a lytic lifecycle. It hijacks a bacteria’s machinery to make millions of copies of itself, eventually bursting the cell apart and killing it. ZoeJ and BPs, on the other hand, could get inside the bacteria. But once there, they just curled up inside its DNA and went dormant. To make them useful for a patient, Hatfull’s team needed to toggle their snooze button into “phage rage” mode, as Steffanie Strathdee, coauthor of The Perfect Predator, calls it.