ISSMAT KASSEM, A microbiologist and assistant professor at the University of Georgia’s Center for Food Safety, has been worried for a while about antibiotic resistance. His research focuses on what he calls hitchhiker genes, short strings of DNA that get passed among bacteria like trading cards. For several years, he had been tracking the worldwide spread of one hitchhiker, a set of genes called mcr that defuse the effectiveness of one of the most valuable antibiotics in medicine: colistin, used on life-threatening infections when other medications fail.

The mcr genes were first found in China in 2015, in E. coli bacteria present in pigs at a slaughterhouse and pork in markets, and also in infections found in hospital patients in two provinces. Ever since, they have been popping up in people, animals, and environmental samples across the globe. That includes a half-dozen sightings in the United States, in patients in Connecticut, Michigan, New Jersey, New York, and Washington State. Every sighting constituted a potential emergency, because if the bacteria carrying mcr spread to other people—or if the genes passed into other pathogens—it could take away one of medicine’s last-resort defenses. But there was no connection between those US patients, and no known event that could explain their infection—and without those pieces of data, no easy way to set up a surveillance scheme to test how widespread its dispersal might be.