Evolution of a Killer

A new study in mBio, a journal published by the American Society for Microbiology, describes the origins of methicillin-resistant Staphylococcus aureus, reports Popular Science's Rebecca Boyle. It started as a bug in humans, but not as a drug-resistant superbug, Boyle says. That came later, when the bug made the jump to pigs. "Researchers traced its evolutionary history by examining 89 genomes from humans, turkeys, chickens and pigs from 19 countries," Boyle says. Because livestock are routinely fed cocktails of antibiotics, hormones, and other drugs, MRSA was able to quickly evolve resistance to drugs when it infected animals, and then made the jump back to humans as a superbug, Boyle says. "It has been thought that humans' antibiotic abuse is the catalyst in superbug genesis, but this new research suggests it's the animals, and the drugs we feed them, that we should worry about," she adds. Study co-author Paul Keim, of Northern Arizona University's Center for Microbial Genetics and Genomics, tells Boyle that "humans have supplied a strong force [for evolution] through the excessive use of antibiotic drugs in farm animal production. It is that inappropriate use of antibiotics that is now coming back to haunt us."