That’s exactly why the Columbia E. coli was so worrying. Over the past decade or two, E. coli has developed resistance to one antibiotic after another. For some infected patients, their last hope is the antibiotic colistin, a toxic substance with potential side effects that include kidney and brain damage. The Columbia E. coli had a mutation in a gene, MCR-1, that confers a terrifying attribute: imperviousness to colistin.

“We’re looking to the shelf for the next antibiotic, and there’s nothing there,” says Erica Shenoy, associate chief of the infection control unit at Massachusetts General Hospital. “We’re facing the specter of patients with infections we can’t treat.”…

The U.S. Centers for Disease Control and Prevention reports that 2 million people a year are sickened in the U.S. by bacteria or fungi resistant to major antibiotics, and that 23,000 die from them. “It’s probably a vast underestimate,” says Karen Hoffman, who heads the Association for Professionals in Infection Control and Epidemiology. “We don’t have a good reporting system for multiresistant organisms, so we don’t really know.” Studies suggest the cost to the U.S. health care system of treating patients with these hardy bugs tops $3 billion a year.