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In 2012, the World Health Organisation classified colistin, the most widely used polymyxin, as being critically important for human health. But that didn't stop farmers around the world, especially in China, from using large quantities of colistin to fatten up pigs and chickens. Now Yi-Yun Liu at the South China Agricultural University in Guangzhou and colleagues have discovered the first known resistance gene for colistin that is able to move freely from one bacterium to another.

"We risk being in a post-antibiotic world," said Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, referring to the urinary tract infection of a 49-year-old Pennsylvania woman who had not traveled within the prior five months. Frieden, speaking at a National Press Club luncheon in Washington, D.C., said the bacteria was resistant to colistin, an antibiotic that is reserved for use against "nightmare bacteria."

Department of Defense researchers disclosed Thursday in a report placed online by the journal Antimicrobial Agents and Chemotherapy that a 49-year-old woman who sought medical care at a military-associated clinic in Pennsylvania last month, with what seemed to be a urinary tract infection, was carrying a strain of E. coli resistant to a wide range of drugs. That turned out to be because the organism carried 15 different genes conferring antibiotic resistance, clustered on two "mobile elements" that can move easily among bacteria. One element included the new, dreaded gene mcr-1.