A mutant strain of E. coli, resistant to even the toughest antibiotics, has been found in the United States, federal health officials said Thursday.

The bacteria, discovered last month in a 49-year-old Pennsylvania woman with a urinary tract infection, contains a gene known as mcr-1, making it resistant even to colistin, a decades-old antibiotic that has increasingly been used as a treatment of last resort against dangerous superbugs.

The discovery -- the first time the strain has been found inside the U.S. -- "heralds the emergence of truly pan-drug resistant bacteria ," according to a report released Thursday by Department of Defense researchers. The woman, now recovered, has a military connection, authorities said without elaborating.