U.S. health officials on Thursday reported the first case in the country of a patient with an infection resistant to all known antibiotics, and expressed grave concern that the superbug could pose serious danger for routine infections if it spreads.

"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 infection was not controlled even by colistin, an antibiotic that is reserved for use against "nightmare bacteria."

The infection was reported Thursday in a study appearing in Antimicrobial Agents and Chemotherapy, a publication of the American Society for Microbiology. It said the superbug itself had first been infected with a tiny piece of DNA called a plasmid, which passed along a gene called mcr-1 that confers resistance to colistin.

"[This] heralds the emergence of truly pan-drug resistant bacteria," said the study, which was conducted by the Walter Reed National Military Medical Center. "To the best of our knowledge, this is the first report of mcr-1 in the USA."

The study said continued surveillance to determine the true frequency of the gene in the United States is critical.