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.

Handout . / Reuters Carbapenem-resistant Enterobacteriaceae (CRE) bacteria is pictured in this medical illustration provided by the Centre of Disease Control and Prevention.

The woman hadn't traveled outside the U.S. in the previous five months, according to the report. Doctors treated the infection using another antibiotic, but said they were alarmed by the discovery of the mcr-1 gene inside the U.S.

"The fear is that this could spread to other bacteria and create the bacterium that would be resistant to everything," Dr. Beth Bell, director of the Centers for Disease Control and Prevention's National Center for Emerging and Zoonotic Infectious Diseases, told ABC News.

"The more we look at drug resistance, the more concerned we are," Dr. Tom Frieden, CDC director, said Thursday. "The medicine cabinet is empty for some patients. It is the end of the road for antibiotics unless we act urgently."

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At least 2 million people become infected in the U.S. each year with antibiotic-resistant bacteria, according to the CDC. Those infections result in at least 23,000 deaths annually.

The Pennsylvania woman's case, while not deadly, is the first U.S. discovery in a human of bacteria resistant to colistin, a drug held in reserve to treat serious infections that resist another major class of antibiotics called carbapenems. Bacteria that could resist colistin and carbapenems would be unstoppable, according to The New York Times.

In a study last year, the CDC warned that drug-resistant infections would continue to rise. And while the medical community has been anticipating the strain's arrival, the troubling part is that "this case seems completely home-grown," according to Dr. Nasia Safdar, an associate professor of infectious diseases at the University of Wisconsin School of Medicine and Public Health.

Multi-drug-resistant bacteria are found elsewhere in the world, frequently in agriculture. This particular colistin-resistant strain, for instance, was first discovered in people and livestock in China in November 2015. Since then, it has turned up in Europe and Canada. The U.S. Department of Agriculture said Thursday it had detected the bacteria in a single sample of a domestic pig's intestine.

When it comes to spreading resistant bacteria, "food-producing animals are of particular concern," the CDC says. "Antibiotics must be used judiciously in humans and animals because both uses contribute to the emergence, persistence, and spread of resistant bacteria."