A 49-year-old woman who came to a clinic in Pennsylvania with symptoms of a urinary tract infection is the first American found to have new and frightening antibiotic-resistant bacteria, Army researchers reported Thursday.

While her particular kind of bacteria, a strain of E. coli, is treatable with some antibiotics, it is resistant to the antibiotic colistin, a drug sometimes used as a last resort for difficult-to-treat infections. What makes the bacteria scarier is their potential to spread colistin resistance to other types of bacteria that are already highly resistant to multiple drugs, infectious-disease experts said.

"It basically shows us that the end of the road isn't very far away for antibiotics - that we may be in a situation where we have patients in our intensive-care units, or patients getting urinary tract infections for which we do not have antibiotics," Tom Frieden, director of the Centers for Disease Control and Prevention, told the Washington Post, which first reported the case, in an interview Thursday.

Philadelphia specialists were more measured.

"It's another warning, not a death star, but a very strong warning that we really do have to be careful with antibiotics and use them optimally," said Neil Fishman, an infectious-disease doctor who is associate chief medical officer of the University of Pennsylvania Health System.

Tom Fekete, an infectious-disease specialist at Temple University Hospital, said he was hopeful about other drugs in the pipeline. As for whether the Pennsylvania case means the antibiotic era is ending: "People have been saying that for about 20 years," he said. "I'm not giving up the ship."

In response to questions about the Pennsylvania woman, Gov. Wolf said Thursday that the state was working closely with the CDC and the Department of Defense, which tested the sample, to "coordinate an appropriate and collaborative response between federal, state, and local entities."

Officials would not say where in the state the patient lives or provide details about her condition.

The Pennsylvania case was reported Thursday in Antimicrobial Agents and Chemotherapy, a publication of the American Society for Microbiology. The authors wrote that the discovery "heralds the emergence of a truly pan-drug resistant bacteria."

Infectious-disease experts have warned for years that overuse of antibiotics is weakening the effectiveness of our defenses against harmful bacteria by creating bugs that are immune to drugs.

Fishman said it was not unusual for doctors to see bacteria that are resistant to all treatments. "We see that all too commonly in the hospital today," he said. Such patients are isolated to keep their germs from spreading.

Colistin is an old antibiotic sometimes used as a last resort against superbugs, including a family of bacteria known as CRE, which health officials have dubbed "nightmare bacteria," the Post reported.

In November, public health officials worldwide reacted with alarm when Chinese and British researchers reported finding the colistin-resistant strain in pigs, raw pork meat, and in a small number of people in China. It was later discovered in Europe and elsewhere.

Bacteria develop antibiotic resistance in two ways. Many acquire mutations in their own genomes that can't be shared with unrelated germs. People can become resistant in this way to colistin during treatment with the drug, Fekete said.

Other bacteria, like the type found in the Pennsylvania woman, rely on a shortcut: They get infected with something called a plasmid, a small piece of DNA, carrying a gene for antibiotic resistance, the Post reported. Plasmids can directly spread resistance genes to other types of bacteria.

Fishman said plasmids are also worrisome because they can carry more than one kind of resistance gene.

The journal report said that Walter Reed National Military Medical Center began testing for colistin resistance in E. coli cases just this month.

Colistin is more than half a century old and can be quite toxic.

Fekete and Fishman said they rarely have to use it and that newer drugs are usually a better bet. Fekete said his hospital probably uses colistin only about once a month. Fishman said it was used four to five times during a recent month when he worked with patients at the Hospital of the University of Pennsylvania. "It's not the antibiotic of last resort," Fishman said. "I hate using this antibiotic."

The state Health Department said it could not release details of its investigation into the Pennsylvania case, but said it might be interviewing the patient and her family, collecting medical samples, and asking about her medical and travel history.

The journal report said she was seen April 26 and had not traveled for five months.

Public health officials say they have been expecting this resistance gene to turn up in the United States.

"This is definitely alarming," David Hyun, a senior officer leading an antibiotic-resistance project at the Pew Charitable Trusts, told the Post. "The fact that we found it in the United States confirms our suspicions and adds urgency to actions we need to work on antibiotic stewardship and surveillance for this type of resistance."

Late last year, as part of a broader budget deal, Congress agreed to give hundreds of millions of dollars to the federal agencies engaged in the battle against antibiotic-resistant bacteria.

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Staff writers Angela Couloumbis and Karen Langley of the Harrisburg bureau contributed to

this article.