Story highlights It is the first U.S. case of a bacterial infection that was resistant to an antibiotic of last resort

The CDC director warns that we may see more of these cases

Dr. Tom Frieden urges new drugs and encourages doctors to stop over-prescribing antibiotics

(CNN) A 49-year-old Pennsylvania woman showed the presence of a rare kind of E. coli infection, the first known case of its kind in the United States. It is a superbug that is resistant to many antibiotics, even Colistin, which doctors use as a last resort when other antibiotics fail.

The case was detailed in a report by the U.S. Department of Defense on Thursday. Centers for Disease Control and Prevention Director Dr. Tom Frieden talked about the case at the National Press Club in Washington.

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The woman went to a clinic in Pennsylvania, and a sample was forwarded to Walter Reed National Military Medical Center. Walter Reed found the bacteria in her urine. There is no indication of how the bacteria got into the woman's system. She had not traveled outside the United States within the past five months.

The CDC and the Pennsylvania Department of Health mobilized immediately to investigate the case and to trace contacts the patient may have had to see whether the bacteria had spread.

The woman was treated and released and has no other medical problems related to the bacteria that we know of, according to Dr. Alex Kallen, a medical officer with the CDC.