According to a report issued on Thursday, the first patient to develop a bacterial infection that carries resistance to an antibiotic used to treat tough infections, has been identified in the US. While the 49-year-old woman has been successfully treated with another antibiotic, researchers are concerned that this resistance could be passed to other bacteria, potentially leading to an outbreak of antibiotic-resistant superbugs.

“It is the end of the road for antibiotics unless we act urgently,” said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention (CDC). Other countries – including China – have encountered multi-drug resistant pathogens, for which no known antibiotic treatments exist.

The Pennsylvania-based woman was diagnosed with a urinary tract infection in April, and was prescribed a course of antibiotics. Diagnostic tests showed that the woman was infected with a strain of E. coli, a common type of gut bacteria that can migrate to the bladder.

Further tests however, revealed that this strain of bacteria was resistant to most antibiotics used as first-line therapies for this type of infection. Even more alarming was the discovery that the bacteria possessed antibiotic resistance genes for the drug, colistin.

listin is a relatively old antibiotic, and most doctors stopped using the by the 1970s due to its unpleasant side effects. It has proven to be effective in recent years however, against bacteria that are resistant to most other over-used antibiotics.

When patients present with infections that are resistant to carbapenems – a class of antibiotics used after other treatments have failed – doctors prescribe colistin. If other pathogens acquire the colistin-resistance genes seen in the Pennsylvania woman, many people could be at risk of developing completely antibiotic-resistant infections.

“This is another piece of a really nasty puzzle that we didn’t want to see here,” said Dr. Beth Bell, who oversees CDC’s emerging infectious diseases programs. According to officials, the woman with the antibiotic resistant infection had not travelled outside the US in recent months. The CDC is now working to identify how and where the woman may have become infected.