A first case of a patient with a bug that is resistant to the antibiotic colistin - known as the drug of last resort - has been detected in the US.
The 49-year-old woman in Pennsylvania has recovered but US health officials warned of the risk of it being "the end of the road" for antibiotics.
Dr Tom Frieden, director of the Centres for Disease Control and Prevention (CDC), told reporters in Washington: "It is the end of the road for antibiotics unless we act urgently."
He added: "We risk being in a post-antibiotic world."
The woman had gone to a military clinic in Pennsylvania in April and was treated for a urinary tract infection. Initial tests found she was infected with E coli bacteria and she did get treated with another antibiotic.
But tests over the last week confirmed the E coli was carrying a gene for resistance against the drug colistin.
The worry is that if bugs develop the colistin-resistance gene, doctors may be out of treatment options.
"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 programmes.
The Associated Press reported that the CDC is working with Pennsylvania health officials to interview the woman and her family to find out how she might have picked up the bug.
The woman had not travelled outside the country recently.
UK health officials have repeatedly warned of the threat of antibiotic resistance and are urging GPs not to prescribe antibiotics unless necessary.