The US has reported its first case of a patient with a bug that is resistant to the antibiotic colistin - known as the drug of last resort.

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."

Read more: Elderly woman dies in Glasgow hospital after contracting Influenza B

He added: "We risk being in a post-antibiotic world."

In the UK in December, Public Health England (PHE) reported it had tested samples and also found resistance to colistin.

Professor Alan Johnson, head of the Department of Healthcare Associated Infection (HCAI) and Antibiotic Resistance at PHE, said: "The mcr-1 gene, recently identified as a cause of resistance to the antibiotic colistin, has been found in a very small number of samples of bacteria - 15 out of 24,000, from humans and food tested in the UK."

The US 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.

Read more: Nurses priced out of homes near Scottish hospitals, warns union

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.

Dr Paul Hoskisson, senior lecturer at the University of Strathclyde, said: "While this study is significant in showing that the mcr-1 gene is truly global, this development is not unexpected.

"The work highlights the worldwide nature of antimicrobial resistant infections - a drug resistance mechanism that was only discovered six months ago in Asia has already been found in samples in the EU and now in a patient in the USA.

Read more: Asking unhealthy would-be parents to delay pregnancy 'would make Scotland world leader'

"The work also highlights the promiscuity of bacteria, which are able to quickly and easily share resistance mechanisms between species. Careful surveillance is now needed to see how far the mcr-1 gene has spread.

"Antimicrobial resistance is inevitable, so we need to better understand how these resistance mechanisms are transmitted in the environment through basic research, if we are to preserve the lifespan of the antibiotics we have left."