Miami (AFP) - A US woman has died from an infection that was resistant to all 26 available antibiotics, health officials said this week, raising new concerns about the rise of dangerous superbugs.

The woman, who was in her 70s, died in Nevada in September, and had recently been hospitalized in India with fractured leg bones, the US Centers for Disease Control and Prevention reported.

The cause of death was sepsis, following infection from a rare bacteria known as carbapenem-resistant Enterobacteriaceae (CRE), which is resistant to all antibiotics available in the United States.

The specific strain of CRE, known as Klebsiella pneumoniae, was isolated from one of her wounds in August.

Tests were negative for the mcr-1 gene -- a great concern to health experts because it makes bacteria resistant to the antibiotic of last resort, colistin.

It was unclear how the woman's infection acquired resistance.

Experts said she had been treated repeatedly in India during the last two years for a femur fracture and hip problems, most recently in June 2016.

Once the bacteria was identified in Nevada, the patient was isolated to prevent the infection from spreading in the hospital.

Postmortem tests showed her infection might have responded to a treatment called fosfomycin, which is not approved in the United States.

Paul Hoskisson, a researcher at the University of Strathclyde, in Scotland, said that several European countries, including Britain, license fosfomycin for intravenous use in such cases.

"This is important because we are seeing increasing numbers of drug-resistant infections, and this is one of the first cases for Klebsiella where no drug options were open to the medical staff."

Multi-drug-resistant Klebsiella pneumoniae has been described by the World Health Organization as "an urgent threat to human health."

According to Nick Thomson, leader of the bacterial genomics and evolution group at the Wellcome Trust Sanger Institute in England, this bacteria is likely to become more and more resistant.

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"The report highlights international travel and treatment overseas as a feature in the introduction of this pan-resistant isolate into the USA," he said.

"Since we live in such an interconnected society, this is important because this isolate represents a truly untreatable infection" which leaves health-care professionals with few options but to seek to prevent further transmission.

Laura Piddock, a professor of microbiology at the University of Birmingham, said the case shows that doctors "need the flexibility to use antibiotics licensed for use in other countries and shown to be active in the laboratory against the patient's infecting bacterium."

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