Strains of bacteria, including E. coli and salmonella that are resistant to the "last resort" antibiotic doctors turn to when all other infection-fighting measures have failed, have turned up in Great Britain, officials say.

On three pig farms and in samples taken from human infections, researchers report detecting bacteria resistant to colistin, which is the "last resort" drug when all other normal antibiotics have failed.

Twelve people have been infected with strains of the two bacteria that carry a gene making them resistant to colistin.

It raises concern over possible global epidemics caused by new infections doctors would have no weapons against, officials say.

The colistin-resistant bacteria were first discovered earlier this year in China, blamed on overuse of antibiotics in agriculture there.

The resistance gene has also been discovered in recent weeks in France, Denmark, Portugal, the Netherlands and in several African and Asian countries.

Medical experts had expected it would take three years for it to spread to the United Kingdom, but UK health agencies began testing for it as a precaution.

Public Health England analyzed 24,000 samples of bacteria it keeps on record, gathered between 2012 and 2015.

Fifteen samples, from both salmonella and E. coli, were found to be resistant to colistin.

That's a very small number of instances, officials said.

"Our assessment is that the public health risk posed by this gene is currently considered very low, but is subject to ongoing review as more information becomes available," says Alan Johnson of Public Health England.

Cooking food properly can kill the bacteria, he says.