A bacterial gene resistant to all antibiotics has appeared in the United States and is worrying epidemiologists.

After two confirmed US cases of a superbug that thwarts a last-resort antibiotic, infectious disease experts say they expect more cases in coming months because the bacterial gene behind it is likely far more widespread than previously believed.

Army scientists in May reported finding E. coli bacteria that harbour a gene that renders the antibiotic colistin useless.

The gene, called mcr-1, was found in a urine sample of a Pennsylvania woman being treated for a urinary tract infection.

On Monday, researchers confirmed preliminary findings that E. coli carrying the same mcr-1 gene were found in a stored bacterial sample of a New York patient who had been treated for an infection last year, as well as in patient samples from nine other countries.

The report came from a global effort called the SENTRY Antimicrobial Surveillance Program, led by Mariana Castanheira of JMI Laboratories based in North Liberty, Iowa.

The mcr-1 superbug has been identified over the past six months in farm animals and people in about 20 countries, including China, Germany and Italy.

Australia has not recorded any cases as yet.

The bacteria can be transmitted by faecal contact and poor hygiene, which suggests a far wider likely presence than the documented cases so far, according to leading infectious disease experts.

Health officials fear the mcr-1 gene, carried by a highly mobile piece of DNA called a plasmid, will soon be found in bacteria already resistant to all or virtually all other types of antibiotics, potentially making infections untreatable.

"You can be sure (mcr-1) is already in the guts of people throughout the United States and will continue to spread," said Dr Brad Spellberg, professor of medicine at the University of Southern California.

Dr David Van Duin, an infectious disease expert at the University of North Carolina in Chapel Hill, said he expects more documented US cases of mcr-1 in coming months because it is already here and will spread from abroad. "We will see a lot more of this gene".

Colistin causes kidney damage, but doctors have opted for it as other antibiotics increasingly fail. Its overuse, especially in overseas farm animals, has allowed bacteria to develop resistance to it.

The concern of many disease experts is that mcr-1 could soon show up in bacteria also resistant to carbapenems, one of the few remaining dependable classes of antibiotics. In that event, with colistin no longer a last-ditch option, some patients would have to rely on their immune systems to fight off infection.

"Within the next two to three years, it's going to be fairly routine for infections to occur in the United States for which we have no (effective) drugs available," Dantas said.

Castanheira also believes mcr-1 will find its way into carbapenem-resistant bacteria, formally known as carbapenem-resistant enterobacteriaceae (CRE).

In an interview, she said the resulting virtually impervious bacterium would likely spread slowly inside the United States because CRE themselves are not yet widespread in the country, giving drugmakers some time to create new antibiotics.