Bacteria resistant to an antibiotic of last resort have been found in pigs, meat and a small number of hospital patients in China, setting off alarm bells for doctors and researchers.

Scientists discovered bacteria with a gene that makes them resistant to an old antibiotic called colistin.

"What they discovered is that by mouth it doesn't work, it doesn't get absorbed. But you could put it through your veins and it's very powerful because we haven't used that antibiotic for a very long time," Dr. Peter Lin, a family physician and medical commentator on CBC News Network, said Thursday.

For doctors, colistin is last line of defence against some infections.

"When they found this gene popping up in China in the animals and in the meats that they were testing and also in the patients, now they're worried because now this germ is now strong against this last line. And so if we don't have another antibiotic to come in, what do we do then?" Lin said.

Worse yet, the Chinese researchers found the new resistance gene, called mcr-1, was easily spread by plasmids, a portable form of DNA.

Prof. Jian-Hua Liu with the South China Agricultural University in Guangzhou and his co-authors found the mcr-1 gene had the potential to spread to bacterial species such as Klebsiella pneumoniae and Pseudomonas aeruginosa, which can cause diseases ranging from pneumonia to serious blood infections.

In Wednesday's online issue of the Lancet Infectious Diseases, the researchers report finding the gene in 166 of 804 pigs at slaughter across four provinces, and from pork and chicken sold in 30 open markets and 27 supermarkets in Guangzhou between 2011 and 2014.

It was also found in 1 per cent of 1,322 samples they tested from hospitalized patients in China, which the researchers called a relatively low proportion.

'Sorry, there is nothing I can do to cure your infection'

"Although currently confined to China, mcr-1 is likely to emulate other resistance genes … and spread worldwide," the researchers wrote. "There is a critical need to re-evaluate the use of polymyxins in animals and for very close international monitoring and surveillance of mcr-1 in human and veterinary medicine."

The discovery of the spreading mcr-1 resistance gene echoes the 2010 discovery of  another so-called "superbug" gene, NDM-1, which emerged in India and rapidly spread around the world.

David Paterson and Patrick Harris from Australia's University of Queensland wrote a commentary in the same journal, titled, "Colistin resistance: a major breach in our last line of defence."

The links between agricultural use of colistin, colistin resistance in slaughtered animals, colistin resistance in food, and colistin resistance in humans are now complete, the pair said.

China leads world

"There have been previous calls for curtailing the use of polymyxins in agriculture. We must all reiterate these appeals and take them to the highest levels of government or face increasing numbers of patients for whom we will need to say, 'Sorry, there is nothing I can do to cure your infection,'" Paterson and Harris wrote.

China is one of the world's largest users and producers of colistin for agriculture and veterinary use.

Worldwide demand for the antibiotic in agriculture is expected to reach almost 12,000 tonnes per year by the end of 2015, rising to 16,500 tonnes by 2021, according to a 2015 report by the QYResearch Medical Research Centre.

Lin and other doctors suggested people can do their part in curbing antibiotic resistance by only taking the drugs when prescribed, taking the full course and returning unused antibiotics to the pharmacy for proper disposal rather than fostering the spread of resistance genes among bacteria in the sewage system.

With files from Reuters and Health Day News