Scientists warned of the "epidemic potential" of deadly and fast-spreading bacteria resistant to last-line antibiotics.
The new superbugs, found in southern China, could erase nearly a century of antibiotic protection against killer diseases born by common germs such as E. coli, the researchers reported in a study.
"These are extremely worrying results," said Jian-Hua Liu, a professor at Southern Agricultural University in Guangzhou and co-author of the study.
Liu and colleagues found a gene, called MCR-1, that makes bacteria resistant to a class of antibiotics, known as polymyxins, used to fight superbugs.
The gene -- detected in common but deadly bacteria such as E. coli and K. pneumoniae, which causes pneumonia and blood disease -- effectively makes these bacteria invincible.
Even worse, MCR-1 allows the bacteria to spread easily from one strain or species to another, said the study, published in Lancet Infectious Diseases.
Until now, rare cases of resistance occurred only through mutation in individual organisms, severely limiting transmission.
"Polymyxins were the last class of antibiotics in which resistance was incapable of spreading from cell to cell," said Liu.
The World Health Organization (WHO) has already warned antimicrobial resistance may result in "a return to the pre-antibiotic era," where infections once easily cured prove fatal.
Most of the 50 to 100 million people who died during the 1918 flu pandemic -- 10 years before the discovery of penicillin -- were killed by bacterial pneumonia, not the flu virus itself.
The superbugs were detected during routine health testing of pigs and chickens in southern China. The animals were found to be carrying bacteria resistant to colistin, an antibiotic widely used in livestock farming.
This prompted researchers to examine E. coli and K. pneumoniae samples collected over a four-year period from pork and chicken sold in dozens of markets across four provinces.
They also analysed lab results from patients at two hospitals in Guangdong and Zhejiang provinces.
More than 20 percent of bacteria in the animal samples, and 15 percent of the raw meat samples, had the telltale MCR-1 gene. It was also found in 16 of the 1,322 specimens taken from hospitals.
The lower infection rate among humans suggests the resistant bacteria passed from animals to people, the study found.
Although currently confined to China, the MCR-1 bacteria were "likely... to spread worldwide", it said.
Experts not involved in the research expressed sharp concern.
"This is a worrying report, as polymyxins are often the last-resort antibiotic to treat serious infections," said Laura Piddock, a professor of microbiology at the University of Birmingham in England.
Other types of drug resistance -- in tuberculosis, for example -- show that "this likely paves the way for it to spread throughout the world," she added.
Some 480,000 people contracted multi-drug resistant tuberculosis in 2014, according to the WHO. The disease killed 190,000 in the same year.
"It is likely inevitable that polymyxin resistance will be added to the arsenal of multi-drug resistant bacteria and that they will spread globally," said Judith Johnson, an expert on emerging pathogens at the University of Florida.
Professor Timothy Walsh of the University of Cardiff, who collaborated on the study, told the BBC News website antibiotics could soon become useless.
"If MCR-1 becomes global -- which is a case of 'when' not 'if' -- and the gene aligns itself with other antibiotic resistance genes, which is inevitable, then we will have very likely reached the start of the post-antibiotic era," he said.
The study will renew debate about the use of colistin in animal husbandry, researchers said.
"The finding that this type of resistance can be shared by different bacteria -- irrespective of whether from food, an animal or a person -- is further evidence that the same drugs should not be used in veterinary and human medicine," Piddock said.
In the European Union, colistin is used in only veterinary medicine. In China, however, it is used routinely to promote growth, especially in pigs.
Nearly 12,000 tons of the drug are used annually in livestock production there, according to Marilyn Roberts, a researcher at the University of Washington School of Public Health in Seattle.