Scientists warn we're moving closer to a "post-antibiotic era"

Scientists have identified a new gene that makes bacteria highly resistant to the "last line" of antibiotics the world has left -- a development one describes as "extremely worrying."

The bacteria was detected in patients and livestock in China. Researchers say the discovery is alarming and warn of its potential to spread globally, leaving doctors without any effective drugs to treat certain infections.

A research team from the South China Agricultural University identified a gene called mcr-1 that is becoming more common in China. It gives bacteria the ability to resist an antibiotic called colistin -- the last resort drug doctors use to wipe out dangerous bacteria. They also discovered that the gene has developed a mechanism to transfer its resistance to other bacteria. The study is published in The Lancet Infectious Diseases.

"These are extremely worrying results," study author Jian-Hua Liu from South China Agricultural University in Guangzhou, China, said in a statement. Colistin and a related antibiotic, polymyxin B, "were the last class of antibiotics in which resistance was incapable of spreading from cell to cell."

The researchers discovered the gene mcr-1 when conducting routine testing of livestock for antimicrobial resistance in China. This prompted them to test samples from pigs at slaughterhouses across four provinces and pork and chicken sold at local open markets and grocery stores between the years 2011 and 2014.

They found the gene in 21 percent of the animals and 15 percent of raw meat samples -- at increasing rates each year. Mcr-1 was also found in 16 E. coli and K. pneumoniae samples taken from 1,322 hospitalized patients.

The study authors said that the relatively low proportion of positive samples taken from humans compared with animals suggests that the colistin resistance originated in livestock and spread to humans. This is most likely because of the drugs' overuse in farm animals to prevent and treat diseases and to fatten them up for market.

Furthermore, though the bacteria that were found in human infections in the study were susceptible to some other drugs, "in the lab, we were able to transfer colistin resistance into organisms that were already highly resistant," study coauthor Jim Spenser, a senior lecturer in microbiology at the University of Bristol in the United Kingdom, told CBS News. "This suggests that it is a case of when, rather than if, this will happen for real."

This could lead to a return to the pre-antibiotic era, he said, where doctors wouldn't be able to treat certain infections and some surgical procedures, such as joint replacements, may be impractical as the risk of associated infection would be too great.

"This isn't going to happen overnight, and the number of infections that can only be treated by colistin is still relatively small, but it highlights the urgent need for new treatments for these organisms, and the limited time that we have to develop them," Spenser said.

The results are disturbing, but not entirely surprising. The World Health Organization has called antibiotic resistance a "global health crisis." In the United States, the Centers for Disease Control and Prevention reports at least 2 million people become infected with bacteria that are resistant to antibiotics and at least 23,000 people die each year as a direct result of these infections.

"The number of antibiotics that are going to be effective in the future keeps getting smaller and smaller as we see antibiotic resistance," David Plunkett, senior staff attorney of the Food Safety Program at the Center for Science in the Public Interest, told CBS News. "Now, this is the last brick in the wall between us and what the WHO is calling a 'post-antibiotic era.'"

And while the latest study was confined to China, Spenser said there are indications that mcr-1 has already spread elsewhere. "A report from Laos and recent entries in the gene sequence databases from Malaysia look like mcr-1," he said. "I strongly suspect that people will look for mcr-1, and find it, in multiple locations."

Experts note that stopping the spread will be difficult. "In the absence of new drugs it will be a case of better surveillance and reporting, being more creative with the drugs we have and controlling colistin use to minimize selection," Spenser said.

In an accompanying editorial, David Paterson and Patrick Harris from the University of Queensland in Australia urge China to stop the use of colistin in agriculture. "This will require substantial political will and we call upon Chinese leaders to act rapidly and decisively," they wrote. "Failure to do so will create a public health problem of major dimensions."