In an alarming discovery, scientists from South China Agricultural University and China Agricultural University have found a new gene that enables bacteria to be highly resistant to humans' last line of antibiotic defense against diseases. They also found evidence showing the gene's potential to easily spread to other kinds of bacteria.

The gene, called mcr-1, makes bacteria resistant to polymyxins, a group of antibiotics typically used against multi-drug resistant gram-negative bacteria. It was found to be widespread among Enterobacteriaceae isolated from patients and animals in south China. Additionally, the mcr-1 gene was found on plasmids, which are easily transferred from one kind of bacteria to another.

The study's authors find the discovery "extremely worrying" because polymyxins, being the last line of antibiotics, are known to cause chromosomal mutations that break down the resistance of gram-negative bacteria against drugs -- until now.

"Our results reveal the emergence of the first polymyxin resistance gene that is readily passed between common bacteria such as Escherichia coli and Klebsiella pneumoniae, suggesting that the progression from extensive drug resistance to pandrug resistance is inevitable," study author Professor Jian-Hua Liu said in a press release.

The researchers found the gene in E. coli strains isolated from a pig in Shanghai, after which they began to collect bacterial samples from pigs from four different provinces. They also collected samples from chicken and pork sold in 30 markets and 27 supermarkets across Guangzhou. Finally, they obtained bacterial samples from patients in Guangdong and Zhejiang hospitals.

The mcr-1 gene was found in E. coli isolated from 166 of 804 animals and 78 of 523 raw meat samples. It was also detected in E. coli and K. pneumoniae isolated from 16 of 1,322 patients.

The researchers found that the mcr-1 gene has a high rate of being copied and transferred particularly among E. coli strains, and it has a high potential to spread to disease causing K. pneumoniae and Pseudomonas aeruginosa, indicating that it can spread to human pathogens.

Study author Professor Jianzhong Shen said that the polymyxin resistance could have originated in animals and transferred to humans, adding that the mcr-1 gene could have developed because of the "increasingly heavy use of colistin in agriculture."

"The emergence of mcr-1 heralds the breach of the last group of antibiotics," the authors said. "Although currently confined to China, mcr-1 is likely to emulate other resistance genes such as blaNDM-1 and spread worldwide. 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 study was published in the online Nov. 18 issue of the journal The Lancet Infectious Diseases.