A new gene (mcr-1) that enables bacteria to be highly resistant to polymyxins, the last line of antibiotic defence we have left, is widespread in Enterobacteriaceae [1] taken from pigs and patients in south China, including strains with epidemic potential, according to new research published in The Lancet Infectious Diseases. The mcr-1 gene was found on plasmids, mobile DNA that can be easily copied and transferred between different bacteria, suggesting an alarming potential to spread and diversify between different bacterial populations.

"These are extremely worrying results. The polymyxins (colistin and polymyxin B) were the last class of antibiotics in which resistance was incapable of spreading from cell to cell. Until now, colistin resistance resulted from chromosomal mutations, making the resistance mechanism unstable and incapable of spreading to other bacteria,"[2] explains author Professor Jian-Hua Liu from South China Agricultural University in Guangzhou, China. 

"Our results reveal the emergence of the first polymyxin resistance gene that is readily passed between common bacteria such as Escherichia coli and Klesbsiella pneumoniae, suggesting that the progression from extensive drug resistance to pandrug resistance is inevitable."[2]

During routine testing of food animals for antimicrobial resistance in China, Liu and colleagues isolated an E coli strain (SHP45) from a pig on an intensive pig farm in Shanghai that showed resistance to colistin that could be transferred to another strain. This prompted the researchers to collect bacteria samples from pigs at slaughter across four provinces, and from pork and chicken sold in 30 open markets and 27 supermarkets across Guangzhou between 2011 and 2014. They also analysed bacteria samples from patients presenting with infections to two hospitals in Guangdong and Zhejiang provinces. Samples were tested for antibiotic susceptibility and the mcr-1 gene using polymerase chain reaction (PCR) and sequencing.