'Superbugs' breach last line of antibiotic defence: study

Resistant bacteria may have breached the last line of antibiotic defence humans have left against the deadly 'superbugs', a new study has warned, posing a global threat from a huge epidemic.

Researchers in China found a 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 taken from pigs and patients in China.

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," said study author Jian-Hua Liu from South China Agricultural University.

"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," Liu said.

During routine testing of food animals for antimicrobial resistance in China, researchers isolated an E coli strain (SHP45) from a pig 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.

The researchers found a high prevalence of the mcr-1 gene in E coli isolates from animal (166 of 804) and raw meat samples (78 of 523).

Worryingly, the proportion of positive samples increased from year to year. Mcr-1 was also found in 16 E coli and K pneumoniae isolates taken from 1,322 hospitalised patients.

The transfer rate (rate at which the mcr-1 gene is copied and transferred between different bacteria) was very high between E coli strains.

Moreover, the researchers found that the mcr-1 gene has the potential to spread into other epidemic pathogenic bacterial species such as K pneumoniae and Pseudomonas aeruginosa, which can cause a variety of diseases from pneumonia to serious blood infections, suggesting that mcr-1 is likely to spread rapidly into human pathogens.

"The emergence of mcr-1 heralds the breach of the last group of antibiotics. Although currently confined to China, mcr-1 is likely to emulate other resistance genes such as blaNDM-1 and spread worldwide," the researchers said.

The study was published in The Lancet Infectious Diseases journal.