Resistance gene emerges against last-defence antibiotic

A gene responsible for resistance to an antibiotic used to treat serious infections has been identified in Escherichia coli samples collected from food products in China.

The MCR-1 gene enables bacteria to be highly resistant to colistin, a strong polymyxin antibiotic used to treat lung and urinary tract infections. Polymyxin antibiotics are often considered the treatments of last resort when other antibiotics have become ineffective after bacteria have developed resistance to them.

During routine antimicrobial-resistance testing of animals in China, researchers identified an E. coli strain (SHP45) that showed resistance to colistin. Upon further analysis, the researchers identified the MCR-1 gene as responsible for the resistance, which had been transferred between bacteria through the process of conjugation.

They also found that the MCR-1 gene, which was found on plasmids, has the potential to spread to other bacterial species such as Klebsiella pneumoniae and Pseudomonas aeruginosa. These bacteria can cause a variety of health conditions in humans, from pneumonia to serious blood infections, suggesting that MCR-1 could potentially spread to human pathogens.

'These are extremely worrying results,' said author Professor Jian-Hua Liu from the South China Agricultural University in Guangzhou. 'Our results reveal the emergence of the first polymyxin resistance gene that is readily passed between common bacteria … suggesting that the progression from extensive drug resistance to pandrug resistance is inevitable.'

To investigate where MCR-1 was widespread, the researchers then tested bacteria samples taken from animals for slaughter and raw meat products sold in markets and supermarkets across several regions in the Guangzhou province. They also tested bacteria samples obtained from patients hospitalised with infections.

They found the MCR-1 gene in E. coli in 15 percent of raw meat samples and 21 percent of animals tested between 2011 and 2014. Furthermore, MCR-1 was also identified in one percent of hospitalised patients.

'Because of the relatively low proportion of positive samples taken from humans compared with animals, it is likely that MCR-1 mediated colistin resistance originated in animals and subsequently spread to humans,' said co-author Professor Jianzhong Shen.

He added, 'The selective pressure imposed by increasingly heavy use of colistin in agriculture in China could have led to the acquisition of MCR-1 by E. coli.'

According to the authors, the emergence of MCR-1 heralds the breach of the last group of antibiotics. Although the study findings are currently confined to China, MCR-1 is likely to emulate other resistance genes and spread worldwide.

In an editorial published in The Lancet Infectious Diseases along with the study, Professor David Paterson and Dr Patrick Harris from the University of Queensland, Australia, wrote: 'The links between agricultural use of colistin, colistin resistance in slaughtered animals, colistin resistance in food, and colistin resistance in human beings are now complete.

'One of the few solutions to uncoupling these connections is limitation or cessation of colistin use in agriculture. This will require substantial political will and we call upon Chinese leaders to act rapidly and decisively. Failure to do so will create a public health problem of major dimensions.'