Scientists have discovered a gene in E. coli that makes it resistant to a class of “last-resort” antibiotics known as polymyxins. The gene can also transfer resistance to other epidemic pathogens such as K. pneumoniae and Pseudomonas aeruginosa. According to research published in The Lancet Infectious Diseases, the mcr-1 gene is widespread in Enterobacteriaceae samples from pigs in south China. During routine testing of food animals for antimicrobial resistance in China, the researchers isolated E. coli strain SHP45 from a pig on a farm in Shanghai that showed resistance to a polymyxin antibiotic called colistin that could be transferred to another strain. This prompted the researchers to collect bacteria samples from pigs at slaughterhouses across four provinces and from pork and chicken sold in 30 open markets and 27 supermarkets across Guangzhou between 2011 and 2014. They also analyzed 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 and raw meat samples, and 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 hospitalized patients. Study co-author Jianzhong Shen from China Agricultural University in Beijing said that because there is a lower proportion of positive samples for the resistance gene in humans than in animals, it’s likely that the resistance originated in animals first. “These are extremely worrying results,” said Jian-Hua Lui, study author 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.” The mcr-1 gene is currently confined to China, but it’s likely to spread worldwide, the researchers wrote, adding that the use of polymyxins in animals needs to be re-evaluated and mcr-1 needs to be closely monitored in human and veterinary medicine around the world. In a linked comment for the journal, David Paterson and Patrick Harris from the University of Queensland in Brisbane, Australia, wrote that, “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.”

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