Chinese researchers at the Key Laboratory of Sichuan Province, Sichuan University have isolated colistin-resistant E coli bacteria from a commercial chicken farm in China.

In the ongoing study, researchers collected rectal swabs from randomly selected chickens in several commercial poultry farms and found that E coli present in the birds carried multiple resistance genes including mcr-1, which is colistin-resistant, as well as one copy of mcr-3. Colistin is an antibiotic used against certain types of bacteria.

This is the first recorded instance of these two genes occurring in a single plasmid. Plasmids are genetic elements that can spread from one bacterium to another and across species, thereby dispersing antibiotic-resistant genes. Sichuan University professor of animal disease prevention and food safety Dr Hongning Wang said: “The coexistence of mcr-1 and mcr-3 in E coli isolates may pose a huge threat to public health.”

The plasmid containing resistance genes, IncP, was also found to house circular pieces of mcr-3 infected DNA. These circular intermediates can encourage integration into other plasmids, thereby increasing the distribution of mcr-3.

Dr Wang said: “This study was originally designed to isolate strains carrying mcr-1 genes, but it is surprising that there are already strains carrying multiple mcr genes in chicken farms. The apparent spread of the same IncP plasmid with one or two mcr genes between different species and a patient, the hospital environment, and animal production is worrying, he said.

Five colistin-resistant mcr genes were discovered in 2016. The most notable was E coli, but Klebsiella pneumoniae, Enterobacteriaceae, and aeromonads were also found

“It is time to let the public understand the serious consequences of the abuse of antibiotics,” Wang added. “If the last line of antibiotics is breached by bacteria, we will find ourselves in the post-antibiotic era.”

Antibiotic abuse occurs when antibiotics are used to promote growth in livestock, or when prescribed to patients unnecessarily. In January, the UK Food Standards Agency found increasingly high levels of the antibiotic-resistant superbug campylobacter in fresh chickens supplied to supermarkets, the most common bacteria known to cause gastroenteritis, or food poisoning.