Reducing antibiotic usage in animal feed is not enough to combat antibiotic resistance

A new study led by the University of Oxford found that natural evolution of antibiotic resistance genes has maintained resistance in bacteria despite a reduction in the use of antibiotics. The new study indicates the importance of understanding the regulatory evolution of resistance genes to strategically combat AMR.

Antimicrobial resistance (AMR) is a serious and growing threat to global health, with 1.2 million people dying each year due to drug-resistant infections. The overuse and misuse of antibiotics is a major driver of AMR, and there is an urgent need to protect the efficacy of ‘last-line’ antibiotics to treat multidrug-resistant infections.

In 2017, the Chinese government banned the use of last-line antibiotic colistin as a growth promotor in animal feed in response to the rapid spread of antibiotic resistant bacteria – Escherichia coli  (E.coli) carrying mobile colistin resistance (MCR) genes. Bacteria carrying MCR genes are resistant to treatment with colistin and cause hard to treat drug-resistant infections in humans and animals.

The ban led to a 90% reduction in colistin consumption, and scientists expected to see a corresponding drop in rates of AMR. However, large-scale surveillance studies across China following the ban found that the decline in the mcr-1 gene was slower than anticipated.

Researchers at the University of Oxford led by Professor Craig Maclean explored this discrepancy and found that the natural evolution of bacteria has maintained the colistin resistance mcr-1 gene despite the reduction in antibiotic usage, testing the idea that the evolution of this gene stabilised colistin resistance in E.coli by optimising mcr-1 expression.