Resistance to the antibiotic of last resort spread from a single Chinese pig farm to human patients on five continents within the space of a decade, scientists have discovered. The study is the first to identify the “shocking” speed with which a gene that gives bacteria such as E. coli and salmonella protection against drugs can jump across borders and from species to species. It could allow public health officials to detect outbreaks of resistance in their early stages while there is still time to suppress them. Anti-microbial resistance (AMR), where potentially deadly pathogens evolve and exchange mutations that shield them against antibiotics, has led to the rise of several strains of “superbug” that are extremely hard to treat even in hospitals.
Dame Sally Davies, the chief medical officer (UK), has warned that the phenomenon could lead to the “end of modern medicine” as routine infections become uncontrollable. The situation has become so dire that doctors have been compelled to bring an old antibiotic known as colistin out of retirement, even though it can cause acute kidney toxicity. The drug has been used sparingly as a last line of defence where all others have failed, particularly against lung infections such as pneumonia and Pseudomonas aeruginosa. In 2015 Public Health England officials found signs of colistin resistance in three samples from pig farms and twelve more from human patients in the UK. The suspicion was that its evolution had been driven by agriculture since livestock farmers often use industrial quantities of human antibiotics to accelerate their animals’ growth and to defend against infection.
A team of scientists led by Francois Balloux, director of the Genetics Institute at University College London, analysed 451 samples of colistin-resistant bacteria from 31 countries to work out where the mutation had come from and how it had travelled around the world. Their DNA detective work showed that the mcr-1 gene, which hardens bacteria against colistin, could be traced back to a single microbe that very probably emerged from the Chinese pig trade in about 2005. The mcr-1 gene then leapt across the boundaries between species in microscopic blobs called plasmids, which bacteria use to exchange fragments of DNA.
Today it is so ubiquitous that it has been found in the water next to Brazilian beaches and in hospitals from South Africa and Saudi Arabia to Germany and Vietnam. “The speed at which mcr-1 spread globally is indeed shocking,” Professor Balloux said. “[But] what worries me even more than the spread of AMR elements would be the spread of virulence elements, which allow infections to spread more readily, such as some we start seeing in the major nosocomial [hospital-derived] pathogen Klebsiella. “I feel we will see the emergence of many more AMR elements . . . We have very few antibiotics that haven’t been breached, and all these tend to have nasty side effects and are thus either last-line or experimental drugs.”
The study is published in the journal Nature Communications.
Credit: Oliver Moody for The Times, 26 March 2018.