Science News

Antibiotic apocalypse

Mon, 23rd Nov 2015

Chris Smith

Bacteria showing resistance to antibiotics of last resort have been discovered in China.

Ironically, in what is officially "World Antibiotic Awareness Week", microbiologists based in Beijing have confirmed the existence of a form of E. coli with resistance to the antibiotic colistin. e-coli

This is a member of a family of drugs called polymyxins, which work by attacking the integrity of the outer cell walls of bugs like E. coli and other so-called gram-negative microbes.

Colistin isn't often prescribed. It's usually held back and used only as an agent of last resort because it tends to have a poor track record of side effects.

However, the emergence in recent years of strains of E. coli and other microbes that are capable of sidestepping our first line defences, which include drugs called carbapenems, means that, increasingly, doctors are relying on last-ditch drugs like colistin. Which makes the discovery in China of colistin-resistant E. coli, in both farm animals and human patients, highly alarming.

The findings, which were published this week in the Lancet by University of Beijing researcher Jianzhang Shen and his colleagues, were made initially during routine screening of meat slaughtered for market.

Subsequent tests of meat tested from retail premises, as well as freshly-slaughtered pigs, showed that between 15-20% of the samples were positive for the resistant bug strain.

The bugs also appear to be circulating in humans too. Of one thousand swabs collected from patients at a hospital, 1% were positive for the resistant bugs. Colistin-resistant bacteria have been detected before, but, critically, on those occasions, the organisms concerned had a mutation - or genetic change - in their main DNA code.

The mutation meant that the bacteria grew more slowly and were also limited in terms of their ability to pass the trait to other bugs, restricting the potential for spread of the resistance.

This time, the resistance mechanism is down to a gene that the Chinese team have called "mcr-1", and it's carried a structure known as a plasmid. This is a tiny additional piece of DNA that bacteria can trade amongst themselves, sharing chemical know-how including mechanisms of antibiotic resistance.

This, studies showed, they were doing very effectively with the mcr-1 gene. The concern is if the agent spreads outside China, assuming it hasn't already. "In the absence of new agents effective against resistant gram negative bacteria, the effect on human health cannot be underestimated," say the team.

Key to the appearance of this potentially deadly strain, they say, is the prolific use of antibiotics in animal husbandry. They cite data showing that more than 12,000 tonnes of colistin will be used in agriculture worldwide this year, much of it in China, to promote animal growth. It's this exposure that is undoubtedly driving the emergence of novel forms of bacterial resistance.


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