Chinese scientists find 2 MCR genes on same plasmid

Scientists in China have isolated a strain of Escherichia coli from commercial poultry that contains two colistin-resistance genes on the same plasmid, according to a study today in Antimicrobial Agents and Chemotherapy.

While multiple colistin resistance genes have been detected since the first, MCR-1, was identified in China in 2015, this is the first time that two of the genes have been found on the same plasmid. These mobile pieces of DNA can share resistance genes within and among different bacterial species and contribute to the rapid spread of colistin-resistant bacteria. Colistin is considered a last-resort antibiotic for multidrug-resistant infections.

The E coli strain, isolated from a chicken cloacal (anal area) swab in Sichuan province in 2015, contained MCR-1 and a variant of MCR-3, named MCR-3.11.

"The coexistence of the mcr-1 and mcr-3 genes in E. coli isolates may pose a huge threat to public health and warrant[s] further investigation," the Sichuan University authors write.

Similar plasmids found in patient, sewage

Whole-genome sequencing revealed that the MCR-1 and MCR-3-variant genes from E coli strain ECSC102 were co-located on an IncP plasmid. When the scientists compared the plasmid with two other MCR-carrying IncP plasmids from Sichuan province—one from an E coli strain isolated from a hospital patient, the other from a strain of Klebsiella pneumoniae recovered from hospital sewage—they found similar sequences, suggesting derivation from a common ancestor.

"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," lead author Hongning Wang, PhD, professor of animal disease prevention and food safety at Sichuan University, said in a news release from the American Society for Microbiology (ASM), the journal's publisher.

In a series of experiments to determine whether the resistance elements could be transferred to other strains of E coli, the scientists found that when both genes were simultaneously present in an E coli strain, resistance to colistin was higher than when either gene was alone in the strain. The initial isolate was also resistant to several other classes of antibiotics.

Since MCR-1 was first reported in E coli isolates from pigs, pork products, and humans in China in November 2015, the gene has been detected in animals, humans, and the environment in more than 30 countries. Scientists have identified several other colistin-resistance genes (MCR-2 through MCR-7) in E coli and other bacterial species, including K pneumoniae, Enterobacter, and aeromonads.

MCR-carrying plasmids also frequently harbor additional antibiotic resistance genes, which raises the possibility of untreatable bacterial infections.

The emergence of colistin resistance genes is thought to be linked to the widespread use of colistin in Chinese agriculture. China and other countries have banned use of the drug in animal feed, but several other countries still allow its use in food animals. Colistin is on the World Health Organization's list of critical antimicrobials for human medicine.

See also:

May 14 Antimicrob Agents Chemother abstract

May 14 ASM press release

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