New Gene Variant of MCR-1 Found in Multidrug-Resistant Pathogen

Researchers discover a gene variant of MCR-1 from the rectal swab of a leukemic child

A new gene variant called mcr-1.2, a functional variant of MCR-1, was detected in a Klebsiella pneumonia strain from a child with leukemia, according to recent data published in Antimicrobial Agents and Chemotherapy.

The researchers discovered the mcr-1.2 gene on a transferable IncX4 plasmid whose structure was similar to plasmids bearing MCR-1 that have previously been found in E. Coli and K. pneumonia from geographically different sites, including Estonia, China, and South Africa.

The strain was isolated in 2014 from a rectal surveillance swab in an Italian child admitted to Pisa University Hospital. A screening test for KPC carbapenemase-producing Enterobacteriaceae was conducted with a direct KPC screening test. The child was receiving a cycle of chemotherapy for acute lymphoblastic leukemia, but did not receive colistin before the first isolation of the strain.

“This is a particularly worrisome development for the future of antimicrobial therapy,” said Gian Maria Rossolini, MD, director of the Clinical Microbiology and Virology Unit at Florence Careggi University Hospital, in a press release. The discovery is concerning because the new resistance mechanism was discovered on a multi-resistant strain of K. pneumonia.

The bacterium may have been transferred from E. coli, which frequently carries mcr genes. The researchers note that this is the first time a mcr-type gene has been found on a “high risk” clone of K. pneumonia, meaning that the clones have the ability to spread into the clinical setting and cause infections. 

“Until now, only a few cases of human infections caused by carbapenemase-producing, mcr-positive, strains of E. coli and K. pneumonia have been reported,” the authors noted.

“Considering that the clinical use of polymyxins is essentially restricted to the treatment of invasive infections caused by carbapenem resistant (CRE) and XDR Gram-negative nonfermenters, the emergence of transferable colistin resistance among CRE is a cause of serious concern, especially in settings of high CRE endemicity.”


Di Pilato V, Arena F, Tascini C, et al. MCR-1.2: a new MCR variant encoded by a transferable plasmid from a colistin-resistant KPC carbapenemase-producing Klebsiella pneumonia of sequence type 512. Antimicrob Agents Chemother. 2016; doi: 10.1128/AAC.01075-16.