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We set out to study the prevalence of the  mcr-1  gene in carbapenemase-producing  Klebsiella pneumoniae  (CPKP) strains, and to determine whether its presence is associated with a fitness cost. A total of 234 clinical CPKP isolates were collected from a tertiary medical center in Taiwan from January 2018 to January 2019. The  mcr-1  and carbapenemase genes were detected by polymerase chain reaction (PCR) followed by Sanger sequencing. The  mcr-1 -positive carbapenemase-producing strain was characterized by whole genome sequencing, a plasmid stability test and a conjugation assay.  In vitro  growth rate and an  in vivo  virulence test were compared between the parental  mcr-1 -positive strain and its  mcr-1  plasmid-cured strain. We identified only one  mcr-1  positive strain (KP2509), co-harboring  bla   KPC–   2  and  bla   OXA–   48 , among 234 (1/234, 0.43%) CPKP strains. KP2509 and its  Escherichia coli mcr-1  transconjugant showed moderate colistin resistance (MIC = 8 mg/L). The  mcr-1  is located on a large conjugative plasmid (317 kb), pKP2509-MCR, with three replicons, IncHI, IncFIB, and IncN. Interestingly, a complete Type IV-A3 CRISPR-Cas system was identified in pKP2509-MCR. Plasmid pKP2509-MCR was highly stable in KP2509 after 270 generation of passage, and the pKP2509-MCR cured strain PC-KP2509 showed similar growth rate and  in vivo  virulence in comparison to KP2509. The prevalence of  mcr-1  in CPKP strains remains low in our center. Notably, we identified a large plasmid with multiple replicons containing both the  mcr-1  and the Type IV-3A CRISPR-Cas genes. The further spread of this highly stable plasmid raises concern that it may promote the increase of  mcr-1  prevalence in CPKP.

Characterization of a mcr-1 and CRISPR-Cas System Co-harboring Plasmid in a Carbapenemase-Producing High-Risk ST11 Klebsiella pneumoniae Strain