Escherichia coli Harboring mcr-1 and blaCTX-M on a Novel IncF Plasmid: First Report of mcr-1 in the United States

The recent discovery of a plasmid-borne colistin resistance gene, mcr-1 , in China heralds the emergence of truly pan-drug-resistant bacteria ([1][1]). The gene has been found primarily in Escherichia coli but has also been identified in other members of the Enterobacteriaceae in human, animal, food

microdilution, and mcr-1 was detected by real-time PCR (6). Wholegenome sequencing (WGS) of MRSN 388634 was performed using a PacBio RS II system and a MiSeq benchtop sequencer.
E. coli MRSN 388634 belonged to sequence type 457 (ST457), a rare E. coli ST first identified in 2008 from a urine culture in the United Kingdom (7). It was subsequently identified from a bloodstream culture in Italy, where it was found to harbor the carbapenemase genes bla KPC-3 and bla CTX-M-55 (8). MRSN 388634 carried 15 antibiotic resistance genes, which were harbored on two plasmids, but no carbapenemases ( Table 2).
The first plasmid, pMR0516mcr, was 225,707 bp in size and belonged to incompatibility group F18:A-:B1 (9). BLAST analysis indicated that pMR0516mcr represented a novel IncF plasmid. Notably, it shares 89 kb of homologous sequence with pHNSHP45-2, a mcr-1-carrying IncHI2 plasmid described by Liu and colleagues (1). This shared sequence contains mcr-1 in association with ISApl1 (1), but in pMR0516mcr it is in a different location and orientation (Fig. 1). pMR0516mcr also carried 7 additional antibiotic resistance genes, including the ESBL gene bla CTX-M-55 ( Table 2). The second plasmid, pMR0416ctx, was ϳ47 kb in size and was assigned to IncN (Table 2). It carried 7 antibiotic resistance genes, including bla CTX-M-14 . A complete description of both plasmids is under preparation.

LETTER TO THE EDITOR
crossmark To the best of our knowledge, this is the first report of mcr-1 in the United States. The epidemiology of MRSN 388634 is noteworthy; the isolate was submitted from a clinic in Pennsylvania, and the patient reported no travel history within the prior 5 months. To date, a further 20 ESBL-producing E. coli isolates from patients at the WRNMMC have tested negative for mcr-1 and have been colistin sensitive. However, as testing has been ongoing for only 3 weeks, it remains unclear what the true prevalence of mcr-1 is in the population. The association between mcr-1 and IncF plasmids is concerning, as these plasmids are vehicles for the dissemination of antibiotic resistance and virulence genes among the Enterobacteriaceae (9). Continued surveillance to determine the true frequency for this gene in the United States is critical.
Nucleotide sequence accession numbers. The Short Read Archive (SRA) file for MRSN 388623 has been deposited at GenBank with accession number SRP075674. The complete sequence of pMR0516mcr has been deposited at GenBank with accession no. KX276657.

ACKNOWLEDGMENTS
This study was funded by the U.S. Army Medical Command, the Global Emerging Infections Surveillance and Response System, and the Defense Medical Research and Development Program. This project was performed as part of a quality improvement and infection control initiative authorized by policy no. 15-042.
We declare that we have no conflicts of interest. The identification of specific products or scientific instrumentation does not constitute endorsement or implied endorsement on the part of the authors, the Department of Defense (DoD), or any component agency. While we generally excise references to products, companies, manufacturers, organizations, etc., in government-produced works, the abstracts produced and other similarly situated research present a special circumstance when such product inclusions become an integral part of the scientific endeavor.
The material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its presentation.
The views expressed in this article are those of the authors and do not reflect the official policy of the Department of Army/Navy/Air Force, Department of Defense, or the U.S. Government.