N. KIEFFER¹, L. POIREL¹, A. BRINK², J. COETZE², A. JAYOL¹, P. NORDMANN<sup>1,3

1</sup>Medical and Molecular Microbiology Unit, Department of Medicine, Faculty of Science, University of Fribourg, Fribourg, Switzerland, ²Department of Clinical Microbiology, Ampath National Laboratory Services, Milpark hospital, Johannesburg, South Africa, ³HFR - Hôpital Cantonal, Fribourg, Switzerland

Emergence of multidrug-resistant (MDR) Enterobacteriacae is increasing worldwide. The ultimate drug to treat patients infected by these MDR Gram negative rods is polymyxins. Polymyxin resistance has been described being associated with mutations in the chromosome, in particular in genes involved in the pathway of lipopolysaccharide (LPS) modifications. The plasmid-born mcr-1 gene was very recently identified to be responsible of a transferable low-level resistance toward colistin and is already reported worldwide. Here we report seven clonally-unrelated Escherichia coli harboring mcr-1 from South Africa. Two plasmids fully sequenced were assembled and the study of the genetic context revealed an identical 2600bp-long DNA sequence defining a mcr-1 cassette. Promoter sequences responsible for the expression of mcr-1 were characterized, including an additional hybrid promoter brought by the insertion sequence ISApl1. These findings describe the diversity of plasmids carrying mcr-1 and highlight the impact of this new resistance gene in human medicine.