P. NORDMANN^1,2, A. JAYOL¹, L. POIREL<sup>1

1</sup>Medical and Molecular Microbiology Unit, Department of Medicine, University of Fribourg , ²HFR - Hôpital Cantonal, 1700 Fribourg, Switzerland

Emergence of colistin resistance among multidrug resistant Gram negative rods are emerging rapidly worldwide. To avoid the development of nosocomial outbreaks due to colistin-resistant gram negatives, an early detection of carriers is mandatory. Colistin-based selective culture media have been developed for screening intrinsic colistin-resistant bacterial species but they are not adapted for screening Gram negatives with acquired resistance to colistin that display low- to high-level resistance to colistin. Therefore, we have developed a selective culture medium, the SuperPolymyxin medium. This medium was prepared using an Eosine Methylene Blue agar (EMB)-based culture medium wherein colistin, daptomycin and amphotericin B were added. Performance of the SuperPolymyxin medium was evaluated with a total of 82 Gram-negative strains from various enterobacterial and non-fermenters species. Seven strains were from intrinsically colistin-resistant species, 44 strains exhibited acquired resistance to colistin (chromosomal mutations, plasmid-mediated mcr-1 gene acquisition or mechanisms not yet known) and 31 strains were colistin-susceptible. MICs of colistin were determined using the broth microdilution method and results were interpreted according to the European Committee on Antimicrobial Susceptibility Testing breakpoints. The basis of the SuperPolymyxin medium is the EMB medium that allows to distinguish between lactose fermenters and lactose non-fermenters. Colistin-resistant strains grew on the SuperPolymyxin medium in 24 to 48 h. The lowest limit of detection was below the cut-off value of 103 CFU/ml for all colistin-resistant strains, whereas the limit of detection of the colistin-susceptible strains was ≥ 106 CFU/ml. The sensitivity and specificity of the SuperPolymyxin medium for selecting colistin-resistant gram negatives were 100% in both cases regardless of the nature of the colistin resistance mechanism (intrinsic, chromosomally or plasmid-encoded) and of its level (high or moderate). The SuperPolymyxin medium is the first screening medium that is aimed to detect intrinsic and acquired colistin resistant Gram negative rods. It will contribute to an early identification of carriers of colistin-resistant strains, therefore preventing and containing outbreaks due to colistin-resistant isolates.