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

Enterobacterial strains resistant to colistin are increasingly reported worldwide. Currently available colistin susceptibility methods are fastidious, time-consuming (24 h) and some methods are not reliable. Therefore, we have developed the Rapid Polymyxin NP test which is based on the detection of bacterial growth in presence of a defined concentration of colistin in a well-defined medium. Growth is evidenced by acid formation related to glucose metabolism observed through a color change (orange to yellow) of a pH indicator (red phenol). A total of 200 enterobacterial strains from varied species were used to evaluate the performance of the Rapid Polymyxin NP test. Five strains were intrinsically resistant to colistin, 130 strains had an acquired mechanism of resistance to colistin (chromosomally-encoded, plasmid-mediated MCR-1 and not yet known), and 65 strains were susceptible to colistin. MICs of colistin were determined using the reference broth microdilution (BMD) method and results were interpreted according to the breakpoints of the European Committee on Antimicrobial Susceptibility Testing. The sensitivity and the specificity of the Polymyxin NP test were excellent, being 99.3 and 95.4 %, respectively, as compared to the BMD method taken as the gold standard. It was rapid (less than 2h) and reproducible. The Rapid Polymyxin NP test was easy to perform, rapid, reliable, cost-effective, sensitive, and specific. It detects colistin-resistant enterobacterial strains from any species regardless the molecular mechanism of resistance to colistin (intrinsic, chromosomic and/or plasmid-mediated). It is a very useful tool for preventing spread with colistin-resistant strains and will change the overall management of those infected / colonized patients.