EC-12 mcr-1.1
In total, 18 isolates belonging to K. pneumoniae (n = 13), 180 E. coli (n = 3) and Pseudomonas aeruginosa (n = 2) were 181 analysed in this study. Strain characteristics of the 18 isolates 182 are described in Table 1 and Supplementary Table S1. Species 183 confirmation was performed by MALDI-TOF (Bruker Daltonik 184 GmbH, Leipzig, Germany). Ethical approval (MRC-01-17-198) 185 was obtained from the Medical Research Centre (MRC), 186 Hamad Medical Corporation (HMC), Doha, Qatar. Initial 187 antimicrobial susceptibility testing was performed by Phoenix 188 using the NMIC/ID-5 panel (BD Biosciences, Heidelberg, 189 Germany) according to the manufacturers recommendations. 190 Briefly, panels were inoculated with 0.5 McFarland pure culture, 191 placed into the instrument and incubated at 35C. The 192 instrument tests panel every 20 min up to 16 h if necessary. 193 MIC values of each antimicrobial agent automatically read as 194 susceptible, intermediate, or resistant (SIR). Isolates showing 195 colistin resistance by Phoenix were consequently confirmed 196 using the SensiTest colistin kit (Liofilchem, Zona Ind. le, Italy), 197 following the manufacturers instructions. Briefly, a suspension 198 of 0.5 McFarland standard was prepared from the tested 199 organism, and then diluted 1:20 in saline. 0.4 ml of this 200 diluted suspension was added to a tube of Muller Hinton broth 201 provided in the kit (solution B). 100 l of solution B was 202 dispensed to the seven wells in a row. Then the plate was 203 incubated at 36  2C for 16-20 h. The lowest concentration 204 of antibiotic that inhibits visible growth was recorded and 205 interpreted according to Clinical and Laboratory Standards 206 Institute (CLSI) guidelines. 207