Evaluation of Commercial Products for Colistin and Polymyxin B Susceptibility Testing for mcr -Positive and Negative Escherichia coli and Klebsiella pneumoniae in China

To evaluate the performance of five widespread commercial products for colistin and polymyxin B susceptibility testing in China for -positive and -negative and . A total of 132 and 83 strains (including 68 -positive and 28 -positive ) were collected. We analysed the performance of colistin susceptibility (with Vitek 2 and Phoenix M50) and the performance of polymyxin B susceptibility (with DL-96II, MA120, and a Polymyxin B Susceptibility Test strip; POL E-strip). Broth microdilution was used as the gold standard. Categorical agreement (CA), essential agreement (EA), major error (ME), and very major error (VME) were calculated for comparisons. For , the total CA, EA, ME, and VME to colistin were as follows: Vitek 2, 98.5%/98.5%/0%/2.9%; and Phoenix M50, 98.5%/97.7%/0%/2.9%. The total CA, EA, ME, and VME to polymyxin B were as follows: POL E-strip, 99.2%/63.6%/1.6%/0%; MA120, 70.0%/-/0%/58.8%; and DL-96II, 80.2%/-/1.6%/36.8%. Only Vitek 2 and Phoenix M50 presented satisfactory performances for -positive . For , the total CA, EA, ME, and VME to colistin were as follows: Vitek 2, 73.2%/72.0%/0%/61.6%; and Phoenix M50, 74.7%/74.7%/0%/58.3%. The total CA, EA, ME, and VME to polymyxin B were as follows: POL E-strip, 91.6%/74.7%/2.1%/16.7%; MA120, 92.8%/-/2.1%/13.9%; and DL-96II, 92.2%/-/2.1%/8.3%. All systems were unsatisfactory for -positive . When the susceptibility of -negative strains was tested, all systems presented excellent performance. Vitek 2 and Phoenix M50 with colistin for showed acceptable performance regardless of expression, while DL-96II, MA120, and the POL E-strip performed worse for -positive strains. Furthermore, greatly affected the performance of all systems with both colistin and polymyxin B for isolates.