Current and emerging polymyxin resistance diagnostics: A systematic review of established and novel detection methods

The emergence of polymyxin resistance, due to transferable mcr genes, threatens public and animal health as there are limited therapeutic options. As polymyxin is one of the last--line antibiotics, there is a need to contain the spread of its resistance to conserve its efficacy. Herein, we describe current and emerging polymyxin resistance diagnostics to inform faster clinical diagnostic choices. A literature search in diverse databases for studies published between 2016 and 2020 was performed. English articles evaluating colistin resistance methods/diagnostics were included. Screening resulted in the inclusion of 93 journal articles. Current colistin resistance diagnostics are either phenotypic or molecular. Broth microdilution is currently the only gold standard for determining colistin MICs (minimum inhibitory concentration). Phenotypic methods comprise of agar--based methods such as CHROMagarT Col--APSE, SuperPolymyxin, ChromID (R) Colistin R, LBJMR and LB medium; manual MIC--determiners viz., UMIC, MICRONAUT MIC--Strip and ComASP Colistin; automated antimicrobial susceptibility testing systems such as BD Phoenix, MICRONAUT--S, MicroScan, Sensititre and Vitek 2; MCR--detectors such as lateral flow immunoassay (LFI) and chelator--based assays including EDTA--and DPA-based tests, that is, combined disk test, modified colistin broth--disk elution (CBDE), Colispot, and Colistin MAC test as well as biochemical colorimetric tests, that is, Rapid Polymyxin NP test and Rapid ResaPolymyxin NP test. Molecular methods only characterize mobile colistin resistance; they include PCR, LAMP and whole--genome sequencing. Due to the faster turnaround time (=3 h), improved sensitivity (84%--100%) and specificity (93.3%--100%) of the Rapid ResaPolymyxin NP test and Fastinov (R), we recommend this test for initial screening of colistin--resistant isolates. This can be followed by CBDE with EDTA or the LFI as they both have 100% sensitivity and a specificity of =94.3% for the rapid screening of mcr genes. However, molecular assays such as LAMP and PCR may be considered in well--equipped clinical laboratories.