Evaluation of Colistin Susceptibility in Carbapenem-Resistant Acinetobacter baumannii Isolates Using Broth Microdilution, MICRONAUT-MIC-Strip, and VITEK MS.

We aimed to analyze the colistin susceptibility using broth microdilution (BMD) and commercially available MICRONAUT-MIC-Strip (MMS) test and VITEK MS in carbapenem-resistant Acinetobacter baumannii (CRAB) isolates. Colistin susceptibility of 194 CRAB isolated from various clinical specimens between December 2020 and 2022 was determined by BMD and commercial MMS method. Minimum inhibitory concentrations of the commercial method were compared with the standard BMD. In the second part of the study, colistin susceptibility of the isolates was tested using the MALDIxin method, which detects modified lipid A, by VITEK MS, MALDI-TOF MS (bioMérieux, France). The presence of mcr-1-5 genes in resistant isolates was investigated using in-house multiplex polymerase chain reaction. Of these, 23 (11.8%) were found to be colistin resistant, whereas 171 (88.2%) isolates were susceptible. MMS categorical agreement was found to be 98.9% and essential agreement was 96.3%. The major error was found to be 1.03%, whereas a very major error was not detected. The MALDIxin test developed according to VITEK MS did not detect mutations responsible for lipopolysaccharide-induced colistin resistance in our isolates (expected peak at 1935-2033 m/z). The mcr-1-5 genes were not detected in our isolates. The MMS test is a reliable alternative method for the detection of colistin susceptibility in CRAB. Colistin resistance of the isolates used in our study was not associated with lipid A modification. Alternative mechanisms, such as efflux pumps, are a possibility. To our knowledge, this is the first VITEK MS-based method that enables rapid detection of colistin resistance in negative ion mode, and further studies are needed to determine colistin resistance in this way.