Performance of Matrix-Assisted Laser Desorption Ionization Time-of-Fight Mass Spectrometry for Rapid Discrimination of Methicillin-Resistant Staphylococcus aureus (MRSA): First Report of a Relation Between Protein Peaks and MRSA spa Type

Dear Editor, Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) is widely applied for the rapid identification of bacterial species [1]. However, an antimicrobial susceptibility test (AST) should be performed by using conventional methods following MALDI-TOF MS, and this delay represents a significant hurdle for the rapid adjustment of administered antimicrobial treatments. To overcome this limitation, many methods to improve MALDI-TOF MS for the production of rapid AST results have been proposed [2–4]. For example, carbapenemase-producing Enterobacteriaceae can be detected by analyzing the MALDI-TOF MS peaks of carbapenems and their metabolites [2, 3]. Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most important multidrug-resistant organism in hospital and community settings. To date, few studies have attempted to differentiate MRSA and methicillin-susceptible S. aureus (MSSA) using MALDI-TOF MS. Despite the subsequent identification of many MRSA-specific peaks, no indisputable evidence has been obtained for reliable markers for MRSA and MSSA discrimination [5–7]. Furthermore, no study has evaluated Korean S. aureus isolates to date. Therefore, we aimed to identify MRSA isolates in Korea using MALDI-TOF MS peak analysis, and evaluated the feasibility of employing this method in clinical microbiology laboratories. This study was approved by institutional review board of Yonsei university health system (Approval number 4-2017-0456) and informed consent was waived due to the retrospective manner of this study. We collected S. aureus isolated from the anterior nares of patients with atopic dermatitis from 2012 to 2013. Of 81 isolates, 47 were MSSA and 34 were MRSA as confirmed by the cefoxitin disc diffusion test according to the CLSI document M100-S25 [8]. All isolates were processed for protein extraction by using Microflex LT (Bruker Daltonics GmbH, Bremen, Germany) according to the manufacturer protocol. In Phase 1, peptide profiles were analyzed by using the QuickClassifier algorithm of ClinProTools 3.0 software (Bruker Daltonics GmbH). This model showed good sensitivity and specificity (Ta-