Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry for Antimicrobial Susceptibility Testing

The advent of matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) in clinical microbiology has dramatically improved the accuracy and speed of diagnostics. However, this progress has mainly been limited to the identification of microorganisms, whereas the practical improvement of antimicrobial susceptibility testing (AST) still lags behind. MALDI-TOF MS-based approaches include the detection of selected resistance mechanisms and the universal phenotypic AST. This minireview focuses on the discussion of those MALDI-TOF MS methods that allow universal growth-based phenotypic AST. The method of minimal profile change concentrations (MPCC) is based on detecting proteome modification in the presence of an antimicrobial. Using stable-isotope labeling, characteristic mass shifts in the presence of an antimicrobial indicate the incorporation of the isotopic labels, and thus the viability and resistance of the microorganism. For MALDI Biotyper antibiotic susceptibility test rapid assay (MBT-ASTRA), microorganisms are incubated with or without an antimicrobial, followed by cell lysis, protein extraction, and transfer of the cell lysate onto a MALDI target plate. Using the internal standard, peak intensities are correlated to the amount of microbial proteins, and the relative microbial growth is calculated. Most recent development in the field is the direct-on-target microdroplet growth assay (DOT-MGA). Here, incubation of microorganisms with antimicrobials takes place directly on spots of a MALDI target in the form of microdroplets. After incubation, nutrient medium is removed by dabbing with absorptive material. Resistant microorganisms grow despite the presence of antimicrobial, and their amplified biomass is detected by MALDI-TOF MS. Finally, an outlook is provided for further assay improvements.