MALDI-TOF mass spectrometry: the quantum leap

Abstract

With this special issue on the matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry, our aim is that infectious diseases specialists and microbiologists not yet familiar with this new tool will appreciate the importance of this ‘magical laser’ for microbial identification. We hope that even specialists will appreciate this overview on this new technology and its applications, since MALDI-TOF mass spectrometry really represents a quantum leap in diagnostic microbiology. In their review article, Emonet et al. thoroughly explain how MALDI-TOF mass spectrometry allows identification of bacterial species available in pure culture. They also compare the different mass spectrometer specifically adapted to routine microbiology laboratories and discuss their respective advantages and disadvantages. Then, they nicely present two other recent mass spectrometry-based tools, i.e. the MALDIresequencing (MALDI-RE) and the association of a new broad-range PCR approach coupled with the analysis of generated amplicons by electrospray ionization mass spectrometry (PCR-ESI-MS), that allows better discrimination of strains and may be performed directly on clinical samples without a culture-based amplification step. In the second article of this special issue, Bizzini and I present the performance of MALDI-TOF mass spectrometry in routine setting. Although current protocols allow accurate identification of most strains routinely isolated in clinical microbiology laboratories, some clades will repeatedly be misidentified due to poor quality of databases or due to the close relatedness of different species (i.e. within the Streptococcus genus; or between E. coli and Shigella species). Despite these limitations, MALDI-TOF mass spectrometry really appears as a quantum leap in diagnostic microbiology that significantly reduce the time to results and costs of microbial identification. The benefit of protein extraction is also discussed in this review as well as some future possible applications such as the search for virulence factors and/or for some antibiotic resistance determinants. In the article by M. Drancourt, the usefulness of MALDITOF mass spectrometry for direct bacterial identification starting from positive blood culture bottles is discussed. This approach was successful in various laboratories using different protocols. In the near future, it will be important to further improve the identification, especially of Gram positive cocci and to precise the clinical impact of such rapid identification of the etiological agents of bacteremia. Then, in the last article of this special issue, P. Murray presents an overview of the usefulness of MALDI-TOF mass spectrometry for taxonomy and epidemiology. Based on the literature currently available, it appears that MALDI-TOF mass spectrometry may be used for microbial identification at the subspecies level and might even prove to be as discriminative then pulse-field gel electrophoresis (PFGE) for some selected species. However, protein extraction and precise protein calibration seems important to obtain reliable results at subspecies, type and subtypes levels. This ‘magic laser’ technology will also have applications in the future in the taxonomy of obligate intracellular bacteria such as members of the Rickettsiales and Chlamydiales order, for which no or little phenotypic traits are available, given their strict intracellular growth. Although current protocols allows accurate identification of most strains routinely isolated in clinical microbiology laboratories, there are still much place for improvement, especially in the quality and completeness of databases and in the quality control of the process. Thus, as an example, it may be important to check the MALDI-TOF microplate for the absence of residual trace of proteins on cleaned microplates. The performance of the extraction step and of the MALDI-TOF mass spectrometer needs although to be daily checked by routinely testing a few selected bacterial ATCC strains with and without a specific extraction step. Thank to this special issue on MALDI-TOF mass spectrometry, you will surf from technical aspects to the application of this quantum leap technology allowing accurate, easy and cheap microbial identification and, to some extent, straightforward typing.