Performance of Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry for Identification of Bacterial Strains Routinely Isolated in a Clinical Microbiology Laboratory

Abstract

Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has recently been introduced in diagnostic microbiology laboratories for the identification of bacterial and yeast strains isolated from clinical samples. In the present study, we prospectively compared MALDI-TOF MS to the conventional phenotypic method for the identification of routine isolates. Colonies were analyzed by MALDI-TOF MS either by direct deposition on the target plate or after a formic acid-acetonitrile extraction step if no valid result was initially obtained. Among 1,371 isolates identified by conventional methods, 1,278 (93.2%) were putatively identified to the species level by MALDI-TOF MS and 73 (5.3%) were identified to the genus level, but no reliable identification was obtained for 20 (1.5%). Among the 1,278 isolates identified to the species level by MALDI-TOF MS, 63 (4.9%) discordant results were initially identified. Most discordant results (42/63) were due to systematic database-related taxonomical differences, 14 were explained by poor discrimination of the MALDI-TOF MS spectra obtained, and 7 were due to errors in the initial conventional identification. An extraction step was required to obtain a valid MALDI-TOF MS identification for 25.6% of the 1,278 valid isolates. In conclusion, our results show that MALDI-TOF MS is a fast and reliable technique which has the potential to replace conventional phenotypic identification for most bacterial strains routinely isolated in clinical microbiology laboratories.In the clinical diagnostic microbiology laboratory, the identification of bacterial or yeast isolates is currently mainly based on phenotypic characteristics, such as growth on different media, colony morphology, Gram stain, and various biochemical reactions. Altogether, these techniques allow the identification of most bacterial isolates with great accuracy, but they are costly and time-consuming.The matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) technique can be used to generate protein fingerprint signatures from whole bacterial cells (9). By comparing these fingerprints to a database of reference spectra by the use of various algorithms, bacteria can be rapidly identified (9). Even though the first study of the use of MS for the identification of bacteria dates back to 1975 and was performed by Anhalt and Fenselau (1), MALDI-TOF MS devices designed for use under routine conditions have only recently been commercially introduced.Over the past few years, this technique has been used in specific studies that have essentially assessed its ability to identify different bacterial genera among Gram-negative rods, such as Escherichia coli and other members of the Enterobacteriaceae family (5, 6); Gram-positive cocci, such as Staphylococcus aureus and streptococci (8, 12); and some Gram-positive rods, such as Bacillus cereus and Listeria species (3, 18). Very recently, the first study to have assessed the performance of MALDI-TOF MS for the identificatio...

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