Evaluation of Matrix-Assisted Laser Desorption Ionization-Time-of-Flight Mass Spectrometry for Identification of Nocardia Species.

The identification of Nocardia species, usually based on biochemical tests together with phenotypic in vitro susceptibility and resistance patterns, is a difficult and lengthy process owing to the slow growth and limited reactivity of these bacteria. In this study, a panel of 153 clinical and reference strains of Nocardia spp. altogether representing 19 different species were characterized by using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS). As reference method for species identification, full-length 16S rRNA gene sequencing and phenotypical biochemical and enzymatic tests were applied. In a first step, a complementary home-made reference database was established from the analysis of 110 Nocardia isolates pre-treated by 30-minutes boiling and extraction, in the MALDI BioTyper software according to the manufacturer's recommendations for microflex measurement (Bruker Daltonik GmbH, Leipzig, Germany) generating a dendrogram with species-specific cluster patterns. In a second step, the MALDI BioTyper database and the generated database were challenged against 43 blind-coded clinical isolates of Nocardia spp. Following addition in the BioTyper Software of the home-made database, MALDI-TOF MS provided reliable identification at the species level for five species for which more than one single isolate was analyzed. A correct identification was achieved for 38 of the 43 isolates (88%) including 34 strains identified to the species level and 4 strains to the genus level according to the manufacturer's log(score) specifications. These data suggest that MALDI-TOF MS has the potential as a rapid (