Comparison of the Vitek MS and Bruker Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry Systems for Identification of Rhodococcus equi and Dietzia spp

Rhodococcus equi causes pyogranulomatous pneumonia in domesticated animals and immunocompromised humans.Dietzia spp. are environmental bacteria that have rarely been associated with human infections.R. equi andDietzia spp. are closely related actinomycetes. Phenotypic discrimination betweenR. equi andDietzia on the basis of their Gram stain morphology and colony appearance is problematic. Matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) is a fast, reliable, and cost-effective method for identification of a wide variety of microorganisms. We have evaluated the performance of Bruker Biotyper versus that of Vitek MS for identification of a collection of 154 isolates identified at the source asR. equi that includes isolates belonging to the genusDietzia . PCR amplification of thechoE gene, encoding a cholesterol oxidase, and 16S rRNA sequencing were considered the reference methods forR. equi identification. Biotyper identified 131 (85.1%) of the 154 isolates at the species level, and this figure increased to 152 (98.7%) when the species cutoff was reduced from a score of ≥2.000 to ≥1.750. Vitek MS correctly identified at the species level 130 (84.4%) isolates as long as bacteria were extracted with ethanol but only 35 (22.7%) isolates when samples were prepared by direct extraction from colonies. The two systems allowed differentiation betweenR. equi andDietzia spp., but identification of allDietzia sp. isolates at the species level needed sequencing of the 16S rRNA gene.