Abstract: 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 and Dietzia spp. are closely related actinomycetes. Phenotypic discrimination between R. equi and Dietzia 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 as R. equi that includes isolates belonging to the genus Dietzia. PCR amplification of the choE gene, encoding a cholesterol oxidase, and 16S rRNA sequencing were considered the reference methods for R. 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 between R. equi and Dietzia spp., but identification of all Dietzia sp. isolates at the species level needed sequencing of the 16S rRNA gene.

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