Practical Utility and Accuracy of Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry for Identification of Corynebacterium Species and Other Medically Relevant Coryneform-Like Bacteria.

Background: Corynebacterium species and gram-positive coryneform-like bacteria (coryneforms) are increasingly reported causes of opportunistic infections in immunocompromised patients. Biochemical identification methods for these bacteria are often inaccurate. We evaluated matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) for routine Corynebacterium and coryneform identification. Methods: A total of 286 Corynebacterium species and coryneforms recovered from patients were identified by MALDI-TOF MS analysis using the Bruker Microflex instrument, Biotyper software version 3.0, and database version 3.1.66 (Bruker Daltonics, Billerica, MA) following formic acid based, direct on plate extraction. The spectral cutoff scores used for interpretation were 2.0 or more for species-level identification and 1.7 or more for genus level. Scores lower than 1.7 were considered as no reliable identification. The results were compared with API Coryne (bioMerieux, Durham, NC) and sequencing of 16S ribosomal RNA genes. Results: Of the 231 Corynebacterium (19 species), 99.6% were correctly identified to the genus level and 88.7% to the species level. Of the 55 coryneforms (14 genera), 90.9% were correctly identified to the genus level and 67.3% to the species level. API Coryne was able to identify 89.2% of Corynebacterium species (species level) and 63.6% of coryneforms (genus level). Conclusions: Rapid on plate testing yielded identification of more Corynebacterium species and related bacteria than biochemical methods.