Abstract
Abstract Background The routine identification to the species level of Nocardia genus by conventional methods is a fastidious and time-consuming process owing to the limited biochemical reactivity of these microorganisms, often requiring 1 or more days to complete identification. Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) is a new technology for definitive and rapid species identification. Methods We evaluated the MALDI-TOF MS for the identification of 44 clinical isolates of Nocardia species in comparison to 16S ribosomal RNA (rRNA) gene sequencing. Nocardia isolates were identified by microbiological examination, phenotypical tests and MALDI-TOF MS and the results were compared by 16S rRNA gene sequencing. Results Of the 44 Nocardia strains, the identification of 28 isolates was determined with MALDI Biotyper database. According to this, 16 isolates (57.1%) of the strain log scores were ≥2. Two (7.1%) were identified to the species level (log scores of ≥2) as Nocardia otitidiscaviarum. The addition of a newly established Nocardia database (16 new Nocardia strains included to the original database) did significantly improve the scores. The results were 43 (97.7%) correct identification to the species level (log scores of ≥2). Conclusions This study showed that the identification of clinical Nocardia isolates by the Bruker MALDI Biotyper is highly reliable, whereas identification rates are generally lower than those for some Gram-negative bacteria and Gram-positive cocci. Based on our data, the identification rates can be improved by validated new database entries and the results can be confirmed with nucleic acid sequence analysis.
Highlights
Nocardia species are isolated from clinical specimens, especially those from immunocompromised patients
This study showed that the identification of clinical Nocardia isolates by the Bruker MALDI Biotyper is highly reliable, whereas identification rates are generally lower than those for some Gram-negative bacteria and Gram-positive cocci
For the analysis of 44 Nocardia species by the Bruker Forty-four clinical isolates were phenotypically characMALDI Biotyper system, all isolates were cultured on terized as Nocardia species based on colony morphology, Hasçelik et al.: Diagnosis of clinical Nocardia species by MALDI-TOF 159
Summary
Nocardia species are isolated from clinical specimens, especially those from immunocompromised patients. Nocardia species are routinely identified by using Gram and modified acid-fast staining and biochemical tests. These tests are cumbersome, timeconsuming processes and require trained staff [4]. 16S ribosomal RNA (rRNA) gene sequencing is considered to be the gold standard method for definitive identification of Nocardia species [5]. These methods are difficult to implement for routine bacterial identification. Methods: We evaluated the MALDI-TOF MS for the identification of 44 clinical isolates of Nocardia species in comparison to 16S ribosomal RNA (rRNA) gene sequencing. The results were 43 (97.7%) correct identification to the species level (log scores of ≥2)
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