Rapid identification of oral anaerobic bacteria cultivated from subgingival biofilm by MALDI-TOF-MS

To facilitate the identification of anaerobes cultivated from periodontal disease, whole cell bacterial identification by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was evaluated.

A total of 84 strains (nine reference strains and 75 recent clinical isolates from 33 patients with aggressive periodontitis) previously identified with phenotypic methods were used. All the references and 10 clinical isolates belonging to the same species as the reference strains were genotypically identified by sequence analysis of the 16S ribosomal RNA gene. All the strains were then analyzed using MALDI-TOF-MS.

The reference strains of anaerobic bacteria used showed characteristic MALDI-TOF-MS spectra with peaks between m/z 2000 and up to about m/z 13,000. On visual inspection, the similarity of spectra produced by strains of a single genus could be recognized. Obvious differences between spectra produced by strains of different species were also easily noticed. The reproducibility of the method was proved by the similarity of spectra belonging to the same species. The spectra of the Prevotella intermedia strains identified with MALDI clustered together and clustered separately from the spectra of Prevotella nigrescens, proving that MALDI-TOF-MS is an accurate method that is capable of separating these two species. The quality of clustering was characterized by calculating an inconsistency coefficient (Mathworks:/Matlab Reference Manual v2007a/, Statistical toolbox).

Our results suggest that MALDI-TOF-MS might become a useful method for the identification of anaerobic bacteria, especially for those that cannot be readily identified by biochemical analysis. It may become an attractive system even for the routine identification of clinical isolates.