The value of MALDI-TOF MS for the identification of clinically relevant anaerobic bacteria in routine laboratories

Between 2010 and 2011, 283 clinically relevant non-duplicate anaerobic isolates were analysed by MALDI-TOF MS and the results were compared with conventional identification. Immediately after isolation, an ethanol precipitation was carried out on isolated colonies and the stabilized samples were anonymized and sent to the laboratory of Bruker Daltonik, Bremen, Germany, where the identification was done using the standard protocol for micro-organism identification on a Microflex LT mass spectrometer equipped with the MALDI Biotyper 3.0 software. Of 283 isolates, 218 (77 %) were identified at species level [log(score) ≥2.0], 31 isolates (10.95 %) were identified at genus level [log(score) 1.7-2.0] and 34 (12 %) gave non-reliable identification [log(score) <1.7]. Out of the 31 isolates with log(score) 1.7-2.0, in the case of 24 isolates the species name given by the MALDI Biotyper was accepted if it was the same as for the classical identification. Of 218 isolates identified at species level, 40 results were discordant with phenotypic identification, and of the 31 isolates identified at genus level according to the manufacturer's score cut-off, four gave results discordant with the phenotypic method. For the 44 discordant results, 16S rRNA gene sequencing confirmed MALDI-TOF MS identification in 41 cases, leaving three isolates (0.7 %) that had been misidentified by MALDI-TOF MS.