Comparison of Direct Colony Method versus Extraction Method for Identification of Gram-Positive Cocci by Use of Bruker Biotyper Matrix-Assisted Laser Desorption Ionization–Time of Flight Mass Spectrometry

Abstract

We evaluated Bruker Biotyper (version 2.0) matrix-assisted laser desorption ionization–time of flight (MALDI-TOF) mass spectrometry (MS) for the identification of 305 clinical isolates of staphylococci, streptococci, and related genera by comparing direct colony testing with preparatory extraction. Isolates were previously identified by use of phenotypic testing and/or 16S rRNA gene sequencing. Manufacturer-specified score cutoffs for genus- and species-level identification were used. After excluding 7 isolates not present in the Biotyper library, the Biotyper correctly identified 284 (95%) and 207 (69%) isolates to the genus and species levels, respectively, using extraction. By using direct colony testing, the Biotyper identified 168 (56%) and 60 (20%) isolates to the genus and species levels, respectively. Overall, more isolates were identified to the genus and species levels with preparatory extraction than with direct colony testing ( P < 0.0001). The analysis was repeated after dividing the isolates into two subgroups, staphylococci, streptococci, and enterococci ( n = 217) and “related genera” ( n = 81). For the former subgroup, the extraction method resulted in the identification of 213 (98%) and 171 (79%) isolates to the genus and species levels, respectively, whereas the direct colony method identified 136 (63%) and 56 (26%) isolates to the genus and species levels, respectively. In contrast, for the subgroup of related genera, the extraction method identified 71 (88%) and 36 (44%) isolates to the genus and species levels, respectively, while the direct colony method identified 32 (40%) and 4 (5%) isolates to the genus and species levels, respectively. For both subgroups, preparatory extraction was superior to direct colony testing for the identification of isolates to the genus and species levels ( P < 0.0001). Preparatory extraction is needed for the identification of a substantial proportion of Gram-positive cocci using the Biotyper method according to manufacturer-specified score cutoffs.