Comparison of MALDI TOF with conventional identification of clinically relevant bacteria.

BACKGROUND: A completely new approach to diagnose microbial agents at least one day earlier based on mass spectrometric analysis becomes possible in the microbiology laboratory: MALDI TOF: matrix-assisted laser desorption/ionisation time-of-flight. Comparison between results of the new procedure with those obtained by conventional testing is mandatory.

METHODS: 204 clinical isolates grown on agar plates were analysed both, by the MALDI TOF Bruker microflex apparatus and by conventional identification using the VITEK II and API systems, both from bioMérieux.

RESULTS: Of the identified isolates, 72 were gram-positive and 130 gram-negative; 2 were yeasts (candida). Concordance was seen with 61/72 (85%) of the Gram-positive bacteria and with 115/130 (88%) of the Gram-negative bacteria. In 27 samples (13.2%), a discrepancy of the species and/or genus was obvious. The discrepancy appeared with 16 gram-negative (12.2%) and with 11 gram-positive germs (15.3%, n.s.). In the latter group, 6 samples showed discordance with Streptococcus pneumoniae (MALDI) and Streptococcus mitis/oralis (conventional identification) constellation. Among gram-negative samples, most differences occurred on the species level only, e.g. Enterobacter cloacae versus Enterobacter kobei. In 5 cases, discordance was major and appeared on the genus level: Enterobacter/Raoultella, Streptococcus/Gemella, Pseumdomonas/Burkholderia, Microbacter/Sphingomonas and Candida/Cryptococcus. The most outstanding difference was Microbacterium arborescens (MALDI TOF) and Sphingomonas paucimobilis (conventional). Molecular biological identification of two Streptococcus mitis group bacteria confirmed the erroneous diagnosis by MALDI TOF of Streptococcus pneumoniae.

CONCLUSION: Good comparability between MALDI TOF analysis and conventional identification procedures (86.8%) but special caution is needed when identifying streptococcal species.