[Identifying bacteria using a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass...

The methods routinely used for bacterial identification in Clinical Microbiology Laboratory, although miniaturized and automated, are still based on the same basic principles as classical identification methods. Nevertheless, technological advances are emerging which could modify these routine methods. We report a comparative study between conventional identification methods and mass spectrometry MALDI-TOF (MS MALDI-TOF) for bacterial identification in the Clinical Microbiology Laboratory.

We analysed 294 facultative anaerobic and aerobic isolates (65 Gram positives and 229 Gram negatives), obtained from different clinical samples, using conventional microbiological methods (Wider, Fco. Soria Melguizo, Madrid, Spain; Vitek-2, APIStaph, API 20 Strep, API Coryne and API NH, bioMérieux, Marcy L'Etoile, France) and an Autoflex III MS with a MALDI-TOF device (Bruker Daltonics GmbH, Leipzig, Germany). Salmonella isolates were also typed by using specific sera. Isolates identified with a confidence rate <95% were checked by using API systems. Isolates which were not accurately identified by API systems were rejected. MS MALDI-TOF identification is automatically scored by the system software between 1 and 3 points. Isolates with scores <1.5 were classified as unreliable. Correlation between both identifications was classified as correlation at the species level, at the genus level or no correlation.

Correlation at the species level in Gram positives was 100%. Correlation in Gram negatives was 87.7% at the species level and 97.7% at the genus level. There was no correlation in 2.2% of Gram negatives studied. Identification failures occurred in the genera Raoultella and Acinetobacter, in Stenotrophomonas maltophilia and in Francisella tularensis.

Bacterial clinical isolates identification obtained by MS MALDI-TOF shows excellent correlation with identification obtained by conventional microbiological methods. Moreover, MS MALDI-TOF allows the identification of bacteria from colonies grown on agar culture plates in just a few minutes, with a very simple methodology and hardly any consumable costs, although the financial costs of purchasing the device can be high.