eJournal ISSN 2196-3460

»Volume 2016 Issue 03 (August)

MALDI-TOF mass spectrometry for reliable identification of bacteria – A validation based on Staphylococcaceae field isolates

J. Rau1, A. Männig1, E. Hiller1, N. Mauder1, 3, C. Wind2, S. Horlacher1, K. Kadlec4, S. Schwarz4, M. Contzen1

1 Chemisches und Veterinäruntersuchungsamt Stuttgart
Schaflandstraße 3/2
70736 Fellbach, Germany
Email: Joerg.Rau@cvuas.bwl.de
2 Chemisches und Veterinäruntersuchungsamt Freiburg
Am Moosweiher 2, 79108 Freiburg, Germany
3 current address: Bruker Daltonik GmbH
Fahrenheitstraße 4, 28359 Bremen, Germany
4 Institute of Farm Animal Genetics, Friedrich-Loeffler-Institut (FLI),
Hoeltystr. 10, 31535 Neustadt-Mariensee, Germany
Keywords:

MALDI-TOF MS, Biotyper, validation, Staphylococcus, Macrococcus

Abstract

Differentiation of microorganisms by matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) is a fast growing application in several fields of microbiology. The comparison of mass-spectra of a microorganism under investigation with spectra in a given database results in a hit list, ranking the best matching spectra. For this approach, commercial databases contain several thousand spectra from microorganisms of a broad taxonomic variety. Until today, the focus of these databases is on microorganisms from clinical microbiology. For application of the technique in the context of food control or animal health, a formal validation procedure must be created for each important target organism. This is especially important for parameters, which were created by the user to supplement the commercial database.

This study describes a workflow and documentation of a validation procedure for MALDI-TOF MS, based predominantly on reliably identified field isolates, representing the main organisms of interest. As an example for a relevant group of microorganisms from a specific environment, the validation of MALDI-TOF MS identification of staphylococcal species, mainly isolated from raw milk, is presented. The results show the reliability obtained with the initial commercial version of Bruker’s MALDI Biotyper (Version 3.1.66, BT 5,989) in comparison with the results obtained with a database version extended by our own additional entries: using the commercial database version 165 (74.3 %) of 222 Staphylococcaceae isolates from 29 species were identified correctly. Only one isolate was incorrectly assigned to Staphylococcus aureus but actually belonged to the just recently described coagulase-positive species Staphylococcus argenteus. For the remaining 56 isolates a species decision was not achieved.

The extension of the commercial database by 22 own entries, including one for Staphylococcus argenteus, resulted in 94.6 % correct identifications. False differentiation results were not obtained with the extended database, while for 5.4 % of the isolates a concluding species decision could not be achieved. For S. aureus, the diagnostically most relevant species, a 100 % match rate was obtained with the commercial and the extended database.

A selection of database entries made for this study can be obtained by exchange via the MALDI-user platform MALDI-UP (http://maldi-up.ua-bw.de).

 

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