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Among coagulase-positive staphylococci (CPS) of companion animal origin, staphylococci composing the Staphylococcus intermedius-group (SIG) are common opportunistic pathogens, capable of causing a wide range of different purulent and toxin-mediated diseases in dogs, cats and horses. While there is a rising rate of methicillin-resistant S. pseudintermedius (MRSP) among microbiological specimens noticeable, phenotypic species identification techniques for the distinct members of the SIG might be more or less imprecise and time-consuming. In recent years, first severe infections with multi-drug resistant MRSP were also reported for humans. A fast and reliable identification of SIG, for instance by use of matrix-assisted laser desorption ionization—time of flight mass spectrometry (MALDI-TOF MS), is necessary to disclose sound identification rates in both veterinary and human medicine.
As a first step, a reliable reference database for spectra associated with the different members of the SIG was created using isolates unambiguously identified by gene-based methods. A total of 27 MALDI-TOF MS spectra were acquired for each isolate of the following species: S. pseudintermedius (n = 43, including 20 MRSP), S. intermedius (n = 5) and S. delphini (n = 12) and a reference library was set up with Bruker Microflex LT together with BIOTYPER 3.0 software (Bruker Daltonics, Bremen, Germany). In a second approach, a broad convenience sample consisting of 200 CPS strains was evaluated using a) the original database content and b) the database after extension with distinct hierarchical clustered reference spectra for 60 SIG.
A significant improvement (average rise of log score value: 0.24) of the SIG identification score values was obtained for 200 SIG. Moreover, for 17 isolates the initial identification as “S. intermedius” changed to “S. pseudintermedius” as best match with improved score values by applying the in-house reference spectra extended database version.
Data presented here highlights the opportunities of sequence-based refinement of the Bruker database content with respect to improvement of MALDI-TOF MS-based bacterial species identification.