Purpose: The ability of MALDI-TOF MS to identify and confirm isolates, directly, from widely used selective agars was evaluated.
Methods: The effect of growth media on the generated spectra and its consequences for species identification were determined by testing 22 target strains, i.e. 5 L. monocytogenes strains, 5 Listeria ivanovii strains, 3 Listeria grayi strains, 3 Listeria welshimeri strains, 3 Listeria innocua strains and 3 Listeria seeligeri strains. Three relevant non-target strains of Bacillus cereus, Bacillus pumilus and Enterococcus faecium were, also, used. The following selective agars were tested: Ottaviani & Agosti, Oxford, Palcam, and Rapid L.mono. The isolates were plotted onto reusable and disposable targets. Three sample preparations were used: HCCA matrix, HCCA combined with formic acid (70%), and an extraction procedure. Mass Spectra Profiles (MSPs) were acquired and analyzed with the MALDI Biotyper Complete Solution.
Results: A total of 375 spectra were generated. All the test isolates were correctly identified; whatever the tested selective agars, targets, or sample preparations. No bias was observed. A subtyping module was also available for use in Listeria species identification. It allowed the use of the simplest sample preparation protocol consisting of the overlaid HCCA matrix, only.
Significance: There was no influence of selective growth media on the identification of Listeria species, and thus, confirmation of L monocytogenes. No culture step on a non-selective agar was required prior to confirmation, and the simplest sample preparation can be used.