Purpose: The objective of this project is to evaluate the ability of MALDI-TOF MS to identify and confirm isolates directly from widely used selective agars, i.e. standard and proprietary media.
Methods: The effect of the growth medium on the generated MALDI-TOF MS Profile Spectra and its consequences for species identification were determined by using a set of relevant target and non-target strains. The following selective agars were tested: XLD, BGA, ASAP, Rapid’Salmonella, ESIA, DFI, mCCDA, Campy Cefex, CampyFood Agar, Rapid’Campylobacter, Ottaviani & Agosti, Oxford, Palcam, and Rapid L.mono. The isolates were plotted onto reusable and disposable targets. A HCCA matrix based sample preparation with and without formic acid (70%) overlaid was used for all the tested strains; an extraction procedure was also run for Listeria spp. confirmation. The MSPs were acquired and analyzed with the Maldi biotyper complete solution.
Results: More than 630 Spectra were generated. All the tested isolate were correctly identified, whatever the tested selective agars, targets, and sample preparations. No bias was observed. A subtyping module is also available to be used for Listeria spp. identification and it allows the use of the simplest sample preparation protocol consisting of the overlaid form of the single HCCA matrix, only.
Significance: There was no influence of selective growth media on the identification and, thus, confirmation of the tested foodborn pathogens by MALDI-Tof MS. Indeed, no culture step on a non-selective agar was required prior confirmation, and the simplest and quickest sample preparation was used.