On-Plate Extraction Method (Formic Acid Overlay)

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The page below is a sample from the LabCE course Microbial Identification Using MALDI-TOF MS. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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On-Plate Extraction Method (Formic Acid Overlay)

The on-plate extraction (or formic acid overlay) method is a supplementary method that can be used to enhance the quality of spectra obtained from microorganisms. For this method, the microorganism is spotted directly onto the plate as previously described. Then, 1 µL of formic acid (70–100%) is applied to the spot and allowed to dry. The CHCA matrix is added and the MALDI-TOF MS is performed per normal protocols. This method has been shown to improve the quality of spectra and result in better identification.11 Some have shown that it improves the identification of gram positive bacteria and it appears to be critical for the identification of yeast.12,13
11. Besséde E, Angla-Gre M, Delagarde Y, Sep Hieng S, Ménard A, Mégraud F. "Matrix-assisted laser-desorption/ionization biotyper: experience in the routine of a University hospital." Clin Microbiol Infect. 2011;17(4):533-538. Available at: http://www.clinicalmicrobiologyandinfection.com/article/S1198-743X(14)63269-9/fulltext.
12. McElvania Tekippe E, Shuey S, Winkler DW, Butler MA, Burnham CAD. "Optimizing identification of clinically relevant Gram-positive organisms by use of the Bruker Biotyper matrix-assisted laser desorption ionization-time of flight mass spectrometry system." J Clin Microbiol. 2013;51(5):1421–1427. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3647943/.
13. Alatoom AA, Cunningham SA, Ihde SM, Mandrekar J, Patel R. "Comparison of direct colony method versus extraction method for identification of gram-positive cocci by use of Bruker Biotyper matrix-assisted laser desorption ionization-time of flight mass spectrometry." J Clin Microbiol. 2011;49(8):2868-73. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3147777/.