MALDI-TOF MS analysis was performed with a MicroFlex LT mass spectrometer (Bruker Daltonik) as reported [5 (link),6 (link),12 (link)]. Each isolated colony was deposited on a MALDI-TOF MS target Microflex (Bruker Daltonik) as above. Then, each colony was overlaid with 2 μL of matrix solution (saturated solution of alpha-cyano-4-hydroxycinnamic acid in 50% acetonitrile and 2.5% tri-fluoracetic-acid), and the matrix-sample was crystallized by air-drying at room temperature, as previously described [5 (link),6 (link),12 (link)]. Two spots were systematically created for each colony. For each assay, a strain of Escherichia coli (DH5 alpha, Bruker Daltonik) was also analyzed for quality control.
The analyses of the obtained spectra were performed using our personal database [5 (link),6 (link)], the Bruker database updated with a laboratory collection of spectra from clinical isolates identified using molecular sequencing (primarily 16S rRNA sequencing) [6 (link)].
The criteria for identification were previously reported [5 (link)]. An isolate was considered correctly identified by MALDI-TOF MS if both spectra had a score ≥1.9 for species identification or ≥1.7 for genus identification [5 (link)].
Finally, positive controls are used in a routine manner for both MALDI-TOF MS systems. Besides, colonies are systematically tested in duplicate. The E. coli positive controls must be correctly identified and a same identification for the duplicate spots of each colony must be obtained using the score of each software package in order to conclude to the identification of microorganism.
The analyses of the obtained spectra were performed using our personal database [5 (link),6 (link)], the Bruker database updated with a laboratory collection of spectra from clinical isolates identified using molecular sequencing (primarily 16S rRNA sequencing) [6 (link)].
The criteria for identification were previously reported [5 (link)]. An isolate was considered correctly identified by MALDI-TOF MS if both spectra had a score ≥1.9 for species identification or ≥1.7 for genus identification [5 (link)].
Finally, positive controls are used in a routine manner for both MALDI-TOF MS systems. Besides, colonies are systematically tested in duplicate. The E. coli positive controls must be correctly identified and a same identification for the duplicate spots of each colony must be obtained using the score of each software package in order to conclude to the identification of microorganism.
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