The matrix-assisted laser desorption ionization timeof-
flight mass spectrometry (MALDI-TOF-MS) analysis
was performed in a Bruker Microflex LT MALDI-TOF-MS
operated in the linear mode and equipped with a 337-
nm nitrogen laser using FlexControl 3.3 software (Bruker
Daltonics). The mass spectra were collected within the
mass range of m/z 2000 to 20,000. The instrument settings
were as follows: ion source 1 at 19.50 kV, ion source 2 at
18.25 kV, lens at 6.49 kV, and an extraction delay time of 4
ns. The instrument was externally calibrated with the
bacterial test standard supplied by Bruker Daltonics. The
bacterial test standard is an Escherichia coli extract
including the additional proteins RNase A and myoglobin.