To prepare the MALDI target plates, 1 mL of the supernatant
containing the bacterial proteins was placed onto
a steel target plate (MSP 96 polished-steel target; Bruker
Daltonics, Bremen, Germany) and allowed to air-dry. The
dried supernatant was overlaid with 1 mL of matrix solution,
which consisted of a-cyano-4-hydroxy-cinnamic acid
(CHCA) dissolved in 50% acetonitrile and 2.5% trifluoroacetic
acid.
For the intact cell method (ICM), the washed bacteria
pellet (w0.1 g)was diluted in 1 mL ofwater and acetonitrile
(1:1 vol/vol). A volume of 1 mL of the obtained solution was
directly spotted on the MALDI plate and the CHCA matrix
was added as described previously.
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.