MALDI-FTMS of Whole-Cell Bacteria

Jeffrey J. Jones, Michael J. Stump, and Charles L. Wilkins 

Identification of Microorganisms by Mass Spectrometry, Edited by Charles L. Wilkins and Jackson O. Lay, Jr. 

Fourier transform mass spectrometry is made possible by the measurement of an AC current produced from the movement of ions within a magnetic field under ultra-high vacuum, commonly referred to as ion cyclotron motion.21 Ion motion, or the frequency of each ion, is recorded to the precision of one thousandth of a Hertz and may last for several seconds, depending on the vacuum conditions. Waveform motion recorded by the mass analyzer is subjected to a Fourier transform to extract ion frequencies that yield the corresponding mass to charge ratios. To a first approximation, motion of a single ion in a magnetic field can be defined by the equation 

3 FUNDAMENTALS OF COMPLEX BIOLOGICAL ANALYSIS 13.3.1 Whole-Cell MALDI-FTMS Analysis 

Approximately 90% to 95% of whole-cell MALDI MS profiles are representative of the ribosomal proteins abundant in rapid growth whole cells. Although the identification of proteins, from whole bacterial cells, by MALDI-TOF MS analysis is ambiguous, at best, due to the low-mass accuracy and resolving power, several researchers realized that many of the observed 

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The results for bacterial whole-cell analysis described here establish the utility of MALDI-FTMS for mass spectral analysis of whole-cell bacteria and (potentially) more complex single-celled organisms. The use of MALDI-measured accurate mass values combined with mass defect plots is rapid, accurate, and simpler in sample preparation then conventional liquid chromatographic methods for bacterial lipid analysis. Intact cell MALDI-FTMS bacterial lipid characterization complements the use of proteomics profiling by mass spectrometry because it relies on accurate mass measurements of chemical species that are not subject to posttranslational modification or proteolytic degradation. 

Another interesting comparison has recently been made between MALDI-TOF MS analysis of whole cells, and MALDI FTMS of the same organisms. This work is reported in greater detail in a dedicated chapter later in this book. It should be noted here that it appears to be much more difficult to obtain spectra from intact bacteria by MALDI FTMS than it is by MALDI-TOF MS. Thus far only a single research group has reported protein-like ions desorbed directly from intact cells by MALDI FTMS. 

An interesting variation on the whole-cell MALDI approach was recently reported in a study aimed more at FTMS than TOF MS, but the results are nevertheless interesting and important to users of both methods for analysis of bacteria 40. Wilkins s group showed both MALDI-TOF and MALDI-FTMS spectra of whole bacteria grown on isotopic media depleted in C13 and N14. Because most bacterial identification protocols involve a culture step prior to analysis, it is possible to manipulate the sample based on control of the growth media. For mass spectral analysis manipulation of the isotope profile... 

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