Journal of the Mass Spectrometry Society of Japan

A rapid and reliable method for the identification and phylogenetic classification of bacteria is described. In recent years, a method for the rapid identification of bacteria at the “species level” using matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) were proposed. In this method, observed mass spectral peaks, which are used as biomarkers, are assigned using the calculated masses of ribosomal subunit proteins based on amino acid sequences registered in protein databases. We applied the proposed method to a variety of samples. including genome sequenced bacteria, and found that the method is reliable and practical for rapidly identifying bacteria at the “species level.” In addition, minor sequence variations in ribosomal subunit proteins at the “strain level” can be clearly detected as mass differences on MALDI mass spectra. Using the mass differences, it was possible to implement a method for the rapid and reliable phylogenetic classification of bacteria at the “strain level.” The method was also compared with analogous classifications using gene analysis.

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Electrospray droplet impact (EDI) secondary ion mass spectrometry (SIMS) is a desorption/ionization technique for mass spectrometry. Highly charged water clusters produced via atmospheric-pressure electrospray are accelerated in a vacuum at several kV and impact the sample, which is deposited on a metal substrate. Two mechanisms for desorption/ionization in the EDI ionization process have been proposed, namely, low-energy and high-energy regimes. In the low-energy regime, the excess charges in the primary droplets play a role in the formation of secondary ions. In the high-energy regime, samples are ionized as the result of the supersonic collision of the primary droplets with the sample. In general, EDI/SIMS is very effective for the formation of [M+H]⁺ and [M-H]^- for a wide variety of molecules. This characteristic of EDI/SIMS is probably due to the formation of the highly acidic H₃O⁺ and highly basic OH^- ions, which are superb protonating and deprotonating reagents, respectively. In addition, EDI produces a much smaller number of hydrogen atoms compared with matrix-assisted laser desorption/ionization (MALDI) and fast atom bombardment (FAB).

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