Using Matrix-Assisted Laser Desorption/Ionization Time of Flight Spectra To Elucidate Species Boundaries by Matching...

A method has been established to map a bacterial colony to the ever-expanding database of publicly available bacterial genomes by means of matrix-assisted laser desorption/ionization (MALDI) spectra. To accomplish this, spectra are mapped to the predicted masses of ∼65 families of mostly ribosomal proteins. Each of the ∼40 000 bacterial strains in the database receives scores, together with tables listing identified protein sequences and how the highest ranking strains are related to one another. The approach was first confirmed with 16 distinct species of bacteria from the Vibrionales whose genome had been sequenced. Identifications of a few species of bacteria from environmental samples from compost, lakes, and streams in Massachusetts are also reported. Most of these organisms map to known species in the Gammaproteobacteria and Firmicutes. The clades of bacteria deducible from shared ribosomal protein sequences do not always correspond well to named bacterial species. Instead, the identifications made by this methodology indicate groupings of organisms that can readily be distinguished by MALDI-TOF and indicate which polymorphisms in highly conserved proteins demarcate the groupings. Successful identifications highlight organism interrelationships that can be deduced from the available genomes, sorting together genomes into new proposed clades typically consistent with relationships deduced from DNA sequence analysis. In contrast, if for a high-quality spectrum from a fresh colony, no group of related organisms receives high scores, one might infer that no closely related genome has yet been deposited into the database.