Bacterial Identification at the Serovar Level by Top-Down Mass Spectrometry

Food safety efforts require serovar and strain-level specificity of Salmonella contamination to trace back bacterial contamination to its source. Detection methods that require selection of probe-based assays are limited by probe selection. In this work we show that the combination of intact protein expression profiles and top-down liquid chromatography-tandem mass spectrometry (LC-MS/MS) facilitates the identification of proteins that result from expressed, serovar-specific non-synonymous single-nucleotide polymorphisms (SNPs). Intact protein expression profiles provide a nontargeted mass spectrometry-based method that facilitates a relatively unbiased snapshot of the expressed proteins in a wide range of bacterial samples and is amenable to both screening and targeted analysis. Such an inherently multiplexed technique facilitates differentiation of closely related bacteria, as well as the detection of un-sequenced or newly acquired SNPs and plasmid proteins that may be specific to a given strain without prior knowledge of the sample. Subsequent identification of expressed proteins, serovar-specific biomarkers, and post-translational modifications by top-down LC-MS/MS is integral to rapid screening development and facilitates collaboration with genome-based methods.