P0442 Discrimination between Escherichia coli and Shigella species using a high resolution MS method Simon New*1 1, Jarkko Räbinä1, Juha Knuuttila1, Suvi Teräslahti1, Naomi Chant1, Pirjo Wacklin1, Soniya Gurung1, Jason Chew1, Mark Hutchins1 1Thermo Fisher Scientific, Vantaa, Finland Background: Shigella sonnei and Shigella flexneri are two organisms responsible for considerable morbidity and mortality through shigellosis - an acute invasive enteric infection. This clinical significance is observed in both developed and developing countries with prevalence of S. sonnei and S. flexneri highest in each geographic location respectively. Differentiating Shigella species from the closely related Escherichia coli remains a diagnostic challenge faced by clinical laboratories today. Biochemical and serological methods can be used to distinguish the species, however these methods can often be time consuming and inconclusive. Moreover, “inactive” (non-lactose fermenting) E. coli proves an even greater diagnostic challenge when using biochemical methods. Due to the high genotypic similarity, identification by molecular methods is difficult with 16S rRNA. Here we report a high resolution mass spectrometry (MS) method using liquid chromatography coupled with OrbitrapTM technology that is able to reliably differentiate between S. sonnei, S. flexneri and E. coli. Materials/methods: The following strains were cultivated in two atmospheric conditions: E. coli, S. sonnei and S. flexneri. Each strain was prepared for high resolution mass spectrometry analysis from each cultivation condition. Analysis of mass spectra of all strains was carried out to achieve classification and the results were compared to characterization achieved using multilocus sequence analysis (MLSA) which involved sequencing and concatenation of multiple housekeeping genes. Results: Using the high resolution mass spectrometry method described, proteoforms obtained from each of the E. coli, S. sonnei and S. flexneri strains were used to cluster and effectively discriminate between the three taxa. The clustering resulting from the mass spectrometry method was in agreement with the phylogeny generated from MLSA analysis. Conclusions: This study showed that high resolution mass spectrometry provides an effective tool for rapidly discriminating between S. sonnei, S. flexneri and E. coli with high accuracy..