Extracellular Proteome\nof a Highly Invasive Multidrug-resistant\nClinical Strain of <i>Acinetobacter baumannii</i>

Abstract

The study of the extracellular proteomes of pathogenic\nbacteria\nis essential for gaining insights into the mechanisms of pathogenesis\nand for the identification of virulence factors. Through the use of\ndifferent proteomic approaches, namely Nano-LC and 2DE combined with\nMALDI-TOF/TOF, we have characterized the extracellular proteome of\na highly invasive, multidrug-resistant strain of <i>A. baumannii</i> (clone AbH12O-A2). This study focused on two main protein fractions\nof the extracellular proteome: proteins that are exported by outer\nmembrane vesicles (OMVs) and freely soluble extracellular proteins\n(FSEPs) present in the culture medium of <i>A. baumannii</i>. Herein, a total of 179 nonredundant proteins were identified in\nthe OMV protein fraction and a total of 148 nonredundant proteins\nwere identified in FSEP fraction. Of the OMV proteins, 39 were associated\nwith pathogenesis and virulence, including proteins associated with\nattachment to host cells (e.g., CsuE, CsuB, CsuA/B) and specialized\nsecretion systems for delivery of virulence factors (e.g., P. pilus\nassembly and FilF), whereas the FSEP fraction possesses extracellular\nenzymes with degradative activity, such as alkaline metalloprotease.\nFurthermore, among the FSEP we have detected at least 18 proteins\nwith a known role in oxidative stress response (e.g., catalase, thioredoxin,\noxidoreductase, superoxide dismutase). Further assays demonstrated\nthat in the presence of FSEPs, bacterial cells withstand much higher\nconcentrations of H₂O₂ showing higher survival\nrate (approximately 2.5 fold) against macrophages. In this study we\nhave identified an unprecedented number of novel extracellular proteins\nof <i>A. baumannii</i> and we provide insight into their\npotential role in relevant processes such as oxidative stress response\nand defense against macrophage attack.