Staphylococcus aureus-derived extracellular vesicles induce an inflammatory response in murine model of mastitis
Résumé
Staphylococcus aureus is one of the main causative agents of mastitis, an inflammation of the mammary gland that affects bovines and small ruminants. The severity of staphylococcal infections is highly variable mainly due to the virulence factors secreted by different strains. Gram-positive bacteria, including S. aureus, produce and release extracellular vesicles (EVs) that play fundamental roles in cell-to-cell communication. In this study we report the isolation and the functional characterization of EVs produced by S. aureus Newbould 305, a bovine strain able to induce chronic mastitis in cows. Vesicles were recovered from the culture supernatants by density gradient ultracentrifugation, visualized by transmission electron microscopy and characterized by Nanoparticle Tracking Analysis (NTA). The mean diameter of purified EVs were 177 nm ± 46 nm with concentration of 3.86 x 1013 vesicles per mL of culture. 222 different proteins involved in a number of bacterial cellular processes were identified in EVs by LC-MS/MS. EVs also contained numerous virulence-associated proteins such as -hemolysin, immunoglobulin G-binding protein and phenol soluble modulins suggesting a physiological role for EVs in pathogenicity. The effect of intra-mammary injections of purified EVs on CD-1 lactating female mice was then evaluated. 24 h post infection, macroscopic signs of inflammation as well as a dose dependent neutrophil recruitment were observed from histological sections of mammary glands of mice treated with EVs. In addition, EVs induced the local level of key pro-inflammatory mediators such as IL-8. The overall response to EVs was comparable to that obtained with heat-inactivated N305 and was milder than the response to live N305. Our findings provide evidence of the importance of the release of EVs as a mechanism of secretion of bacterial effectors in the pathogenesis of mastitis and correspond to the first work characterizing EVs of a strain of veterinary interest.