Extracellular vesicles are ubiquitously released by all forms of life. The roles of Gram-negative outer membrane vesicles (OMVs) in inter-bacterial communication and host-pathogen interactions are well described in the literature. In contrast, research surrounding Gram-positive membrane vesicles (MVs) is still in its infancy. There is limited knowledge regarding the pathogenic properties of Gram-positive MVs, including their contents, biological functions and role in cellular communication.
This study characterises MVs produced by the Gram-positive pathogen, Staphylococcus aureus, and elucidates their ability to induce an innate immune response. Transmission electron microscopy and NanoSight particle tracking analysis (NTA) were used to characterise the amount and size of MVs produced by S. aureus at mid-log and early stationary phase. Qubit fluorometric quantification demonstrated that S. aureus MVs contain DNA, RNA and proteins. Analysis of MV-derived proteins by Western immunoblot revealed that MVs contain a range of proteins of varying molecular weight. Furthermore, high molecular weight DNA was found to be associated with MVs, while examination of MV-derived RNA using a bioanalyser identified a range of RNA including small RNA. This may suggest that MV-derived RNA has a potential role in regulation of translation of target RNA through RNA-RNA interactions.
Investigation into the immunogenicity of S. aureus MVs revealed that they were capable of inducing the production of the pro-inflammatory cytokine interleukin-8 by epithelial cells. Furthermore, using reporter cell lines, we showed that DNA, RNA and lipoproteins associated with S. aureus MVs are immunogenic, as they activate Toll-like receptors (TLR) 9, 7 and 2, respectively. This suggests that S. aureus MVs may be involved in mediating a pro-inflammatory response in the host through their immunogenic contents.
This study reveals that S. aureus MVs contain a range of cargo including nucleic acids and proteins, and may play a role in host immune modulation. The findings facilitate the understanding of a previously uncharacterised mechanism of S. aureus pathogenesis and sheds light on the role of MVs in driving inflammation in S. aureus infections.