Bacterial Outer Membrane Vesicles: Research Advances from Biogenesis Mechanisms to Engineered Applications

Bacterial outer membrane vesicles (OMVs) are spherical structures made up of a double layer, they are each nanostructured (20-300 nm), and they are released from all populations of Gram-negative bacteria. The purpose of this review is to synthesize a comprehensive summary of the current state of knowledge about OMV biogenesis, function in biology, and application to biomedical engineering. Using these three known biogenesis mechanisms as a basis for this review, we discuss the mechanisms of OMV biogenesis that have been described as conserved: (1) disruption of outer membrane-peptidoglycan links. (2) periplasmic stress-driven adaptive release is associated with bilayer lipid asymmetry and the use of signaling molecules. OMVs are considered to be "public goods" for the microbe, allowing for nutrient acquisition, resistance to antibiotics, and the potential for horizontal gene transfer between microbes. OMVs exhibit a different duality at the interface of the pathogen host, where the pathogenic OMV is the delivery vehicle for virulence factors and pathogen-associated molecular patterns (PAMPs) leading to host immune response, while the symbiotic OMV (e.g., those produced by Bacteroides fragilis (Bact. fragilis)) promote regulatory T cell differentiation and mucosal tolerance. The review also addresses the various techniques currently available to isolate OMVs (e.g., ultracentrifugation and size-exclusion chromatographic techniques) and presents engineered/alloying strategies (e.g., genetic modifications to tolR/msbB and surface functionalization) to enhance the viability, safety, and specificity of OMVs for biomedical delivery. Finally, the review addresses significant obstacles related to standardization, batch variation, and in vivo safety associated with synthetic or personalized therapeutics based on OMVs, providing some recommendations for future research in this area.