Extracellular Vesicle Biogenesis and Functions in Gram-Positive Bacteria

Abstract

Extracellular vesicles (EVs) are membrane-derived lipid bilayers secreted by bacteria and eukaryotic cells. Bacterial membrane vesicles were discovered over 60 years ago and have been extensively studied in Gram-negative bacteria. During their production, EVs are loaded with proteins, nucleic acids, and various compounds that are subsequently released into the environment. Depending on the packaged cargo, EVs have a broad spectrum of action and are involved in pathogenesis, antibiotic resistance, nutrient uptake, and nucleic acid transfer. Due to differences in cell wall structure, EVs in Gram-positive bacteria have been disregarded for decades, and our understanding of their biogenesis and host cell interaction is incomplete. Recently, studies on bacteria such as Staphylococcus aureus, Streptococcus spp., Bacillus subtilis, and Mycobacterium spp. have demonstrated EV production in Gram-positive bacteria and shown the great importance EVs have in Gram-positive bacterial physiology and disease progression. Here, we review the latest findings on the biogenesis and functions of EVs from Gram-positive bacteria and identify key areas for future research.

Keywords: EVs; Gram-positive bacteria; OMV; extracellular vesicles; membrane vesicles.

FIG 1

Biogenesis and functions of Gram-positive EVs. Key steps are shown in bold. Red font indicates the hypothetical involvement of proteins or RNAs that have yet to be demonstrated. Turgor pressure promotes membrane curvature, aided by the overall negative charge due to phospholipid enrichment. A selective sorting mechanism occurs during vesiculogenesis, driven by electrostatic interactions within EVs. The involvement of curvature-recognizing proteins is also possible. Specific proteins, such as the αPSMs in S. aureus, facilitate the liberation of EVs from the plasma membrane. Autolysins and antibiotics that inhibit penicillin-binding proteins (PBPs) loosen the peptidoglycan and thus promote the release of EVs. The cargo can be discharged directly into the environment. This process can facilitate increased antibiotic resistance if, for example, antibiotic resistance proteins (e.g., β-lactamase [BlaZ]) have been preincorporated into EVs. Once EVs come into contact with host cells, surface-associated molecules, such as lipopolysaccharides, can interact with Toll-like receptor 2 (TLR-2) and trigger EV endocytosis. Direct membrane fusion, as described for Gram-negative OMVs, is also a potential delivery mechanism for Gram-positive EVs. Cargo released inside the cell can have a wide variety of effects, such as activation or inhibition of transcription factors (TFs), disruption of host cell membranes (Hla, PSMs, listeriolysin O [LLO], or pneumolysin A [PNL-A]), production and release of cytokines, and cell death. Finally, as described for Gram-negative bacteria, the presence of RNA in Gram-positive bacterial EVs could interfere with host cell protein synthesis and/or gene regulation by a base-pairing mechanism. Alternatively, prokaryote-specific RNA modifications may trigger immune system activation. (Created with BioRender.com.)