Engineered bacterial outer membrane vesicles with enhanced functionality

Engineered ClearColi™-derived outer membrane vesicles as functional carriers for development of HIV-1 therapeutic vaccine candidate

Bacteria-derived outer membrane vesicles (OMVs) can be engineered to incorporate foreign antigens. This study explored the potential of ClearColi™-derived OMVs as a natural adjuvant and a carrier (recombinant OMVs or rOMVs) for development of an innovative therapeutic vaccine candidate harboring HIV-1 Nef and Nef-Tat antigens. Herein, the rOMVs containing CytolysinA (ClyA)-Nef and ClyA-Nef-Tat fusion proteins were isolated from ClearColi™ strain. The presence of Nef and Nef-Tat proteins on their surface (rOMVNef and rOMVNef-Tat) was confirmed by western blotting after proteinase K treatment. Immune responses induced by Nef and Nef-Tat proteins emulsified with Montanide® ISA720 or mixed with OMVs, and also rOMVNef and rOMVNef-Tat were investigated in BALB/c mice. Additionally, the potency of splenocytes exposed to single-cycle replicable (SCR) HIV-1 virions was assessed for the secretion of cytokines in vitro. Our findings showed that the rOMVs as an antigen carrier (rOMVNef and rOMVNef-Tat) induced higher levels of IgG2a, IFN-γ and granzyme B compared to OMVs as an adjuvant (Nef + OMV and Nef-Tat + OMV), and also Montanide® ISA720 (Nef + Montanide and Nef-Tat + Montanide). Moreover, IFN-γ level in splenocytes isolated from mice immunized with rOMVNef-Tat was higher than other regimens after exposure to SCR virions. Generally, ClearColi™-derived rOMVs can serve as potent carriers for developing effective vaccines against HIV-1 infection.

In Silico Design of CT26 Polytope and its Surface Display by ClearColi™-Derived Outer Membrane Vesicles as a Cancer Vaccine Candidate Against Colon Carcinoma

Simultaneous targeting of several mutations can be useful in colorectal cancer (CRC) due to its heterogeneity and presence of somatic mutations. As CT26 mutations and expression profiles resemble those of human CRC, we focused on designing a polyepitope vaccine based on CT26 neoepitopes. Due to its low immunogenicity, outer membrane vesicles (rOMV) as an antigen delivery system and adjuvant was applied. Herein, based on previous experimental and our in silico studies four CT26 neoepitopes with the ability to bind MHC-I and MHC-II, TCR, and induce IFN-α production were selected. To increase their immunogenicity, the gp70 and PADRE epitopes were added. The order of the neoepitopes was determined through 3D structure analysis using ProSA, Verify 3D, ERRAT, and Ramachandran servers. The stable peptide-protein docking between the selected epitopes and MHC alleles strengthen our prediction. The CT26 polytope vaccine sequence was fused to the C-terminal of cytolysin A (ClyA) anchor protein and rOMVs were isolated from endotoxin-free ClearColi™ strain. The results of the C-ImmSim server showed that the ClyA-CT26 polytope vaccine could induce T and B cells immunity.The ClyA-CT26 polytope was characterized as a soluble, stable, immunogen, and non-allergen vaccine and optimized for expression in ClearColi™ 24 h after induction with 1 mM IPTG at 25 °C. Western blot analysis confirmed the expression of ClyA-CT26 polytope by ClearColi™ and also on ClearColi™-derived rOMVs. In conclusion, we found that ClearColi™-derived rOMVs with CT26 polytope can deliver CRC neoantigens and induce antitumor immunity, but in vivo immunological studies are needed to confirm vaccine efficacy.

Reconciling the Cooperative-Competitive Patterns among Tumor and Immune Cells for Triple-Negative Breast Cancer Treatment Using Multimodule Nanocomplexes

Targeting the competitive-cooperative relationships among tumor cells and various immune cells can efficiently reverse the immune-dysfunction microenvironment to boost the immunotherapies for the triple-negative breast cancer treatment. Hence, a bacterial outer membrane vesicle-based nanocomplex is designed for specifically targeting malignant cells and immune cells to reconcile the relationships based on metabolic-immune crosstalk. By uniquely utilizing the property of charge-reversal polymers to realize function separation, the nanocomplexes could synergistically regulate tumor cells and immune cells. This approach could reshape the immunosuppressive competition-cooperation pattern into one that is immune-responsive, showcasing significant potential for inducing tumor remission in TNBC models.

The proteome of bacterial membrane vesicles in Escherichia coli-a time course comparison study in two different media

Introduction

Bacteria inhabit the in- and outside of the human body, such as skin, gut or the oral cavity where they play an innoxious, beneficial or even pathogenic role. It is well known that bacteria can secrete membrane vesicles (MVs) like eukaryotic cells with extracellular vesicles (EVs). Several studies indicate that bacterial membrane vesicles (bMVs) play a crucial role in microbiome-host interactions. However, the composition of such bMVs and their functionality under different culture conditions are still largely unknown.

Methods

To gain a better insight into bMVs, we investigated the composition and functionality of E. coli (DSM 105380) bMVs from the culture media Lysogeny broth (LB) and RPMI 1640 throughout the different phases of growth (lag-, log- and stationary-phase). bMVs from three time points (8 h, 54 h, and 168 h) and two media (LB and RPMI 1640) were isolated by ultracentrifugation and analyzed using nanoparticle tracking analysis (NTA), cryogenic electron microscopy (Cryo-EM), conventional transmission electron microscopy (TEM) and mass spectrometry-based proteomics (LC-MS/MS). Furthermore, we examined pro-inflammatory cytokines IL-1β and IL-8 in the human monocyte cell line THP-1 upon bMV treatment.

Results

Particle numbers increased with inoculation periods. The bMV morphologies in Cryo-EM/TEM were similar at each time point and condition. Using proteomics, we identified 140 proteins, such as the common bMV markers OmpA and GroEL, present in bMVs isolated from both media and at all time points. Additionally, we were able to detect growth-condition-specific proteins. Treatment of THP-1 cells with bMVs of all six groups lead to significantly high IL-1β and IL-8 expressions.

Conclusion

Our study showed that the choice of medium and the duration of culturing significantly influence both E. coli bMV numbers and protein composition. Our TEM/Cryo-EM results demonstrated the presence of intact E. coli bMVs. Common E. coli proteins, including OmpA, GroEL, and ribosome proteins, can consistently be identified across all six tested growth conditions. Furthermore, our functional assays imply that bMVs isolated from the six groups retain their function and result in comparable cytokine induction.

Outer membrane vesicles as a platform for the discovery of antibodies to bacterial pathogens

Bacterial outer membrane vesicles (OMVs) are nanosized spheroidal particles shed by gram-negative bacteria that contain biomolecules derived from the periplasmic space, the bacterial outer membrane, and possibly other compartments. OMVs can be purified from bacterial culture supernatants, and by genetically manipulating the bacterial cells that produce them, they can be engineered to harbor cargoes and/or display molecules of interest on their surfaces including antigens that are immunogenic in mammals. Since OMV bilayer-embedded components presumably maintain their native structures, OMVs may represent highly useful tools for generating antibodies to bacterial outer membrane targets. OMVs have historically been utilized as vaccines or vaccine constituents. Antibodies that target bacterial surfaces are increasingly being explored as antimicrobial agents either in unmodified form or as targeting moieties for bactericidal compounds. Here, we review the properties of OMVs, their use as immunogens, and their ability to elicit antibody responses against bacterial antigens. We highlight antigens from bacterial pathogens that have been successfully targeted using antibodies derived from OMV-based immunization and describe opportunities and limitations for OMVs as a platform for antimicrobial antibody development. KEY POINTS: • Outer membrane vesicles (OMVs) of gram-negative bacteria bear cell-surface molecules • OMV immunization allows rapid antibody (Ab) isolation to bacterial membrane targets • Review and analysis of OMV-based immunogens for antimicrobial Ab development.

Bacteria-derived extracellular vesicles: endogenous roles, therapeutic potentials and their biomimetics for the treatment and prevention of sepsis

Sepsis is one of the medical conditions with a high mortality rate and lacks specific treatment despite several years of extensive research. Bacterial extracellular vesicles (bEVs) are emerging as a focal target in the pathophysiology and treatment of sepsis. Extracellular vesicles (EVs) derived from pathogenic microorganisms carry pathogenic factors such as carbohydrates, proteins, lipids, nucleic acids, and virulence factors and are regarded as "long-range weapons" to trigger an inflammatory response. In particular, the small size of bEVs can cross the blood-brain and placental barriers that are difficult for pathogens to cross, deliver pathogenic agents to host cells, activate the host immune system, and possibly accelerate the bacterial infection process and subsequent sepsis. Over the years, research into host-derived EVs has increased, leading to breakthroughs in cancer and sepsis treatments. However, related approaches to the role and use of bacterial-derived EVs are still rare in the treatment of sepsis. Herein, this review looked at the dual nature of bEVs in sepsis by highlighting their inherent functions and emphasizing their therapeutic characteristics and potential. Various biomimetics of bEVs for the treatment and prevention of sepsis have also been reviewed. Finally, the latest progress and various obstacles in the clinical application of bEVs have been highlighted.

Analysis of Bacterial Extracellular Vesicles by Immunogold Transmission Electron Microscopy

Bacterial extracellular vesicles (BEVs) have emerged as mediators of transkingdom communication with numerous potential biotechnological applications. As such, investigation of BEV's protein composition holds promise to uncover new biological mechanisms, such as in microbiome-host communication or pathogen infection. Additionally, bioengineering of BEV protein composition can enhance their therapeutic potential. However, accurate assessment of BEV protein cargo is limited by their nanometer size, which precludes light microscopy imaging, as well as by co-isolation of protein impurities during separation processes. A solution to these challenges is found in immunogold transmission electron microscopy (TEM), which combines antibody-based labeling with direct visualization of BEVs. Several challenges are commonly encountered during immunogold TEM analysis of BEVs, most notably inefficient antibody labeling and poor contrast. Here, we present an optimized protocol for immunogold TEM analysis of BEVs that overcomes such challenges.

Outer Membrane Vesicle Vaccine Platforms

Outer membrane vesicles (OMVs) are spontaneously released by many gram-negative bacteria during their growth and constitute an important virulence factor for bacteria, helping them to survive through harsh environmental conditions. Native OMVs, naturally-released from bacteria, are produced at a level too low for vaccine manufacturing, requiring chemical treatment (detergent-extracted) or genetic manipulation, resulting in generalized modules for membrane antigens (GMMAs). Over the years, the nature and properties of OMVs have made them a viable platform for vaccine development. There are a few licensed OMV vaccines mainly for the prevention of meningitis caused by Neisseria meningitidis serogroup B (MenB) and Haemophilus influenzae type b (Hib). There are several candidates in clinical development against other gram-negative organisms from which the OMVs are derived, but also against heterologous targets in which the OMVs are used as carriers (e.g. coronavirus disease 2019 [COVID-19]). The use of OMVs for targets other than those from which they are derived is a major advancement in OMV technology, improving its versatility by being able to deliver protein or polysaccharide antigens. Other advances include the range of genetic modifications that can be made to improve their safety, reduce reactogenicity, and increase immunogenicity and protective efficacy. However, significant challenges remain, such as identification of general tools for high-content surface expression of heterologous proteins on the OMV surface. Here, we outline the progress of OMV vaccines to date, particularly discussing licensed OMV-based vaccines and candidates in clinical development. Recent trends in preclinical research are described, mainly focused on genetic manipulation and chemical conjugation for the use of OMVs as carriers for heterologous protein and polysaccharide antigens. Remaining challenges with the use of OMVs and directions for future research are also discussed.

On-Demand Vaccine Production via Dock-and-Display of Biotinylated Antigens on Bacterial Extracellular Vesicles

Engineered outer membrane vesicles (OMVs) derived from Gram-negative bacteria are a promising vaccine technology for developing immunity against diverse pathogens. However, antigen display on OMVs can be challenging to control and highly variable due to bottlenecks in protein expression and localization to the bacterial host cell's outer membrane, especially for bulky and complex antigens. Here, we describe methods related to a universal vaccine technology called AvidVax (avidin-based vaccine antigen crosslinking) for rapid and simplified assembly of antigens on the exterior of OMVs during vaccine development. The AvidVax platform involves remodeling the OMV surface with multiple copies of a synthetic antigen-binding protein (SNAP), which is an engineered fusion protein comprised of an outer membrane scaffold protein linked to a biotin-binding protein. The resulting SNAPs enable efficient decoration of OMVs with a molecularly diverse array of biotinylated subunit antigens, including globular and membrane proteins, glycans and glycoconjugates, haptens, lipids, nucleic acids, and short peptides. We detail the key steps in the AvidVax vaccine production pipeline including preparation and isolation of SNAP-OMVs, biotinylation and enrichment of vaccine antigens, and formulation and characterization of antigen-loaded SNAP-OMVs.

Bacterial outer membrane vesicles provide an alternative pathway for trafficking of Escherichia coli O157 type III secreted effectors to epithelial cells

IMPORTANCE

Bacteria can package protein cargo into nanosized membrane blebs that are shed from the bacterial membrane and released into the environment. Here, we report that a type of pathogenic bacteria called enterohemorrhagic Escherichia coli O157 (EHEC) uses their membrane blebs (outer membrane vesicles) to package components of their type 3 secretion system and send them into host cells, where they can manipulate host signaling pathways including those involved in infection response, such as immunity. Usually, EHEC use a needle-like apparatus to inject these components into host cells, but packaging them into membrane blebs that get taken up by host cells is another way of delivery that can bypass the need for a functioning injection system.

High performance anion exchange chromatography purification of probiotic bacterial extracellular vesicles enhances purity and anti-inflammatory efficacy

Bacterial extracellular vesicles (BEVs), including outer membrane vesicles, have emerged as a promising new class of vaccines and therapeutics to treat cancer and inflammatory diseases, among other applications. However, clinical translation of BEVs is hindered by a current lack of scalable and efficient purification methods. Here, we address downstream BEV biomanufacturing limitations by developing a method for orthogonal size- and charge-based BEV enrichment using tangential flow filtration (TFF) in tandem with high performance anion exchange chromatography (HPAEC). The data show that size-based separation coisolated protein contaminants, whereas size-based TFF with charged-based HPAEC dramatically improved purity of BEVs produced by probiotic Gram-negative Escherichia coli and Gram-positive lactic acid bacteria (LAB). Escherichia coli BEV purity was quantified using established biochemical markers while improved LAB BEV purity was assessed via observed potentiation of anti-inflammatory bioactivity. Overall, this work establishes orthogonal TFF + HPAEC as a scalable and efficient method for BEV purification that holds promise for future large-scale biomanufacturing of therapeutic BEV products.

Bacterial Membrane Vesicles for In Vitro Catalysis

The use of biological systems in manufacturing and medical applications has seen a dramatic rise in recent years as scientists and engineers have gained a greater understanding of both the strengths and limitations of biological systems. Biomanufacturing, or the use of biology for the production of biomolecules, chemical precursors, and others, is one particular area on the rise as enzymatic systems have been shown to be highly advantageous in limiting the need for harsh chemical processes and the formation of toxic products. Unfortunately, biological production of some products can be limited due to their toxic nature or reduced reaction efficiency due to competing metabolic pathways. In nature, microbes often secrete enzymes directly into the environment or encapsulate them within membrane vesicles to allow catalysis to occur outside the cell for the purpose of environmental conditioning, nutrient acquisition, or community interactions. Of particular interest to biotechnology applications, researchers have shown that membrane vesicle encapsulation often confers improved stability, solvent tolerance, and other benefits that are highly conducive to industrial manufacturing practices. While still an emerging field, this review will provide an introduction to biocatalysis and bacterial membrane vesicles, highlight the use of vesicles in catalytic processes in nature, describe successes of engineering vesicle/enzyme systems for biocatalysis, and end with a perspective on future directions, using selected examples to illustrate these systems' potential as an enabling tool for biotechnology and biomanufacturing.

Targeted miR-34a delivery with PD1 displayed bacterial outer membrane vesicles-coated zeolitic imidazolate framework nanoparticles for enhanced tumor therapy

MicroRNA (miRNA) has been widely used as an effective gene drug for tumor therapy, but its chemical instability limited its therapeutic application in vivo. In this research, we fabricate an efficient miRNA nano-delivery system using zeolitic imidazolate framework-8 (ZIF-8) coated with bacterial outer membrane vesicles (OMVs), aimed for cancer treatment. The acid-sensitive ZIF-8 core enables this system to encapsulate miRNA and release them from lysosome quickly and efficiently in the target cells. The OMVs engineered to display programmed death receptor 1 (PD1) on the surface provides a specific tumor-targeting capability. Using a murine breast cancer model, we show that this system has high miRNA delivery efficiency and accurate tumor targeting. Moreover, the miR-34a payloads in carriers can further synergize with immune activation and checkpoint inhibition triggered by OMV-PD1 to enhance tumor therapeutic efficacy. Overall, this biomimetic nano-delivery platform provides a powerful tool for the intracellular delivery of miRNA and has great potential in RNA-based cancer therapeutic applications.

Emerging therapeutic role of gut microbial extracellular vesicles in neurological disorders

Extracellular vesicles (EVs) serve as cell-to-cell and inter-organ communicators by conveying proteins and nucleic acids with regulatory functions. Emerging evidence shows that gut microbial-released EVs play a pivotal role in the gut-brain axis, bidirectional communication, and crosstalk between the gut and the brain. Increasing pre-clinical and clinical evidence suggests that gut bacteria-released EVs are capable of eliciting distinct signaling to the brain with the ability to cross the blood-brain barrier, exerting regulatory function on brain cells such as neurons, astrocytes, and microglia, via their abundant and diversified protein and nucleic acid cargo. Conversely, EVs derived from certain species of bacteria, particularly from gut commensals with probiotic properties, have recently been shown to confer distinct therapeutic effects on various neurological disorders. Thus, gut bacterial EVs may be both a cause of and therapy for neuropathological complications. This review marshals the basic, clinical, and translational studies that significantly contributed to our up-to-date knowledge of the therapeutic potential of gut microbial-derived EVs in treating neurological disorders, including strokes, Alzheimer's and Parkinson's disease, and dementia. The review also discusses the newer insights in recent studies focused on developing superior therapeutic microbial EVs via genetic manipulation and/or dietary intervention.

Detoxified synthetic bacterial membrane vesicles as a vaccine platform against bacteria and SARS-CoV-2

The development of vaccines based on outer membrane vesicles (OMV) that naturally bud off from bacteria is an evolving field in infectious diseases. However, the inherent inflammatory nature of OMV limits their use as human vaccines. This study employed an engineered vesicle technology to develop synthetic bacterial vesicles (SyBV) that activate the immune system without the severe immunotoxicity of OMV. SyBV were generated from bacterial membranes through treatment with detergent and ionic stress. SyBV induced less inflammatory responses in macrophages and in mice compared to natural OMV. Immunization with SyBV or OMV induced comparable antigen-specific adaptive immunity. Specifically, immunization with Pseudomonas aeruginosa-derived SyBV protected mice against bacterial challenge, and this was accompanied by significant reduction in lung cell infiltration and inflammatory cytokines. Further, immunization with Escherichia coli-derived SyBV protected mice against E. coli sepsis, comparable to OMV-immunized group. The protective activity of SyBV was driven by the stimulation of B-cell and T-cell immunity. Also, SyBV were engineered to display the SARS-CoV-2 S1 protein on their surface, and these vesicles induced specific S1 protein antibody and T-cell responses. Collectively, these results demonstrate that SyBV may be a safe and efficient vaccine platform for the prevention of bacterial and viral infections.

High performance anion exchange chromatography purification of probiotic bacterial extracellular vesicles enhances purity and anti-inflammatory efficacy

Bacterial extracellular vesicles (BEVs), including outer membrane vesicles (OMVs), have emerged as a promising new class of vaccines and therapeutics to treat cancer and inflammatory diseases, among other applications. However, clinical translation of BEVs is hindered by a current lack of scalable and efficient purification methods. Here, we address downstream BEV biomanufacturing limitations by developing a method for orthogonal size- and charge-based BEV enrichment using tangential flow filtration (TFF) in tandem with high performance anion exchange chromatography (HPAEC). The data show that size-based separation co-isolated protein contaminants, whereas size-based TFF with charged-based HPAEC dramatically improved purity of BEVs produced by probiotic Gram-negative Escherichia coli and Gram-positive lactic acid bacteria (LAB). E. coli BEV purity was quantified using established biochemical markers while improved LAB BEV purity was assessed via observed potentiation of anti-inflammatory bioactivity. Overall, this work establishes orthogonal TFF + HPAEC as a scalable and efficient method for BEV purification that holds promise for future large-scale biomanufacturing of therapeutic BEV products.

Functionalization of OMVs for Biocatalytic Applications

Outer membrane vesicles (OMVs) are miniature versions of gram-negative bacteria that contain almost the same content as their parent cells, particularly in terms of membrane composition. Using OMVs as biocatalysts is a promising approach due to their potential benefits, including their ability to be handled similarly to bacteria while lacking potentially pathogenic organisms. To employ OMVs as biocatalysts, they must be functionalized with immobilized enzymes to the OMV platform. Various enzyme immobilization techniques are available, including surface display and encapsulation, each with advantages and disadvantages depending on the objectives. This review provides a concise yet comprehensive overview of these immobilization techniques and their applications in utilizing OMVs as biocatalysts. Specifically, we discuss the use of OMVs in catalyzing the conversion of chemical compounds, their role in polymer degradation, and their performance in bioremediation.

Engineered Bacterial Outer Membrane Vesicles with Lipidated Heterologous Antigen as an Adjuvant-Free Vaccine Platform for Streptococcus suis

Bacterial outer membrane vesicles (OMVs) are considered a promising vaccine platform for their high built-in adjuvanticity and ability to efficiently induce immune responses. OMVs can be engineered with heterologous antigens based on genetic engineering strategies. However, several critical issues should still be validated, including optimal exposure to the OMV surface, increased production of foreign antigens, nontoxicity, and induction of powerful immune protection. In this study, engineered OMVs with the lipoprotein transport machinery (Lpp) were designed to present SaoA antigen as a vaccine platform against Streptococcus suis. The results suggest that Lpp-SaoA fusions can be delivered on the OMV surface and do not have significant toxicity. Moreover, they can be engineered as lipoprotein and significantly accumulated in OMVs at high levels, thus accounting for nearly 10% of total OMV proteins. Immunization with OMVs containing Lpp-SaoA fusion antigen induced strong specific antibody responses and high levels of cytokines, as well as a balanced Th1/Th2 immune response. Furthermore, the decorated OMV vaccination significantly enhanced microbial clearance in a mouse infection model. It was found that antiserum against lipidated OMVs significantly promoted the opsonophagocytic uptake of S. suis in RAW246.7 macrophages. Lastly, OMVs engineered with Lpp-SaoA induced 100% protection against a challenge with 8× the 50% lethal dose (LD50) of S. suis serotype 2 and 80% protection against a challenge with 16× the LD50 in mice. Altogether, the results of this study provide a promising versatile strategy for the engineering of OMVs and suggest that Lpp-based OMVs may be a universal adjuvant-free vaccine platform for important pathogens. IMPORTANCE Bacterial outer membrane vesicles (OMVs) have become a promising vaccine platform due to their excellent built-in adjuvanticity properties. However, the location and amount of the expression of the heterologous antigen in the OMVs delivered by the genetic engineering strategies should be optimized. In this study, we exploited the lipoprotein transport pathway to engineer OMVs with heterologous antigen. Not only did lapidated heterologous antigen accumulate in the engineered OMV compartment at high levels, but also it was engineered to be delivered on the OMV surface, thus leading to the optimal activation of antigen-specific B cells and T cells. Immunization with engineered OMVs induced a strong antigen-specific antibodies in mice and conferred 100% protection against S. suis challenge. In general, the data of this study provide a versatile strategy for the engineering of OMVs and suggest that OMVs engineered with lipidated heterologous antigens may be a vaccine platform for significant pathogens.

A modular vaccine platform enabled by decoration of bacterial outer membrane vesicles with biotinylated antigens

Engineered outer membrane vesicles (OMVs) derived from Gram-negative bacteria are a promising technology for the creation of non-infectious, nanoparticle vaccines against diverse pathogens. However, antigen display on OMVs can be difficult to control and highly variable due to bottlenecks in protein expression and localization to the outer membrane of the host cell, especially for bulky and/or complex antigens. Here, we describe a universal approach for avidin-based vaccine antigen crosslinking (AvidVax) whereby biotinylated antigens are linked to the exterior of OMVs whose surfaces are remodeled with multiple copies of a synthetic antigen-binding protein (SNAP) comprised of an outer membrane scaffold protein fused to a biotin-binding protein. We show that SNAP-OMVs can be readily decorated with a molecularly diverse array of biotinylated subunit antigens, including globular and membrane proteins, glycans and glycoconjugates, haptens, lipids, and short peptides. When the resulting OMV formulations are injected in mice, strong antigen-specific antibody responses are observed that depend on the physical coupling between the antigen and SNAP-OMV delivery vehicle. Overall, these results demonstrate AvidVax as a modular platform that enables rapid and simplified assembly of antigen-studded OMVs for application as vaccines against pathogenic threats.

Extracellular vesicles: emerging anti-cancer drugs and advanced functionalization platforms for cancer therapy

Increasing evidences show that unmodified extracellular vesicles (EVs) derived from various cells can effectively inhibit the malignant progression of different types of tumors by delivering the bioactive molecules. Therefore, EVs are expected to be developed as emerging anticancer drugs. Meanwhile, unmodified EVs as an advanced and promising nanocarrier that is frequently used in targeted delivery therapeutic cargos and personalized reagents for the treatment and diagnosis of cancer. To improve the efficacy of EV-based treatments, researchers are trying to engineering EVs as an emerging nanomedicine translational therapy platform through biological, physical and chemical approaches, which can be broaden and altered to enhance their therapeutic capability. EVs loaded with therapeutic components such as tumor suppressor drugs, siRNAs, proteins, peptides, and conjugates exhibit significantly enhanced anti-tumor effects. Moreover, the design and preparation of tumor-targeted modified EVs greatly enhance the specificity and effectiveness of tumor therapy, and these strategies are expected to become novel ideas for tumor precision medicine. This review will focus on reviewing the latest research progress of functionalized EVs, clarifying the superior biological functions and powerful therapeutic potential of EVs, for researchers to explore new design concepts based on EVs and build next-generation nanomedicine therapeutic platforms.

Membrane-Binding Biomolecules Influence the Rate of Vesicle Exchange between Bacteria

The exchange of bacterial extracellular vesicles facilitates molecular exchange between cells, including the horizontal transfer of genetic material. Given the implications of such transfer events on cell physiology and adaptation, some bacterial cells have likely evolved mechanisms to regulate vesicle exchange. Past work has identified mechanisms that influence the formation of extracellular vesicles, including the production of small molecules that modulate membrane structure; however, whether these mechanisms also modulate vesicle uptake and have an overall impact on the rate of vesicle exchange is unknown. Here, we show that membrane-binding molecules produced by microbes influence both the formation and uptake of extracellular vesicles and have the overall impact of increasing the vesicle exchange rate within a bacterial coculture. In effect, production of compounds that increase vesicle exchange rates encourage gene exchange between neighboring cells. The ability of several membrane-binding compounds to increase vesicle exchange was demonstrated. Three of these compounds, nisin, colistin, and polymyxin B, are antimicrobial peptides added at sub-inhibitory concentrations. These results suggest that a potential function of exogenous compounds that bind to membranes may be the regulation of vesicle exchange between cells. IMPORTANCE The exchange of bacterial extracellular vesicles is one route of gene transfer between bacteria, although it was unclear if bacteria developed strategies to modulate the rate of gene transfer within vesicles. In eukaryotes, there are many examples of specialized molecules that have evolved to facilitate the production, loading, and uptake of vesicles. Recent work with bacteria has shown that some small molecules influence membrane curvature and induce vesicle formation. Here, we show that similar compounds facilitate vesicle uptake, thereby increasing the overall rate of vesicle exchange within bacterial populations. The addition of membrane-binding compounds, several of them antibiotics at subinhibitory concentrations, to a bacterial coculture increased the rate of horizontal gene transfer via vesicle exchange.

Bacterial extracellular vesicle applications in cancer immunotherapy

Cancer therapy is undergoing a paradigm shift toward immunotherapy focusing on various approaches to activate the host immune system. As research to identify appropriate immune cells and activate anti-tumor immunity continues to expand, scientists are looking at microbial sources given their inherent ability to elicit an immune response. Bacterial extracellular vesicles (BEVs) are actively studied to control systemic humoral and cellular immune responses instead of using whole microorganisms or other types of extracellular vesicles (EVs). BEVs also provide the opportunity as versatile drug delivery carriers. Unlike mammalian EVs, BEVs have already made it to the clinic with the meningococcal vaccine (Bexsero®). However, there are still many unanswered questions in the use of BEVs, especially for chronic systemically administered immunotherapies. In this review, we address the opportunities and challenges in the use of BEVs for cancer immunotherapy and provide an outlook towards development of BEV products that can ultimately translate to the clinic.

Outer Membrane Vesicles of Actinobacillus pleuropneumoniae Exert Immunomodulatory Effects on Porcine Alveolar Macrophages

Outer membrane vesicles (OMVs) are spontaneously released by Gram-negative bacteria, including Actinobacillus pleuropneumoniae, which causes contagious pleuropneumonia in pigs and leads to considerable economic losses in the swine industry worldwide. A. pleuropneumoniae OMVs have previously been demonstrated to contain Apx toxins and proteases, as well as antigenic proteins. Nevertheless, comprehensive characterizations of their contents and interactions with host immune cells have not been made. Understanding the protein compositions and immunomodulating ability of A. pleuropneumoniae OMVs could help illuminate their biological functions and facilitate the development of OMV-based applications. In the current investigation, we comprehensively characterized the proteome of native A. pleuropneumoniae OMVs. Moreover, we qualitatively and quantitatively compared the OMV proteomes of a wild-type strain and three mutant strains, in which relevant genes were disrupted to increase OMV production and/or produce OMVs devoid of superantigen PalA. Furthermore, the interaction between A. pleuropneumoniae OMVs and porcine alveolar macrophages was also characterized. Our results indicate that native OMVs spontaneously released by A. pleuropneumoniae MIDG2331 appeared to dampen the innate immune responses by porcine alveolar macrophages stimulated by either inactivated or live parent cells. The findings suggest that OMVs may play a role in manipulating the porcine defense during the initial phases of the A. pleuropneumoniae infection. IMPORTANCE Owing to their built-in adjuvanticity and antigenicity, bacterial outer membrane vesicles (OMVs) are gaining increasing attention as potential vaccines for both human and animal use. OMVs released by Actinobacillus pleuropneumoniae, an important respiratory pathogen in pigs, have also been investigated for vaccine development. Our previous studies have shown that A. pleuropneumoniae secretes OMVs containing multiple immunogenic proteins. However, immunization of pigs with these vesicles was not able to relieve the pig lung lesions induced by the challenge with A. pleuropneumoniae, implying the elusive roles that A. pleuropneumoniae OMVs play in host-pathogen interaction. Here, we showed that A. pleuropneumoniae secretes OMVs whose yield and protein content can be altered by the deletion of the nlpI and palA genes. Furthermore, we demonstrate that A. pleuropneumoniae OMVs dampen the immune responses in porcine alveolar macrophages stimulated by A. pleuropneumoniae cells, suggesting a novel mechanism that A. pleuropneumoniae might use to evade host defense.

Bacterial outer membrane vesicles as a candidate tumor vaccine platform

Cancer represents a serious concern for human life and health. Due to drug resistance and the easy metastasis of tumors, there is urgent need to develop new cancer treatment methods beyond the traditional radiotherapy, chemotherapy, and surgery. Bacterial outer membrane vesicles (OMVs) are a type of double-membrane vesicle secreted by Gram-negative bacteria in the process of growth and life, and play extremely important roles in the survival and invasion of those bacteria. In particular, OMVs contain a large number of immunogenic components associated with their parent bacterium, which can be used as vaccines, adjuvants, and vectors to treat diseases, especially in presenting tumor antigens or targeted therapy with small-molecule drugs. Some OMV-based vaccines are already on the market and have demonstrated good therapeutic effect on the corresponding diseases. OMV-based vaccines for cancer are also being studied, and some are already in clinical trials. This paper reviews bacterial outer membrane vesicles, their interaction with host cells, and their applications in tumor vaccines.

Engineering bacterial membrane nanovesicles for improved therapies in infectious diseases and cancer

Research on bacterial membrane vesicles (BMVs) is an emerging topic, and the goal is to address whether BMVs can bring translational tools to improve current therapies. In this review, we provided the updated studies on BMVs including their production, their types, and therapeutic regimens for treating infectious diseases and cancers. We described several platforms of BMVs, such as outer membrane vesicles (OMVs), inner membrane vesicles (IMVs) and double membrane vesicles (DMVs), and those structures were produced from Gram-negative or Gram-positive bacteria. We also discussed how to engineer and formulate new and novel BMVs using chemical, physical, and genetic methods. For therapies, we analyzed current methods for loading drugs in BMVs and discussed their limitations. Finally, we reviewed several therapeutic platforms of BMVs that have been exploited in improving the treatments of infectious diseases and cancers. Although BMVs offer the promising biomedical applications, it is needed to develop rigorous approaches and methods to generate reproducible and scalable drug delivery systems for translation.

Pseudomonas aeruginosa: pathogenesis, virulence factors, antibiotic resistance, interaction with host, technology advances and emerging therapeutics

Pseudomonas aeruginosa (P. aeruginosa) is a Gram-negative opportunistic pathogen that infects patients with cystic fibrosis, burn wounds, immunodeficiency, chronic obstructive pulmonary disorder (COPD), cancer, and severe infection requiring ventilation, such as COVID-19. P. aeruginosa is also a widely-used model bacterium for all biological areas. In addition to continued, intense efforts in understanding bacterial pathogenesis of P. aeruginosa including virulence factors (LPS, quorum sensing, two-component systems, 6 type secretion systems, outer membrane vesicles (OMVs), CRISPR-Cas and their regulation), rapid progress has been made in further studying host-pathogen interaction, particularly host immune networks involving autophagy, inflammasome, non-coding RNAs, cGAS, etc. Furthermore, numerous technologic advances, such as bioinformatics, metabolomics, scRNA-seq, nanoparticles, drug screening, and phage therapy, have been used to improve our understanding of P. aeruginosa pathogenesis and host defense. Nevertheless, much remains to be uncovered about interactions between P. aeruginosa and host immune responses, including mechanisms of drug resistance by known or unannotated bacterial virulence factors as well as mammalian cell signaling pathways. The widespread use of antibiotics and the slow development of effective antimicrobials present daunting challenges and necessitate new theoretical and practical platforms to screen and develop mechanism-tested novel drugs to treat intractable infections, especially those caused by multi-drug resistance strains. Benefited from has advancing in research tools and technology, dissecting this pathogen's feature has entered into molecular and mechanistic details as well as dynamic and holistic views. Herein, we comprehensively review the progress and discuss the current status of P. aeruginosa biophysical traits, behaviors, virulence factors, invasive regulators, and host defense patterns against its infection, which point out new directions for future investigation and add to the design of novel and/or alternative therapeutics to combat this clinically significant pathogen.

Polymerized porin as a novel delivery platform for coronavirus vaccine

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), seriously threatens human life and health. The correct folding and polymerization of the receptor-binding domain (RBD) protein of coronavirus in Escherichia coli may reduce the cost of SARS-CoV-2 vaccines. In this study, we constructed this nanopore by using the principle of ClyA porin polymerization triggered by the cell membrane. We used surfactants to "pick" the ClyA-RBD nanopore from the bacterial outer membrane. More importantly, the polymerized RBD displayed on the ClyA-RBD polymerized porin (RBD-PP) already displays some correct spatial conformational epitopes that can induce neutralizing antibodies. The nanostructures of RBD-PP can target lymph nodes and promote antigen uptake and processing by dendritic cells, thereby effectively eliciting the production of anti-SARS-CoV-2 neutralizing antibodies, systemic cellular immune responses, and memory T cells. We applied this PP-based vaccine platform to fabricate an RBD-based subunit vaccine against SARS-CoV-2, which will provide a foundation for the development of inexpensive coronavirus vaccines. The development of a novel vaccine delivery system is an important part of innovative drug research. This novel PP-based vaccine platform is likely to have additional applications, including other viral vaccines, bacterial vaccines, tumor vaccines, drug delivery, and disease diagnosis.

Bacterial membrane vesicles for vaccine applications

Vaccines have been highly successful in the management of many diseases. However, there are still numerous illnesses, both infectious and noncommunicable, for which there are no clinically approved vaccine formulations. While there are unique difficulties that must be overcome in the case of each specific disease, there are also a number of common challenges that have to be addressed for effective vaccine development. In recent years, bacterial membrane vesicles (BMVs) have received increased attention as a potent and versatile vaccine platform. BMVs are inherently immunostimulatory and are able to activate both innate and adaptive immune responses. Additionally, BMVs can be readily taken up and processed by immune cells due to their nanoscale size. Finally, BMVs can be modified in a variety of ways, including by genetic engineering, cargo loading, and nanoparticle coating, in order to create multifunctional platforms that can be leveraged against different diseases. Here, an overview of the interactions between BMVs and immune cells is provided, followed by discussion on the applications of BMV vaccine nanotechnology against bacterial infections, viral infections, and cancers.

Rapid Surface Display of mRNA Antigens by Bacteria-Derived Outer Membrane Vesicles for a Personalized Tumor Vaccine

Therapeutic mRNA vaccination is an attractive approach to trigger antitumor immunity. However, the mRNA delivery technology for customized tumor vaccine is still limited. In this work, bacteria-derived outer membrane vesicles (OMVs) are employed as an mRNA delivery platform by genetically engineering with surface decoration of RNA binding protein, L7Ae, and lysosomal escape protein, listeriolysin O (OMV-LL). OMV-LL can rapidly adsorb box C/D sequence-labelled mRNA antigens through L7Ae binding (OMV-LL-mRNA) and deliver them into dendritic cells (DCs), following by the cross-presentation via listeriolysin O-mediated endosomal escape. OMV-LL-mRNA significantly inhibits melanoma progression and elicits 37.5% complete regression in a colon cancer model. OMV-LL-mRNA induces a long-term immune memory and protects the mice from tumor challenge after 60 days. In summary, this platform provides a delivery technology distinct from lipid nanoparticles (LNPs) for personalized mRNA tumor vaccination, and with a "Plug-and-Display" strategy that enables its versatile application in mRNA vaccines.

Cytolysin A (ClyA): A Bacterial Virulence Factor with Potential Applications in Nanopore Technology, Vaccine Development, and Tumor Therapy

Cytolysin A (ClyA) is a pore-forming toxin that is produced by some bacteria from the Enterobacteriaceae family. This review provides an overview of the current state of knowledge regarding ClyA, including the prevalence of the encoding gene and its transcriptional regulation, the secretion pathway used by the protein, and the mechanism of protein assembly, and highlights potential applications of ClyA in biotechnology. ClyA expression is regulated at the transcriptional level, primarily in response to environmental stressors, and ClyA can exist stably both as a soluble monomer and as an oligomeric membrane complex. At high concentrations, ClyA induces cytolysis, whereas at low concentrations ClyA can affect intracellular signaling. ClyA is secreted in outer membrane vesicles (OMVs), which has important implications for biotechnology applications. For example, the native pore-forming ability of ClyA suggests that it could be used as a component of nanopore-based technologies, such as sequencing platforms. ClyA has also been exploited in vaccine development owing to its ability to present antigens on the OMV surface and provoke a robust immune response. In addition, ClyA alone or OMVs carrying ClyA fusion proteins have been investigated for their potential use as anti-tumor agents.

Engineered Bacteria Enhance Immunotherapy and Targeted Therapy through Stromal Remodeling of Tumors

Desmoplastic solid tumors are characterized by the rapid build-up of extracellular matrix (ECM) macromolecules, such as hyaluronic acid (HA). The resulting physiological barrier prevents the infiltration of immune cells and also impedes the delivery of anticancer agents. The development of a hypervesiculating Escherichia coli Nissle (ΔECHy) based tumor targeting bacterial system capable of distributing a fusion peptide, cytolysin A (ClyA)-hyaluronidase (Hy) via outer membrane vesicles (OMVs) is reported. The capability of targeting hypoxic tumors, manufacturing recombinant proteins in situ and the added advantage of an on-site OMV based distribution system makes the engineered bacterial vector a unique candidate for peptide delivery. The HA degrading potential of Hy for stromal modulation is combined with the cytolytic activity of ClyA followed by testing it within syngeneic cancer models. ΔECHy is combined with immune checkpoint antibodies and tyrosine kinase inhibitors (TKIs) to demonstrate that remodeling the tumor stroma results in the improvement of immunotherapy outcomes and enhancing the efficacy of biological signaling inhibitors. The biocompatibility of ΔECHy is also investigated to show that the engineered bacteria are effectively cleared, elicit minimal inflammatory and immune responses, and therefore could be a reliable candidate as a live biotherapeutic.

Nanotechnology-Employed Bacteria-Based Delivery Strategy for Enhanced Anticancer Therapy

Bacteria and their derivatives (membrane vesicles, MVs) exhibit great advantages for targeting hypoxic tumor cores, strong penetration ability and activating immune responses, holding great potential as auspicious candidates for therapeutic and drug-delivery applications. However, the safety issues and low therapeutic efficiency by single administration still need to be solved. To further optimize their performance and to utilize their natural abilities, scientists have strived to modify bacteria with new moieties on their surface while preserving their advantages. The aim of this review is to give a comprehensive overview of a non-genetic engineering modification strategy that can be used to optimize the bacteria with nanomaterials and the design strategy that can be used to optimize MVs for better targeted therapy. Here, the advantages and disadvantages of these processes and their applicability for the development of bacteria-related delivery system as antitumor therapeutic agents are discussed. The prospect and the challenges of the above targeted delivery system are also proposed.

Surface Engineering of Escherichia coli-Derived OMVs as Promising Nano-Carriers to Target EGFR-Overexpressing Breast Cancer Cells

Bacterial outer membrane vesicles (OMVs) have recently drawn a great deal of attention due to their therapeutic efficiency and ability to target specific cells. In the present study, we sought to probe engineered OMVs as novel and promising carriers to target breast cancer cells. Following the fusion of the affi_EGFR-GALA structure to the C-terminal of ClyA as an anchor protein, the ClyA-affi_EGFR-GALA construct was successfully expressed on the surface of ∆msbB/∆pagP E. coli W3110-derived OMVs. Morphological features of the engineered and wild-type OMVs were identical. The engineered OMVs induced no endotoxicity, cytotoxicity, or immunogenicity, indicating the safety of their application. These OMVs could specifically bind to EGF receptors of MDA-MB-468 cells expressing high levels of EGFR and not to those with low levels of EGFR (HEK293T cells). Interestingly, despite a lower binding affinity of the engineered OMVs relative to the positive control Cetuximab, it was strong enough to identify these cells. Moreover, confocal microscopy revealed no uptake of the modified OMVs by the EGFR-overexpressing cells in the presence of EGFR competitors. These results suggest that OMVs might internalize into the cells with EGF receptors, as no OMVs entered the cells with any EGFR expression or those pretreated with EGF or Cetuximab. Regarding the EGFR-binding affinity of the engineered OMVs and their cellular uptake, they are presented here as a potential carrier for cell-specific drug delivery to treat a wide variety of cancer cells. Interestingly, the engineered OMVs are capable of reaching the cytoplasm while escaping the endosome due to the incorporation of a fusogenic GALA peptide in the construct.

Membrane Vesicles of Pectobacterium as an Effective Protein Secretion System

Bacteria of genus Pectobacterium are Gram-negative rods of the family Pectobacteriaceae. They are the causative agent of soft rot diseases of crops and ornamental plants. However, their virulence mechanisms are not yet fully elucidated. Membrane vesicles (MVs) are universally released by bacteria and are believed to play an important role in the pathogenicity and survival of bacteria in the environment. Our study investigates the role of MVs in the virulence of Pectobacterium. The results indicate that the morphology and MVs production depend on growth medium composition. In polygalacturonic acid (PGA) supplemented media, Pectobacterium produces large MVs (100–300 nm) and small vesicles below 100 nm. Proteomic analyses revealed the presence of pectate degrading enzymes in the MVs. The pectate plate test and enzymatic assay proved that those enzymes are active and able to degrade pectates. What is more, the pathogenicity test indicated that the MVs derived from Pectobacterium were able to induce maceration of Zantedeschia sp. leaves. We also show that the MVs of β-lactamase producing strains were able to suppress ampicillin activity and permit the growth of susceptible bacteria. Those findings indicate that the MVs of Pectobacterium play an important role in host-pathogen interactions and niche competition with other bacteria. Our research also sheds some light on the mechanism of MVs production. We demonstrate that the MVs production in Pectobacterium strains, which overexpress a green fluorescence protein (GFP), is higher than in wild-type strains. Moreover, proteomic analysis revealed that the GFP was present in the MVs. Therefore, it is possible that protein sequestration into MVs might not be strictly limited to periplasmic proteins. Our research highlights the importance of MVs production as a mechanism of cargo delivery in Pectobacterium and an effective secretion system.

Robust Immune Response Induced by Schistosoma mansoni TSP-2 Antigen Coupled to Bacterial Outer Membrane Vesicles

Purpose

The use of adjuvants can significantly strengthen a vaccine’s efficacy. We sought to explore the immunization efficacy of bacterial outer membrane vesicles (OMVs) displaying the Schistosoma mansoni antigen, SmTSP-2, through a biotin-rhizavidin coupling approach. The rationale is to exploit the nanoparticulate structure and the adjuvant properties of OMVs to induce a robust antigen-specific immune response, in light of developing new vaccines against S. mansoni.

Materials and Methods

OMVs were obtained from Neisseria lactamica and conjugated with biotin. The recombinant SmTSP-2 in fusion with the biotin-binding protein rhizavidin (rRzvSmTSP-2) was produced in E. coli and coupled to biotinylated OMVs to generate an OMV complex displaying SmTSP-2 on the membrane surface (OMV:rSmTSP-2). Transmission electron microscopy (TEM) and dynamic light scattering analysis were used to determine particle charge and size. The immunogenicity of the vaccine complex was evaluated in C57BL/6 mice.

Results

The rRzvSmTSP-2 protein was successfully coupled to biotinylated OMVs and purified by size-exclusion chromatography. The OMV:rSmTSP-2 nanoparticles showed an average size of 200 nm, with zeta potential around – 28 mV. Mouse Bone Marrow Dendritic Cells were activated by the nanoparticles as determined by increased expression of the co-stimulatory molecules CD40 and CD86, and the proinflammatory cytokines (TNF-α, IL-6 and IL-12) or IL-10. Splenocytes of mice immunized with OMV:rSmTSP-2 nanoparticles reacted to an in vitro challenge with SmTSP-2 with an increased production of IL-6, IL-10 and IL-17 and displayed a higher number of CD4+ and CD8+ T lymphocytes expressing IFN-γ, IL-4 and IL-2, compared to mice immunized with the antigen alone. Immunization of mice with OMV:rSmTSP-2 induced a 100-fold increase in specific anti-SmTSP-2 IgG antibody titers, as compared to the group receiving the recombinant rSmTSP-2 protein alone or even co-administered with unconjugated OMV.

Conclusion

Our results demonstrate that the SmTSP-2 antigen coupled with OMVs is highly immunogenic in mice, supporting the potential effectiveness of this platform for improved antigen delivery in novel vaccine strategies.

A Novel Immunomodulator Delivery Platform Based on Bacterial Biomimetic Vesicles for Enhanced Antitumor Immunity

T cell activation-induced cell death (AICD) during tumor pathogenesis is a tumor immune escape process dependent on dendritic cells (DCs). Proper immune-modulatory therapies effectively inhibit tumor-specific CD8+ T cell exhaustion and enhance antitumor immune responses. Here, high-pressure homogenization is utilized to drive immunomodulator IL10-modified bacteria to extrude through the gap and self-assemble into bacterial biomimetic vesicles exposing IL10 (IL10-BBVs) on the surface with high efficiency. IL10-BBVs efficiently target DCs in tumor-draining lymph nodes and thus increase the interaction between IL10 on BBVs and IL10R on DCs to suppress AICD and mitigate CD8+ T cell exhaustion specific to tumor antigens. Two subcutaneous peripheral injections of IL10-BBVs 1 week apart in tumor-bearing mice effectively increase systemic and intratumoral proportions of CD8+ T cells to suppress tumor growth and metastasis. Tumor-specific antigen E7 is enclosed into the periplasm of IL10-BBVs (IL10-E7-BBVs) to realize concurrent actions of the immunomodulator IL10 and the tumor antigen human papillomavirus (HPV) 16E7 in lymph nodes, further enhancing the antitumor effects mediated by CD8+ T cells. The development of this modified BBV delivery platform will expand the application of bacterial membranes and provide novel immunotherapeutic strategies for tumor treatment.

Bacterial Outer Membrane Vesicles as Antibiotic Delivery Vehicles

Bacterial outer membrane vesicles (OMVs) are nanometer-scale, spherical vehicles released by Gram-negative bacteria into their surroundings throughout growth. These OMVs have been demonstrated to play key roles in pathogenesis by delivering certain biomolecules to host cells, including toxins and other virulence factors. In addition, this biomolecular delivery function enables OMVs to facilitate intra-bacterial communication processes, such as quorum sensing and horizontal gene transfer. The unique ability of OMVs to deliver large biomolecules across the complex Gram-negative cell envelope has inspired the use of OMVs as antibiotic delivery vehicles to overcome transport limitations. In this review, we describe the advantages, applications, and biotechnological challenges of using OMVs as antibiotic delivery vehicles, studying both natural and engineered antibiotic applications of OMVs. We argue that OMVs hold great promise as antibiotic delivery vehicles, an urgently needed application to combat the growing threat of antibiotic resistance.

Bacterial Outer Membrane Vesicles as a Versatile Tool in Vaccine Research and the Fight against Antimicrobial Resistance

Gram-negative bacteria include a number of pathogens that cause disease in humans and animals. Although antibiotics are still effective in treating a considerable range of infections caused by Gram-negative bacteria, the alarming increase of antimicrobial resistance (AMR) induced by excessive use of antibiotics has raised global concerns. Therefore, alternative strategies must be developed to prevent and treat bacterial infections and prevent the advent of a postantibiotic era. Vaccines, one of the greatest achievements in the history of medical science, hold extraordinary potential to prevent bacterial infections and thereby reduce the need for antibiotics. Novel bacterial vaccines are urgently needed, however, and outer membrane vesicles (OMVs), naturally produced by Gram-negative bacteria, represent a promising and versatile tool that can be employed as adjuvants, antigens, and delivery platforms in the development of vaccines against Gram-negative bacteria. Here, we provide an overview of the many roles OMVs can play in vaccine development and the mechanisms behind these applications. Methods to improve OMV yields and a comparison of different strategies for OMV isolation aiming at cost-effective production of OMV-based vaccines are also reviewed.

Bioengineered bacterial vesicles for optoacoustics-guided phototherapy

Genetically engineered bacterial outer membrane vesicles (OMVs) offer promising applications for gene therapy, immunotherapy, and vaccine delivery. Importantly, OMVs are biocompatible, biodegradable, and easy to engineer and produce on a large scale. In this chapter, we discuss the development and application of bioengineered OMVs for optoacoustics-guided phototherapy applications (theranostics). We provide detailed protocols for OMVs preparation, characterization, and in vitro and in vivo validation. The engineered OMVs carry the biopolymer melanin, which generates a strong optoacoustic (OA) signal and intense heat upon absorption of near-infrared (NIR) light, enabling optoacoustics-guided cancer diagnosis and photothermal therapy in vivo.

Neisseria meningitidis Factor H Binding Protein Surface Exposure on Salmonella Typhimurium GMMA Is Critical to Induce an Effective Immune Response against Both Diseases

GMMA, outer membrane vesicles resulting from hyperblebbing mutated bacterial strains, are a versatile vaccine platform for displaying both homologous and heterologous antigens. Periplasmic expression is a popular technique for protein expression in the lumen of the blebs. However, the ability of internalized antigens to induce antibody responses has not been extensively investigated. Herein, the Neisseria meningitidis factor H binding protein (fHbp) was heterologously expressed in the lumen of O-antigen positive (OAg+) and O-antigen negative (OAg−) Salmonella Typhimurium GMMA. Only the OAg− GMMA induced an anti-fHbp IgG response in mice if formulated on Alum, although it was weak and much lower compared to the recombinant fHbp. The OAg− GMMA on Alum showed partial instability, with possible exposure of fHbp to the immune system. When we chemically conjugated fHbp to the surface of both OAg+ and OAg− GMMA, these constructs induced a stronger functional response compared to the fHbp immunization alone. Moreover, the OAg+ GMMA construct elicited a strong response against both the target antigens (fHbp and OAg), with no immune interference observed. This result suggests that antigen localization on GMMA surface can play a critical role in the induction of an effective immune response and can encourage the development of GMMA based vaccines delivering key protective antigens on their surface.

Outer membrane vesicles as versatile tools for therapeutic approaches

Budding of the bacterial surface results in the formation and secretion of outer membrane vesicles, which is a conserved phenomenon observed in Gram-negative bacteria. Recent studies highlight that these sphere-shaped facsimiles of the donor bacterium's surface with enclosed periplasmic content may serve multiple purposes for their host bacterium. These include inter- and intraspecies cell-cell communication, effector delivery to target cells and bacterial adaptation strategies. This review provides a concise overview of potential medical applications to exploit outer membrane vesicles for therapeutic approaches. Due to the fact that outer membrane vesicles resemble the surface of their donor cells, they represent interesting nonliving candidates for vaccine development. Furthermore, bacterial donor species can be genetically engineered to display various proteins and glycans of interest on the outer membrane vesicle surface or in their lumen. Outer membrane vesicles also possess valuable bioreactor features as they have the natural capacity to protect, stabilize and enhance the activity of luminal enzymes. Along these features, outer membrane vesicles not only might be suitable for biotechnological applications but may also enable cell-specific delivery of designed therapeutics as they are efficiently internalized by nonprofessional phagocytes. Finally, outer membrane vesicles are potent modulators of our immune system with pro- and anti-inflammatory properties. A deeper understanding of immunoregulatory effects provoked by different outer membrane vesicles is the basis for their possible future applications ranging from inflammation and immune response modulation to anticancer therapy.

Bacterial extracellular vesicles: Understanding biology promotes applications as nanopharmaceuticals

Extracellular vesicle (EV)-mediated communication between proximal and distant cells is a highly conserved characteristic in all of the life domains, including bacteria. These vesicles that contain a variety of biomolecules, such as proteins, lipids, nucleic acids, and small-molecule metabolites play a key role in the biology of bacteria. They are one of the key underlying mechanisms behind harmful or beneficial effects of many pathogenic, symbiont, and probiotic bacteria. These nanoscale EVs mediate extensive crosstalk with mammalian cells and deliver their cargos to the host. They are stable in physiological condition, can encapsulate diverse biomolecules and nanoparticles, and their surface could be engineered with available technologies. Based on favorable characteristics of bacterial vesicles, they can be harnessed for designing a diverse range of therapeutics and diagnostics for treatment of disorders including tumors and resistant infections. However, technical limitations for their production, purification, and characterization must be addressed in future studies.

Extracellular vesicles as antigen carriers for novel vaccination avenues

Antigen delivery has always been a challenge in scientific practice of vaccine formulation. Yet, mammalian extracellular vesicles (EVs) or bacterial membrane vesicles (MVs) provide an innovative avenue for safe and effective delivery of antigenic material. They include intrinsically loaded antigens from EV-secreting cells or extrinsically loaded antigens onto pre-formed vesicles. Interestingly, many studies shed light on potential novel anti-cancer vaccination immunotherapy for therapeutic applications from mammalian cell host-derived EVs, as well as conventional vaccination for prophylactic applications using bacterial cell-derived MVs against infectious diseases. Here, we discuss the rationale, status quo and potential for both vaccine applications using EVs.

Fight bacteria with bacteria: Bacterial membrane vesicles as vaccines and delivery nanocarriers against bacterial infections

Bacterial membrane vesicles (MVs) are particles secreted by bacteria with diameter of 20-400 nm. The pathogen-associated molecular patterns (PAMPs) present on the surface of MVs are capable of activating human immune system, leading to non-specific immune response and specific immune response. Due to the immunostimulatory properties and proteoliposome nanostructures, MVs have been increasingly explored as vaccines or delivery systems for the prevention and treatment of bacterial infections. Herein, the recent progresses of MVs for antibacterial applications are reviewed to provide an overview of MVs vaccines and MVs-related delivery systems. In addition, the safety issues of bacterial MVs are discussed to demonstrate their potential for clinical translation. In the end of this review, the challenges of bacterial MVs as vaccines and delivery systems for clinical applications are highlighted with the purpose of predicting future research directions in this field.

Bacterial Extracellular Vesicles and the Gut-Microbiota Brain Axis: Emerging Roles in Communication and Potential as Therapeutics

Bacterial extracellular vesicles (BEVs) have emerged as candidate signaling vectors for long-distance interkingdom communication within the gut-microbiota brain axis. Most bacteria release these nanosized vesicles, capable of signaling to the brain via their abundant protein and small RNA cargo, possibly directly via crossing the blood-brain barrier. BEVs have been shown to regulate brain gene expression and induce pathology at most stages of neuroinflammation and neurodegeneration, and thus they may play a causal role in diseases such as Alzheimer's, Parkinson's, and depression/anxiety. On the other hand, BEVs have intrinsic therapeutic properties that may be relevant to probiotic therapy and can also be engineered to function as drug delivery vehicles and vaccines. Thus, BEVs may be both a cause of and solution to neuropathological conditions. In this review, current knowledge of the physiological roles of BEVs as well as state of the art pertaining to the development of therapeutic BEVs in the context of the microbiome-gut-brain axis are summarized.

Outer Membrane Vesicles (OMVs) Produced by Gram-Negative Bacteria: Structure, Functions, Biogenesis, and Vaccine Application

Gram-negative bacteria produce outer membrane vesicles (OMVs) with 10 to 300 nm of diameter. The contribution of OMVs to bacterial pathogenesis is a topic of great interest, and their capacity to be combined with antigens impact in the future to the development of vaccines.

Immunomodulatory roles and novel applications of bacterial membrane vesicles

Bacteria release extracellular vesicles (EVs) known as bacterial membrane vesicles (BMVs) during their normal growth. Gram-negative bacteria produce BMVs termed outer membrane vesicles (OMVs) that are composed of a range of biological cargo and facilitate numerous bacterial functions, including promoting pathogenesis and mediating disease in the host. By contrast, less is understood about BMVs produced by Gram-positive bacteria, which are referred to as membrane vesicles (MVs), however their contribution to mediating bacterial pathogenesis has recently become evident. In this review, we summarise the mechanisms whereby BMVs released by Gram-negative and Gram-positive bacteria are produced, in addition to discussing their key functions in promoting bacterial survival, mediating pathogenesis and modulating host immune responses. Furthermore, we discuss the mechanisms whereby BMVs produced by both commensal and pathogenic organisms can enter host cells and interact with innate immune receptors, in addition to how they modulate host innate and adaptive immunity to promote immunotolerance or drive the onset and progression of disease. Finally, we highlight current and emerging applications of BMVs in vaccine design, biotechnology and cancer therapeutics.

A GMMA-CPS-Based Vaccine for Non-Typhoidal Salmonella

Non-typhoidal Salmonella are a major cause of gastroenteritis worldwide, as well as causing bloodstream infections in sub-Saharan Africa with a high fatality rate. No vaccine is currently available for human use. Current vaccine development strategies are focused on capsular polysaccharides (CPS) present on the surface of non-typhoidal Salmonella. This study aimed to boost the amount of CPS purified from S. Typhimurium for immunization trials. Random mutagenesis with Tn10 transposon increased the production of CPS colanic acid, by 10-fold compared to wildtype. Immunization with colanic acid or colanic acid conjugated to truncated glycoprotein D or inactivated diphtheria toxin did not induce a protective immune response in mice. However, immunization with Generalized Modules for Membrane Antigens (GMMAs) isolated from colanic acid overproducing isolates reduced Salmonella colonization in mice. Our results support the development of a GMMA-CPS-based vaccine against non-typhoidal Salmonella.

Pathogenesis Mediated by Bacterial Membrane Vesicles

The release of extracellular vesicles (EVs) is a process conserved across the three domains of life. Amongst prokaryotes, EVs produced by Gram-negative bacteria, termed outer membrane vesicles (OMVs), were identified more than 50 years ago and a wealth of literature exists regarding their biogenesis, composition and functions. OMVs have been implicated in benefiting numerous metabolic functions of their parent bacterium. Additionally, OMVs produced by pathogenic bacteria have been reported to contribute to pathology within the disease setting. By contrast, the release of EVs from Gram-positive bacteria, known as membrane vesicles (MVs), has only been widely accepted within the last decade. As such, there is a significant disproportion in knowledge regarding MVs compared to OMVs. Here we provide an overview of the literature regarding bacterial membrane vesicles (BMVs) produced by pathogenic and commensal bacteria. We highlight the mechanisms of BMV biogenesis and their roles in assisting bacterial survival, in addition to discussing their functions in promoting disease pathologies and their potential use as novel therapeutic strategies.