Cysteine depletion causes oxidative stress and triggers outer membrane vesicle release by Neisseria meningitidis; implications for vaccine development

Bioprocessing strategies for enhanced probiotic extracellular vesicle production: culture condition modulation

Probiotic extracellular vesicles are biochemically active structures responsible for biological effects elicited by probiotic bacteria. Lactobacillus spp., which are abundant in the human body (e.g., gut), are known to have anti-inflammatory and antimicrobial properties, and are commonly used in food products, supplements, and in discovery research. There is increasing evidence that Lactobacillus-derived extracellular vesicles (LREVs) have potent immunomodulatory capacity that is superior to probiotics themselves. However, key mechanistic insights into the process that controls production and thus, the function of LREVs, are lacking. Currently, it is unknown how the probiotic culture microenvironment orchestrates the type, yield and function of LREVs. Here, we investigated how multifactor modulation of the biomanufacturing process controls the yield and biological functionality of the LREVs. To achieve this, we selected Lacticaseibacillus rhamnosus as the candidate probiotic, initially cultivated under traditional culture conditions, i.e., 100% broth concentration and pH 5.5. Subsequently, we systematically modified the culture conditions of the probiotic by adjusting three critical process parameters: (1) culture medium pH (pH 3.5, 5.5 and 7.5), (2) growth time (48 and 72 h), and (3) broth concentration (50% and 10% of original broth concentration). EVs were then isolated separately from each condition. The critical quality attributes (CQA) of LREVs, including physical characteristics (size, distribution, concentration) and biological composition (protein, carbohydrate, lipid), were analysed. Functional impacts of LREVs on human epidermal keratinocytes and Staphylococcus aureus were also assessed as CQA. Our findings show that the production of LREVs is influenced by environmental stresses induced by the culture conditions. Factors like broth concentration, pH levels, and growth time significantly impact stress levels in L. rhamnosus, affecting both the production and composition of LREVs. Additionally, we have observed that LREVs are non-toxicity for keratinocytes, the major cell type of the epidermis, and possess antimicrobial properties against S. aureus, a common human skin pathogen. These properties are prerequisites for the potential application of EVs to treat skin conditions, including infected wounds. However, the functionality of LREVs depends on the culture conditions and stress levels experienced by L. rhamnosus during production. Understanding this relationship between the culture microenvironment, probiotic stress response, and LREV characteristics, can lead to the biomanufacturing of customised probiotic-derived EVs for various medical and industrial applications.

Covalent coupling of functionalized outer membrane vesicles (OMVs) to gold nanoparticles

Outer membrane vesicle-functionalized nanoparticles (OMV-NPs) have attracted significant interest, especially regarding drug delivery applications and vaccines. Here, we report on novel OMV-NPs by applying bioorthogonal click reaction for encapsulating gold nanoparticles (NPs) within outer membrane vesicles (OMVs) by covalent coupling. For this purpose, outer membrane protein A (OmpA), abundant in large numbers (due to 100,000 copies/cell [1]) in OMVs, was modified via the incorporation of the unnatural amino acid p-azidophenylalanine. The azide group was covalently coupled to alkyne-functionalized NPs after incorporation into OmpA. A simplified procedure using low-speed centrifugation (1,000 x g) was developed for preparing OMV-NPs. The OMV-NPs were characterized by zeta potential, Laurdan-based lipid membrane dynamics studies, and the enzymatic activity of functionalized OMVs with surface-displayed nicotinamide adenine dinucleotide oxidase (Nox). In addition, OMVs from attenuated bacteria (ClearColiTM BL21(DE3), E. coli F470) with surface-displayed Nox or antibody fragments were prepared and successfully coupled to AuNPs. Finally, OMV-NPs displaying single-chain variable fragments from a monoclonal antibody directed against epidermal growth factor receptor were applied to demonstrate the feasibility of OMV-NPs for tumor cell targeting.

Transcending membrane barriers: advances in membrane engineering to enhance the production capacity of microbial cell factories

Microbial cell factories serve as pivotal platforms for the production of high-value natural products, which tend to accumulate on the cell membrane due to their hydrophobic properties. However, the limited space of the cell membrane presents a bottleneck for the accumulation of these products. To enhance the production of intracellular natural products and alleviate the burden on the cell membrane caused by product accumulation, researchers have implemented various membrane engineering strategies. These strategies involve modifying the membrane components and structures of microbial cell factories to achieve efficient accumulation of target products. This review summarizes recent advances in the application of membrane engineering technologies in microbial cell factories, providing case studies involving Escherichia coli and yeast. Through these strategies, researchers have not only improved the tolerance of cells but also optimized intracellular storage space, significantly enhancing the production efficiency of natural products. This article aims to provide scientific evidence and references for further enhancing the efficiency of similar cell factories.

Bacterial extracellular vesicles: biotechnological perspective for enhanced productivity

Bacterial extracellular vesicles (BEVs) are non-replicative nanostructures released by Gram-negative and Gram-positive bacteria as a survival mechanism and inter- and intraspecific communication mechanism. Due to BEVs physical, biochemical, and biofunctional characteristics, there is interest in producing and using them in developing new therapeutics, vaccines, or delivery systems. However, BEV release is typically low, limiting their application. Here, we provide a biotechnological perspective to enhance BEV production, highlighting current strategies. The strategies include the production of hypervesiculating strains through gene modification, bacteria culture under stress conditions, and artificial vesicles production. We discussed the effect of these production strategies on BEVs types, morphology, composition, and activity. Furthermore, we summarized general aspects of BEV biogenesis, functional capabilities, and applications, framing their current importance and the need to produce them in abundance. This review will expand the knowledge about the range of strategies associated with BEV bioprocesses to increase their productivity and extend their application possibilities.

BACTERIAL EXTRACELLULAR VESICLES IN THE REGULATION OF INFLAMMATORY RESPONSE AND HOST-MICROBE INTERACTIONS

ABSTRACT

Extracellular vesicles (EVs) are a new revelation in cross-kingdom communication, with increasing evidence showing the diverse roles of bacterial EVs (BEVs) in mammalian cells and host-microbe interactions. Bacterial EVs include outer membrane vesicles released by gram-negative bacteria and membrane vesicles generated from gram-positive bacteria. Recently, BEVs have drawn attention for their potential as biomarkers and therapeutic tools because they are nano-sized and can deliver bacterial cargo into host cells. Importantly, exposure to BEVs significantly affects various physiological and pathological responses in mammalian cells. Herein, we provide a comprehensive overview of the various effects of BEVs on host cells (i.e., immune cells, endothelial cells, and epithelial cells) and inflammatory/infectious diseases. First, the biogenesis and purification methods of BEVs are summarized. Next, the mechanisms and pathways identified by BEVs that stimulate either proinflammatory or anti-inflammatory responses are highlighted. In addition, we discuss the mechanisms by which BEVs regulate host-microbe interactions and their effects on the immune system. Finally, this review focuses on the contribution of BEVs to the pathogenesis of sepsis/septic shock and their therapeutic potential for the treatment of sepsis.

Engineered bacterial outer membrane vesicles: a versatile bacteria-based weapon against gastrointestinal tumors

Outer membrane vesicles (OMVs) are nanoscale lipid bilayer structures released by gram-negative bacteria. They share membrane composition and properties with their originating cells, making them adept at traversing cellular barriers. These OMVs have demonstrated exceptional membrane stability, immunogenicity, safety, penetration, and tumor-targeting properties, which have been leveraged in developing vaccines and drug delivery systems. Recent research efforts have focused on engineering OMVs to increase production yield, reduce cytotoxicity, and improve the safety and efficacy of treatment. Notably, gastrointestinal (GI) tumors have proven resistant to several traditional oncological treatment strategies, including chemotherapy, radiotherapy, and targeted therapy. Although immune checkpoint inhibitors have demonstrated efficacy in some patients, their usage as monotherapy remains limited by tumor heterogeneity and individual variability. The immunogenic and modifiable nature of OMVs makes them an ideal design platform for the individualized treatment of GI tumors. OMV-based therapy enables combination therapy and optimization of anti-tumor effects. This review comprehensively summarizes recent advances in OMV engineering for GI tumor therapy and discusses the challenges in the clinical translation of emerging OMV-based anti-tumor therapies.

Cysteine and resistance to oxidative stress: implications for virulence and antibiotic resistance

Reactive oxygen species (ROS), including the superoxide radical anion (O2•-), hydrogen peroxide (H2O2), and the hydroxyl radical (•HO), are inherent components of bacterial metabolism in an aerobic environment. Bacteria also encounter exogenous ROS, such as those produced by the host cells during the respiratory burst. As ROS have the capacity to damage bacterial DNA, proteins, and lipids, detoxification of ROS is critical for bacterial survival. It has been recently recognised that low-molecular-weight (LMW) thiols play a central role in this process. Here, we review the emerging role of cysteine in bacterial resistance to ROS with a link to broader elements of bacterial lifestyle closely associated with cysteine-mediated oxidative stress response, including virulence and antibiotic resistance.

Gut-bacteria derived membrane vesicles and host metabolic health: a narrative review

The intestinal microbiota, consisting of an estimated 10^10-10^11 organisms, regulate physiological processes involved in digestion, metabolism, and immunity. Surprisingly, these intestinal microorganisms have been found to influence tissues that are not directly in contact with the gut, such as adipose tissue, the liver, skeletal muscle, and the brain. This interaction takes place even when intestinal barrier function is uncompromised. An increasing body of evidence suggests that bacterial membrane vesicles (bMVs), in addition to bacterial metabolites such as short-chain fatty acids, are able to mediate effects of the microbiota on these host tissues. The ability of bMVs to dissipate from the intestinal lumen into systemic circulation hereby facilitates the transport and presentation of bacterial components and metabolites to host organs. Importantly, there are indications that the interaction between bMVs and tissues or immune cells may play a role in the etiology of (chronic metabolic) disease. For example, the gut-derived bMV-mediated induction of insulin resistance in skeletal muscle cells and pro-inflammatory signaling by adipocytes possibly underlies diseases such as type 2 diabetes and obesity. Here, we review the current knowledge on bMVs in the microbiota's effects on host energy/substrate metabolism with a focus on etiological roles in the onset and progression of metabolic disease. We furthermore illustrate that vesicle production by bacterial microbiota could potentially be modulated through lifestyle intervention to improve host metabolism.

Proteome and immune responses of extracellular vesicles derived from macrophages infected with the periodontal pathogen Tannerella forsythia

Periodontitis is a chronic inflammatory disease caused by periodontal pathogens in subgingival plaque and is associated with systemic inflammatory diseases. Extracellular vesicles (EVs) released from host cells and pathogens carry a variety of biological molecules and are of interest for their role in disease progression and as diagnostic markers. In the present study, we analysed the proteome and inflammatory response of EVs derived from macrophages infected with Tannerella forsythia, a periodontal pathogen. The EVs isolated from the cell conditioned medium of T. forsythia-infected macrophages were divided into two distinct vesicles, macrophage-derived EVs and T. forsythia-derived OMVs, by size exclusion chromatography combined with density gradient ultracentrifugation. Proteome analysis showed that in T. forsythia infection, macrophage-derived EVs were enriched with pro-inflammatory cytokines and inflammatory mediators associated with periodontitis progression. T. forsythia-derived OMVs harboured several known virulence factors, including BspA, sialidase, GroEL and various bacterial lipoproteins. T. forsythia-derived OMVs induced pro-inflammatory responses via TLR2 activation. In addition, we demonstrated that T. forsythia actively released OMVs when T. forsythia encountered macrophage-derived soluble molecules. Taken together, our results provide insight into the characterisation of EVs derived from cells infected with a periodontal pathogen.

Calcium phosphate nanoparticles conjugated with outer membrane vesicle of Riemerella anatipestifer for vaccine development in ducklings

Biodegradable calcium phosphate nanoparticles offer a viable substitute for traditional adjuvants such as aluminum in vaccine production. Calcium phosphate nanoparticle adjuvanted with outer membrane vesicle (OMV) of gram negative bacteria may induce efficient immune response in the host. The present study was carried out to evaluate the potential of a mucosal vaccine formulation of calcium phosphate (CAP) nanoparticle using OMV of Riemerella anatipestifer (RA) as antigen against New Duck disease in ducks. The work was initiated with isolation, identification of RA, followed by OMV production and extraction. The CAP-OMV nanoparticle was prepared and characterized. The efficacy of the vaccine formulation and toxicity were studied in ducks. The average OMV yield in terms of protein concentration was found to be 122.33 ± 3.48 mg per liter of BHI broth. In SDS-PAGE, isolated OMVs exhibited presence of 16 distinct protein bands with molecular weight ranging from 142.1 to 12.1 kDa. Seven protein bands of 74.1, 69.3, 55.5, 50.6, 45.6, 25.1 and 13.1 kDa were detected relatively more distinct. The major bands detected in our findings were 42 kDa, 37 kDa and 16 kDa that corresponds to OmpA, OmpH, P6 respectively. The mean zeta size (±SD) and potential of the nanoparticle were 246.20 ± 0.53 nm and -25.60 ± 5.97 respectively. In transmission electron microscopy (TEM), the nanoparticles exhibited an average diameter of 129.80 ± 11.10 nm and displayed spherical morphology. The median protective dose (PD50) of CAP-OMV nanoparticle was 1881.10 μg of protein. Group I ducks received 3762 μg of protein (entrapped protein in CAP-OMV nanoparticle) via intra nasal route and it showed the highest serum IgG and secretory IgA level than other immunized groups. All experimental ducks were challenged with 10 × LD50 on 35 days of post primary immunization. Group I showed 100 % survivability in the challenge study. No gross and biochemical indication of acute or chronic toxicity were recorded. In conclusion, our results suggest that CAP-OMV nanoparticle can be a safe and efficient mucosal vaccine delivery system for RA, eliciting strong immune response in the host.

Bacterial extracellular vesicles: Emerging nanoplatforms for biomedical applications

Bacterial extracellular vesicles (BEVs) are nanosized lipid bilayers generated from membranes that are filled with components derived from bacteria. BEVs are important for the physiology, pathogenicity, and interactions between bacteria and their hosts as well. BEVs represent an important mechanism of transport and interaction between cells. Recent advances in biomolecular nanotechnology have enabled the desired properties to be engineered on the surface of BEVs and decoration with desired and diverse biomolecules and nanoparticles, which have potential biomedical applications. BEVs have been the focus of various fields, including nanovaccines, therapeutic agents, and drug delivery vehicles. In this review, we delineate the fundamental aspects of BEVs, including their biogenesis, cargo composition, function, and interactions with host cells. We comprehensively summarize the factors influencing the biogenesis of BEVs. We further highlight the importance of the isolation, purification, and characterization of BEVs because they are essential processes for potential benefits related to host-microbe interactions. In addition, we address recent advancements in BEVs in biomedical applications. Finally, we provide conclusions and future perspectives as well as highlight the remaining challenges of BEVs for different biomedical applications.

Diet, commensal microbiota-derived extracellular vesicles, and host immunity

The gut microbiota has co-evolved with its host, and commensal bacteria can influence both the host's immune development and function. Recently, a role has emerged for bacterial extracellular vesicles (BEVs) as potent immune modulators. BEVs are nanosized membrane vesicles produced by all bacteria, possessing the membrane characteristics of the originating bacterium and carrying an internal cargo that may include nucleic acid, proteins, lipids, and metabolites. Thus, BEVs possess multiple avenues for regulating immune processes, and have been implicated in allergic, autoimmune, and metabolic diseases. BEVs are biodistributed locally in the gut, and also systemically, and thus have the potential to affect both the local and systemic immune responses. The production of gut microbiota-derived BEVs is regulated by host factors such as diet and antibiotic usage. Specifically, all aspects of nutrition, including macronutrients (protein, carbohydrates, and fat), micronutrients (vitamins and minerals), and food additives (the antimicrobial sodium benzoate), can regulate BEV production. This review summarizes current knowledge of the powerful links between nutrition, antibiotics, gut microbiota-derived BEV, and their effects on immunity and disease development. It highlights the potential of targeting or utilizing gut microbiota-derived BEV as a therapeutic intervention.

Bordetella pertussis and outer membrane vesicles

Bordetella pertussis is the causative agent of a respiratory infection called pertussis (whooping cough) that can be fatal in newborns and infants. The pathogen produces a variety of antigenic compounds which alone or simultaneously can damage various host cells. Despite the availability of pertussis vaccines and high vaccination coverage around the world, a resurgence of the disease has been observed in many countries. Reasons for the increase in pertussis cases may include increased awareness, improved diagnostic techniques, low vaccine efficacy, especially acellular vaccines, and waning immunity. Many efforts have been made to develop more effective strategies to fight against B. pertussis and one of the strategies is the use of outer membrane vesicles (OMVs) in vaccine formulations. OMVs are attracting great interest as vaccine platforms since they can carry immunogenic structures such as toxins and LPS. Many studies have been carried out with OMVs from different B. pertussis strains and they revealed promising results in the animal challenge and human preclinical model. However, the composition of OMVs differs in terms of isolation and purification methods, strains, culture, and stress conditions. Although the vesicles from B. pertussis represent an attractive pertussis vaccine candidate, further studies are needed to advance clinical research for next-generation pertussis vaccines. This review summarizes general information about pertussis, the history of vaccines against the disease, and the immune response to these vaccines, with a focus on OMVs. We discuss progress in developing an OMV-based pertussis vaccine platform and highlight successful applications as well as potential challenges and gaps.

Microbiota and plant-derived vesicles that serve as therapeutic agents and delivery carriers to regulate metabolic syndrome

The gut is a fundamental organ in controlling human health. Recently, researches showed that substances in the intestine can alter the course of many diseases through the intestinal epithelium, especially intestinal flora and exogenously ingested plant vesicles that can be transported over long distances to various organs. This article reviews the current knowledge on extracellular vesicles in modulating gut homeostasis, inflammatory response and numerous metabolic disease that share obesity as a co-morbidity. These complex systemic diseases that are difficult to cure, but can be managed by some bacterial and plant vesicles. Vesicles, due to their digestive stability and modifiable properties, have emerged as novel and targeted drug delivery vehicles for effective treatment of metabolic diseases.

Production and purification of bacterial membrane vesicles for biotechnology applications: Challenges and opportunities

Bacterial membrane vesicles (BMVs) are bi-layered nanostructures derived from Gram-negative and Gram-positive bacteria. Among other pathophysiological roles, BMVs are critical messengers in intercellular communication. As a result, BMVs are emerging as a promising technology for the development of numerous therapeutic applications. Despite the remarkable progress in unveiling BMV biology and functions in recent years, their successful isolation and purification have been limited. Several challenges related to vesicle purity, yield, and scalability severely hamper the further development of BMVs for biotechnology and clinical applications. This review focuses on the current technologies and methodologies used in BMV production and purification, such as ultracentrifugation, density-gradient centrifugation, size-exclusion chromatography, ultrafiltration, and precipitation. We also discuss the current challenges related to BMV isolation, large-scale production, storage, and stability that limit their application. More importantly, the present work explains the most recent strategies proposed for overcoming those challenges. Finally, we summarize the ongoing applications of BMVs in the biotechnological field.

Outer membrane vesicles as biomimetic vaccine carriers against infections and cancers

In the last decade, nanoparticle-based therapeutic modalities have emerged as promising treatment options for cancer and infectious diseases. To improve prognosis, chemotherapeutic and antimicrobial drugs must be delivered selectively to the target sites. Researchers have increasingly focused their efforts on improving drug delivery, with a particular emphasis on cancer and infectious diseases. When drugs are administered systemically, they become diluted and can diffuse to all tissues but only until the immune system intervenes and quickly removes them from circulation. To enhance and prolong the systemic circulation of drugs, nanocarriers have been explored and used; however, nanocarriers have a major drawback in that they can trigger immune responses. Numerous nanocarriers for optimal drug delivery have been developed using innovative and effective biointerface technologies. Autologous cell-derived drug carriers, such as outer membrane vesicles (OMVs), have demonstrated improved bioavailability and reduced toxicity. Thus, this study investigates the use of biomimetic OMVs as biomimetic vaccine carriers against infections and cancers to improve our understanding in the field of nanotechnology. In addition, discussion on the advantages, disadvantages, and future prospects of OMVs will also be explored. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Engineered bacterial membrane vesicles are promising carriers for vaccine design and tumor immunotherapy

Bacterial membrane vesicles (BMVs) have emerged as novel and promising platforms for the development of vaccines and immunotherapeutic strategies against infectious and noninfectious diseases. The rich microbe-associated molecular patterns (MAMPs) and nanoscale membrane vesicle structure of BMVs make them highly immunogenic. In addition, BMVs can be endowed with more functions via genetic and chemical modifications. This article reviews the immunological characteristics and effects of BMVs, techniques for BMV production and modification, and the applications of BMVs as vaccines or vaccine carriers. In summary, given their versatile characteristics and immunomodulatory properties, BMVs can be used for clinical vaccine or immunotherapy applications.

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.

Bacterial membrane vesicle functions, laboratory methods, and applications

Bacterial membrane vesicles are cupped-shaped structures formed by bacteria in response to environmental stress, genetic alteration, antibiotic exposure, and others. Due to the structural similarities shared with the producer organism, they can retain certain characteristics like stimulating immune responses. They are also able to carry molecules for long distances, without changes in the concentration and integrity of the molecule. Bacteria originally secrete membrane vesicles for gene transfer, excretion, cell to cell interaction, pathogenesis, and protection against phages. These functions are unique and have several innovative applications in the pharmaceutical industry that have attracted both scientific and commercial interest.This led to the development of efficient methods to artificially stimulate vesicle production, purification, and manipulation in the lab at nanoscales. Also, for specific applications, engineering methods to impart pathogen antigens against specific diseases or customization as cargo vehicles to deliver payloads to specific cells have been reported. Many applications of bacteria membrane vesicles are in cancer drugs, vaccines, and adjuvant development with several candidates in clinical trials showing promising results. Despite this, applications in therapy and commercialization stay timid probably due to some challenges one of which is the poor understanding of biogenesis mechanisms. Nevertheless, so far, bacterial membrane vesicles seem to be a reliable and cost-efficient technology with several therapeutic applications. Research toward characterizing more membrane vesicles, genetic engineering, and nanotechnology will enable the scope of applications to widen. This might include solutions to other currently faced medical and healthcare-related challenges.

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.

Differential Packaging Into Outer Membrane Vesicles Upon Oxidative Stress Reveals a General Mechanism for Cargo Selectivity

Selective cargo packaging into bacterial extracellular vesicles has been reported and implicated in many biological processes, however, the mechanism behind the selectivity has remained largely unexplored. In this study, proteomic analysis of outer membrane (OM) and OM vesicle (OMV) fractions from enterotoxigenic E. coli revealed significant differences in protein abundance in the OMV and OM fractions for cultures shifted to oxidative stress conditions. Analysis of sequences of proteins preferentially packaged into OMVs showed that proteins with oxidizable residues were more packaged into OMVs in comparison with those retained in the membrane. In addition, the results indicated two distinct classes of OM-associated proteins were differentially packaged into OMVs as a function of peroxide treatment. Implementing a Bayesian hierarchical model, OM lipoproteins were determined to be preferentially exported during stress whereas integral OM proteins were preferentially retained in the cell. Selectivity was determined to be independent of transcriptional regulation of the proteins upon oxidative stress and was validated using randomly selected protein candidates from the different cargo classes. Based on these data, a hypothetical functional and mechanistic basis for cargo selectivity was tested using OmpA constructs. Our study reveals a basic mechanism for cargo selectivity into OMVs that may be useful for the engineering of OMVs for future biotechnological applications.

Vaccination with meningococcal outer membrane vesicles carrying Borrelia OspA protects against experimental Lyme borreliosis

Currently there is no human vaccine against Lyme borreliosis, and most research focuses on recombinant protein vaccines, as such a vaccine has been proven to be successful in the past. The expression of recombinant antigens in meningococcal Outer Membrane Vesicles (OMVs), with the OMV functioning both as adjuvant and delivery vehicle, greatly enhances their potential. Immunization studies in mice have shown that OMV-based vaccines can protect against various pathogens and an OMV-based meningococcal vaccine is approved and available for human use. Because of its surface localization in Borrelia and the detailed knowledge regarding its immunogenicity and structure, OspA was chosen as a suitable lipoprotein to be tested as an OMV-based vaccine against Lyme borreliosis. We have previously shown that the OMV-OspA vaccine was immunogenic in mice and here we assessed the efficacy of OMV-OspA. We generated a second-generation OMV-OspA vaccine and vaccinated C3H/HeN mice with (EDTA extracted) meningococcal OMVs expressing OspA from B. burgdorferi strain B31. The adjuvant effect of empty OMVs on recombinant OspA was tested as well. We subsequently challenged mice with a subcutaneous injection of B. burgdorferi. Average antibody end-point titers against the OspA-OMV construct were high, although lower compared to the antibodies raised against recombinant OspA. Interestingly, antibody titers between recombinant OspA adjuvanted with aluminum hydroxide and recombinant OspA with OMV as adjuvant were comparable. Finally, qPCR and culture data show that both the OspA-OMV and the vaccine based on recombinant OspA with OMV as adjuvant provided significant, yet partial protection, against Borrelia infection. OMV-based vaccines using Borrelia (lipo)proteins are an easy and feasible vaccination method protecting against B. burgdorferi infection and could be a promising strategy in humans.

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.

Outer Membrane Vesicle Induction and Isolation for Vaccine Development

Gram-negative bacteria release vesicular structures from their outer membrane, so called outer membrane vesicles (OMVs). OMVs have a variety of functions such as waste disposal, communication, and antigen or toxin delivery. These vesicles are the promising structures for vaccine development since OMVs carry many surface antigens that are identical to the bacterial surface. However, isolation is often difficult and results in low yields. Several methods to enhance OMV yield exist, but these do affect the resulting OMVs. In this review, our current knowledge about OMVs will be presented. Different methods to induce OMVs will be reviewed and their advantages and disadvantages will be discussed. The effects of the induction and isolation methods used in several immunological studies on OMVs will be compared. Finally, the challenges for OMV-based vaccine development will be examined and one example of a successful OMV-based vaccine will be presented.

Staphylococcus aureus Extracellular Vesicles: A Story of Toxicity and the Stress of 2020

Staphylococcus aureus generates and releases extracellular vesicles (EVs) that package cytosolic, cell-wall associated, and membrane proteins, as well as glycopolymers and exoproteins, including alpha hemolysin, leukocidins, phenol-soluble modulins, superantigens, and enzymes. S. aureus EVs, but not EVs from pore-forming toxin-deficient strains, were cytolytic for a variety of mammalian cell types, but EV internalization was not essential for cytotoxicity. Because S. aureus is subject to various environmental stresses during its encounters with the host during infection, we assessed how these exposures affected EV production in vitro. Staphylococci grown at 37 °C or 40 °C did not differ in EV production, but cultures incubated at 30 °C yielded more EVs when grown to the same optical density. S. aureus cultivated in the presence of oxidative stress, in iron-limited media, or with subinhibitory concentrations of ethanol, showed greater EV production as determined by protein yield and quantitative immunoblots. In contrast, hyperosmotic stress or subinhibitory concentrations of erythromycin reduced S. aureus EV yield. EVs represent a novel S. aureus secretory system that is affected by a variety of stress responses and allows the delivery of biologically active pore-forming toxins and other virulence determinants to host cells.

Membrane Vesicle Production as a Bacterial Defense Against Stress

Membrane vesicles are the nano-sized vesicles originating from membranes. The production of membrane vesicles is a common feature among bacteria. Depending on the bacterial growth phase and environmental conditions, membrane vesicles show diverse characteristics. Various physiological and ecological roles have been attributed to membrane vesicles under both homeostatic and stressful conditions. Pathogens encounter several stressors during colonization in the hostile environment of host tissues. Nutrient deficiency, the presence of antibiotics as well as elements of the host’s immune system are examples of stressors threatening pathogens inside their host. To combat stressors and survive, pathogens have established various defensive mechanisms, one of them is production of membrane vesicles. Pathogens produce membrane vesicles to alleviate the destructive effects of antibiotics or other types of antibacterial treatments. Additionally, membrane vesicles can also provide benefits for the wider bacterial community during infections, through the transfer of resistance or virulence factors. Hence, given that membrane vesicle production may affect the activities of antibacterial agents, their production should be considered when administering antibacterial treatments. Besides, regarding that membrane vesicles play vital roles in bacteria, disrupting their production may suggest an alternative strategy for battling against pathogens. Here, we aim to review the stressors encountered by pathogens and shed light on the roles of membrane vesicles in increasing pathogen adaptabilities in the presence of stress-inducing factors.

Abiotic stressors impact outer membrane vesicle composition in a beneficial rhizobacterium: Raman spectroscopy characterization

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria have roles in cell-to-cell signaling, biofilm formation, and stress responses. Here, the effects of abiotic stressors on OMV contents and composition from biofilm cells of the plant health-promoting bacterium Pseudomonas chlororaphis O6 (PcO6) are examined. Two stressors relevant to this root-colonizing bacterium were examined: CuO nanoparticles (NPs)-a potential fertilizer and fungicide- and H2O2-released from roots during plant stress responses. Atomic force microscopy revealed 40-300 nm diameter OMVs from control and stressed biofilm cells. Raman spectroscopy with linear discriminant analysis (LDA) was used to identify changes in chemical profiles of PcO6 cells and resultant OMVs according to the cellular stressor with 84.7% and 83.3% accuracies, respectively. All OMVs had higher relative concentrations of proteins, lipids, and nucleic acids than PcO6 cells. The nucleic acid concentration in OMVs exhibited a cellular stressor-dependent increase: CuO NP-induced OMVs > H2O2-induced OMVs > control OMVs. Biochemical assays confirmed the presence of lipopolysaccharides, nucleic acids, and protein in OMVs; however, these assays did not discriminate OMV composition according to the cellular stressor. These results demonstrate the sensitivity of Raman spectroscopy using LDA to characterize and distinguish cellular stress effects on OMVs composition and contents.

Continuous production of Neisseria meningitidis outer membrane vesicles

Outer membrane vesicles (OMVs) are nanoparticles secreted by Gram-negative bacteria that can be used for diverse biotechnological applications. Interesting applications have been developed, where OMVs are the basis of drug delivery, enzyme carriers, adjuvants, and vaccines. Historically, OMV research has mainly focused on vaccines. Therefore, current OMV production processes have been based on batch processes. The production of OMVs in batch mode is characterized by relatively low yields and high costs. Transition of OMV production processes from batch to continuous processes could increase the volumetric productivity, reduce the production and capital costs, and result in a higher quality product. Here, we study the continuous production of Neisseria meningitidis OMVs to improve volumetric productivity. Continuous cultivation of N. meningitidis resulted in a steady state with similar high OMV concentrations as are reached in current batch processes. The steady state was reproducible and could be maintained for at least 600 h. The volumetric productivity of a continuous culture reached 4.0 × 1014 OMVs per liter culture per day, based on a dilution rate of 1/day. The tested characteristics of the OMVs did not change during the experiments showing feasibility of a continuous production process for the production of OMVs for any application.

Natural and engineered bacterial outer membrane vesicles

Bacterial outer membrane vesicle (OMV) is a kind of spherical lipid bilayer nanostructure naturally secreted by bacteria, which has diverse functions such as intracellular and extracellular communication, horizontal gene transfer, transfer of contents to host cells, and eliciting an immune response in host cells. In this review, several methods including ultracentrifugation and precipitation for isolating OMVs were summarized. The latest progresses of OMVs in biomedical fields, especially in vaccine development, cancer treatment, infection control, and bioimaging and detection were also summarized in this review. We highlighted the importance of genetic engineering for the safe and effective application and in facilitating the rapid development of OMVs. Finally, we discussed the bottleneck problems about OMVs in preparation and application at present and put forward our own suggestions about them. Some perspectives of OMVs in biomedical field were also provided.

Spontaneously released Neisseria meningitidis outer membrane vesicles as vaccine platform: production and purification

Outer membrane vesicles (OMVs) are nanoparticles produced by Gram-negative bacteria that can be used as vaccines. The application of OMVs as vaccine component can be expanded by expressing heterologous antigens on OMVs, creating an OMV-based antigen presenting platform. This study aims to develop a production process for such OMV-based vaccines and studies a production method based on meningococcal OMVs that express heterologous antigens on their surface. As a proof of concept, the Borrelia burgdorferi antigens OspA and OspC were expressed on Neisseria meningitidis OMVs to create a concept anti-Lyme disease vaccine. Production of OMVs released in the culture supernatant was induced by high dissolved oxygen concentrations and purification was based on scalable unit operations. A crude recovery of 90 mg OMV protein could be obtained per liter culture. Expressing heterologous antigens on the OMVs did result in minor reduction of bacterial growth, while OMV production remained constant. The antigen expression did not alter the OMV characteristics. This study shows that production of well characterized OMVs containing heterologous antigens is possible with high yields by combining high oxygen concentrations with an optimized purification process. It is concluded that heterologous OMVs show potential as a vaccine platform.

Sulfate depletion triggers overproduction of phospholipids and the release of outer membrane vesicles by Neisseria meningitidis

Outer membrane vesicles (OMVs) produced by bacteria are interesting vaccine candidates. OMVs are nanoparticles that contain many immunogenic components, are self-adjuvating, and non-replicative. Despite recent insights in the biogenesis of OMVs, there is no consensus on a conserved mechanism of OMV release and the OMV yield from bacterial cultures remains low. For Neisseria meningitidis, a Gram-negative human pathogen causing meningitis and sepsis, a feasible OMV production method based on triggering OMV release by cysteine depletion has been described. In this study, we investigated the mechanism behind this external trigger for OMV release to improve the production process. Since enhanced OMV release upon cysteine depletion was associated with oxidative stress and redox responses, we investigate the influence of more oxidized sulfur sources on OMV release. We show that N. meningitidis grows similarly on sulfate, the most oxidized sulfur source, and OMV release is triggered by sulfur depletion in general. Sulfate depletion induced increased release of OMVs over cysteine depletion. Proteomics showed that sulfur depletion resulted in oxidative stress responses and upregulated phospholipid and LPS biosynthesis. Furthermore, OMVs produced by sulfur depletion were enriched in phospholipids. Mechanistically, we hypothesize that sulfur depletion results in overproduction of phospholipids causing increased bulging of the outer membrane and subsequent OMV release.

PmrC (EptA) and CptA Negatively Affect Outer Membrane Vesicle Production in Citrobacter rodentium

Although OMVs secreted by Gram-negative bacteria fulfill multiple functions, the molecular mechanism of OMV biogenesis remains ill defined. Our group has previously shown that PmrC (also known as EptA) and CptA maintain OM integrity and provide resistance to iron toxicity and antibiotics in the murine pathogen Citrobacter rodentium. In several enteric bacteria, these proteins modify the lipid A and core regions of lipopolysaccharide with phosphoethanolamine moieties. Here, we show that these proteins also repress OMV production in response to environmental iron in C. rodentium. These data support the emerging understanding that lipopolysaccharide modifications are important regulators of OMV biogenesis in Gram-negative bacteria.

Outer membrane vesicles (OMVs) are naturally produced by Gram-negative bacteria by a bulging of the outer membrane (OM) and subsequent release into the environment. By serving as vehicles for various cargos, including proteins, nucleic acids and small metabolites, OMVs are central to interbacterial interactions and both symbiotic and pathogenic host bacterial interactions. However, despite their importance, the mechanism of OMV formation remains unclear. Recent evidence indicates that covalent modifications of lipopolysaccharides (LPS) influence OMV biogenesis. Several enteric bacteria modify LPS with phosphoethanolamine (pEtN) using the iron-regulated PmrC (EptA) and CptA pEtN transferases. In wild-type Citrobacter rodentium, the presence of increasing subtoxic concentrations of iron was found to stimulate OMV production 4- to 9-fold above baseline. C. rodentium uses the two-component system PmrAB to sense and adapt to environmental iron. Compared to the wild type, the C. rodentium ΔpmrAB strain exhibited heightened OMV production at similar iron concentrations. PmrAB regulates transcription of pmrC (also known as eptA) and cptA. OMV production in strains lacking either pmrC (eptA) or cptA was similarly increased in comparison to that of the wild type. Importantly, plasmid complementation of C. rodentium strains with either pmrC (eptA) or cptA resulted in a drastic inhibition of OMV production. Finally, we showed that β-lactamase and CroP, two enzymes found in the C. rodentium periplasm and outer membrane (OM), respectively, are associated with OMVs. These data suggest a novel mechanism by which C. rodentium and possibly other Gram-negative bacteria can negatively affect OMV production through the PmrAB-regulated genes pmrC (eptA) and cptA.

IMPORTANCE Although OMVs secreted by Gram-negative bacteria fulfill multiple functions, the molecular mechanism of OMV biogenesis remains ill defined. Our group has previously shown that PmrC (also known as EptA) and CptA maintain OM integrity and provide resistance to iron toxicity and antibiotics in the murine pathogen Citrobacter rodentium. In several enteric bacteria, these proteins modify the lipid A and core regions of lipopolysaccharide with phosphoethanolamine moieties. Here, we show that these proteins also repress OMV production in response to environmental iron in C. rodentium. These data support the emerging understanding that lipopolysaccharide modifications are important regulators of OMV biogenesis in Gram-negative bacteria.

The Meningococcal Cysteine Transport System Plays a Crucial Role in Neisseria meningitidis Survival in Human Brain Microvascular Endothelial Cells

Neisseria meningitidis colonizes at a nasopharynx of human as a unique host and has many strains that are auxotrophs for amino acids for their growth. To cause invasive meningococcal diseases (IMD) such as sepsis and meningitis, N. meningitidis passes through epithelial and endothelial barriers and infiltrates into blood and cerebrospinal fluid as well as epithelial and endothelial cells. However, meningococcal nutrients, including cysteine, become less abundant when it more deeply infiltrates the human body even during inflammation, such that N. meningitidis has to acquire nutrients in order to survive/persist, disseminate, and proliferate in humans. This was the first study to examine the relationship between meningococcal cysteine acquisition and the pathogenesis of meningococcal infections. The results of the present study provide insights into the mechanisms by which pathogens with auxotrophs acquire nutrients in hosts and may also contribute to the development of treatments and prevention strategies for IMD.

While Neisseria meningitidis typically exists in an asymptomatic nasopharyngeal carriage state, it may cause potentially lethal diseases in humans, such as septicemia or meningitis, by invading deeper sites in the body. Since the nutrient compositions of human cells are not always conducive to meningococci, N. meningitidis needs to exploit nutrients from host environments. In the present study, the utilization of cysteine by the meningococcal cysteine transport system (CTS) was analyzed for the pathogenesis of meningococcal infections. A N. meningitidis strain deficient in one of the three cts genes annotated as encoding cysteine-binding protein (cbp) exhibited approximately 100-fold less internalization into human brain microvascular endothelial cells (HBMEC) than the wild-type strain. This deficiency was restored by complementation with the three cts genes together, and the infectious phenotype of HBMEC internalization correlated with cysteine uptake activity. However, efficient accumulation of ezrin was observed beneath the cbp mutant. The intracellular survival of the cbp mutant in HBMEC was markedly reduced, whereas equivalent reductions of glutathione concentrations and of resistance to reactive oxygens species in the cbp mutant were not found. The cbp mutant grew well in complete medium but not in synthetic medium supplemented with less than 300 μM cysteine. Taking cysteine concentrations in human cells and other body fluids, including blood and cerebrospinal fluid, into consideration, the present results collectively suggest that the meningococcal CTS is crucial for the acquisition of cysteine from human cells and participates in meningococcal nutrient virulence.

Cysteine biosynthesis in Neisseria species

The principal mechanism of reducing sulfur into organic compounds is via the synthesis of l-cysteine. Cysteine is used for protein and glutathione synthesis, as well as being the primary sulfur source for a variety of other molecules, such as biotin, coenzyme A, lipoic acid and more. Glutathione and other cysteine derivatives are important for protection against the oxidative stress that pathogenic bacteria such as Neisseria gonorrhoeae and Neisseria meningitidis encounter during infection. With the alarming rise of antibiotic-resistant strains of N. gonorrhoeae, the development of inhibitors for the future treatment of this disease is critical, and targeting cysteine biosynthesis enzymes could be a promising approach for this. Little is known about the transport of sulfate and thiosulfate and subsequent sulfate reduction and incorporation into cysteine in Neisseria species. In this review we investigate cysteine biosynthesis within Neisseria species and examine the differences between species and with other bacteria. Neisseria species exhibit different arrangements of cysteine biosynthesis genes and have slight differences in how they assimilate sulfate and synthesize cysteine, while, most interestingly, N. gonorrhoeae by virtue of a genome deletion, lacks the ability to reduce sulfate to bisulfide for incorporation into cysteine, and as such uses the thiosulfate uptake pathway for the synthesis of cysteine.

High dissolved oxygen tension triggers outer membrane vesicle formation by Neisseria meningitidis

Background

Outer membrane vesicles (OMVs) are nanoparticles released by Gram-negative bacteria and can be used as vaccines. Often, detergents are used to promote release of OMVs and to remove the toxic lipopolysaccharides. Lipopolysaccharides can be detoxified by genetic modification such that vesicles spontaneously produced by bacteria can be directly used as vaccines. The use of spontaneous OMVs has the advantage that no separate extraction step is required in the purification process. However, the productivity of spontaneous OMVs by bacteria at optimal growth conditions is low. One of many methods for increasing OMV formation is to reduce the linkage of the outer membrane to the peptidoglycan layer by knocking out the rmpM gene. A previous study showed that for Neisseria meningitidis this resulted in release of more OMVs. Furthermore, cysteine depletion was found to trigger OMV release and at the same time cause reduced growth and oxidative stress responses. Here we study the effect of growth rate and oxidative stress on OMV release.

Results

First, we identified using chemostat and accelerostat cultures of N. meningitidis that increasing the growth rate from 0.03 to 0.18 h⁻¹ has a limited effect on OMV productivity. Thus, we hypothesized that oxidative stress is the trigger for OMV release and that oxidative stress can be introduced directly by increasing the dissolved oxygen tension of bacterial cultures. Slowly increasing oxygen concentrations in a N. meningitidis changestat showed that an increase from 30 to 150% air saturation improved OMV productivity four-fold. Batch cultures controlled at 100% air saturation increased OMV productivity three-fold over batch cultures controlled at 30% air saturation.

Conclusion

Increased dissolved oxygen tension induces the release of outer membrane vesicles in N. meningitidis cultures. Since oxygen concentration is a well-controlled process parameter of bacterial cultures, this trigger can be applied as a convenient process parameter to induce OMV release in bacterial cultures. Improved productivity of OMVs not only improves the production costs of OMVs as vaccines, it also facilitates the use of OMVs as adjuvants, enzyme carriers, or cell-specific drug delivery vehicles.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1007-7) contains supplementary material, which is available to authorized users.

Antibiotic treatment modulates protein components of cytotoxic outer membrane vesicles of multidrug-resistant clinical strain, Acinetobacter baumannii DU202

Background

Outer membrane vesicles (OMVs) of Acinetobacter baumannii are cytotoxic and elicit a potent innate immune response. OMVs were first identified in A. baumannii DU202, an extensively drug-resistant clinical strain. Herein, we investigated protein components of A. baumannii DU202 OMVs following antibiotic treatment by proteogenomic analysis.

Methods

Purified OMVs from A. baumannii DU202 grown in different antibiotic culture conditions were screened for pathogenic and immunogenic effects, and subjected to quantitative proteomic analysis by one-dimensional electrophoresis and liquid chromatography combined with tandem mass spectrometry (1DE-LC-MS/MS). Protein components modulated by imipenem were identified and discussed.

Results

OMV secretion was increased > twofold following imipenem treatment, and cytotoxicity toward A549 human lung carcinoma cells was elevated. A total of 277 proteins were identified as components of OMVs by imipenem treatment, among which β-lactamase OXA-23, various proteases, outer membrane proteins, β-barrel assembly machine proteins, peptidyl-prolyl cis–trans isomerases and inherent prophage head subunit proteins were significantly upregulated.

Conclusion

In vitro stress such as antibiotic treatment can modulate proteome components in A. baumannii OMVs and thereby influence pathogenicity.

Electronic supplementary material

The online version of this article (10.1186/s12014-018-9204-2) contains supplementary material, which is available to authorized users.

Outer Membrane Vesicles: Current Status and Future Direction of These Novel Vaccine Adjuvants

Adjuvants have been of great interest to vaccine formulation as immune-stimulators. Prior to the recent research in the field of immune stimulation, conventional adjuvants utilized for aluminum-based vaccinations dominated the adjuvant market. However, these conventional adjuvants have demonstrated obvious defects, including poor protective efficiency and potential side effects, which hindered their widespread circulation. Outer membrane vesicles (OMVs) naturally exist in gram-negative bacteria and are capable of engaging innate and adaptive immunity and possess intrinsic adjuvant capacity. They have shown tremendous potential for adjuvant application and have recently been successfully applied in various vaccine platforms. Adjuvants could be highly effective with the introduction of OMVs, providing complete immunity and with the benefits of low toxicity; further, OMVs might also be designed as an advanced mucosal delivery vehicle for use as a vaccine carrier. In this review, we discuss adjuvant development, and provide an overview of novel OMV adjuvants and delivery vehicles. We also suggest future directions for adjuvant research. Overall, we believe that OMV adjuvants would find high value in vaccine formulation in the future.

Versatile effects of bacterium-released membrane vesicles on mammalian cells and infectious/inflammatory diseases

Gram-negative bacterium-released outer-membrane vesicles (OMVs) and Gram-positive bacterium-released membrane vesicles (MVs) share significant similarities with mammalian cell-derived MVs (eg, microvesicles and exosomes) in terms of structure and their biological activities. Recent studies have revealed that bacterial OMVs/MVs could (1) interact with immune cells to regulate inflammatory responses, (2) transport virulence factors (eg, enzymes, DNA and small RNAs) to host cells and result in cell injury, (3) enhance barrier function by stimulating the expression of tight junction proteins in intestinal epithelial cells, (4) upregulate the expression of endothelial cell adhesion molecules, and (5) serve as natural nanocarriers for immunogenic antigens, enzyme support and drug delivery. In addition, OMVs/MVs can enter the systemic circulation and induce a variety of immunological and metabolic responses. This review highlights the recent advances in the understanding of OMV/MV biogenesis and their compositional remodeling. In addition, interactions between OMVs/MVs and various types of mammalian cells (ie, immune cells, epithelial cells, and endothelial cells) and their pathological/preventive effects on infectious/inflammatory diseases are summarized. Finally, methods for engineering OMVs/MVs and their therapeutic potential are discussed.

Amino acid deprivation and central carbon metabolism regulate the production of outer membrane vesicles and tubes by Francisella

Francisella tularensis is a highly virulent Gram-negative bacterial pathogen that causes the zoonotic disease tularemia. F. novicida, a model tularemia strain, produces spherical outer membrane vesicles (OMV), as well as novel tubular vesicles and extensions of the cell surface. These OMV and tubes (OMV/T) are produced in a regulated manner and contain known virulence factors. Mechanisms by which bacterial vesicles are produced and regulated are not well understood. We performed a genetic screen in F. novicida to decipher the molecular basis for regulated OMV/T formation, and identified both hypo- and hyper-vesiculating mutants. Mutations in fumA and tktA, involved in central carbon metabolism, and in FTN_0908 and FTN_1037, of unknown function, resulted in severe defects in OMV/T production. Cysteine deprivation was identified as the signal that triggers OMV/T formation in F. novicida during growth in rich medium. We also found that fully virulent F. tularensis produces OMV/T in a similarly regulated manner. Further analysis revealed that OMV/T production is responsive to deprivation of essential amino acids in addition to cysteine, and that the hypo-vesiculating mutants are defective in responding to this signal. Thus, amino acid starvation, such as encountered by Francisella during host cell invasion, regulates the production of membrane-derived structures.

Genomic, Transcriptomic, and Phenotypic Analyses of Neisseria meningitidis Isolates from Disease Patients and Their Household Contacts

Neisseria meningitidis causes meningococcal disease but is frequently carried in the throats of healthy individuals; the factors that determine whether invasive disease develops are not completely understood. We carried out detailed studies of isolates, collected from patients and their household contacts, to identify differences between commensal throat isolates and those that caused invasive disease. Though isolates were identical by laboratory typing methods, we uncovered many differences in their genomes, in gene expression, and in their interactions with host cells. In particular, we found that several carriage isolates had lost their type IV pili, a surprising finding since pili are often described as essential for colonization. However, loss of type IV pili correlated with reduced secretion of a proinflammatory cytokine, TNF-α, when meningococci were cocultured with human bronchial epithelial cells; hence, the loss of pili could provide an advantage to meningococci, by resulting in a dampened localized host immune response.

Neisseria meningitidis (meningococcus) can cause meningococcal disease, a rapidly progressing and often fatal disease that can occur in previously healthy children. Meningococci are found in healthy carriers, where they reside in the nasopharynx as commensals. While carriage is relatively common, invasive disease, associated with hypervirulent strains, is a comparatively rare event. The basis of increased virulence in some strains is not well understood. New Zealand suffered a protracted meningococcal disease epidemic, from 1991 to 2008. During this time, a household carriage study was carried out in Auckland: household contacts of index meningococcal disease patients were swabbed for isolation of carriage strains. In many households, healthy carriers harbored strains identical, as determined by laboratory typing, to the ones infecting the associated patient. We carried out more-detailed analyses of carriage and disease isolates from a select number of households. We found that isolates, although indistinguishable by laboratory typing methods and likely closely related, had many differences. We identified multiple genome variants and transcriptional differences between isolates. These studies enabled the identification of two new phase-variable genes. We also found that several carriage strains had lost their type IV pili and that this loss correlated with reduced tumor necrosis factor alpha (TNF-α) expression when cultured with epithelial cells. While nonpiliated meningococcal isolates have been previously found in carriage strains, this is the first evidence of an association between type IV pili from meningococci and a proinflammatory epithelial response. We also identified potentially important metabolic differences between carriage and disease isolates, including the sulfate assimilation pathway.

IMPORTANCENeisseria meningitidis causes meningococcal disease but is frequently carried in the throats of healthy individuals; the factors that determine whether invasive disease develops are not completely understood. We carried out detailed studies of isolates, collected from patients and their household contacts, to identify differences between commensal throat isolates and those that caused invasive disease. Though isolates were identical by laboratory typing methods, we uncovered many differences in their genomes, in gene expression, and in their interactions with host cells. In particular, we found that several carriage isolates had lost their type IV pili, a surprising finding since pili are often described as essential for colonization. However, loss of type IV pili correlated with reduced secretion of a proinflammatory cytokine, TNF-α, when meningococci were cocultured with human bronchial epithelial cells; hence, the loss of pili could provide an advantage to meningococci, by resulting in a dampened localized host immune response.

Immunomodulatory role for membrane vesicles released by THP-1 macrophages and respiratory pathogens during macrophage infection

Background

During infection, inflammation is partially driven by the release of mediators which facilitate intercellular communication. Amongst these mediators are small membrane vesicles (MVs) that can be released by both host cells and Gram-negative and -positive bacteria. Bacterial membrane vesicles are known to exert immuno-modulatory and -stimulatory actions. Moreover, it has been proposed that host cell-derived vesicles, released during infection, also have immunostimulatory properties. In this study, we assessed the release and activity of host cell-derived and bacterial MVs during the first hours following infection of THP-1 macrophages with the common respiratory pathogens non-typeable Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pneumoniae, and Pseudomonas aeruginosa.

Results

Using a combination of flow cytometry, tunable resistive pulse sensing (TRPS)-based analysis and electron microscopy, we demonstrated that the release of MVs occurs by both host cells and bacteria during infection. MVs released during infection and bacterial culture were found to induce a strong pro-inflammatory response by naive THP-1 macrophages. Yet, these MVs were also found to induce tolerance of host cells to secondary immunogenic stimuli and to enhance bacterial adherence and the number of intracellular bacteria.

Conclusions

Bacterial MVs may play a dual role during infection, as they can both trigger and dampen immune responses thereby contributing to immune defence and bacterial survival.

Tiny but mighty: bacterial membrane vesicles in food biotechnological applications

Membrane vesicle (MV) production is observed in all domains of life. Evidence of MV production accumulated in recent years among bacterial species involved in fermentation processes. These studies revealed MV composition, biological functions and properties, which made us recognize the potential of MVs in food applications as delivery vehicles of various compounds to other bacteria or the human host. Moreover, MV producing strains can deliver benefits as probiotics or starters in fermentation processes. Next to the natural production of MVs, we also highlight possible methods for artificial generation of bacterial MVs and cargo loading to enhance their applicability. We believe that a more in-depth understanding of bacterial MVs opens new avenues for their exploitation in biotechnological applications.

Gram-negative bacterial membrane vesicle release in response to the host-environment: different threats, same trick?

Bacteria are confronted with a multitude of stressors when occupying niches within the host. These stressors originate from host defense mechanisms, other bacteria during niche competition or result from physiological challenges such as nutrient limitation. To counteract these stressors, bacteria have developed a stress-induced network to mount the adaptations required for survival. These stress-induced adaptations include the release of membrane vesicles from the bacterial envelope. Membrane vesicles can provide bacteria with a plethora of immediate and ultimate benefits for coping with environmental stressors. This review addresses how membrane vesicles aid Gram-negative bacteria to cope with host-associated stress factors, focusing on vesicle biogenesis and the physiological functions. As many of the pathways, that drive vesicle biogenesis, confer we propose that shedding of membrane vesicles by Gram-negative bacteria entails an integrated part of general stress responses.

High throughput nanoparticle tracking analysis for monitoring outer membrane vesicle production

Outer membrane vesicles (OMVs) are spherical membrane nanoparticles released by Gram-negative bacteria. OMVs can be quantified in complex matrices by nanoparticle tracking analysis (NTA). NTA can be performed in static mode or with continuous sample flow that results in analysis of more particles in a smaller time-frame. Flow measurements must be performed manually despite the availability of a sample changer on the NanoSight system. Here we present a method for automated measurements in flow mode. OMV quantification in flow mode results in lower variance in particle quantification (coefficient of variation (CV) of 6%, CV static measurements of 14%). Sizing of OMVs was expected to be less favorable in flow mode due to the increased movement of the particles. However, we observed a CV of 3% in flow mode and a CV of 8% in static measurements. Flow rates of up to 5 µL/min displayed correct size and particle measurements, however, particle concentration was slightly lower than in static measurements. The automated method was used to assess OMV release of batch cultures of Neisseria meningitidis. The bacteria released more OMVs in stationary growth phase, while the size of the vesicles remained constant throughout the culture. Taken together, this study shows that automated measurements in flow mode can be established with advanced scripting to reduce the workload for the user.

The Hfq regulon of Neisseria meningitidis

The conserved RNA‐binding protein, Hfq, has multiple regulatory roles within the prokaryotic cell, including promoting stable duplex formation between small RNAs and mRNAs, and thus hfq deletion mutants have pleiotropic phenotypes. Previous proteome and transcriptome studies of Neisseria meningitidis have generated limited insight into differential gene expression due to Hfq loss. In this study, reversed‐phase liquid chromatography combined with data‐independent alternate scanning mass spectrometry (LC‐MS^E) was utilized for rapid high‐resolution quantitative proteomic analysis to further elucidate the differentially expressed proteome of a meningococcal hfq deletion mutant. Whole‐cell lysates of N. meningitidis serogroup B H44/76 wild‐type (wt) and H44/76Δhfq (Δhfq) grown in liquid growth medium were subjected to tryptic digestion. The resulting peptide mixtures were separated by liquid chromatography (LC) prior to analysis by mass spectrometry (MS^E). Differential expression was analyzed by Student's t‐test with control for false discovery rate (FDR). Reliable quantitation of relative expression comparing wt and Δhfq was achieved with 506 proteins (20%). Upon FDR control at q ≤ 0.05, 48 up‐ and 59 downregulated proteins were identified. From these, 81 were identified as novel Hfq‐regulated candidates, while 15 proteins were previously found by SDS/PAGE/MS and 24 with microarray analyses. Thus, using LC‐MS^E we have expanded the repertoire of Hfq‐regulated proteins. In conjunction with previous studies, a comprehensive network of Hfq‐regulated proteins was constructed and differentially expressed proteins were found to be involved in a large variety of cellular processes. The results and comparisons with other gram‐negative model systems, suggest still unidentified sRNA analogs in N. meningitidis.

Interspecies Communication between Pathogens and Immune Cells via Bacterial Membrane Vesicles

The production of extracellular vesicles is a universal mechanism for intercellular communication that is conserved across kingdoms. Prokaryotes secrete 50–250 nm membrane vesicles (MVs) in a manner that is regulated by environmental stress and is thought to promote survival. Since many types of host-derived stress are encountered during infection, this implies an important role for MV secretion in bacterial pathogenesis. Accordingly, MVs produced by gram-positive and gram-negative pathogens contain toxins, virulence factors, and other molecules that promote survival in the host. However, recent studies have also shown that bacterial MVs are enriched for molecules that stimulate innate and adaptive immune responses. As an example, MVs may serve multiple, important roles in regulating the host response to Mycobacterium tuberculosis (Mtb), an intracellular pathogen that infects lung macrophages and resides within modified phagosomes. Previously, we demonstrated that Mtb secretes MVs during infection that may modulate infected and uninfected immune cells. Our present data demonstrates that Mtb MVs inhibit the functions of macrophages and T cells, but promote Major Histocompatibility Complex (MHC) class II antigen presentation by dendritic cells. We conclude that bacterial MVs serve dual and opposing roles in the activation of and defense against host immune responses to Mtb and other bacterial pathogens. We also propose that MV secretion is a central mechanism for interspecies communication between bacteria and host cells during infection.