Outer membrane vesicle production by Escherichia coli is independent of membrane instability

Commensal and Pathogenic Bacterial-Derived Extracellular Vesicles in Host-Bacterial and Interbacterial Dialogues: Two Sides of the Same Coin

Extracellular vesicles (EVs) cause effective changes in various domains of life. These bioactive structures are essential to the bidirectional organ communication. Recently, increasing research attention has been paid to EVs derived from commensal and pathogenic bacteria in their potential role to affect human disease risk for cancers and a variety of metabolic, gastrointestinal, psychiatric, and mental disorders. The present review presents an overview of both the protective and harmful roles of commensal and pathogenic bacteria-derived EVs in host-bacterial and interbacterial interactions. Bacterial EVs could impact upon human health by regulating microbiota–host crosstalk intestinal homeostasis, even in distal organs. The importance of vesicles derived from bacteria has been also evaluated regarding epigenetic modifications and applications. Generally, the evaluation of bacterial EVs is important towards finding efficient strategies for the prevention and treatment of various human diseases and maintaining metabolic homeostasis.

Interaction with mammalian enteric viruses alters outer membrane vesicle production and content by commensal bacteria

Intestinal commensal bacteria contribute to maintaining gut homeostasis. Disruptions to the commensal flora are linked to the development and persistence of disease. The importance of these organisms is further demonstrated by the widespread ability of enteric viruses to exploit commensal bacteria to enhance viral infection. These viruses interact directly with commensal bacteria, and while the impact of this interaction on viral infection is well described for several viruses, the impact on the commensal bacteria has yet to be explored. In this article, we demonstrate, for the first time, that enteric viruses alter the gene expression and phenotype of individual commensal bacteria. Human and murine norovirus interaction with bacteria resulted in genome-wide differential gene expression and marked changes in the surface architecture of the bacterial cells. Furthermore, the interaction of the virus with bacteria led to increased production of smaller outer membrane vesicles (OMVs). Enhanced production of smaller vesicles was also observed when noroviruses were incubated with other commensal bacteria, indicating a potentially broad impact of norovirus interaction. The vesicle production observed in the in vivo model followed a similar trend where an increased quantity of smaller bacterial vesicles was observed in stool collected from virus-infected mice compared to mock-infected mice. Furthermore, changes in vesicle size were linked to changes in protein content and abundance, indicating that viral binding induced a shift in the mechanism of the OMV biogenesis. Collectively, these data demonstrate that enteric viruses induce specific changes in bacterial gene expression, leading to changes in bacterial extracellular vesicle production that can potentially impact host responses to infection.

Incorporation of DNA to Bacterial Vesicles by in Escherichia coli

Membrane vesicles (MVs) are released by various prokaryotes and play a role in the delivery of various cell-cell interaction factors. Recent studies have determined that these vesicles are capable of functioning as mediators of horizontal gene transfer. Outer membrane vesicles (OMVs) are a type of MV that is released by Gram-negative bacteria and primarily composed of outer membrane and periplasm components; however, it remains largely unknown why DNA is contained within OMVs. Our study aimed to understand the mechanism by which DNA that is localized in the cytoplasm is incorporated into OMVs in Gram-negative bacteria. We compared DNA associated with OMVs using Escherichia coli BW25113 cells harboring the non-conjugative, non-mobilized, and high-copy plasmid pUC19 and its hypervesiculating mutants that included ΔnlpI, ΔrseA, and ΔtolA. Plasmid copy per vesicle was increased in OMVs derived from ΔnlpI, in which peptidoglycan (PG) breakdown and synthesis are altered. When supplemented with 1% glycine to inhibit PG synthesis, both OMV formation and plasmid copy per vesicle were increased in the wild type. The bacterial membrane condition test indicated that membrane permeability was increased in the presence of glycine at the late exponential phase, in which cell lysis did not occur. Additionally, quick-freeze deep-etch and replica electron microscopy observations revealed that outer-inner membrane vesicles (O-IMVs) are formed in the presence of glycine. Thus, two proposed routes for DNA incorporation into OMVs under PG-damaged conditions are suggested. These routes include DNA leakage due to increased membrane permeation and O-IMV formation. Additionally, our findings contribute to a greater understanding of the vesicle-mediated horizontal gene transfer that occurs in nature and the utilization of MVs for DNA cargo.

GMMA as a 'plug and play' technology to tackle infectious disease to improve global health: context and perspectives for the future

INTRODUCTION

Generalized-Modules-for-Membrane-Antigens (GMMA) is a technology platform developed to design outer membrane vesicle (OMV)-based vaccines. GMMA are basically OMVs derived from a bacterial strain specifically engineered to obtain a fit-for-purpose and affordable vaccine by potentiating, or deleting, expression of specific genes. OMVs can be used as a carrier for antigens by inducing their expression on them, with the aim to improve antigen immunogenicity and design multivalent combination vaccines.

AREAS COVERED

We expanded this finding to show that the chemical conjugation of different proteic and/or polysaccharidic antigens, to GMMA, is a methodology complementary to the genetic manipulation to obtain highly effective combination vaccines. Here we discuss our findings with a specific focus on the impact that GMMA technology can have on global health, as this technology platform is particularly suited to support the development of affordable vaccines for low-income countries.

EXPERT OPINION

We believe that it is critical to elucidate the mode of action of GMMA immunogenicity and have provided a summarized description of the immunological questions to be addressed in the near future. The improved knowledge of GMMA might lead to designing more effective and safer GMMA-based vaccines to tackle the most serious vaccine-preventable diseases.

Enhancement of membrane vesicle production by disrupting the degP gene in Meiothermus ruber H328

The phenomenon of membrane vesicle (MV) production is known to be common to all bacterial cells. Although MVs are expected to be employed in a variety of applications, improving MV productivity is essential for applications. Since the deletion of the degP gene, a periplasmic dual-function protease and chaperone, in Escherichia coli has successfully improved MV production capacity, we tried to enhance MV productivity in the thermophilic M. ruber H328 by deleting the degP gene. One gene (mrH_0331) was selected for degP gene from the H328 genome and we constructed the mutant strain ∆degP by deleting the degP gene of the H328 strain that was replaced with the htk gene showing thermophilic kanamaycin resistance by homologous recombination. The mutant strain ∆degP exhibited smooth growth but a lower level of turbidity at 60 °C although there was no difference in growth at 55 °C between the wild strain and the mutant strain. Finally, we have confirmed that incubation at 60 °C increases MV in the mutant strain ∆degP strain about fivefold by using two fluorescent dyes, DiI and FM4-64, which is followed by TEM analysis. The deletion of the degP gene presumably causes an increase in denatured proteins at 60 °C, leading to enhanced MV production. Meanwhile, the S-layer protein included in the outer membrane of the H328 strain increased in the MV fraction prepared from the mutant cells incubated at 60 °C. This indicates that this method is effective for MV production and that degP deletion enhances it in strain H328.

PMAP-36 reduces the innate immune response induced by Bordetella bronchiseptica-derived outer membrane vesicles

Host defense peptides (HDPs), such as cathelicidins, are small, cationic, amphipathic peptides and represent an important part of the innate immune system. Most cathelicidins, including the porcine PMAP-36, are membrane active and disrupt the bacterial membrane. For example, a chicken cathelicidin, CATH-2, has been previously shown to disrupt both Escherichia coli membranes and to release, at sub-lethal concentrations, outer membrane vesicles (OMVs). Since OMVs are considered promising vaccine candidates, we sought to investigate the effect of sub-bactericidal concentrations of PMAP-36 on both OMV release by a porcine strain of Bordetella bronchiseptica and on the modulation of immune responses to OMVs. PMAP-36 treatment of bacteria resulted in a slight increase in OMV release. The characteristics of PMAP-36-induced OMVs were compared with those of spontaneously released OMVs and OMVs induced by heat treatment. The stability of both PMAP-36- and heat-induced OMVs was decreased compared to spontaneous OMVs, as shown by dynamic light scattering. Furthermore, treatment of bacteria with PMAP-36 or heat resulted in an increase in negatively charged phospholipids in the resulting OMVs. A large increase in lysophospholipid content was observed in heat-induced OMVs, which was at least partially due to the activity of the outer-membrane phospholipase A (OMPLA). Although PMAP-36 was detected in OMVs isolated from PMAP-36-treated bacteria, the immune response of porcine bone-marrow-derived macrophages to these OMVs was similar as those against spontaneous or heat-induced OMVs. Therefore, the effect of PMAP-36 addition after OMV isolation was investigated. This did decrease cytokine expression of OMV-stimulated macrophages. These results indicate that PMAP-36 is a promising molecule to attenuate undesirable immune responses, for instance in vaccines.

Architects of their own environment: How membrane proteins shape the Gram-negative cell envelope

Gram-negative bacteria are surrounded by a complex multilayered cell envelope, consisting of an inner and an outer membrane, and separated by the aqueous periplasm, which contains a thin peptidoglycan cell wall. These bacteria employ an arsenal of highly specialized membrane protein machineries to ensure the correct assembly and maintenance of the membranes forming the cell envelope. Here, we review the diverse protein systems, which perform these functions in Escherichia coli, such as the folding and insertion of membrane proteins, the transport of lipoproteins and lipopolysaccharide within the cell envelope, the targeting of phospholipids, and the regulation of mistargeted envelope components. Some of these protein machineries have been known for a long time, yet still hold surprises. Others have only recently been described and some are still missing pieces or yet remain to be discovered.

Bacteria from Infectious Particles to Cell Based Anticancer Targeted Drug Delivery Systems

Bacterial ghosts (BGs) are empty cell envelopes of nonliving evacuated bacterial cells. They are free from their cytoplasmic contents; however, they sustain their cellular 3D morphology and antigenic structures, counting on bioadhesive properties. Lately, they have been tested as an advanced drug delivery system (DDS) for different materials like DNA, peptides, or drugs, either single components or combinations. Different studies have revealed that, BG DDS were paid the greatest attention in recent years. The current review explores the impact of BGs on the field of drug delivery and drug targeting. BGs have a varied area of applications, including vaccine and tumor therapy. Moreover, the use of BGs, their synthesis, their uniqueness as a delivery system and application principles in cancer are discussed. Furthermore, the safety issues of BGs and stability aspects of using ghost bacteria as delivery systems are discussed. Future perspective efforts that must be followed for this important system to continue to grow are important and promising.

Glycine Induction Method: Effective Production of Immunoactive Bacterial Membrane Vesicles with Low Endotoxin Content

Bacteria are known to release nanometer scale proteoliposomes termed bacterial membrane vesicles (MVs), and it is considered that native and bioengineered MVs would be applicable for development of acellular vaccines and novel drug delivery systems in medical settings. However, important considerations for manufacturing purposes include the varied productivity of MV among bacterial species and strains, as well as endotoxicity levels due to the lipopolysaccharide component. The method for MV induction using glycine described here is simple and provides a solution to these problems. Glycine weakens bacterial peptidoglycans and significantly increases bacterial MV formation, while the relative endotoxin activity of glycine-induced MVs is extremely reduced as compared to that of noninduced MVs. Nevertheless, glycine-induced MVs elicit strong immune responses at levels nearly equivalent to those of noninduced MVs. Taken together, the present method for induction by glycine is convenient for research studies of bacterial MVs and has potential for use in medical applications including vaccine development.

Biomimetic Nanoparticles Coated with Bacterial Outer Membrane Vesicles as a New-Generation Platform for Biomedical Applications

The biomedical field is currently reaping the benefits of research on biomimetic nanoparticles (NPs), which are synthetic nanoparticles fabricated with natural cellular materials for nature-inspired biomedical applications. These camouflage NPs are capable of retaining not only the physiochemical properties of synthetic nanoparticles but also the original biological functions of the cellular materials. Accordingly, NPs coated with cell-derived membrane components have achieved remarkable growth as prospective biomedical materials. Particularly, bacterial outer membrane vesicle (OMV), which is a cell membrane coating material for NPs, is regarded as an important molecule that can be employed in several biomedical applications, including immune response activation, cancer therapeutics, and treatment for bacterial infections with photothermal activity. The currently available cell membrane-coated NPs are summarized in this review. Furthermore, the general features of bacterial OMVs and several multifunctional NPs that could serve as inner core materials in the coating strategy are presented, and several methods that can be used to prepare OMV-coated NPs (OMV-NPs) and their characterization are highlighted. Finally, some perspectives of OMV-NPs in various biomedical applications for future potential breakthrough are discussed. This in-depth review, which includes potential challenges, will encourage researchers to fabricate innovative and improvised, new-generation biomimetic materials through future biomedical applications.

The extracellular vesicle generation paradox: a bacterial point of view

All bacteria produce secreted vesicles that carry out a variety of important biological functions. These extracellular vesicles can improve adaptation and survival by relieving bacterial stress and eliminating toxic compounds, as well as by facilitating membrane remodeling and ameliorating inhospitable environments. However, vesicle production comes with a price. It is energetically costly and, in the case of colonizing pathogens, it elicits host immune responses, which reduce bacterial viability. This raises an interesting paradox regarding why bacteria produce vesicles and begs the question as to whether the benefits of producing vesicles outweigh their costs. In this review, we discuss the various advantages and disadvantages associated with Gram-negative and Gram-positive bacterial vesicle production and offer perspective on the ultimate score. We also highlight questions needed to advance the field in determining the role for vesicles in bacterial survival, interkingdom communication, and virulence.

Microbiota-derived extracellular vesicles in interkingdom communication in the gut

The intestine is fundamental in controlling human health. Intestinal epithelial and immune cells are continuously exposed to millions of microbes that greatly impact on intestinal epithelial barrier and immune function. This microbial community, known as gut microbiota, is now recognized as an important partner of the human being that actively contribute to essential functions of the intestine but also of distal organs. In the gut ecosystem, bidirectional microbiota-host communication does not involve direct cell contacts. Both microbiota and host-derived extracellular vesicles (EVs) are key players of such interkingdom crosstalk. There is now accumulating body of evidence that bacterial secreted vesicles mediate microbiota functions by transporting and delivering into host cells effector molecules that modulate host signalling pathways and cell processes. Consequently, vesicles released by the gut microbiota may have great influence on health and disease. Here we review current knowledge on microbiota EVs and specifically highlight their role in controlling host metabolism, intestinal barrier integrity and immune training.

Bacterial Extracellular Vesicles in Gastrointestinal Tract Cancer: An Unexplored Territory

Bacterial extracellular vesicles are membrane-enclosed, lipid bi-layer nanostructures that carry different classes of biomolecules, such as nucleic acids, lipids, proteins, and diverse types of small molecular metabolites, as their cargo. Almost all of the bacteria in the gut secrete extracellular vesicles to assist them in competition, survival, material exchange, host immune modulation, infection, and invasion. The role of gut microbiota in the development, progression, and pathogenesis of gastrointestinal tract (GIT) cancer has been well documented. However, the possible involvement of bacterial extracellular vesicles (bEVs) in GIT cancer pathophysiology has not been given due attention. Studies have illustrated the ability of bEVs to cross physiological barriers, selectively accumulate near tumor cells, and possibly alter the tumor microenvironment (TME). A systematic search of original published works related to bacterial extracellular vesicles on gastrointestinal cancer was performed for this review. The current systemic review outlines the possible impact of gut microbiota derived bEVs in GIT cancer in light of present-day understanding. The necessity of using advanced sequencing technologies, such as genetic, proteomic, and metabolomic investigation methodologies, to facilitate an understanding of the interrelationship between cancer-associated bacterial vesicles and gastrointestinal cancer is also emphasized. We further discuss the clinical and pharmaceutical potential of bEVs, along with future efforts needed to understand the mechanism of interaction of bEVs in GIT cancer pathogenesis.

Potential Applications of Microparticulate-Based Bacterial Outer Membrane Vesicles (OMVs) Vaccine Platform for Sexually Transmitted Diseases (STDs): Gonorrhea, Chlamydia, and Syphilis

Sexually transmitted diseases (STDs) are a major global health issue. Approximately 250 million new cases of STDs occur each year globally. Currently, only three STDs (human papillomavirus (HPV), hepatitis A, and hepatitis B) are preventable by vaccines. Vaccines for other STDs, including gonorrhea, chlamydia, and syphilis, await successful development. Currently, all of these STDs are treated with antibiotics. However, the efficacy of antibiotics is facing growing challenge due to the emergence of bacterial resistance. Therefore, alternative therapeutic approaches, including the development of vaccines against these STDs, should be explored to tackle this important global public health issue. Mass vaccination could be more efficient in reducing the spread of these highly contagious diseases. Bacterial outer membrane vesicle (OMV) is a potential antigen used to prevent STDs. OMVs are released spontaneously during growth by many Gram-negative bacteria. They present a wide range of surface antigens in native conformation that possess interesting properties such as immunogenicity, adjuvant potential, and the ability to be taken up by immune cells, all of which make them an attractive target for application as vaccines against pathogenic bacteria. The major challenge associated with the use of OMVs is its fragile structure and stability. However, a particulate form of the vaccine could be a suitable delivery system that can protect the antigen from degradation by a harsh acidic or enzymatic environment. The particulate form of the vaccine can also act as an adjuvant by itself. This review will highlight some practical methods for formulating microparticulate OMV-based vaccines for STDs.

On the Offensive: the Role of Outer Membrane Vesicles in the Successful Dissemination of New Delhi Metallo-β-lactamase (NDM-1)

The emergence and worldwide dissemination of carbapenemase-producing Gram-negative bacteria are a major public health threat. Metallo-β-lactamases (MBLs) represent the largest family of carbapenemases. Regrettably, these resistance determinants are spreading worldwide. Among them, the New Delhi metallo-β-lactamase (NDM-1) is experiencing the fastest and largest geographical spread. NDM-1 β-lactamase is anchored to the bacterial outer membrane, while most MBLs are soluble, periplasmic enzymes. This unique cellular localization favors the selective secretion of active NDM-1 into outer membrane vesicles (OMVs). Here, we advance the idea that NDM-containing vesicles serve as vehicles for the local dissemination of NDM-1. We show that OMVs with NDM-1 can protect a carbapenem-susceptible strain of Escherichia coli upon treatment with meropenem in a Galleria mellonella infection model. Survival curves of G. mellonella revealed that vesicle encapsulation enhances the action of NDM-1, prolonging and favoring bacterial protection against meropenem inside the larva hemolymph. We also demonstrate that E. coli cells expressing NDM-1 protect a susceptible Pseudomonas aeruginosa strain within the larvae in the presence of meropenem. By using E. coli variants engineered to secrete variable amounts of NDM-1, we demonstrate that the protective effect correlates with the amount of NDM-1 secreted into vesicles. We conclude that secretion of NDM-1 into OMVs contributes to the survival of otherwise susceptible nearby bacteria at infection sites. These results disclose that OMVs play a role in the establishment of bacterial communities, in addition to traditional horizontal gene transfer mechanisms.

Phage-Mediated Explosive Cell Lysis Induces the Formation of a Different Type of O-IMV in Shewanella vesiculosa M7T

Shewanella vesiculosa M7^T is a cold-adapted Antarctic bacterium that has a great capacity to secrete membrane vesicles (MVs), making it a potentially excellent model for studying the vesiculation process. S. vesiculosa M7^T undergoes a blebbing mechanism to produce different types of MVs, including outer membrane vesicles and outer-inner membrane vesicles (O-IMVs). More recently, other mechanisms have been considered that could lead to the formation of O-IMVs derived from prophage-mediated explosive cell lysis in other bacteria, but it is not clear if they are of the same type. The bacterial growth phase could also have a great impact on the type of MVs, although there are few studies on the subject. In this study, we used high-resolution flow cytometry, transmission electron microscopy, and cryo-electron microscopy (Cryo-EM) analysis to determine the amount and types of MVs S. vesiculosa M7^T secreted during different growth phases. We show that MV secretion increases during the transition from the late exponential to the stationary phase. Moreover, prophage-mediated explosive cell lysis is activated in S. vesiculosa M7^T, increasing the heterogeneity of both single- and double-layer MVs. The sequenced DNA fragments from the MVs covered the entire genome, confirming this explosive cell lysis mechanism. A different structure and biogenesis mechanisms for the explosive cell lysis-derived double-layered MVs was observed, and we propose to name them explosive O-IMVs, distinguishing them from the blebbing O-IMVs; their separation is a first step to elucidate their different functions. In our study, we used for the first time sorting by flow cytometry and Cryo-EM analyses to isolate bacterial MVs based on their nucleic acid content. Further improvements and implementation of bacterial MV separation techniques is essential to develop more in-depth knowledge of MVs.

Structure-Function and Industrial Relevance of Bacterial Aminopeptidase P

Aminopeptidase P (APPro, E.C 3.4.11.9) cleaves N-terminal amino acids from peptides and proteins where the penultimate residue is proline. This metal-ion-dependent enzyme shares a similar fold, catalytic mechanism, and substrate specificity with methionine aminopeptidase and prolidase. It adopts a canonical pita bread fold that serves as a structural basis for the metal-dependent catalysis and assembles as a tetramer in crystals. Similar to other metalloaminopeptidase, APPro requires metal ions for its maximal enzymatic activity, with manganese being the most preferred cation. Microbial aminopeptidase possesses unique characteristics compared with aminopeptidase from other sources, making it a great industrial enzyme for various applications. This review provides a summary of recent progress in the study of the structure and function of aminopeptidase P and describes its various applications in different industries as well as its significance in the environment.

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.

Aberrant Membrane Structures in Hypervesiculating Escherichia coli Strain ΔmlaE ΔnlpI Visualized by Electron Microscopy

Escherichia coli produces extracellular vesicles called outer membrane vesicles (OMVs) by releasing a part of its outer membrane. We previously reported that the combined deletion of nlpI and mlaE, related to envelope structure and phospholipid accumulation in the outer leaflet of the outer membrane, respectively, resulted in the synergistic increase of OMV production. In this study, the analysis of ΔmlaEΔnlpI cells using quick-freeze, deep-etch electron microscopy (QFDE-EM) revealed that plasmolysis occurred at the tip of the long axis in cells and that OMVs formed from this tip. Plasmolysis was also observed in the single-gene knockout mutants ΔnlpI and ΔmlaE. This study has demonstrated that plasmolysis was induced in the hypervesiculating mutant E. coli cells. Furthermore, intracellular vesicles and multilamellar OMV were observed in the ΔmlaEΔnlpI cells. Meanwhile, the secretion of recombinant green fluorescent protein (GFP) expressed in the cytosol of the ΔmlaEΔnlpI cells was more than 100 times higher than that of WT and ΔnlpI, and about 50 times higher than that of ΔmlaE in the OMV fraction, suggesting that cytosolic components were incorporated into outer-inner membrane vesicles (OIMVs) and released into the extracellular space. Additionally, QFDE-EM analysis revealed that ΔmlaEΔnlpI sacculi contained many holes noticeably larger than the mean radius of the peptidoglycan (PG) pores in wild-type (WT) E. coli. These results suggest that in ΔmlaEΔnlpI cells, cytoplasmic membrane materials protrude into the periplasmic space through the peptidoglycan holes and are released as OIMVs.

Disruption of the Pseudomonas aeruginosa Tat system perturbs PQS-dependent quorum sensing and biofilm maturation through lack of the Rieske cytochrome bc1 sub-unit

Extracellular DNA (eDNA) is a major constituent of the extracellular matrix of Pseudomonas aeruginosa biofilms and its release is regulated via pseudomonas quinolone signal (PQS) dependent quorum sensing (QS). By screening a P. aeruginosa transposon library to identify factors required for DNA release, mutants with insertions in the twin-arginine translocation (Tat) pathway were identified as exhibiting reduced eDNA release, and defective biofilm architecture with enhanced susceptibility to tobramycin. P. aeruginosa tat mutants showed substantial reductions in pyocyanin, rhamnolipid and membrane vesicle (MV) production consistent with perturbation of PQS-dependent QS as demonstrated by changes in pqsA expression and 2-alkyl-4-quinolone (AQ) production. Provision of exogenous PQS to the tat mutants did not return pqsA, rhlA or phzA1 expression or pyocyanin production to wild type levels. However, transformation of the tat mutants with the AQ-independent pqs effector pqsE restored phzA1 expression and pyocyanin production. Since mutation or inhibition of Tat prevented PQS-driven auto-induction, we sought to identify the Tat substrate(s) responsible. A pqsA::lux fusion was introduced into each of 34 validated P. aeruginosa Tat substrate deletion mutants. Analysis of each mutant for reduced bioluminescence revealed that the primary signalling defect was associated with the Rieske iron-sulfur subunit of the cytochrome bc₁ complex. In common with the parent strain, a Rieske mutant exhibited defective PQS signalling, AQ production, rhlA expression and eDNA release that could be restored by genetic complementation. This defect was also phenocopied by deletion of cytB or cytC₁. Thus, either lack of the Rieske sub-unit or mutation of cytochrome bc₁ genes results in the perturbation of PQS-dependent autoinduction resulting in eDNA deficient biofilms, reduced antibiotic tolerance and compromised virulence factor production.

Pseudomonas aeruginosa is a highly adaptable human pathogen responsible for causing chronic biofilm-associated infections. Biofilms are highly refractory to host defences and antibiotics and thus difficult to eradicate. The biofilm extracellular matrix incorporates extracellular DNA (eDNA). This stabilizes biofilm architecture and helps confer tolerance to antibiotics. Since mechanisms that control eDNA release are not well understood, we screened a P. aeruginosa mutant bank for strains with defects in eDNA release and discovered a role for the twin-arginine translocation (Tat) pathway that exports folded proteins across the cytoplasmic membrane. Perturbation of the Tat pathway resulted in defective biofilms susceptible to antibiotic treatment as a consequence of perturbed pseudomonas quinolone (PQS) signalling. This resulted in the failure to produce or release biofilm components including eDNA, phenazines and rhamnolipids as well as microvesicles. Furthermore, we discovered that perturbation of PQS signalling was a consequence of the inability of tat mutants to translocate the Rieske subunit of the cytochrome bc₁ complex involved in electron transfer and energy transduction. Given the importance of PQS signalling and the Tat system to virulence and biofilm maturation in P. aeruginosa, our findings underline the potential of the Tat system as a drug target for novel antimicrobial agents.

Outer Membrane Vesicles Protect Gram-Negative Bacteria against Host Defense Peptides

Host defense peptides (HDPs) are part of the innate immune system and constitute a first line of defense against invading pathogens. They possess antimicrobial activity against a broad spectrum of pathogens. However, pathogens have been known to adapt to hostile environments. Therefore, the bacterial response to treatment with HDPs was investigated. Previous observations suggested that sublethal concentrations of HDPs increase the release of outer membrane vesicles (OMVs) in Escherichia coli. First, the effects of sublethal treatment with HDPs CATH-2, PMAP-36, and LL-37 on OMV release of several Gram-negative bacteria were analyzed. Treatment with PMAP-36 and CATH-2 induced release of OMVs, but treatment with LL-37 did not. The OMVs were further characterized with respect to morphological properties. The HDP-induced OMVs often had disc-like shapes. The beneficial effect of bacterial OMV release was studied by determining the susceptibility of E. coli toward HDPs in the presence of OMVs. The minimal bactericidal concentration was increased in the presence of OMVs. It is concluded that OMV release is a means of bacteria to dispose of HDP-affected membrane. Furthermore, OMVs act as a decoy for HDPs and thereby protect the bacterium. IMPORTANCE Antibiotic resistance is a pressing problem and estimated to be a leading cause of mortality by 2050. Antimicrobial peptides, also known as host defense peptides (HDPs), and HDP-derived antimicrobials have potent antimicrobial activity and high potential as alternatives to antibiotics due to low resistance development. Some resistance mechanisms have developed in bacteria, and complete understanding of bacterial defense against HDPs will aid their use in the clinic. This study provides insight into outer membrane vesicles (OMVs) as potential defense mechanisms against HDPs, which will allow anticipation of unforeseen resistance to HDPs in clinical use and possibly prevention of bacterial resistance by the means of OMVs.

The Attenuated Protective Effect of Outer Membrane Vesicles Produced by a mcr-1 Positive Strain on Colistin Sensitive Escherichia coli

Resistance to colistin, especially mobilized colistin resistance (mcr), is a serious threat to public health since it may catalyze a return of the “pre-antibiotic era”. Outer membrane vesicles (OMVs) play a role in antibiotic resistance in various ways. Currently, how OMVs participate in mcr-1-mediated colistin resistance has not been established. In this study, we showed that both OMVs from the mcr-1 negative and positive Escherichia coli (E. coli) strains conferred dose-dependent protection from colistin. However, OMVs from the mcr-1 positive strain conferred attenuated protection when compared to the OMVs of a mcr-1 negative strain at the same concentration. The attenuated protective effect of OMVs was related to the reduced ability to absorb colistin from the environment, thus promoting the killing of colistin sensitive E. coli strains. Lipid A modified with phosphoethanolamine was presented in the OMVs of the mcr-1 positive E. coli strain and resulted in decreased affinity to colistin and less protection. Meanwhile, E. coli strain carrying the mcr-1 gene packed more unmodified lipid A in OMVs and kept more phosphoethanolamine modified lipid A in the bacterial cells. Our study provides a first glimpse of the role of OMVs in mcr-1 -mediated colistin resistance.

Metallo-β-lactamases in the Age of Multidrug Resistance: From Structure and Mechanism to Evolution, Dissemination, and Inhibitor Design

Antimicrobial resistance is one of the major problems in current practical medicine. The spread of genes coding for resistance determinants among bacteria challenges the use of approved antibiotics, narrowing the options for treatment. Resistance to carbapenems, last resort antibiotics, is a major concern. Metallo-β-lactamases (MBLs) hydrolyze carbapenems, penicillins, and cephalosporins, becoming central to this problem. These enzymes diverge with respect to serine-β-lactamases by exhibiting a different fold, active site, and catalytic features. Elucidating their catalytic mechanism has been a big challenge in the field that has limited the development of useful inhibitors. This review covers exhaustively the details of the active-site chemistries, the diversity of MBL alleles, the catalytic mechanism against different substrates, and how this information has helped developing inhibitors. We also discuss here different aspects critical to understand the success of MBLs in conferring resistance: the molecular determinants of their dissemination, their cell physiology, from the biogenesis to the processing involved in the transit to the periplasm, and the uptake of the Zn(II) ions upon metal starvation conditions, such as those encountered during an infection. In this regard, the chemical, biochemical and microbiological aspects provide an integrative view of the current knowledge of MBLs.

Production of Rainbow Colorants by Metabolically Engineered Escherichia coli

There has been much interest in producing natural colorants to replace synthetic colorants of health concerns. Escherichia coli has been employed to produce natural colorants including carotenoids, indigo, anthocyanins, and violacein. However, production of natural green and navy colorants has not been reported. Many natural products are hydrophobic, which are accumulated inside or on the cell membrane. This causes cell growth limitation and consequently reduces production of target chemicals. Here, integrated membrane engineering strategies are reported for the enhanced production of rainbow colorants-three carotenoids and four violacein derivatives-as representative hydrophobic natural products in E. coli. By integration of systems metabolic engineering, cell morphology engineering, inner- and outer-membrane vesicle formation, and fermentation optimization, production of rainbow colorants are significantly enhanced to 322 mg L-1 of astaxanthin (red), 343 mg L-1 of β-carotene (orange), 218 mg L-1 of zeaxanthin (yellow), 1.42 g L-1 of proviolacein (green), 0.844 g L-1 of prodeoxyviolacein (blue), 6.19 g L-1 of violacein (navy), and 11.26 g L-1 of deoxyviolacein (purple). The membrane engineering strategies reported here are generally applicable to microbial production of a broader range of hydrophobic natural products, contributing to food, cosmetic, chemical, and pharmaceutical industries.

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.

Extracellular Vesicle Analysis by Paper Spray Ionization Mass Spectrometry

Paper spray ionization mass spectrometry (PSI-MS) is a direct MS analysis technique with several reported bacterial metabolomics applications. As with most MS-based bacterial studies, all currently reported PSI-MS bacterial analyses have focused on the chemical signatures of the cellular unit. One dimension of the bacterial metabolome that is often lost in such analyses is the exometabolome (extracellular metabolome), including secreted metabolites, lipids, and peptides. A key component of the bacterial exometabolome that is gaining increased attention in the microbiology and biomedical communities is extracellular vesicles (EVs). These excreted structures, produced by cells in all domains of life, contain a variety of biomolecules responsible for a wide array of cellular functions, thus representing a core component of the bacterial secreted metabolome. Although previously examined using other MS approaches, no reports currently exist for a PSI-MS analysis of bacterial EVs, nor EVs from any other organism (exosomes, ectosomes, etc.). PSI-MS holds unique analytical strengths over other commonly used MS platforms and could thus provide an advantageous approach to EV metabolomics. To address this, we report a novel application representing, to our knowledge, the first PSI-MS analysis of EVs from any organism (using the human gut resident Oxalobacter formigenes as the experimental model, a bacterium whose EVs were never previously investigated). In this report, we show how we isolated and purified EVs from bacterial culture supernatant by EV-specific affinity chromatography, confirmed and characterized these vesicles by nanoparticle tracking analysis, analyzed the EV isolate by PSI-MS, and identified a panel of EV-derived metabolites, lipids, and peptides. This work serves as a pioneering study in the field of MS-based EV analysis and provides a new, rapid, sensitive, and economical approach to EV metabolomics.

Reaching out in anticipation: bacterial membrane extensions represent a permanent investment in polysaccharide sensing and utilization

Outer membrane extensions are common in many marine bacteria. However, the function of these surface enlargements or extracellular compartments is poorly understood. Using a combined approach of microscopy and subproteome analyses, we therefore examined Pseudoalteromonas distincta ANT/505, an Antarctic polysaccharide degrading gamma-proteobacterium. P. distincta produced outer membrane vesicles (MV) and vesicle chains (VC) on polysaccharide and non-polysaccharide carbon sources during the exponential and stationary growth phase. Surface structures of carbohydrate-grown cells were equipped with increased levels of highly substrate-specific proteins. At the same time, proteins encoded in all other polysaccharide degradation-related genomic regions were also detected in MV and VC samples under all growth conditions, indicating a basal expression. In addition, two alkaline phosphatases were highly abundant under non-limiting phosphate conditions. Surface structures may thus allow rapid sensing and fast responses in nutritionally deprived environments. It may also facilitate efficient carbohydrate processing and reduce loss of substrates and enzymes by diffusion as important adaptions to the aquatic ecosystem.

"One for All": Functional Transfer of OMV-Mediated Polymyxin B Resistance From Salmonella enterica sv. Typhi ΔtolR and ΔdegS to Susceptible Bacteria

The appearance of multi-resistant strains has contributed to reintroducing polymyxin as the last-line therapy. Although polymyxin resistance is based on bacterial envelope changes, other resistance mechanisms are being reported. Outer membrane vesicles (OMVs) are nanosized proteoliposomes secreted from the outer membrane of Gram-negative bacteria. In some bacteria, OMVs have shown to provide resistance to diverse antimicrobial agents either by sequestering and/or expelling the harmful agent from the bacterial envelope. Nevertheless, the participation of OMVs in polymyxin resistance has not yet been explored in S. Typhi, and neither OMVs derived from hypervesiculating mutants. In this work, we explored whether OMVs produced by the hypervesiculating strains Salmonella Typhi ΔrfaE (LPS synthesis), ΔtolR (bacterial envelope) and ΔdegS (misfolded proteins and σ ^E activation) exhibit protective properties against polymyxin B. We found that the OMVs extracted from S. Typhi ΔtolR and ΔdegS protect S. Typhi WT from polymyxin B in a concentration-depending manner. By contrast, the protective effect exerted by OMVs from S. Typhi WT and S. Typhi ΔrfaE is much lower. This effect is achieved by the sequestration of polymyxin B, as assessed by the more positive Zeta potential of OMVs with polymyxin B and the diminished antibiotic's availability when coincubated with OMVs. We also found that S. Typhi ΔtolR exhibited an increased MIC of polymyxin B. Finally, we determined that S. Typhi ΔtolR and S. Typhi ΔdegS, at a lesser level, can functionally and transiently transfer the OMV-mediated polymyxin B resistance to susceptible bacteria in cocultures. This work shows that mutants in genes related to OMVs biogenesis can release vesicles with improved abilities to protect bacteria against membrane-active agents. Since mutations affecting OMV biogenesis can involve the bacterial envelope, mutants with increased resistance to membrane-acting agents that, in turn, produce protective OMVs with a high vesiculation rate (e.g., S. Typhi ΔtolR) can arise. Such mutants can functionally transfer the resistance to surrounding bacteria via OMVs, diminishing the effective concentration of the antimicrobial agent and potentially favoring the selection of spontaneous resistant strains in the environment. This phenomenon might be considered the source for the emergence of polymyxin resistance in an entire bacterial community.

Improving cell-free glycoprotein synthesis by characterizing and enriching native membrane vesicles

Cell-free gene expression (CFE) systems from crude cellular extracts have attracted much attention for biomanufacturing and synthetic biology. However, activating membrane-dependent functionality of cell-derived vesicles in bacterial CFE systems has been limited. Here, we address this limitation by characterizing native membrane vesicles in Escherichia coli-based CFE extracts and describing methods to enrich vesicles with heterologous, membrane-bound machinery. As a model, we focus on bacterial glycoengineering. We first use multiple, orthogonal techniques to characterize vesicles and show how extract processing methods can be used to increase concentrations of membrane vesicles in CFE systems. Then, we show that extracts enriched in vesicle number also display enhanced concentrations of heterologous membrane protein cargo. Finally, we apply our methods to enrich membrane-bound oligosaccharyltransferases and lipid-linked oligosaccharides for improving cell-free N-linked and O-linked glycoprotein synthesis. We anticipate that these methods will facilitate on-demand glycoprotein production and enable new CFE systems with membrane-associated activities.

Role of Microbiota-Derived Extracellular Vesicles in Gut-Brain Communication

Human intestinal microbiota comprise of a dynamic population of bacterial species and other microorganisms with the capacity to interact with the rest of the organism and strongly influence the host during homeostasis and disease. Commensal and pathogenic bacteria coexist in homeostasis with the intestinal epithelium and the gastrointestinal tract’s immune system, or GALT (gut-associated lymphoid tissue), of the host. However, a disruption to this homeostasis or dysbiosis by different factors (e.g., stress, diet, use of antibiotics, age, inflammatory processes) can cause brain dysfunction given the communication between the gut and brain. Recently, extracellular vesicles (EVs) derived from bacteria have emerged as possible carriers in gut-brain communication through the interaction of their vesicle components with immune receptors, which lead to neuroinflammatory immune response activation. This review discusses the critical role of bacterial EVs from the gut in the neuropathology of brain dysfunctions by modulating the immune response. These vesicles, which contain harmful bacterial EV contents such as lipopolysaccharide (LPS), peptidoglycans, toxins and nucleic acids, are capable of crossing tissue barriers including the blood-brain barrier and interacting with the immune receptors of glial cells (e.g., Toll-like receptors) to lead to the production of cytokines and inflammatory mediators, which can cause brain impairment and behavioral dysfunctions.

Relationship Between Membrane Vesicles, Extracellular ATP and Biofilm Formation in Antarctic Gram-Negative Bacteria

Biofilms offer a safe environment that favors bacterial survival; for this reason, most pathogenic and environmental bacteria live integrated in biofilm communities. The development of biofilms is complex and involves many factors, which need to be studied in order to understand bacterial behavior and control biofilm formation when necessary. We used a collection of cold-adapted Antarctic Gram-negative bacteria to study whether their ability to form biofilms is associated with a capacity to produce membrane vesicles and secrete extracellular ATP. In most of the studied strains, no correlation was found between biofilm formation and these two factors. Only Shewanella vesiculosa M7^T secreted high levels of extracellular ATP, and its membrane vesicles caused a significant increase in the speed and amount of biofilm formation. In this strain, an important portion of the exogenous ATP was contained in membrane vesicles, where it was protected from apyrase treatment. These results confirm that ATP influences biofilm formation. Although the role of extracellular ATP in prokaryotes is still not well understood, the metabolic cost of its production suggests it has an important function, such as a role in biofilm formation. Thus, the liberation of extracellular ATP through membrane vesicles and its function deserve further study.

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.

Comparative Analysis of Outer Membrane Vesicle Isolation Methods With an Escherichia coli tolA Mutant Reveals a Hypervesiculating Phenotype With Outer-Inner Membrane Vesicle Content

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria are mediators of cell survival and pathogenesis by facilitating virulence factor dissemination and resistance to antimicrobials. Studies of OMV properties often focus on hypervesiculating Escherichia coli mutants that have increased OMV production when compared to their corresponding wild-type (WT) strains. Currently, two conventional techniques, ultracentrifugation (UC) and ultradiafiltration (UF), are used interchangeably to isolate OMVs, however, there is concern that each technique may inadvertently alter the properties of isolated OMVs during study. To address this concern, we compared two OMV isolation methods, UC and UF, with respect to final OMV quantities, size distributions, and morphologies using a hypervesiculating Escherichia coli K-12 ΔtolA mutant. Nanoparticle tracking analysis (NTA) indicated that UC techniques result in lower vesicle yields compared to UF. However, UF permitted isolation of OMVs with smaller average sizes than UC, highlighting a potential OMV isolation size bias by each technique. Cryo-transmission electron microscopy (cryo-TEM) visualization of isolated OMVs revealed distinct morphological differences between WT and ΔtolA OMVs, where ΔtolA OMVs isolated by either UC or UF method possessed a greater proportion of OMVs with two or more membranes. Proteomic OMV analysis of WT and ΔtolA OMVs confirmed that ΔtolA enhances inner plasma membrane carryover in multi-lamellar OMVs. This study demonstrates that UC and UF are useful techniques for OMV isolation, where UF may be preferable due to faster isolation, higher OMV yields and enrichment of smaller sized vesicles.

Outer Membrane Vesicles of Gram-Negative Bacteria: An Outlook on Biogenesis

Outer membrane vesicles (OMVs) from Gram-negative bacteria were first described more than 50 years ago. However, the molecular mechanisms involved in biogenesis began to be studied only in the last few decades. Presently, the biogenesis and molecular mechanisms for their release are not completely known. This review covers the most recent information on cellular components involved in OMV biogenesis, such as lipoproteins and outer membrane proteins, lipopolysaccharide, phospholipids, quorum-sensing molecules, and flagella.

RNA release via membrane vesicles in Pseudomonas aeruginosa PAO1 is associated with the growth phase

Pseudomonas aeruginosa PAO1 membrane vesicles (MVs) are known to play a role in cell-to-cell communication. Several studies have shown that the MV composition and physicochemical properties vary according to the bacterial growth stage, but the impact this might have on the externalization of RNA via MVs has not been addressed. Therefore, a study to characterize the RNA content from MVs retrieved at different growth phases was conducted. First, the transcriptome analyses revealed a higher abundance of around 300 RNA species in MVs when compared with the cells. The vesiculation rate along the growth curve was determined, showing that the release of MVs increased during the transition to the stationary phase, whereas it decreased in the late stationary phase. RNA-seq of MVs retrieved along the transition to the stationary phase demonstrated that the RNA cargo of vesicles did not vary. However, the amount of smaller RNAs (<200 nt) inside MVs retrieved in the late exponential phase was higher than in the stationary phase MVs. These results indicate that the externalization of RNA via MVs occurs during late exponential phase and implies the secretion of different types of MVs during growth.

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.

Extracellular heme recycling and sharing across species by novel mycomembrane vesicles of a Gram-positive bacterium

Microbes spontaneously release membrane vesicles (MVs), which play roles in nutrient acquisition and microbial interactions. Iron is indispensable for microbes, but is a difficult nutrient to acquire. However, whether MVs are also responsible for efficient iron uptake and therefore involved in microbial interaction remains to be elucidated. Here, we used a Gram-positive strain, Dietzia sp. DQ12-45-1b, to analyze the function of its MVs in heme-iron recycling and sharing between species. We determined the structure and constituent of MVs and showed that DQ12-45-1b releases MVs originating from the mycomembrane. When comparing proteomes of MVs between iron-limiting and iron-rich conditions, we found that under iron-limiting conditions, heme-binding proteins are enriched. Next, we proved that MVs participate in extracellular heme capture and transport, especially in heme recycling from environmental hemoproteins. Finally, we found that the heme carried in MVs is utilized by multiple species, and we further verified that membrane fusion efficiency and species evolutionary distance determine heme delivery. Together, our findings strongly suggest that MVs act as a newly identified pathway for heme recycling, and represent a public good shared between phylogenetically closely related species.

Real-time tracking of bacterial membrane vesicles reveals enhanced membrane traffic upon antibiotic exposure

Membrane vesicles are ubiquitous carriers of molecular information. A broad understanding of the biological functions of membrane vesicles in bacteria remains elusive because of the imaging challenges during real-time in vivo experiments. Here, we provide a quantitative analysis of the motion of individual vesicles in living microbes using fluorescence microscopy, and we show that while vesicle free diffusion in the intercellular space is rare, vesicles mostly disperse along the bacterial surfaces. Most remarkably, when bacteria are challenged with low doses of antibiotics, vesicle production and traffic, quantified by instantaneous vesicle speeds and total traveled distance per unit time, are significantly enhanced. Furthermore, the enhanced vesicle movement is independent of cell clustering properties but rather is associated with a reduction of the density of surface appendages in response to antibiotics. Together, our results provide insights into the emerging field of spatial organization and dynamics of membrane vesicles in microcolonies.

Protective plant immune responses are elicited by bacterial outer membrane vesicles

Bacterial outer membrane vesicles (OMVs) perform a variety of functions in bacterial survival and virulence. In mammalian systems, OMVs activate immune responses and are exploited as vaccines. However, little work has focused on the interactions of OMVs with plant hosts. Here, we report that OMVs from Pseudomonas syringae and P. fluorescens activate plant immune responses that protect against bacterial and oomycete pathogens. OMV-mediated immunomodulatory activity from these species displayed different sensitivity to biochemical stressors, reflecting differences in OMV content. Importantly, OMV-mediated plant responses are distinct from those triggered by conserved bacterial epitopes or effector molecules alone. Our study shows that OMV-induced protective immune responses are independent of the T3SS and protein, but that OMV-mediated seedling growth inhibition largely depends on proteinaceous components. OMVs provide a unique opportunity to understand the interplay between virulence and host response strategies and add a new dimension to consider in host-microbe interactions.

The role that bacterial outer membrane vesicles (OMVs) play in plant-microbe interactions is poorly characterized. McMillan et al. show that OMVs elicit plant immune responses that protect against pathogens. This study also reveals a use for OMVs as tools to probe the plant immune system.

Outer membrane vesicle vaccines

Outer Membrane Vesicles (OMV) have received increased attention in recent years as a vaccine platform against bacterial pathogens. OMV from Neisseria meningitidis serogroup B have been extensively explored. Following the success of the MeNZB OMV vaccine in controlling an outbreak of N. meningitidis B in New Zealand, additional research and development resulted in the licensure of the OMV-containing four-component 4CMenB vaccine, Bexsero. This provided broader protection against multiple meningococcal B strains. Advances in the field of genetic engineering have permitted further improvements in the platform resulting in increased yields, reduced endotoxicity and decoration with homologous and heterologous antigens to enhance immuno genicity and provide broader protection. The OMV vaccine platform has been extended to many other pathogens. In this review, we discuss progress in the development of the OMV vaccine delivery platform, highlighting successful applications, together with potential challenges and gaps.

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.

Complex Interaction between Resident Microbiota and Misfolded Proteins: Role in Neuroinflammation and Neurodegeneration

Neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and Creutzfeldt-Jakob disease (CJD) are brain conditions affecting millions of people worldwide. These diseases are associated with the presence of amyloid-β (Aβ), alpha synuclein (α-Syn) and prion protein (PrP) depositions in the brain, respectively, which lead to synaptic disconnection and subsequent progressive neuronal death. Although considerable progress has been made in elucidating the pathogenesis of these diseases, the specific mechanisms of their origins remain largely unknown. A body of research suggests a potential association between host microbiota, neuroinflammation and dementia, either directly due to bacterial brain invasion because of barrier leakage and production of toxins and inflammation, or indirectly by modulating the immune response. In the present review, we focus on the emerging topics of neuroinflammation and the association between components of the human microbiota and the deposition of Aβ, α-Syn and PrP in the brain. Special focus is given to gut and oral bacteria and biofilms and to the potential mechanisms associating microbiome dysbiosis and toxin production with neurodegeneration. The roles of neuroinflammation, protein misfolding and cellular mediators in membrane damage and increased permeability are also discussed.

Development of a Simple and Rapid Method for In Situ Vesicle Detection in Cultured Media

Extracellular membrane vesicles (EMVs) are biogenic secretory lipidic vesicles that play significant roles in intercellular communication related to human diseases and bacterial pathogenesis. They are being investigated for their possible use in diagnosis, vaccines, and biotechnology. However, the existing methods suffer from a number of issues. High-speed centrifugation, a widely used method to collect EMVs, may cause structural artifacts. Immunostaining methods require several steps and thus the separation and detection of EMVs from the secretory cells is time-consuming. Furthermore, detection of EMVs using these methods requires specific and costly antibodies. To tackle these problems, development of a simple and rapid detection method for the EMVs in the cultured medium without separation from the secretory cells is a pressing task. In this study, we focused on the Gram-negative bacterium Shewanella vesiculosa HM13, which produces a large amount of EMVs including a cargo protein with high purity, as a model. Curvature-sensing peptides were used for EMV-detection tools. FAAV, a peptide derived from sorting nexin protein 1, selectively binds to the EMVs even in the presence of the secretory cells in the complex cultured medium. FAAV can fully detect the EMVs within a few minutes, and the resistance of FAAV to proteases enables it to withstand prolonged use in the cultured medium. Fluorescence/Förster resonance energy transfer was used to develop a method to detect changes in the amount of the EMVs with high sensitivity. Overall, our results indicate the potential applicability of FAAV for in situ EMV detection in cultured media.

Immunization With Outer Membrane Vesicles Derived From Major Outer Membrane Protein-Deficient Salmonella Typhimurium Mutants for Cross Protection Against Salmonella Enteritidis and Avian Pathogenic Escherichia coli O78 Infection in Chickens

Colibacillosis is an economically important infectious disease in poultry, caused by avian pathogenic Escherichia coli (APEC). Salmonella enterica serovar Enteritidis (S. Enteritidis) is a major cause of food-borne diseases in human circulated through poultry-derived products, including meat and chicken eggs. Vaccine control is the mainstream approach for combating these infections, but it is difficult to create a vaccine for the broad-spectrum protection of poultry due to multiple serotypes of these pathogens. Our previous studies have shown that outer membrane vesicles (OMVs) derived from S. enterica serovar Typhimurium mutants with a remodeled outer membrane could induce cross-protection against heteroserotypic Salmonella infection. Therefore, in this study, we further evaluated the potential of broad-spectrum vaccines based on major outer membrane protein (OMP)-deficient OMVs, including ΔompA, ΔompC, and ΔompD, and determined the protection effectiveness of these candidate vaccines in murine and chicken infection models. The results showed that ΔompA led to an increase in the production of OMVs. Notably, ΔompAΔompCΔompD OMVs showed significantly better cross-protection against S. enterica serovar Choleraesuis, S. Enteritidis, APEC O78, and Shigella flexneri 2a than did other omp-deficient OMVs, with the exception of ΔompA OMVs. Subsequently, we verified the results in the chicken model, in which ΔompAΔompCΔompD OMVs elicited significant cross-protection against S. Enteritidis and APEC O78 infections. These findings further confirmed the feasibility of improving the immunogenicity of OMVs by remodeling the outer membrane and provide a new perspective for the development of broad-spectrum vaccines based on OMVs.

Multilamellar and Multivesicular Outer Membrane Vesicles Produced by a Buttiauxella agrestis tolB Mutant

Outer membrane vesicles (OMVs) are naturally released from Gram-negative bacteria and play important roles in various biological functions. Released vesicles are not uniform in shape, size, or characteristics, and little is known about this diversity of OMVs. Here, we show that deletion of tolB, which encodes a part of the Tol-Pal system, leads to the production of multiple types of vesicles and increases overall vesicle production in the high-vesicle-forming Buttiauxella agrestis type strain JCM 1090. The ΔtolB mutant produced small OMVs and multilamellar/multivesicular OMVs (M-OMVs) as well as vesicles with a striking similarity to the wild type. M-OMVs, previously undescribed, contained triple-lamellar membrane vesicles and multiple vesicle-incorporating vesicles. Ultracentrifugation enabled the separation and purification of each type of OMV released from the ΔtolB mutant, and visualization by quick-freeze deep-etch and replica electron microscopy indicated that M-OMVs are composed of several lamellar membranes. Visualization of intracellular compartments of ΔtolB mutant cells showed that vesicles were accumulated in the broad periplasm, which is probably due to the low linkage between the outer and inner membranes attributed to the Tol-Pal defect. The outer membrane was invaginating inward by wrapping a vesicle, and the precursor of M-OMVs existed in the cell. Thus, we demonstrated a novel type of bacterial OMV and showed that unconventional processes enable the B. agrestis ΔtolB mutant to form unique vesicles.IMPORTANCE Membrane vesicle (MV) formation has been recognized as a common mechanism in prokaryotes, and MVs play critical roles in intercellular interaction. However, a broad range of MV types and their multiple production processes make it difficult to gain a comprehensive understanding of MVs. In this work, using vesicle separation and electron microscopic analyses, we demonstrated that diverse types of outer membrane vesicles (OMVs) were released from an engineered strain, Buttiauxella agrestis JCM 1090^T ΔtolB mutant. We also discovered a previously undiscovered type of vesicle, multilamellar/multivesicular outer membrane vesicles (M-OMVs), which were released by this mutant using unconventional processes. These findings have facilitated considerable progress in understanding MV diversity and expanding the utility of MVs in biotechnological applications.

Enhanced floc formation by degP-deficient Escherichia coli cells in the presence of glycerol

Flocculation is an aggregation phenomenon of microbial cells in which they form flocs or flakes. In this study, it was found that addition of glycerol to a complex glucose medium promoted spontaneous floc formation by an Escherichia coli degP-deficient mutant strain (ΔdegP) in a dose-dependent manner. In the presence of 10% (v/v) glycerol, the amount of floc formation (quantified as floc protein) reached its maximum value (230 mg/L), five times that in its absence. 10% (v/v) glycerol was the limit concentration that does not inhibit cell growth of ΔdegP strain. Glycerol was not consumed by ΔdegP cells during floc formation. To provide media having nearly the same viscosity as that containing 10% (v/v) glycerol, carboxymethyl cellulose (CMC) or polyvinylpyrrolidone (PVP) were added to medium as viscosifying agents. Floc formation was not promoted by increasing the medium viscosity with CMC or PVP. However, addition of ethylene glycol also significantly promoted floc formation in the same manner as glycerol. Addition of short-chain polyols decreased the number of viable ΔdegP cells in the floc structure and enhanced outer membrane vesicle (OMV) production by ΔdegP cells; polyols-induced damage on the outer membrane of ΔdegP cells may contribute to the promoted floc formation.

Glycine significantly enhances bacterial membrane vesicle production: a powerful approach for isolation of LPS-reduced membrane vesicles of probiotic Escherichia coli

Bacterial membrane vesicles (MVs) have attracted strong interest in recent years as novel nanoparticle delivery platforms. Glycine is known to induce morphological changes in the outer layer of bacteria. We report here that glycine dramatically facilitates MV production in a flagella-deficient mutant of the non-pathogenic probiotic Escherichia coli strain Nissle 1917. Supplementation of culture medium with 1.0% glycine induced cell deformation at the early exponential phase, eventually followed by quasi-lysis during the late exponential to stationary phase. Glycine supplementation also significantly increased the number of MVs with enlarged particle size and altered the protein profile with an increase in the inner membrane and cytoplasmic protein contents as compared to non-induced MVs. Of note, the endotoxin activity of glycine-induced MVs was approximately eightfold or sixfold lower than that of non-induced MVs when compared at equal protein or lipid concentrations respectively. Nevertheless, glycine-induced MVs efficiently induced both immune responses in a mouse macrophage-like cell line and adjuvanticity in an intranasal vaccine mouse model, comparable to those of non-induced MVs. We propose that the present method of inducing MV production with glycine can be used for emerging biotechnological applications of MVs that have immunomodulatory activities, while dramatically reducing the presence of endotoxins.