An update on our understanding of Gram-positive bacterial membrane vesicles: discovery, functions, and applications

Extracellular vesicles (EVs) are nano-sized particles released from cells into the extracellular environment, and are separated from eukaryotic cells, bacteria, and other organisms with cellular structures. EVs alter cell communication by delivering their contents and performing various functions depending on their cargo and release into certain environments or other cells. The cell walls of Gram-positive bacteria have a thick peptidoglycan layer and were previously thought to be unable to produce EVs. However, recent studies have demonstrated that Gram-positive bacterial EVs are crucial for health and disease. In this review, we have summarized the formation, composition, and characteristics of the contents, resistance to external stress, participation in immune regulation, and other functions of Gram-positive bacterial EVs, as well as their application in clinical diagnosis and treatment, to provide a new perspective to further our understanding of Gram-positive bacterial EVs.

Roles of bacterial extracellular vesicles in systemic diseases

Accumulating evidence suggests that in various systems, not all bidirectional microbiota-host interactions involve direct cell contact. Bacterial extracellular vesicles (BEVs) may be key participants in this interkingdom crosstalk. BEVs mediate microbiota functions by delivering effector molecules that modulate host signaling pathways, thereby facilitating host-microbe interactions. BEV production during infections by both pathogens and probiotics has been observed in various host tissues. Therefore, these vesicles released by microbiota may have the ability to drive or inhibit disease pathogenesis in different systems within the host. Here, we review the current knowledge of BEVs and particularly emphasize their interactions with the host and the pathogenesis of systemic diseases.

Bacterial Membrane Vesicles: Physiological Roles, Infection Immunology, and Applications

Bacterial or fungal membrane vesicles, traditionally considered as microbial metabolic wastes, are secreted mainly from the outer membrane or cell membrane of microorganisms. However, recent studies have shown that these vesicles play essential roles in direct or indirect communications among microorganisms and between microorganisms and hosts. This review aims to provide an updated understanding of the physiological functions and emerging applications of bacterial membrane vesicles, with a focus on their biogenesis, mechanisms of adsorption and invasion into host cells, immune stimulatory effects, and roles in the much-concerned problem of bacterial resistance. Additionally, the potential applications of these vesicles as biomarkers, vaccine candidates, and drug delivery platforms are discussed.

Microbe-host interactions: structure and functions of Gram-negative bacterial membrane vesicles

Bacteria-host interaction is a common, relevant, and intriguing biological phenomena. The host reacts actively or passively to the bacteria themselves, their products, debris, and so on, through various defense systems containing the immune system, the bacteria communicate with the local or distal tissues of the host via their own surface antigens, secreted products, nucleic acids, etc., resulting in relationships of attack and defense, adaptation, symbiosis, and even collaboration. The significance of bacterial membrane vesicles (MVs) as a powerful vehicle for the crosstalk mechanism between the two is growing. In the recent decade, the emergence of MVs in microbial interactions and a variety of bacterial infections, with multiple adhesions to host tissues, cell invasion and evasion of host defense mechanisms, have brought MVs to the forefront of bacterial pathogenesis research. Whereas MVs are a complex combination of molecules not yet fully understood, research into its effects, targeting and pathogenic components will advance its understanding and utilization. This review will summarize structural, extraction and penetration information on several classes of MVs and emphasize the role of MVs in transport and immune response activation. Finally, the potential of MVs as a therapeutic method will be highlighted, as will future research prospects.

Apilactobacillus kunkeei releases RNA-associated membrane vesicles and proteinaceous nanoparticles

Extracellularly released particles, including membrane vesicles, have increasingly been recognized as important for bacterial community functions and host-interaction processes, but their compositions and functional roles differ between species and also between strains of the same species. In this study, we have determined the composition of membrane vesicles and protein particles identified in the cell-free pellets of two strains of Apilactobacillus kunkeei, a defensive symbiont of honeybees. The membrane vesicles were separated from the extracellular particles using density gradient ultracentrifugation. The peaks of the RNA and protein distributions were separated from each other and the highest concentration of RNA was observed in the fractions that contained the membrane vesicles while the highest protein concentration coincided with the fractions that contained extracellular particles. A comparative proteomics analysis by LC-MS/MS showed that 37 proteins with type-I signal peptides were consistently identified across the fractionated samples obtained from the cell-free pellets, of which 29 were orthologs detected in both strains. Functional predictions of the extracellular proteins revealed the presence of glycoside hydrolases, glycosyltransferases, giant proteins and peptidases. The extracellular transcriptomes mapped to a broad set of genes with a similar functional profile as the whole cell transcriptome. This study provides insights into the composition of membrane vesicles and extracellular proteins of a bee-associated symbiont.

A Mildly Acidic Environment Alters Pseudomonas aeruginosa Virulence and Causes Remodeling of the Bacterial Surface

Pseudomonas aeruginosa is a versatile pathogen that resists environmental stress, such as suboptimal pH. As a result of exposure to environmental stress, P. aeruginosa shows an altered virulence-related phenotype. This study investigated the modifications that P. aeruginosa undertakes at a mildly low pH (pH 5.0) compared with the bacteria grown in a neutral medium (pH 7.2). Results indicated that in a mildly acidic environment, expression of two-component system genes (phoP/phoQ and pmrA/pmrB), lipid A remodeling genes such as arnT and pagP and virulence genes, i.e., pqsE and rhlA, were induced. Moreover, lipid A of the bacteria grown at a mildly low pH is modified by adding 4-amino-arabinose (l-Ara4N). Additionally, the production of virulence factors such as rhamnolipid, alginate, and membrane vesicles is significantly higher in a mildly low-pH environment than in a neutral medium. Interestingly, at a mildly low pH, P. aeruginosa produces a thicker biofilm with higher biofilm biomass. Furthermore, studies on inner membrane viscosity and permeability showed that a mildly low pH causes a decrease in the inner membrane permeability and increases its viscosity. Besides, despite the importance of PhoP, PhoQ, PmrA, and PmrB in Gram-negative bacteria for responding to low pH stress, we observed that the absence of each of these two-component systems does not meaningfully impact the remodeling of the P. aeruginosa envelope. Given that P. aeruginosa is likely to encounter mildly acidic environments during infection in its host, the alterations that the bacterium undertakes under such conditions must be considered in designing antibacterial strategies against P. aeruginosa. IMPORTANCE P. aeruginosa encounters environments with acidic pH when establishing infections in hosts. The bacterium develops an altered phenotype to tolerate a moderate decrease in the environmental pH. At the level of the bacterial envelope, modified lipid A composition and a reduction of the bacterial inner membrane permeability and fluidity are among the changes P. aeruginosa undergoes at a mildly low pH. Also, the bacterium is more likely to form biofilm in a mildly acidic environment. Overall, these alterations in the P. aeruginosa phenotype put obstacles in the way of antibacterial activities. Thus, considering physiological changes in the bacterium at low pH helps design and implement antimicrobial approaches against this hostile microorganism.

Red Blood Cell Deformability Is Expressed by a Set of Interrelated Membrane Proteins

Red blood cell (RBC) deformability, expressing their ability to change their shape, allows them to minimize their resistance to flow and optimize oxygen delivery to the tissues. RBC with reduced deformability may lead to increased vascular resistance, capillary occlusion, and impaired perfusion and oxygen delivery. A reduction in deformability, as occurs during RBC physiological aging and under blood storage, is implicated in the pathophysiology of diverse conditions with circulatory disorders and anemias. The change in RBC deformability is associated with metabolic and structural alterations, mostly uncharacterized. To bridge this gap, we analyzed the membrane protein levels, using mass spectroscopy, of RBC with varying deformability determined by image analysis. In total, 752 membrane proteins were identified. However, deformability was positively correlated with the level of only fourteen proteins, with a highly significant inter-correlation between them. These proteins are involved in membrane rafting and/or the membrane-cytoskeleton linkage. These findings suggest that the reduction of deformability is a programmed (not arbitrary) process of remodeling and shedding of membrane fragments, possibly mirroring the formation of extracellular vesicles. The highly significant inter-correlation between the deformability-expressing proteins infers that the cell deformability can be assessed by determining the level of a few, possibly one, of them.

Activation of the toll-like receptor 2 signaling pathway by GAPDH from bacterial strain RD055328

We characterized the membrane vesicle fraction (RD-MV fraction) from bacterial strain RD055328, which is related to members of the genus Companilactobacillus and Lactiplantibacillus plantarum. RD-MVs and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) were detected in the RD-MV fraction. Immunoglobulin A (IgA) was produced by Peyer's patch cells following the addition of the RD-MV fraction. In the presence of the RD-MV fraction, RAW264 cells produced the pro-inflammatory cytokine IL-6. Recombinant GAPDH probably induced the production of IL-6 by RAW264 cells via superficial toll-like receptor 2 (TLR2) recognition. A confocal laser scanning microscopy image analysis indicated that RD-MVs and GAPDH were taken up by RAW264 cells. GAPDH wrapped around RAW264 cells. We suggest that GAPDH from strain RD055328 enhanced the production of IgA by acquired immune cells via the production of IL-6 by innate immune cells through TLR2 signal transduction.

Membrane Vesicles as Drug Delivery Systems: Source, Preparation, Modification, Drug Loading, In Vivo Administration and Biodistribution, and Application in Various Diseases

Bioinspired (or biologically inspired) drug delivery systems (DDSs) have been intensively studied in the last decades. As bioinspired DDSs, membrane vesicles, including extracellular vesicles (EVs) released from eukaryotic cells, outer membrane vesicles (OMVs) from bacteria, cell-bound membrane vesicles (CBMVs) isolated in situ from cell surfaces, membrane vesicles reorganized after the isolation of the plasma membrane of cells, and others have been rapidly developed and are attracting more and more attention. Most recently, a collection of 25 papers on the advances in membrane vesicle-based drug delivery systems was published in a Special Issue of Pharmaceutics entitled "Advances of membrane vesicles in drug delivery systems". These papers cover many related topics including the source, preparation, modification, drug loading, and in vivo administration and biodistribution of membrane vesicles (mainly extracellular vesicles or exosomes and bacterial outer membrane vesicles), as well as application of membrane vesicles as DDSs in the treatment of various diseases.

Iron Delivery through Membrane Vesicles in Corynebacterium glutamicum

Bacterial cells form and release membrane vesicles (MVs) originating from cellular membranes. In recent years, many biological functions of bacterial MVs have been identified. Here, we show that MVs derived from Corynebacterium glutamicum, a model organism for mycolic acid-containing bacteria, can mediate iron acquisition and other phylogenetically related bacteria. Lipid/protein analysis and iron quantification assay indicate that C. glutamicum MVs formed by outer mycomembrane blebbing can load ferric iron (Fe3+) as its cargo. Iron-loaded C. glutamicum MVs promoted the growth of producer bacteria in iron-limited liquid media. MVs were received by C. glutamicum cells, suggesting a direct transfer of iron to the recipient cells. Cross-feeding of C. glutamicum MVs with phylogenetically close (Mycobacterium smegmatis and Rhodococcus erythropolis) or distant (Bacillus subtilis) bacteria indicated that C. glutamicum MVs could be received by the different species tested, while iron uptake is limited to M. smegmatis and R. erythropolis. In addition, our results indicate that iron loading on MVs in C. glutamicum does not depend on membrane-associated proteins or siderophores, which is different from what has been shown in other mycobacterial species. Our findings illustrate the biological importance of MV-associated extracellular iron for C. glutamicum growth and suggest its ecological impact on selected members of microbial communities. IMPORTANCE Iron is an essential element of life. Many bacteria have developed iron acquisition systems, such as siderophores, for external iron uptake. Corynebacterium glutamicum, a soil bacterium known for its potential for industrial applications, was shown to lack the ability to produce extracellular, low-molecular-weight iron carriers, and it remains elusive how this bacterium acquires iron. Here, we demonstrated that MVs released from C. glutamicum cells could act as extracellular iron carriers that mediate iron uptake. Although MV-associated proteins or siderophores have been shown to play critical roles in MV-mediated iron uptake by other mycobacterial species, the iron delivery through C. glutamicum MVs is not dependent on these factors. Moreover, our results suggest that there is an unidentified mechanism that determines the species specificity of MV-mediated iron acquisition. Our results further demonstrated the important role of MV-associated iron.

The Two Faces of Bacterial Membrane Vesicles: Pathophysiological Roles and Therapeutic Opportunities

Bacterial membrane vesicles (MVs) are nanosized lipid particles secreted by lysis or blebbing mechanisms from Gram-negative and -positive bacteria. It is becoming increasingly evident that MVs can promote antimicrobial resistance but also provide versatile opportunities for therapeutic exploitation. As non-living facsimiles of parent bacteria, MVs can carry multiple bioactive molecules such as proteins, lipids, nucleic acids, and metabolites, which enable them to participate in intra- and interspecific communication. Although energetically costly, the release of MVs seems beneficial for bacterial fitness, especially for pathogens. In this review, we briefly discuss the current understanding of diverse MV biogenesis routes affecting MV cargo. We comprehensively highlight the physiological functions of MVs derived from human pathogens covering in vivo adaptation, colonization fitness, and effector delivery. Emphasis is given to recent findings suggesting a vicious cycle of MV biogenesis, pathophysiological function, and antibiotic therapy. We also summarize potential therapeutical applications, such as immunotherapy, vaccination, targeted delivery, and antimicrobial potency, including their experimental validation. This comparative overview identifies common and unique strategies for MV modification used along diverse applications. Thus, the review summarizes timely aspects of MV biology in a so far unprecedented combination ranging from beneficial function for bacterial pathogen survival to future medical applications.

Membrane vesicle delivery of a streptococcal M protein disrupts the blood-brain barrier by inducing autophagic endothelial cell death

M family proteins are critical virulence determinants of Streptococci. Streptococcus equi subsp. zooepidemicus (SEZ) are Group C streptococci that cause meningitis in animals and humans. SzM, the M protein of SEZ, has been linked to SEZ brain invasion. Here, we demonstrate that SzM is important in SEZ disruption of the blood-brain barrier (BBB). SEZ release SzM-bound membrane vesicles (MVs), and endocytosis of these vesicles by human brain endothelial microvascular cells (hBMECs) results in SzM-dependent cytotoxicity. Furthermore, administration of SzM-bound MVs disrupted the murine BBB. A CRISPR screen revealed that SzM cytotoxicity in hBMECs depends on PTEN-related activation of autophagic cell death. Pharmacologic inhibition of PTEN activity prevented SEZ disruption of the murine BBB and delayed mortality. Our data show that MV delivery of SzM to host cells plays a key role in SEZ pathogenicity and suggests that MV delivery of streptococcal M family proteins is likely a common streptococcal virulence mechanism.

Different culture media and purification methods unveil the core proteome of Propionibacterium freudenreichii-derived extracellular vesicles

Bacterial extracellular vesicles (EVs) are natural lipidic nanoparticles implicated in intercellular communication. Although EV research focused mainly on pathogens, the interest in probiotic-derived EVs is now rising. One example is Propionibacterium freudenreichii, which produces EVs with anti-inflammatory effects on human epithelial cells. Our previous study with P. freudenreichii showed that EVs purified by size exclusion chromatography (SEC) displayed variations in protein content according to bacterial growth conditions. Considering these content variations, we hypothesized that a comparative proteomic analysis of EVs recovered in different conditions would elucidate whether a representative vesicular proteome existed, possibly providing a robust proteome dataset for further analysis. Therefore, P. freudenreichii was grown in two culture media, and EVs were purified by sucrose density gradient ultracentrifugation (UC). Microscopic and size characterization confirmed EV purification, while shotgun proteomics unveiled that they carried a diverse set of proteins. A comparative analysis of the protein content of UC- and SEC-derived EVs, isolated from cultures either in UF (cow milk ultrafiltrate medium) or YEL (laboratory yeast extract lactate medium), showed that EVs from all these conditions shared 308 proteins. This EV core proteome was notably enriched in proteins related to immunomodulation. Moreover, it showed distinctive features, including highly interacting proteins, compositional biases for some specific amino acids, and other biochemical parameters. Overall, this work broadens the toolset for the purification of P. freudenreichii-derived EVs, identifies a representative vesicular proteome, and enumerates conserved features in vesicular proteins. These results hold the potential for providing candidate biomarkers of purification quality, and insights into the mechanisms of EV biogenesis and cargo sorting.

In vitro validation of an in vivo phenotyping drug cocktail for major drug transporters in humans

PURPOSE

Cocktails of transporter probe drugs are used in vivo to assess transporter activity and respective drug-drug interactions. An inhibitory effect of components on transporter activities should be ruled out. Here, for a clinically tested cocktail consisting of adefovir, digoxin, metformin, sitagliptin, and pitavastatin, inhibition of major transporters by individual probe substrates was investigated in vitro.

METHODS

Transporter transfected HEK293 cells were used in all evaluations. Cell-based assays were applied for uptake by human organic cation transporters 1/2 (hOCT1/2), organic anion transporters 1/3 (hOAT1/3), multidrug and toxin extrusion proteins 1/2K (hMATE1/2K), and organic anion transporter polypeptide 1B1 (hOATP1B1). For P-glycoprotein (hMDR1) a cell-based efflux assay was used whereas an inside-out vesicle-based assay was used for the bile salt export pump (hBSEP). All assays used standard substrates and established inhibitors (as positive controls). Inhibition experiments using clinically achievable concentrations of potential perpetrators at the relevant transporter expression site were carried out initially. If there was a significant effect, the inhibition potency (K_i) was studied in detail.

RESULTS

In the inhibition tests, only sitagliptin had an effect and reduced hOCT1- and hOCT2- mediated metformin uptake and hMATE2K mediated MPP⁺ uptake by more than 70%, 80%, and 30%, respectively. The ratios of unbound C_max (observed clinically) to K_i of sitagliptin were low with 0.009, 0.03, and 0.001 for hOCT1, hOCT2, and hMATE2K, respectively.

CONCLUSION

The inhibition of hOCT2 in vitro by sitagliptin is in agreement with the borderline inhibition of renal metformin elimination observed clinically, supporting a dose reduction of sitagliptin in the cocktail.

Hydrogen/deuterium exchange-mass spectrometry of integral membrane proteins in native-like environments: current scenario and the way forward

Integral membrane proteins (IMPs) perform a range of diverse functions and their dysfunction underlies numerous pathological conditions. Consequently, IMPs constitute most drug targets, and the elucidation of their mechanism of action has become an intense field of research. Historically, IMP studies have relied on their extraction from membranes using detergents, which have the potential to perturbate their structure and dynamics. To circumnavigate this issue, an array of membrane mimetics has been developed that aim to reconstitute IMPs into native-like lipid environments that more accurately represent the biological membrane. Hydrogen/deuterium exchange-mass spectrometry (HDX-MS) has emerged as a versatile tool for probing protein dynamics in solution. The continued development of HDX-MS methodology has allowed practitioners to investigate IMPs using increasingly native-like membrane mimetics, and even pushing the study of IMPs into the in vivo cellular environment. Consequently, HDX-MS has come of age and is playing an ever-increasingly important role in the IMP structural biologist toolkit. In the present mini-review, we discuss the evolution of membrane mimetics in the HDX-MS context, focusing on seminal publications and recent innovations that have led to this point. We also discuss state-of-the-art methodological and instrumental advancements that are likely to play a significant role in the generation of high-quality HDX-MS data of IMPs in the future.

Extracellular Vesicles of Probiotics: Shedding Light on the Biological Activity and Future Applications

For many decades, the proper functioning of the human body has become a leading scientific topic. In the course of numerous experiments, a striking impact of probiotics on the human body has been documented, including maintaining the physiological balance of endogenous microorganisms, regulating the functioning of the immune system, enhancing the digestive properties of the host, and preventing or alleviating the course of many diseases. Recent research, especially from the last decade, shows that this health-benefiting activity of probiotics is largely conditioned by the production of extracellular vesicles. Although the importance of extracellular vesicles in the virulence of many live-threatening pathogens is widely described in the literature, much less is known with respect to the health-promoting effect of extracellular vesicles secreted by non-pathogenic microorganisms, including probiotics. Based on this, in the current review article, we decided to collect the latest literature data on the health-inducing properties of extracellular vesicles secreted by probiotics. The characteristics of probiotics' extracellular vesicles will be extended by the description of their physicochemical properties and the proteome in connection with the biological activities exhibited by these structures.

A Universal Strategy to Promote Secretion of G+/G- Bacterial Extracellular Vesicles and Its Application in Host Innate Immune Responses

Both Gram-positive and Gram-negative bacteria release nanosized extracellular vesicles called membrane vesicles (MVs, 20-400 nm), which have great potential in various biomedical applications due to their abilities to deliver effector molecules and induce therapeutic responses. To fully utilize bacterial MVs for therapeutic purposes, regulated and enhanced production of MVs would be highly advantageous. In this study, we developed a universal method to enhance MV yields in both G+/G- bacteria through an autonomous controlled peptidoglycan hydrolase (PGase) expression system. A significant increase (9.37-fold) of MV concentration was observed in engineered E. coli Nissle 1917 compared to the wild-type. With the help of this autonomous system, for the first time we experimentally confirmed horizontal gene transfer and nutrient acquisition in a cocultured bacterial consortium. Furthermore, the engineered probiotic E. coli strains with high yield of MVs showed higher activation of the innate immune responses in human embryonic kidney 293T (HEK293T) and human colorectal carcinoma cells (HCT116), thereby demonstrating the great potential of engineering probiotics in immunology and further living therapeutics in humans.

Hcp1-loaded staphylococcal membrane vesicle vaccine protects against acute melioidosis

Burkholderia pseudomallei is the causal agent of melioidosis, a deadly tropical infectious disease that lacks a vaccine. On the basis of the attenuated Staphylococcus aureus RN4220-Δagr (RN), we engineered the RN4220-Δagr/pdhB-hcp1 strain (RN-Hcp1) to generate B. pseudomallei hemolysin-coregulated protein 1 (Hcp1)-loaded membrane vesicles (^hcp1MVs). The immunization of BALB/c mice with ^hcp1MVs mixed with adjuvant by a three-dose regimen increased the serum specific IgG production. The serum levels of inflammatory factors, including TNF-α and IL-6, in ^hcp1MV-vaccinated mice were comparable with those in PBS-challenged mice. The partial adjuvant effect of staphylococcal MVs was observed with the elevation of specific antibody titer in ^hcp1MV-vaccinated mice relative to those that received the recombinant Hcp1 protein (rHcp1) or MVs derived from RN strain (^ΔagrMVs). The ^hcp1MVs/adjuvant vaccine protected 70% of mice from lethal B. pseudomallei challenge. Immunization with ^hcp1MVs only protected 60% of mice, whereas vaccination with rHcp1 or ^ΔagrMVs conferred no protection. Moreover, mice that received ^hcp1MVs/adjuvant and ^hcp1MVs immunization had low serum TNF-α and IL-6 levels and no inflammatory infiltration in comparison with other groups. In addition, all surviving mice in ^hcp1MVs/adjuvant and ^hcp1MVs groups exhibited no culturable bacteria in their lungs, livers, and spleens five days postinfection. Overall, our data highlighted a new strategy for developing B. pseudomallei vaccine and showed that Hcp1-incorporated staphylococcal MV is a promising candidate for the prevention of acute melioidosis.

Extracellular vesicle-mediated regulation of macrophage polarization in bacterial infections

Extracellular vesicles (EVs) are nanoscale membrane-enveloped vesicles secreted by prokaryotic and eukaryotic cells, which are commonly defined as membrane vesicles (MVs) and exosomes, respectively. They play critical roles in the bacteria-bacteria and bacteria-host interactions. In infectious diseases caused by bacteria, as the first line of defense against pathogens, the macrophage polarization mode commonly determines the success or failure of the host's response to pathogen aggression. M1-type macrophages secrete pro-inflammatory factors that support microbicidal activity, while alternative M2-type macrophages secrete anti-inflammatory factors that perform an antimicrobial immune response but partially allow pathogens to replicate and survive intracellularly. Membrane vesicles (MVs) released from bacteria as a distinctive secretion system can carry various components, including bacterial effectors, nucleic acids, or lipids to modulate macrophage polarization in host-pathogen interaction. Similar to MVs, bacteria-infected macrophages can secrete exosomes containing a variety of components to manipulate the phenotypic polarization of "bystander" macrophages nearby or long distance to differentiate into type M1 or M2 to regulate the course of inflammation. Exosomes can also repair tissue damage associated with the infection by upregulating the levels of anti-inflammatory factors, downregulating the pro-inflammatory factors, and regulating cellular biological behaviors. The study of the mechanisms by which EVs modulate macrophage polarization has opened new frontiers in delineating the molecular machinery involved in bacterial pathogenesis and challenges in providing new strategies for diagnosis and therapy.

Enhancement of IgA production by membrane vesicles derived from Bifidobacterium longum subsp. infantis

Immunoglobulin A (IgA) is involved in the maintenance of gut homeostasis. Although the oral administration of bifidobacteria increases the amount of fecal IgA, the effects of bifidobacteria on intestinal immunity remain unclear. We found and characterized membrane vesicles (MVs) derived from Bifidobacterium longum subsp. infantis toward host immune cells. Bifidobacterium infantis MVs consisted of a cytoplasmic membrane, and extracellular solute-binding protein (ESBP) was specifically detected. In the presence of B. infantis MVs or recombinant ESBP, RAW264 cells produced the pro-inflammatory cytokine IL-6. IgA was produced by Peyer's patches cells following the addition of B. infantis MVs. Therefore, ESBP of B. infantis MVs is involved in the production of IgA by acquired immune cells via the production of IL-6 by innate immune cells.

Methicillin-Resistant Staphylococcus aureus Membrane Vesicles Inhibit the Proliferation and Induce the Apoptosis of Epithelial Cells

Staphylococcus aureus, or methicillin-resistant Staphylococcus aureus (MRSA), is the predominant pathogen in skin and soft tissue infections (SSTIs), and MRSA membrane vesicles (MVs) play a pivotal role in bacterial pathogenesis and the modulation of the host immune response. We aimed to investigate the interaction between MRSA MVs and epithelial cells. In this study, MVs were isolated from an MRSA culture supernatant using the ELD method, comprising an electrophoretic technique used in combination with a 300 kDa cut-off dialysis bag. The proteomic analysis of the MRSA MVs via mass spectrometry showed that shared and distinct proteins exist in the MVs from clinical MRSA isolates with different genetic backgrounds, such as health-care-associated MRSA (HA-MRSA) and community-associated MRSA (CA-MRSA). These MRSA MVs were found to suppress the proliferation and increase the apoptosis of HaCaT cells. We conducted qPCR array, quantitative real-time PCR (qRT-PCR), and Western blotting (WB) analyses, and the results indicated that BCL2 antagonist/killer 1 (Bak1) may be involved in the apoptosis of HaCaT epithelial cells. Our findings suggest that MRSA MVs inhibit the proliferation and induce the apoptosis of epithelial cells.

Membrane Vesicles Derived from Gut Microbiota and Probiotics: Cutting-Edge Therapeutic Approaches for Multidrug-Resistant Superbugs Linked to Neurological Anomalies

Multidrug-resistant (MDR) superbugs can breach the blood-brain barrier (BBB), leading to a continuous barrage of pro-inflammatory modulators and induction of severe infection-related pathologies, including meningitis and brain abscess. Both broad-spectrum or species-specific antibiotics (β-lactamase inhibitors, polymyxins, vancomycin, meropenem, plazomicin, and sarecycline) and biocompatible poly (lactic-co-glycolic acid) (PLGA) nanoparticles have been used to treat these infections. However, new therapeutic platforms with a broad impact that do not exert off-target deleterious effects are needed. Membrane vesicles or extracellular vesicles (EVs) are lipid bilayer-enclosed particles with therapeutic potential owing to their ability to circumvent BBB constraints. Bacteria-derived EVs (bEVs) from gut microbiota are efficient transporters that can penetrate the central nervous system. In fact, bEVs can be remodeled via surface modification and CRISPR/Cas editing and, thus, represent a novel platform for conferring protection against infections breaching the BBB. Here, we discuss the latest scientific research related to gut microbiota- and probiotic-derived bEVs, and their therapeutic modifications, in terms of regulating neurotransmitters and inhibiting quorum sensing, for the treatment of neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases. We also emphasize the benefits of probiotic-derived bEVs to human health and propose a novel direction for the development of innovative heterologous expression systems to combat BBB-crossing pathogens.

Spontaneous Prophage Induction Contributes to the Production of Membrane Vesicles by the Gram-Positive Bacterium Lacticaseibacillus casei BL23

The formation of membrane vesicles (MVs) by Gram-positive bacteria has gained increasing attention over the last decade. Recently, models of vesicle formation have been proposed and involve the digestion of the cell wall by prophage-encoded or stress-induced peptidoglycan (PG) hydrolases and the inhibition of PG synthesis by β-lactam antibiotics. The impact of these mechanisms on vesicle formation is largely dependent on the strain and growth conditions. To date, no information on the production of vesicles by the lactobacilli family has been reported. Here, we aimed to characterize the MVs released by the Gram-positive bacteria Lacticaseibacillus casei BL23 and also investigated the mechanisms involved in vesicle formation. Using electron microscopy, we established that the size of the majority of L. casei BL23 vesicles ranged from 50 to 100 nm. Furthermore, we showed that the vesicles were released consistently throughout the growth of the bacteria in standard culture conditions. The protein composition of the vesicles released in the supernatant was identified and a significant number of prophage proteins was detected. Moreover, using a mutant strain harboring a defective PLE2 prophage, we were able to show that the spontaneous and mitomycin-triggered induction of the prophage PLE2 contribute to the production of MVs by L. casei BL23. Finally, we also demonstrated the influence of prophages on the membrane integrity of bacteria. Overall, our results suggest a key role of the prophage PLE2 in the production of MVs by L. casei BL23 in the absence or presence of genotoxic stress.

Extracellular vesicle miRNAs promote the intestinal microenvironment by interacting with microbes in colitis

Inflammatory bowel disease (IBD) is a global disease with no cure. Disruption of the microbial ecosystem is considered to be an important cause of IBD. Extracellular vesicles (EVs) are vital participants in cell-cell and cell-organism communication. Both host-derived EVs and bacteria-derived membrane vesicles (OMVs) contribute to homeostasis in the intestine. However, the roles of EVs-miRNAs and MVs in host-microbe interactions in colitis remain unclear. In the present study, the animal model of colitis was established by dextran sulfate sodium (DSS) to investigate the changes of miRNAs in colonic EVs from colitis. Several miRNAs were significantly altered in colitis EVs. miR-181b-5p transplantation inhibited M1 macrophage polarization and promoted M2 polarization to reduce the levels of inflammation both in acute and remission of chronic colitis. miR-200b-3p could interact with bacteria and regulate the composition of the microbiota, which contributed to intestinal barrier integrity and homeostasis. Notably, MVs from normal feces could effectively reverse the composition of the intestinal microbiota, restore the intestinal barrier and rescue colitis, and BMVs from colitis would also have similar effects after miR-200b-3p treatment. Our results preliminarily identify a vesicle-based host-microbe interaction cycle in colitis and provide new ideas for colitis treatment.