Autolysis-mediated membrane vesicle formation in Bacillus subtilis

Horizontal gene transfer and beyond: the delivery of biological matter by bacterial membrane vesicles to host and bacterial cells

Membrane vesicles (MVs) are produced in all domains of life. In eukaryotes, extracellular vesicles have been shown to mediate the horizontal transfer of biological material between cells [1]. Therefore, bacterial MVs are also thought to mediate horizontal material transfer to host cells and other bacteria, especially in the context of cell stress. In this review, we discuss the mechanisms of bacterial MV production, evidence that their contents can be trafficked to host cells and other bacteria, and the biological relevance of horizontal material transfer by bacterial MVs.

Influences of bacterial extracellular vesicles on macrophage immune functions

Bacterial extracellular vesicles (EVs) are crucial mediators of information transfer between bacteria and host cells. Macrophages, as key effector cells in the innate immune system, have garnered widespread attention for their interactions with bacterial EVs. Increasing evidence indicates that bacterial EVs can be internalized by macrophages through multiple pathways, thereby influencing their immune functions. These functions include inflammatory responses, antimicrobial activity, antigen presentation, and programmed cell death. Therefore, this review summarizes current research on the interactions between bacterial EVs and macrophages. This will aid in the deeper understanding of immune modulation mediated by pathogenic microorganisms and provide a basis for developing novel antibacterial therapeutic strategies.

Microbial extracellular vesicles contribute to antimicrobial resistance

With the escalating global antimicrobial resistance crisis, there is an urgent need for innovative strategies against drug-resistant microbes. Accumulating evidence indicates microbial extracellular vesicles (EVs) contribute to antimicrobial resistance. Therefore, comprehensively elucidating the roles and mechanisms of microbial EVs in conferring resistance could provide new perspectives and avenues for novel antimicrobial approaches. In this review, we systematically examine current research on antimicrobial resistance involving bacterial, fungal, and parasitic EVs, delineating the mechanisms whereby microbial EVs promote resistance. Finally, we discuss the application of bacterial EVs in antimicrobial therapy.

Staphylococcus aureus membrane vesicles: an evolving story

Staphylococcus aureus is an important bacterial pathogen that causes a wide variety of human diseases in community and hospital settings. S. aureus employs a diverse array of virulence factors, both surface-associated and secreted, to promote colonization, infection, and immune evasion. Over the past decade, a growing body of research has shown that S. aureus generates extracellular membrane vesicles (MVs) that package a variety of bacterial components, many of which are virulence factors. In this review, we summarize recent advances in our understanding of S. aureus MVs and highlight their biogenesis, cargo, and potential role in the pathogenesis of staphylococcal infections. Lastly, we present some emerging questions in the field.

Bacterial extracellular vesicles: biotechnological perspective for enhanced productivity

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

Reducing Cellular Autolysis of Bacillus subtilis to Improve Keratinase Production

Bacillus subtilis has been shown to be an excellent expression host for keratinases due to its powerful secretion system. However, cellular autolysis limits its production capacity. Here, we selected seven genes with significantly upregulated transcript levels from 15 genes associated with cellular autolysis as knockout targets by qRT-PCR and constructed a total of 127 strains to reduce cellular autolysis. Among them, the biomass of B. subtilis BSΔXLPC-ker deficient in xpf, lytC, pcfA, and cwlC increased by 57%. This was confirmed by cell staining, green fluorescent protein imaging, and extracellular nucleic acid leakage assay. Keratinase activity was increased by 1.46-fold in the 5 L fermenter. In addition, the activities of nattokinase and subtilisin E were also increased by 1.50-fold and 1.43-fold, respectively, in the modified chassis cells, which further confirms the generalizability of the strategy. Thus, reducing cellular autolysis to increase the ability of B. subtilis to produce subtilisins is promising.

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.

Proteomic analysis of Staphylococcus aureus exposed to bacteriocin XJS01 and its bio-preservative effect on raw pork loins

Antibacterial mechanism of bacteriocins against foodborne S. aureus is still to be explored, particularly in proteomics, and a deep and comprehensive study on application of bacteriocins for preservation of raw pork is required. Here, proteomic mechanism of Lactobacillus salivarius bacteriocin XJS01 against foodborne S. aureus 2612:1606BL1486 (S. aureus_26) and its preservation effect on raw pork loins stored at 4 °C for 12 days was investigated. The results showed that 301 differentially abundant proteins (DAPs) were identified between XJS01-treated and -free groups (control group) using Tandem mass tag (TMT) quantitative proteomics technology, which were primarily involved in amino acids and carbohydrate metabolism, cytolysis, defense response, cell apoptosis, cell killing, adhesion, and oxygen utilization of S. aureus_26. Bacterial secretion system (SRP) and cationic antimicrobial peptide resistance may be key pathways to maintain protein secretion and counteract the deleterious effects on S. aureus_26 caused by XJS01. In addition, XJS01 could significantly improve the preservation of raw pork loins by the evaluation results of sensory and antibacterial activity on the meat surface. Overall, this study showed that XJS01 induced a complex organism response in S. aureus, and it could be potential pork preservative.

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.

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.

Bacterial extracellular vesicles and their interplay with the immune system

The mammalian intestinal tract harbors trillions of microorganisms confined within this space by mucosal barriers. Despite these barriers, bacterial components may still be found elsewhere in the body, even in healthy subjects. Bacteria can release small lipid-bound particles, also named bacterial extracellular vesicles (bEV). While bacteria themselves cannot normally penetrate the mucosal defense, bEVs may infiltrate the barrier and disseminate throughout the body. The extremely diverse cargo that bEVs can carry, depending on their parent species, strain, and growth conditions, grant them an equally broad potential to interact with host cells and influence immune functions. Herein, we review the current knowledge of processes underlying the uptake of bEVs by mammalian cells, and their effect on the immune system. Furthermore, we discuss how bEVs could be targeted and manipulated for diverse therapeutic purposes.

A model industrial workhorse: Bacillus subtilis strain 168 and its genome after a quarter of a century

The vast majority of genomic sequences are automatically annotated using various software programs. The accuracy of these annotations depends heavily on the very few manual annotation efforts that combine verified experimental data with genomic sequences from model organisms. Here, we summarize the updated functional annotation of Bacillus subtilis strain 168, a quarter century after its genome sequence was first made public. Since the last such effort 5 years ago, 1168 genetic functions have been updated, allowing the construction of a new metabolic model of this organism of environmental and industrial interest. The emphasis in this review is on new metabolic insights, the role of metals in metabolism and macromolecule biosynthesis, functions involved in biofilm formation, features controlling cell growth, and finally, protein agents that allow class discrimination, thus allowing maintenance management, and accuracy of all cell processes. New 'genomic objects' and an extensive updated literature review have been included for the sequence, now available at the International Nucleotide Sequence Database Collaboration (INSDC: AccNum AL009126.4).

Composition and functions of bacterial membrane vesicles

Extracellular vesicles are produced by species across all domains of life, suggesting that vesiculation represents a fundamental principle of living matter. In Gram-negative bacteria, membrane vesicles (MVs) can originate either from blebs of the outer membrane or from endolysin-triggered explosive cell lysis, which is often induced by genotoxic stress. Although less is known about the mechanisms of vesiculation in Gram-positive and Gram-neutral bacteria, recent research has shown that both lysis and blebbing mechanisms also exist in these organisms. Evidence has accumulated over the past years that different biogenesis routes lead to distinct types of MV with varied structure and composition. In this Review, we discuss the different types of MV and their potential cargo packaging mechanisms. We summarize current knowledge regarding how MV composition determines their various functions including support of bacterial growth via the disposal of waste material, nutrient scavenging, export of bioactive molecules, DNA transfer, neutralization of phages, antibiotics and bactericidal functions, delivery of virulence factors and toxins to host cells and inflammatory and immunomodulatory effects. We also discuss the advantages of MV-mediated secretion compared with classic bacterial secretion systems and we introduce the concept of quantal secretion.

Methylarsenite is a broad-spectrum antibiotic disrupting cell wall biosynthesis and cell membrane potential

Methylarsenite (MAs(III)), a product of arsenic biomethylation or bioreduction of methylarsenate (MAs(V)), has been proposed as a primitive antibiotic. However, the antibacterial property and the bactericidal mechanism of MAs(III) remain largely unclear. In this study, we found that MAs(III) is highly toxic to 14 strains of bacteria, especially against 9 strains of Gram-positive bacteria with half maximal inhibitory concentration (IC50) in the sub micromolar range for Staphyloccocus aureus, Microbacterium sp., Pseudarthrobacter siccitolerans and several Bacillus species. In a co-culture of B. subtilis 168 and MAs(III)-producer Enterobacter sp. CZ-1, the later reduced non-toxic MAs(V) to highly toxic MAs(III) to kill the former and gain a competitive advantage. MAs(III) induced autolysis of B. subtilis 168. Deletion of the autolysins LytC, LytD, LytE, and LytF suppressed MAs(III)-induced autolysis in B. subtilis 168. Transcriptomic analysis showed that MAs(III) downregulated the expression of the major genes involved in the biosynthesis of the cell wall peptidoglycan. Overexpression of an UDP-N-acetylglucosamine enolpyruvyl transferase gene murAA alleviated MAs(III)-induced autolysis in B. subtilis 168. MAs(III) disrupted the membrane potential of B. subtilis 168 and caused severe membrane damage. The results suggest that MAs(III) is a broad-spectrum antibiotic preferentially against Gram-positive bacteria by disrupting the cell wall biosynthesis pathway and cell membrane potential.

Membrane vesicles released by Lacticaseibacillus casei BL23 inhibit the biofilm formation of Salmonella Enteritidis

Biofilms represent a major concern in the food industry and healthcare. The use of probiotic bacteria and their derivatives as an alternative to conventional treatments to fight biofilm development is a promising option that has provided convincing results in the last decades. Recently, membrane vesicles (MVs) produced by probiotics have generated considerable interest due to the diversity of roles they have been associated with. However, the antimicrobial activity of probiotic MVs remains to be studied. In this work, we showed that membrane vesicles produced by Lacticaseibacillus casei BL23 (LC-MVs) exhibited strong antibiofilm activity against Salmonella enterica serovar Enteritidis (S. Enteritidis) without affecting bacterial growth. Furthermore, we found that LC-MVs affected the early stages of S. Enteritidis biofilm development and prevented attachment of bacteria to polystyrene surfaces. Importantly, LC-MVs did not impact the biomass of already established biofilms. We also demonstrated that the antibiofilm activity depended on the proteins associated with the LC-MV fraction. Finally, two peptidoglycan hydrolases (PGHs) were found to be associated with the antibiofilm activity of LC-MVs. Overall, this work allowed to identify the antibiofilm properties of LC-MVs and paved the way for the use of probiotic MVs against the development of negative biofilms.

[Study on biofilm formation and heterogeneity in Clostridium perfringens]

Many bacteria form biofilms and survive in the actual environment. Biofilms are not only a major form of bacteria but are also involved in tolerance to environmental stresses and antibiotics, suggesting the association with bacterial pathogenesis. Cells within biofilms display phenotypic heterogeneity; thus, even bacteria, unicellular organisms, can functionally differentiate and show multicellular behavior. Therefore, it is necessary to understand bacteria as a population to control their survival and pathogenesis in the actual environment. Previously, we found that Clostridium perfringens, an anaerobic pathogenic bacterium, form different structures in different temperatures and phenotypic heterogeneity on biofilm matrix gene expression within the biofilm. In this article, I summarize the results of our research on biofilms and their heterogeneity, the mechanisms of post-transcriptional gene expression regulation of virulence genes, and bacteria-host interactions mediated by extracellular membrane vesicles.

Research Progress on the Effect of Autolysis to Bacillus subtilis Fermentation Bioprocess

Bacillus subtilis is a gram-positive bacterium, a promising microorganism due to its strong extracellular protein secretion ability, non-toxic, and relatively mature industrial fermentation technology. However, cell autolysis during fermentation restricts the industrial application of B. subtilis. With the fast advancement of molecular biology and genetic engineering technology, various advanced procedures and gene editing tools have been used to successfully construct autolysis-resistant B. subtilis chassis cells to manufacture various biological products. This paper first analyses the causes of autolysis in B. subtilis from a mechanistic perspective and outlines various strategies to address autolysis in B. subtilis. Finally, potential strategies for solving the autolysis problem of B. subtilis are foreseen.

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.

A pathway independent multi-modular ordered control system based on thermosensors and CRISPRi improves bioproduction in Bacillus subtilis

Dynamic regulation is an effective strategy for control of gene expression in microbial cell factories. In some pathway contexts, several metabolic modules must be controlled in a time dependent or ordered manner to maximize production, while the creation of genetic circuits with ordered regulation capacity still remains a great challenge. In this work, we develop a pathway independent and programmable system that enables multi-modular ordered control of metabolism in Bacillus subtilis. First, a series of thermosensors were created and engineered to expand their thresholds. Then we designed single-input-multi-output circuits for ordered control based on the use of thermosensors with different transition points. Meanwhile, a repression circuit was constructed by combining CRISPRi-based NOT gates. As a proof-of-concept, these genetic circuits were applied for multi-modular ordered control of 2′-fucosyllactose (2′-FL) biosynthesis, resulting in a production of 1839.7 mg/l in shake flask, which is 5.16-times that of the parental strain. In a 5-l bioreactor, the 2′-FL titer reached 28.2 g/l with down-regulation of autolysis. Taken together, this work provides programmable and versatile thermosensitive genetic toolkits for dynamic regulation in B. subtilis and a multi-modular ordered control framework that can be used to improve metabolic modules in other chassis cells and for other compounds.

Essential role of membrane vesicles for biological activity of the bacteriocin micrococcin P1

Bacterial membrane vesicles (MVs) have recently gained much attention and have been shown to carry a wide diversity of secreted bacterial components. However, it is poorly understood whether MV carriage is an indispensable requirement for a cargo's function. Bacteriocins as weapons of bacterial warfare shape the composition of microbial communities. Many bacteriocins have pronounced hydrophobicity that is imposed by their mechanism of action, but how they diffuse through aqueous environments to reach their target competitors is not known. Here we show that antimicrobial competitive activity of an exemplary hydrophobic bacteriocin of the thiopeptide antibiotic family, micrococcin P1 (MP1), is dependent on incorporation into MVs, which were found to carry MP1 at high concentrations. In contrast, MP1 without MV association was poorly active due to low solubility. Furthermore, we provide previously unavailable evidence that MVs fuse with a Gram-positive bacterium's cytoplasmic membrane, in this case to deliver a bacteriocin to its intracellular target. Our findings demonstrate how bacteria overcome the problem associated with secreting hydrophobic small molecules and delivering them to their target and show that MVs have a key function in bacterial warfare. Furthermore, our study provides hitherto rare evidence that MVs provide an essential rather than merely accessory function in bacterial physiology.

Controllable secretion of multilayer vesicles driven by microbial polymer accumulation

Membrane vesicles (MVs) are formed in various microorganisms triggered by physiological and environmental phenomena. In this study, we have discovered that the biogenesis of MV took place in the recombinant cell of Escherichia coli BW25113 strain that intracellularly accumulates microbial polyester, polyhydroxybutyrate (PHB). This discovery was achieved as a trigger of foam formation during the microbial PHB fermentation. The purified MVs were existed as a mixture of outer MVs and outer/inner MVs, revealed by transmission electron microscopy. It should be noted that there was a good correlation between MV formation and PHB production level that can be finely controlled by varying glucose concentrations, suggesting the causal relationship in both supramolecules artificially produced in the microbial platform. Notably, the controllable secretion of MV was governed spatiotemporally through the morphological change of the E. coli cells caused by the PHB intracellular accumulation. Based on a hypothesis of PHB internal-pressure dependent envelope-disorder induced MV biogenesis, here we propose a new Polymer Intracellular Accumulation-triggered system for MV Production (designated "PIA-MVP") with presenting a mechanistic model for MV biogenesis. The PIA-MVP is a promising microbial platform that will provides us with a significance for further study focusing on biopolymer capsulation and cross-membrane transportation for different application purposes.

The Role of Bacterial Membrane Vesicles in Human Health and Disease

Bacterial membrane vesicles (MVs) are nanoparticles derived from the membrane components of bacteria that transport microbial derived substances. MVs are ubiquitous across a variety of terrestrial and marine environments and vary widely in their composition and function. Membrane vesicle functional diversity is staggering: MVs facilitate intercellular communication by delivering quorum signals, genetic information, and small molecules active against a variety of receptors. MVs can deliver destructive virulence factors, alter the composition of the microbiota, take part in the formation of biofilms, assist in the uptake of nutrients, and serve as a chemical waste removal system for bacteria. MVs also facilitate host-microbe interactions including communication. Released in mass, MVs overwhelm the host immune system and injure host tissues; however, there is also evidence that vesicles may take part in processes which promote host health. This review will examine the ascribed functions of MVs within the context of human health and disease.

Proteome analysis of the Gram-positive fish pathogen Renibacterium salmoninarum reveals putative role of membrane vesicles in virulence

Bacterial kidney disease (BKD) is a chronic bacterial disease affecting both wild and farmed salmonids. The causative agent for BKD is the Gram-positive fish pathogen Renibacterium salmoninarum. As treatment and prevention of BKD have proven to be difficult, it is important to know and identify the key bacterial proteins that interact with the host. We used subcellular fractionation to report semi-quantitative data for the cytosolic, membrane, extracellular, and membrane vesicle (MV) proteome of R. salmoninarum. These data can aid as a backbone for more targeted experiments regarding the development of new drugs for the treatment of BKD. Further analysis was focused on the MV proteome, where both major immunosuppressive proteins P57/Msa and P22 and proteins involved in bacterial adhesion were found in high abundance. Interestingly, the P22 protein was relatively enriched only in the extracellular and MV fraction, implicating that MVs may play a role in host–pathogen interaction. Compared to the other subcellular fractions, the MVs were also relatively enriched in lipoproteins and all four cell wall hydrolases belonging to the New Lipoprotein C/Protein of 60 kDa (NlpC/P60) family were detected, suggesting an involvement in the formation of the MVs.

A Single Shot of Vesicles

Bacteria communicate through signaling molecules that coordinate group behavior. Hydrophobic signals that do not diffuse in aqueous environments are used as signaling molecules by several bacteria. However, limited information is currently available on the mechanisms by which these molecules are transported between cells. Membrane vesicles (MVs) with diverse functions play important roles in the release and delivery of hydrophobic signaling molecules, leading to differences in the dynamics of signal transportation from those of free diffusion. Studies on Paracoccus denitrificans, which produces a hydrophobic long-chain N-acyl homoserine lactone (AHL), showed that signals were loaded into MVs at a concentration with the potential to trigger the quorum sensing (QS) response with a "single shot" to the cell. Furthermore, stimulating the formation of MVs increased the release of signals from the cell; therefore, a basic understanding of MV formation is important. Novel findings revealed the formation of MVs through different routes, resulting in the production of different types of MVs. Methods such as high-speed atomic force microscopy (AFM) phase imaging allow the physical properties of MVs to be analyzed at a nanometer resolution, revealing their heterogeneity. In this special minireview, we introduce the role of MVs in bacterial communication and highlight recent findings on MV formation and their physical heterogeneity by referring to our research. We hope that this minireview will provide basic information for understanding the functionality of MVs in ecological systems.

Regulate the hydrophobic motif to enhance the non-classical secretory expression of Pullulanase PulA in Bacillus subtilis

Bacillus subtilis has been widely used as a prokaryotic host for the secretory expression of heterologous proteins. In this study, a pullulanase (PulA) from Anoxybacillus sp. LM18-11 was firstly identified to be expressed in Bacillus subtilis 1A751 through non-classical secretion pathway. Results showed that both the N- and C-terminal regions of PulA were essential for its soluble expression. To explore its specific structural basis of secretion in B. subtilis, we revealed a hydrophobic motif A501-H507 which is vital for the secretion of the whole protein of PulA. Through a series of site-specific mutagenesis, the triple-sites mutants R503E/I506E/H507E and R503E/I506Y/H507E showed the highest extracellular activity (160.07 U/mL) and total activity (243.37 U/mL) which was 1.71 times and 1.55 times higher than those of PulA. The highest secretion rate of mutant I506E/H507E was more than 50% which was 34.72% higher comparing with that of PulA. The glutamic acid substitution on these three key surface sites which decreased the surface hydrophobicity of that region was confirmed to be beneficial to improve the secretory expression of PulA. This novel discovery for the secretory expression of PulA in B. subtilis would make a new perspective on regulating a kind of non-classical secretion in B. subtilis.

Engineered Remolding and Application of Bacterial Membrane Vesicles

Bacterial membrane vesicles (MVs) are produced by both Gram-positive and Gram-negative bacteria during growth in vitro and in vivo. MVs are nanoscale vesicular structures with diameters ranging from 20 to 400 nm. MVs incorporate bacterial lipids, proteins, and often nucleic acids, and can effectively stimulate host immune response against bacterial infections. As vaccine candidates and drug delivery systems, MVs possess high biosafety owing to the lack of self-replication ability. However, wild-type bacterial strains have poor MV yield, and MVs from the wild-type strains may be harmful due to the carriage of toxic components, such as lipopolysaccharides, hemolysins, enzymes, etc. In this review, we summarize the genetic modification of vesicle-producing bacteria to reduce MV toxicity, enhance vesicle immunogenicity, and increase vesicle production. The engineered MVs exhibit broad applications in vaccine designs, vaccine delivery vesicles, and drug delivery systems.