Release of the type I secreted alpha-haemolysin via outer membrane vesicles from Escherichia coli

Multivariate Analysis of Individual Bacterial Outer Membrane Vesicles Using Fluorescence Microscopy

Gram-negative bacteria produce outer membrane vesicles (OMVs) that play a critical role in cell-cell communication and virulence. OMVs have emerged as promising therapeutic agents for various biological applications such as vaccines and targeted drug delivery. However, the full potential of OMVs is currently constrained by inherent heterogeneities, such as size and cargo differences, and traditional ensemble assays are limited in their ability to reveal OMV heterogeneity. To overcome this issue, we devised an innovative approach enabling the identification of various characteristics of individual OMVs. This method, employing fluorescence microscopy, facilitates the detection of variations in size and surface markers. To demonstrate our method, we utilize the oral bacterium Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) which produces OMVs with a bimodal size distribution. As part of its virulence, A. actinomycetemcomitans secretes leukotoxin (LtxA) in two forms: soluble and surface associated with the OMVs. We observed a correlation between the size and toxin presence where larger OMVs were much more likely to possess LtxA compared to the smaller OMVs. In addition, we noted that, among the smallest OMVs (<100 nm diameter), the fractions that are toxin positive range from 0 to 30%, while the largest OMVs (>200 nm diameter) are between 70 and 100% toxin positive.

Heterogeneity of Size and Toxin Distribution in Aggregatibacter actinomycetemcomitans Outer Membrane Vesicles

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium associated with localized aggressive periodontitis as well as some systemic diseases. The strains of A. actinomycetemcomitans most closely associated with disease produce more of a secreted leukotoxin (LtxA) than isolates from healthy carriers, suggesting a key role for this toxin in disease progression. LtxA is released into the bacterial cytosol in a free form as well as in association with the surface of outer membrane vesicles (OMVs). We previously observed that the highly leukotoxic A. actinomycetemcomitans strain JP2 produces two populations of OMVs: a highly abundant population of small (<100 nm) OMVs and a less abundant population of large (>300 nm) OMVs. Here, we have developed a protocol to isolate the OMVs produced during each specific phase of growth and used this to demonstrate that small OMVs are produced throughout growth and lack LtxA, while large OMVs are produced only during the exponential phase and are enriched with LtxA. Our results indicate that surface-associated DNA drives the selective sorting of LtxA into large OMVs. This study provides valuable insights into the observed heterogeneity of A. actinomycetemcomitans vesicles and emphasizes the importance of understanding these variations in the context of bacterial pathogenesis.

Expression of accessory genes in Salmonella requires the presence of the Gre factors

Genomic studies with Salmonella enterica serovar Typhimurium reveal a crucial role of horizontal gene transfer (HGT) in the acquisition of accessory cellular functions involved in host-interaction. Many virulence genes are located in genomic islands, plasmids and prophages. GreA and GreB proteins, Gre factors, interact transiently with the RNA polymerase alleviating backtracked complexes during transcription elongation. The overall effect of Gre factors depletion in Salmonella expression profile was studied. Both proteins are functionally redundant since only when both Gre factors were depleted a major effect in gene expression was detected. Remarkably, the accessory gene pool is particularly sensitive to the lack of Gre factors, with 18.6% of accessory genes stimulated by the Gre factors versus 4.4% of core genome genes. Gre factors involvement is particularly relevant for the expression of genes located in genomic islands. Our data reveal that Gre factors are required for the expression of accessory genes.

A molecular toolkit for heterologous protein secretion across Bacteroides species

Bacteroides species are abundant and prevalent stably colonizing members of the human gut microbiota, making them a promising chassis for developing long-term interventions for chronic diseases. Engineering these bacteria as on-site production and delivery vehicles for biologic drugs or diagnostics, however, requires efficient heterologous protein secretion tools, which are currently lacking. To address this limitation, we systematically investigated methods to enable heterologous protein secretion in Bacteroides using both endogenous and exogenous secretion systems. Here, we report a collection of secretion carriers that can export functional proteins across multiple Bacteroides species at high titers. To understand the mechanistic drivers of Bacteroides secretion, we characterized signal peptide sequence features as well as post-secretion extracellular fate and cargo size limit of protein cargo. To increase titers and enable flexible control of protein secretion, we developed a strong, self-contained, inducible expression circuit. Finally, we validated the functionality of our secretion carriers in vivo in a mouse model. This toolkit should enable expanded development of long-term living therapeutic interventions for chronic gastrointestinal disease.

The Prc and CtpA proteases modulate cell-surface signaling activity and virulence in Pseudomonas aeruginosa

Cell-surface signaling (CSS) is a signal transfer system of Gram-negative bacteria that produces the activation of an extracytoplasmic function σ factor (σECF) in the cytosol in response to an extracellular signal. Activation requires the regulated and sequential proteolysis of the σECF-associated anti-σ factor, and the function of the Prc and RseP proteases. In this work, we have identified another protease that modulates CSS activity, namely the periplasmic carboxyl-terminal processing protease CtpA. CtpA functions upstream of Prc in the proteolytic cascade and seems to prevent the Prc-mediated proteolysis of the CSS anti-σ factor. Importantly, using zebrafish embryos and the A549 lung epithelial cell line as hosts, we show that mutants in the rseP and ctpA proteases of the human pathogen Pseudomonas aeruginosa are considerably attenuated in virulence while the prc mutation increases virulence likely by enhancing the production of membrane vesicles.

Identifying size-dependent toxin sorting in bacterial outer membrane vesicles

Gram-negative bacteria produce outer membrane vesicles (OMVs) that play a critical role in cell-cell communication and virulence. Despite being isolated from a single population of bacteria, OMVs can exhibit heterogeneous size and toxin content, which can be obscured by assays that measure ensemble properties. To address this issue, we utilize fluorescence imaging of individual OMVs to reveal size-dependent toxin sorting. Our results showed that the oral bacterium Aggregatibacter actinomycetemcomitans (A.a.) produces OMVs with a bimodal size distribution, where larger OMVs were much more likely to possess leukotoxin (LtxA). Among the smallest OMVs (< 100 nm diameter), the fraction that are toxin positive ranges from 0-30%, while the largest OMVs (> 200 nm diameter) are between 70-100% toxin positive. Our single OMV imaging method provides a non-invasive way to observe OMV surface heterogeneity at the nanoscale level and determine size-based heterogeneities without the need for OMV fraction separation.

Exosomes: from biology to immunotherapy in infectious diseases

Exosomes are extracellular vesicles derived from the endosomal compartment, which are released by all kinds of eukaryotic and prokaryotic organisms. These vesicles contain a variety of biomolecules that differ both in quantity and type depending on the origin and cellular state. Exosomes are internalized by recipient cells, delivering their content and thus contributing to cell-cell communication in health and disease. During infections exosomes may exert a dual role, on one hand, they can transmit pathogen-related molecules mediating further infection and damage, and on the other hand, they can protect the host by activating the immune response and reducing pathogen spread. Selective packaging of pathogenic components may mediate these effects. Recently, quantitative analysis of samples by omics technologies has allowed a deep characterization of the proteins, lipids, RNA, and metabolite cargoes of exosomes. Knowledge about the content of these vesicles may facilitate their therapeutic application. Furthermore, as exosomes have been detected in almost all biological fluids, pathogenic or host-derived components can be identified in liquid biopsies, making them suitable for diagnosis and prognosis. This review attempts to organize the recent findings on exosome composition and function during viral, bacterial, fungal, and protozoan infections, and their contribution to host defense or to pathogen spread. Moreover, we summarize the current perspectives and future directions regarding the potential application of exosomes for prophylactic and therapeutic purposes.

Outer Membrane Vesicles: Biogenesis, Functions, and Issues

This review focuses on nonlytic outer membrane vesicles (OMVs), a subtype of bacterial extracellular vesicles (BEVs) produced by Gram-negative organisms focusing on the mechanisms of their biogenesis, cargo, and function. Throughout, we highlight issues concerning the characterization of OMVs and distinguishing them from other types of BEVs. We also highlight the shortcomings of commonly used methodologies for the study of BEVs that impact the interpretation of their functionality and suggest solutions to standardize protocols for OMV studies.

Involvement of Bacterial Extracellular Membrane Nanovesicles in Infectious Diseases and Their Application in Medicine

Bacterial extracellular membrane nanovesicles (EMNs) are attracting the attention of scientists more and more every year. These formations are involved in the pathogenesis of numerous diseases, among which, of course, the leading role is occupied by infectious diseases, the causative agents of which are a range of Gram-positive and Gram-negative bacteria. A separate field for the study of the role of EMN is cancer. Extracellular membrane nanovesicles nowadays have a practical application as vaccine carriers for immunization against many infectious diseases. At present, the most essential point is their role in stimulating immune response to bacterial infections and tumor cells. The possibility of nanovesicles' practical use in several disease treatments is being evaluated. In our review, we listed diseases, focusing on their multitude and diversity, for which EMNs are essential, and also considered in detail the possibilities of using EMNs in the therapy and prevention of various pathologies.

Breaking down the cell wall: Still an attractive antibacterial strategy

Since the advent of penicillin, humans have known about and explored the phenomenon of bacterial inhibition via antibiotics. However, with changes in the global environment and the abuse of antibiotics, resistance mechanisms have been selected in bacteria, presenting huge threats and challenges to the global medical and health system. Thus, the study and development of new antimicrobials is of unprecedented urgency and difficulty. Bacteria surround themselves with a cell wall to maintain cell rigidity and protect against environmental insults. Humans have taken advantage of antibiotics to target the bacterial cell wall, yielding some of the most widely used antibiotics to date. The cell wall is essential for bacterial growth and virulence but is absent from humans, remaining a high-priority target for antibiotic screening throughout the antibiotic era. Here, we review the extensively studied targets, i.e., MurA, MurB, MurC, MurD, MurE, MurF, Alr, Ddl, MurI, MurG, lipid A, and BamA in the cell wall, starting from the very beginning to the latest developments to elucidate antimicrobial screening. Furthermore, recent advances, including MraY and MsbA in peptidoglycan and lipopolysaccharide, and tagO, LtaS, LspA, Lgt, Lnt, Tol-Pal, MntC, and OspA in teichoic acid and lipoprotein, have also been profoundly discussed. The review further highlights that the application of new methods such as macromolecular labeling, compound libraries construction, and structure-based drug design will inspire researchers to screen ideal antibiotics.

Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient ‘contact weapons’ that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

In Silico Prediction and Design of Uropathogenic Escherichia coli Alpha-Hemolysin Generate a Soluble and Hemolytic Recombinant Toxin

The secretion of α-hemolysin by uropathogenic Escherichia coli (UPEC) is commonly associated with the severity of urinary tract infections, which makes it a predictor of poor prognosis among patients. Accordingly, this toxin has become a target for diagnostic tests and therapeutic interventions. However, there are several obstacles associated with the process of α-hemolysin purification, therefore limiting its utilization in scientific investigations. In order to overcome the problems associated with α-hemolysin expression, after in silico prediction, a 20.48 kDa soluble α-hemolysin recombinant denoted rHlyA was constructed. This recombinant is composed by a 182 amino acid sequence localized in the aa542–723 region of the toxin molecule. The antigenic determinants of the rHlyA were estimated by bioinformatics analysis taking into consideration the tertiary form of the toxin, epitope analysis tools, and solubility inference. The results indicated that rHlyA has three antigenic domains localized in the aa555–565, aa600–610, and aa674–717 regions. Functional investigation of rHlyA demonstrated that it has hemolytic activity against sheep red cells, but no cytotoxic effect against epithelial bladder cells. In summary, the results obtained in this study indicate that rHlyA is a soluble recombinant protein that can be used as a tool in studies that aim to understand the mechanisms involved in the hemolytic and cytotoxic activities of α-hemolysin produced by UPEC. In addition, rHlyA can be applied to generate monoclonal and/or polyclonal antibodies that can be utilized in the development of diagnostic tests and therapeutic interventions.

Gut Microbiota Extracellular Vesicles as Signaling Molecules Mediating Host-Microbiota Communications

Over the past decade, gut microbiota dysbiosis has been linked to many health disorders; however, the detailed mechanism of this correlation remains unclear. Gut microbiota can communicate with the host through immunological or metabolic signalling. Recently, microbiota-released extracellular vesicles (MEVs) have emerged as significant mediators in the intercellular signalling mechanism that could be an integral part of microbiota-host communications. MEVs are small membrane-bound vesicles that encase a broad spectrum of biologically active compounds (i.e., proteins, mRNA, miRNA, DNA, carbohydrates, and lipids), thus mediating the horizontal transfer of their cargo across intra- and intercellular space. In this study, we provide a comprehensive and in-depth discussion of the biogenesis of microbial-derived EVs, their classification and routes of production, as well as their role in inter-bacterial and inter-kingdom signaling.

Bacterial extracellular vesicles: Understanding biology promotes applications as nanopharmaceuticals

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

Analysis of genes encoding for proteolytic enzymes and cytotoxic proteins as virulence factors of Piscirickettsia salmonis in SHK-1 cells

Piscirickettsia salmonis is the causative agent of Piscirickettsiosis, a systemic disease generating high mortality rates in farmed salmon cultures of southern Chile. Proteolytic enzymes are important virulence factors since they play a key role in bacterial invasion and proliferation within the host. Bacteria growing in muscle tissues are known to secrete proteases, but no proteolytic enzymes have been described in P. salmonis to date. A battery of putative protease genes was found in the genomes and available strains of P. salmonis by bioinformatics analyses, and their identity was established through comparison with protease genes in databases. The transcript levels of five candidate genes were analysed by in vitro infection and qPCR. All strains were found to generate protease activity to varying degrees, and this was significantly increased when bacteria infected a salmon cell line. Gene expression of several types of proteases was also evidenced, with the highest levels corresponding to the type 1 secretion system (T1SS), which is also involved in the transport of haemolysin A, although transcripts with significant levels of peptidase M4 (thermolysin) and CLP protease were also found.

Eco-evolutionary feedbacks mediated by bacterial membrane vesicles

Bacterial membrane vesicles (BMVs) are spherical extracellular organelles whose cargo is enclosed by a biological membrane. The cargo can be delivered to distant parts of a given habitat in a protected and concentrated manner. This review presents current knowledge about BMVs in the context of bacterial eco-evolutionary dynamics among different environments and hosts. BMVs may play an important role in establishing and stabilizing bacterial communities in such environments; for example, bacterial populations may benefit from BMVs to delay the negative effect of certain evolutionary trade-offs that can result in deleterious phenotypes. BMVs can also perform ecosystem engineering by serving as detergents, mediators in biochemical cycles, components of different biofilms, substrates for cross-feeding, defense systems against different dangers and enzyme-delivery mechanisms that can change substrate availability. BMVs further contribute to bacteria as mediators in different interactions, with either other bacterial species or their hosts. In short, BMVs extend and deliver phenotypic traits that can have ecological and evolutionary value to both their producers and the ecosystem as a whole.

Non-infectious outer membrane vesicles derived from Brucella abortus S19Δper as an alternative acellular vaccine protects mice against virulent challenge

The prime human and animal safety issues accentuate the search of promising newer alternative vaccine candidates to resolve complications associated with the live attenuated Brucella abortus strain19 (S19) vaccine. Outer membrane vesicles (OMVs S19 Δper) extracted from Brucella abortus S19Δper (S19Δper) as an alternative subunit vaccine candidate has been explored in the present study as OMVs are endowed with immunogenic molecules, including LPS and outer membrane proteins (OMPs) and do not cause infection by virtue of being an acellular entity. The LPS defective S19Δper released a higher amount of OMVs than its parent strain S19. Under transmission electron microscopy (TEM), OMVs were seen as nano-sized outward bulge from the surface of Brucella. Dynamic light scattering analysis of OMVs revealed that OMVs S19Δper showed the less polydispersity index (PDI) than OMVs S19 pointing towards relatively more homogenous OMVs populations. Both OMVs S19Δper and OMVs S19 with or without booster dose and S19 vaccine were used for immunization of mice and subsequently challenged with 2 × 10⁵ CFU virulent Brucella abortus strain 544 (S544) to assess protective efficacy of vaccines. The less splenic weight index and less S544 count in OMVs immunized mice in comparison to unimmunized mice after S544 challenge clearly indicated good protective efficacy of OMVs. OMVs S19 Δper induced relatively high titer of IgG than OMVs S19 but conferred nearly equal protection against brucellosis. An ELISA based determination of IgG and its isotype response, Cytometric Bead Array (CBA) based quantitation of serum cytokines and FACS based enumeration of CD4⁺ and CD8⁺ T cells revealed high titer of IgG, production of both Th1 (IgG2a) and Th2 (IgG1) related antibodies, stimulation of IL-2, TNF (Th1) and IL-4, IL-6, IL-10 (Th2) cytokines, and induced T cell response suggested that OMVs S19Δper elicited Th1 and Th2 type immune response and ensured protection against S544 challenge in murine model.

Various Facets of Pathogenic Lipids in Infectious Diseases: Exploring Virulent Lipid-Host Interactome and Their Druggability

Lipids form an integral, structural, and functional part of all life forms. They play a significant role in various cellular processes such as membrane fusion, fission, endocytosis, protein trafficking, and protein functions. Interestingly, recent studies have revealed their more impactful and critical involvement in infectious diseases, starting with the manipulation of the host membrane to facilitate pathogenic entry. Thereafter, pathogens recruit specific host lipids for the maintenance of favorable intracellular niche to augment their survival and proliferation. In this review, we showcase the lipid-mediated host pathogen interplay in context of life-threatening viral and bacterial diseases including the recent SARS-CoV-2 infection. We evaluate the emergent lipid-centric approaches adopted by these pathogens, while delineating the alterations in the composition and organization of the cell membrane within the host, as well as the pathogen. Lastly, crucial nexus points in their interaction landscape for therapeutic interventions are identified. Lipids act as critical determinants of bacterial and viral pathogenesis by altering the host cell membrane structure and functions.

Oral and intestinal bacterial exotoxins: Potential linked to carcinogenesis

Growing evidence suggests that imbalances in resident microbes (dysbiosis) can promote chronic inflammation, immune-subversion, and production of carcinogenic metabolites, thus leading to neoplasia. Yet, evidence to support a direct link of individual bacteria species to human sporadic cancer is still limited. This chapter focuses on several emerging bacterial toxins that have recently been characterized for their potential oncogenic properties toward human orodigestive cancer and the presence of which in human tissue samples has been documented. These include cytolethal distending toxins produced by various members of gamma and epsilon Proteobacteria, Dentilisin from mammalian oral Treponema, Pasteurella multocida toxin, two Fusobacterial toxins, FadA and Fap2, Bacteroides fragilis toxin, colibactin, cytotoxic necrotizing factors and α-hemolysin from Escherichia coli, and Salmonella enterica AvrA. It was clear that these bacterial toxins have biological activities to induce several hallmarks of cancer. Some toxins directly interact with DNA or chromosomes leading to their breakdowns, causing mutations and genome instability, and others modulate cell proliferation, replication and death and facilitate immune evasion and tumor invasion, prying specific oncogene and tumor suppressor pathways, such as p53 and β-catenin/Wnt. In addition, most bacterial toxins control tumor-promoting inflammation in complex and diverse mechanisms. Despite growing laboratory evidence to support oncogenic potential of selected bacterial toxins, we need more direct evidence from human studies and mechanistic data from physiologically relevant experimental animal models, which can reflect chronic infection in vivo, as well as take bacterial-bacterial interactions among microbiome into consideration.

Aggregatibacter actinomycetemcomitans leukotoxin: From mechanism to targeted anti-toxin therapeutics

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium associated with localized aggressive periodontitis, as well as other systemic diseases. This organism produces a number of virulence factors, all of which provide some advantage to the bacterium. Several studies have demonstrated that clinical isolates from diseased patients, particularly those of African descent, frequently belong to specific clones of A. actinomycetemcomitans that produce significantly higher amounts of a protein exotoxin belonging to the repeats-in-toxin (RTX) family, leukotoxin (LtxA), whereas isolates from healthy patients harbor minimally leukotoxic strains. This finding suggests that LtxA might play a key role in A. actinomycetemcomitans pathogenicity. Because of this correlation, much work over the past 30 years has been focused on understanding the mechanisms by which LtxA interacts with and kills host cells. In this article, we review those findings, highlight the remaining open questions, and demonstrate how knowledge of these mechanisms, particularly the toxin's interactions with lymphocyte function-associated antigen-1 (LFA-1) and cholesterol, enables the design of targeted anti-LtxA strategies to prevent/treat disease.

Secretion and Delivery of Intestinal Pathogenic Escherichia coli Virulence Factors via Outer Membrane Vesicles

Outer membrane vesicles (OMVs) are nanoscale proteoliposomes secreted from the cell envelope of all Gram-negative bacteria. Originally considered as an artifact of the cell wall, OMVs are now recognized as a general secretion system, which serves to improve the fitness of bacteria and facilitate bacterial interactions in polymicrobial communities as well as interactions between the microbe and the host. In general, OMVs are released in increased amounts from pathogenic bacteria and have been found to harbor much of the contents of the parental bacterium. They mainly encompass components of the outer membrane and the periplasm including various virulence factors such as toxins, adhesins, and immunomodulatory molecules. Numerous studies have clearly shown that the delivery of toxins and other virulence factors via OMVs essentially influences their interactions with host cells. Here, we review the OMV-mediated intracellular deployment of toxins and other virulence factors with a special focus on intestinal pathogenic Escherichia coli. Especially, OMVs ubiquitously produced and secreted by enterohemorrhagic E. coli (EHEC) appear as a highly advanced mechanism for secretion and simultaneous, coordinated and direct delivery of bacterial virulence factors into host cells. OMV-associated virulence factors are not only stabilized by the association with OMVs, but can also often target previously unknown target structures and perform novel activities. The toxins are released by OMVs in their active forms and are transported via cell sorting processes to their specific cell compartments, where they can develop their detrimental effects. OMVs can be considered as bacterial “long distance weapons” that attack host tissues and help bacterial pathogens to establish the colonization of their biological niche(s), impair host cell function, and modulate the defense of the host. Thus, OMVs contribute significantly to the virulence of the pathogenic bacteria.

Characterization of Aspergillus fumigatus Extracellular Vesicles and Their Effects on Macrophages and Neutrophils Functions

Extracellular vesicles (EVs) has been considered an alternative process for intercellular communication. EVs release by filamentous fungi and the role of vesicular secretion during fungus-host cells interaction remain unknown. Here, we identified the secretion of EVs from the pathogenic filamentous fungus, Aspergillus fumigatus. Analysis of the structure of EVs demonstrated that A. fumigatus produces round shaped bilayer structures ranging from 100 to 200 nm size, containing ergosterol and a myriad of proteins involved in REDOX, cell wall remodeling and metabolic functions of the fungus. We demonstrated that macrophages can phagocytose A. fumigatus EVs. Phagocytic cells, stimulated with EVs, increased fungal clearance after A. fumigatus conidia challenge. EVs were also able to induce the production of TNF-α and CCL2 by macrophages and a synergistic effect was observed in the production of these mediators when the cells were challenged with the conidia. In bone marrow-derived neutrophils (BMDN) treated with EVs, there was enhancement of the production of TNF-α and IL-1β in response to conidia. Together, our results demonstrate, for the first time, that A. fumigatus produces EVs containing a diverse set of proteins involved in fungal physiology and virulence. Moreover, EVs are biologically active and stimulate production of inflammatory mediators and fungal clearance.

Fosfomycin Protects Mice From Staphylococcus aureus Pneumonia Caused by α-Hemolysin in Extracellular Vesicles by Inhibiting MAPK-Regulated NLRP3 Inflammasomes

α-Hemolysin (Hla) is a significant virulence factor in Staphylococcus aureus (S. aureus)-caused infectious diseases such as pneumonia. Thus, to prevent the production of Hla when treating S. aureus infection, it is necessary to choose an antibiotic with good antibacterial activity and effect. In our study, we observed that Fosfomycin (FOM) at a sub-inhibitory concentration inhibited expression of Hla. Molecular dynamics demonstrated that FOM bound to the binding sites LYS 154 and ASP 108 of Hla, potentially inhibiting Hla. Furthermore, we verified that staphylococcal membrane-derived vesicles (SMVs) contain Hla and that FOM treatment significantly reduced the production of SMVs and Hla. Based on our pharmacological inhibition analysis, ERK and p38 activated NLRP3 inflammasomes. Moreover, FOM inhibited expression of MAPKs and NLRP3 inflammasome-related proteins in S. aureus as well as SMV-infected human macrophages (MΦ) and alveolar epithelial cells. In vivo, SMVs isolated from S. aureus DU1090 (an isogenic Hla deletion mutant) or the strain itself caused weaker inflammation than that of its parent strain 8325-4. FOM also significantly reduced the phosphorylation levels of ERK and P38 and expression of NLRP3 inflammasome-related proteins. In addition, FOM decreased MPO activity, pulmonary vascular permeability and edema formation in the lungs of mice with S. aureus-caused pneumonia. Taken together, these data indicate that FOM exerts protective effects against S. aureus infection in vitro and in vivo by inhibiting Hla in SMVs and blocking ERK/P38-mediated NLRP3 inflammasome activation by Hla.

Analysis of the Escherichia coli extracellular vesicle proteome identifies markers of purity and culture conditions

Bacteria release nano-sized extracellular vesicles (EVs) into the extracellular milieu. Bacterial EVs contain molecular cargo originating from the parent bacterium and have important roles in bacterial survival and pathogenesis. Using 8-plex iTRAQ approaches, we profiled the EV proteome of two Escherichia coli strains, uropathogenic (UPEC) 536 and probiotic Nissle 1917. For these strains, we compared the proteome of crude input EVs prepared by ultracentrifugation alone with EVs purified by either density gradient centrifugation (DGC) or size exclusion chromatography (SEC). We further compared the proteome of EVs from bacterial cultures that were grown in iron-restricted (R) and iron-supplemented (RF) conditions. Overall, outer membrane components were highly enriched, and bacterial inner membrane components were significantly depleted in both UPEC and Nissle EVs, in keeping with an outer membrane origin. In addition, we found enrichment of ribosome-related Gene Ontology terms in UPEC EVs and proteins involved in glycolytic processes and ligase activity in Nissle EVs. We have identified that three proteins (RbsB of UPEC in R; YoeA of UPEC in RF; BamA of Nissle in R) were consistently enriched in the DGC- and SEC-purified EV samples in comparison to their crude input EV, whereas conversely the 60 kDa chaperonin GroEL was enriched in the crude input EVs for both UPEC and Nissle in R condition. Such proteins may have utility as technical markers for assessing the purity of E. coli EV preparations. Several proteins were changed in their abundance depending on the iron availability in the media. Data are available via ProteomeXchange with identifier PXD011345. In summary, we have undertaken a comprehensive characterization of the protein content of E. coli EVs and found evidence of specific EV cargos for physiological activity and conserved protein cargo that may find utility as markers in the future.

Abbreviation: DGC: density gradient centrifugation; DTT: 1,4-dithiothreitol; EV: extracellular vesicles; FDR: false discovery rate; GO: Gene Ontology; R: iron-restricted; RF: iron-supplemented; iTRAQ: isobaric tags for relative and absolute quantitation; OMV: outer membrane vesicle; SWATH-MS: sequential window acquisition of all theoretical mass spectra; SEC: size exclusion chromatography.

Variation in hemolysin A expression between uropathogenic Escherichia coli isolates determines NLRP3-dependent vs. -independent macrophage cell death and host colonization

Uropathogenic Escherichia coli (UPEC) is the major cause of urinary tract infections (UTIs). The multidrug-resistant E. coli sequence type 131 (ST131) clone is a serious threat to human health, yet its effects on immune responses are not well understood. Here we screened a panel of ST131 isolates, finding that only strains expressing the toxin hemolysin A (HlyA) killed primary human macrophages and triggered maturation of the inflammasome-dependent cytokine IL-1β. Using a representative strain, the requirement for the hlyA gene in these responses was confirmed. We also observed considerable heterogeneity in levels of cell death initiated by different HlyA^+ve ST131 isolates, and this correlated with secreted HlyA levels. Investigation into the biological significance of this variation revealed that an ST131 strain producing low levels of HlyA initiated cell death that was partly dependent on the nod-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome, with this response being associated with a host-protective role in a mouse UTI model. When the same ST131 strain was engineered to overexpress high HlyA levels, macrophage cell death occurred even when NLRP3 function was abrogated, and bladder colonization was significantly increased. Thus, variation in HlyA expression in UPEC affects mechanisms by which macrophages die, as well as host susceptibility vs. resistance to colonization.-Murthy, A. M. V., Sullivan, M. J., Nhu, N. T. K., Lo, A. W., Phan, M.-D., Peters, K. M., Boucher, D., Schroder, K., Beatson, S. A., Ulett, G. C., Schembri, M. A., Sweet, M. J. Variation in hemolysin A expression between uropathogenic Escherichia coli isolates determines NLRP3-dependent vs. -independent macrophage cell death and host colonization.

Secretion of Salmonella Pathogenicity Island 1-Encoded Type III Secretion System Effectors by Outer Membrane Vesicles in Salmonella enterica Serovar Typhimurium

Outer membrane vesicles (OMVs) are spherical membranous structures released by Gram-negative bacteria. Several bacterial pathogens utilize OMVs as vehicles for the delivery of virulence factors into host cells. Results of our previous study on proteomic analysis revealed that OMVs isolated from Salmonellaenterica serovar Typhimurium had virulence effectors that are known to be translocated by Salmonella pathogenicity island 1 (SPI-1)-encoded type III secretion system (T3SS1) into the host cell. In the present study, immunoblot analysis confirmed the secretion of the six T3SS1 effector proteins, namely SipB and SipC (translocators of T3SS1), and SipA, SopA, SopB, and SopE2 (effectors translocated by T3SS1), by OMVs. Results of proteinase K treatment revealed the localization of these T3SS1 effector proteins on the outer surface of OMVs. SipC and SopE2 were secreted by OMVs independent of the three secretion systems T3SS1, T3SS2, and flagella, signifying OMVs to be an alternative delivery system to T3SSs. T3SS1 effectors SipA, SipC, and SopE2 were internalized into the cytoplasm of the host cell by OMVs independent of cellular Salmonella–host cell contact. In epithelial cells, addition of OMVs harboring T3SS1 effectors stimulated the production of F-actin, thereby complementing the attenuated invasion of ΔsopE2 into host cells. These results suggest that S. Typhimurium might exploit OMVs as a long-distance vehicle to deliver T3SS1 effectors into the cytoplasm of the host cell independent of bacteria–host cell interaction.

Proteins in the periplasmic space and outer membrane vesicles of Rhizobium etli CE3 grown in minimal medium are largely distinct and change with growth phase

Rhizobium etli CE3 grown in succinate-ammonium minimal medium (MM) excreted outer membrane vesicles (OMVs) with diameters of 40 to 100 nm. Proteins from the OMVs and the periplasmic space were isolated from 6 and 24 h cultures and identified by proteome analysis. A total of 770 proteins were identified: 73.8 and 21.3 % of these occurred only in the periplasm and OMVs, respectively, and only 4.9 % were found in both locations. The majority of proteins found in either location were present only at 6 or 24 h: in the periplasm and OMVs, only 24 and 9 % of proteins, respectively, were present at both sampling times, indicating a time-dependent differential sorting of proteins into the two compartments. The OMVs contained proteins with physiologically varied roles, including Rhizobium adhering proteins (Rap), polysaccharidases, polysaccharide export proteins, auto-aggregation and adherence proteins, glycosyl transferases, peptidoglycan binding and cross-linking enzymes, potential cell wall-modifying enzymes, porins, multidrug efflux RND family proteins, ABC transporter proteins and heat shock proteins. As expected, proteins with known periplasmic localizations (phosphatases, phosphodiesterases, pyrophosphatases) were found only in the periplasm, along with numerous proteins involved in amino acid and carbohydrate metabolism and transport. Nearly one-quarter of the proteins present in the OMVs were also found in our previous analysis of the R. etli total exproteome of MM-grown cells, indicating that these nanoparticles are an important mechanism for protein excretion in this species.

Aggregatibacter actinomycetemcomitans Leukotoxin Is Delivered to Host Cells in an LFA-1-Indepdendent Manner When Associated with Outer Membrane Vesicles

The Gram-negative bacterium, Aggregatibacter actinomycetemcomitans, has been associated with localized aggressive periodontitis (LAP). In particular, highly leukotoxic strains of A. actinomycetemcomitans have been more closely associated with this disease, suggesting that LtxA is a key virulence factor for A. actinomycetemcomitans. LtxA is secreted across both the inner and outer membranes via the Type I secretion system, but has also been found to be enriched within outer membrane vesicles (OMVs), derived from the bacterial outer membrane. We have characterized the association of LtxA with OMVs produced by the highly leukotoxic strain, JP2, and investigated the interaction of these OMVs with host cells to understand how LtxA is delivered to host cells in this OMV-associated form. Our results demonstrated that a significant fraction of the secreted LtxA exists in an OMV-associated form. Furthermore, we have discovered that in this OMV-associated form, the toxin is trafficked to host cells by a cholesterol- and receptor-independent mechanism in contrast to the mechanism by which free LtxA is delivered. Because OMV-associated toxin is trafficked to host cells in an entirely different manner than free toxin, this study highlights the importance of studying both free and OMV-associated forms of LtxA to understand A. actinomycetemcomitans virulence.

Dysregulated hemolysin liberates bacterial outer membrane vesicles for cytosolic lipopolysaccharide sensing

Inflammatory caspase-11/4/5 recognize cytosolic LPS from invading Gram-negative bacteria and induce pyroptosis and cytokine release, forming rapid innate antibacterial defenses. Since extracellular or vacuole-constrained bacteria are thought to rarely access the cytoplasm, how their LPS are exposed to the cytosolic sensors is a critical event for pathogen recognition. Hemolysin is a pore-forming bacterial toxin, which was generally accepted to rupture cell membrane, leading to cell lysis. Whether and how hemolysin participates in non-canonical inflammasome signaling remains undiscovered. Here, we show that hemolysin-overexpressed enterobacteria triggered significantly increased caspase-4 activation in human intestinal epithelial cell lines. Hemolysin promoted LPS cytosolic delivery from extracellular bacteria through dynamin-dependent endocytosis. Further, we revealed that hemolysin was largely associated with bacterial outer membrane vesicles (OMVs) and induced rupture of OMV-containing vacuoles, subsequently increasing LPS exposure to the cytosolic sensor. Accordingly, overexpression of hemolysin promoted caspase-11 dependent IL-18 secretion and gut inflammation in mice, which was associated with restricting bacterial colonization in vivo. Together, our work reveals a concept that hemolysin promotes noncanonical inflammasome activation via liberating OMVs for cytosolic LPS sensing, which offers insights into innate immune surveillance of dysregulated hemolysin via caspase-11/4 in intestinal antibacterial defenses.

Sensing of lipopolysaccharide (LPS) in the cytosol triggers non-canonical inflammasome-mediated innate responses. Recent work revealed that bacterial outer membrane vesicles (OMVs) enables LPS to access the cytosol for extracellular bacteria. However, since intracellular OMVs are generally constrained in endosomes, how OMV-derived LPS gain access to the cytosol remains unknown. Here, we reported that hemolysin largely bound with OMVs and entered cells through dynamin-dependent endocytosis. Intracellular hemolysin significantly impaired OMVs-constrained vacuole integrity and increased OMV-derived LPS exposure to the cytosolic sensor, which promoted non-canonical inflammasome activation and restricted bacterial gut infections. This work reveals that dysregulated hemolysin promotes non-canonical inflammasome activation and alerts host immune recognition, providing insights into the more sophisticated biological functions of hemolysin upon infection.

The Escherichia coli Type III Secretion System 2 Has a Global Effect on Cell Surface

Many strains of Escherichia coli carry a 29,250-bp ETT2 pathogenicity island (PAI), which includes genes predicted to encode type III secretion system (T3SS) components. Because it is similar to the Salmonella pathogenicity island 1 (SPI-1) system, encoding a T3SS in Salmonella enterica, it was assumed that ETT2 also encodes a secretion system injecting effectors into host cells. This assumption was checked in E. coli serotype O2-associated with urinary tract infections and septicemia-which has an intact ETT2 gene cluster, in contrast to most strains in which this cluster carries deletions and mutations. A proteomic search did not reveal any putative secreted effector. Instead, the majority of the secreted proteins were identified as flagellar proteins. A deletion of the ETT2 gene cluster significantly reduced the secretion of flagellar proteins, resulting in reduced motility. There was also a significant reduction in the transcriptional level of flagellar genes, indicating that ETT2 affects the synthesis, rather than secretion, of flagellar proteins. The ETT2 deletion also resulted in additional major changes in secretion of fimbrial proteins and cell surface proteins, resulting in relative resistance to detergents and hydrophobic antibiotics (novobiocin), secretion of large amounts of outer membrane vesicles (OMVs), and altered multicellular behavior. Most important, the ETT2 deletion mutants were sensitive to serum. These major changes indicate that the ETT2 gene cluster has a global effect on cell surface and physiology, which is especially important for pathogenicity, as it contributes to the ability of the bacteria to survive serum and cause sepsis.IMPORTANCE Drug-resistant extraintestinal pathogenic E. coli (ExPEC) strains are major pathogens, especially in hospital- and community-acquired infections. They are the major cause of urinary tract infections and are often involved in septicemia with high mortality. ExPEC strains are characterized by broad-spectrum antibiotic resistance, and development of a vaccine is not trivial because the ExPEC strains include a large number of serotypes. It is therefore important to understand the virulence factors that are involved in pathogenicity of ExPEC and identify new targets for development of antibacterial drugs or vaccines. Such a target could be ETT2, a unique type III secretion system present (complete or in parts) in many ExPEC strains. Here, we show that this system has a major effect on the bacterial surface-it affects sensitivity to drugs, motility, and secretion of extracellular proteins and outer membrane vesicles. Most importantly, this system is important for serum resistance, a prerequisite for septicemia.

Association of Vibrio cholerae 569B outer membrane vesicles with host cells occurs in a GM1-independent manner

The primary virulence factor of Vibrio cholerae, cholera toxin (CT), initiates a pathway in epithelial cells that leads to the severe diarrhoea characteristic of cholera. Secreted CT binds to GM1 on the surface of host cells to facilitate internalisation. Many bacterial toxins, including CT, have been shown to be additionally delivered via outer membrane vesicles (OMVs). A fraction of the closely related heat labile toxin produced by enterotoxigenic Escherichia coli has been demonstrated to reside on the surface of OMVs, where it binds GM1 to facilitate OMV internalisation by host cells. In this work, we investigated whether OMV-associated CT is likewise trafficked to host cells in a GM1-dependent mechanism. We demonstrated that a majority of CT is secreted in its OMV-associated form and is located exclusively inside the vesicle. Therefore, the toxin is unable to bind GM1 on the host cell surface, and the OMVs are trafficked to the host cells in a GM1-independent mechanism. These findings point to a secondary, noncompeting mechanism for secretion and delivery of CT, beyond its well-studied secretion via a Type II secretion system and underscore the importance of focusing future studies on understanding this GM1-independent delivery mechanism to fully understand Vibrio cholerae pathogenesis.

Modulation of gene transcription and epigenetics of colon carcinoma cells by bacterial membrane vesicles

Interactions between bacteria and colon cancer cells influence the transcription of the host cell. Yet is it undetermined whether the bacteria itself or the communication between the host and bacteria is responsible for the genomic changes in the eukaryotic cell. Now, we have investigated the genomic and epigenetic consequences of co-culturing colorectal carcinoma cells with membrane vesicles from pathogenic bacteria Vibrio cholerae and non-pathogenic commensal bacteria Escherichia coli. Our study reveals that membrane vesicles from pathogenic and commensal bacteria have a global impact on the gene expression of colon-carcinoma cells. The changes in gene expression correlate positively with both epigenetic changes and chromatin accessibility of promoters at transcription start sites of genes induced by both types of membrane vesicles. Moreover, we have demonstrated that membrane vesicles obtained only from V. cholerae induced the expression of genes associated with epithelial cell differentiation. Altogether, our study suggests that the observed genomic changes in host cells might be due to specific components of membrane vesicles and do not require communication by direct contact with the bacteria.

A new role for Zinc limitation in bacterial pathogenicity: modulation of α-hemolysin from uropathogenic Escherichia coli

Metal limitation is a common situation during infection and can have profound effects on the pathogen’s success. In this report, we examine the role of zinc limitation in the expression of a virulence factor in uropathogenic Escherichia coli. The pyelonephritis isolate J96 carries two hlyCABD operons that encode the RTX toxin α-hemolysin. While the coding regions of both operons are largely conserved, the upstream sequences, including the promoters, are unrelated. We show here that the two hlyCABD operons are differently regulated. The hly_II operon is efficiently silenced in the presence of zinc and highly expressed when zinc is limited. In contrast, the hly_I operon does not respond to zinc limitation. Genetic studies reveal that zinc-responsive regulation of the hly_II operon is controlled by the Zur metalloregulatory protein. A Zur binding site was identified in the promoter sequence of the hly_II operon, and we observe direct binding of Zur to this promoter region. Moreover, we find that Zur regulation of the hly_II operon modulates the ability of E. coli J96 to induce a cytotoxic response in host cell lines in culture. Our report constitutes the first description of the involvement of the zinc-sensing protein Zur in directly modulating the expression of a virulence factor in bacteria.

Immunization with outer membrane vesicles displaying conserved surface polysaccharide antigen elicits broadly antimicrobial antibodies

Many microbial pathogens produce a β-(1→6)-linked poly-N-acetyl-d-glucosamine (PNAG) surface capsule, including bacterial, fungal, and protozoan cells. Broadly protective immune responses to this single conserved polysaccharide antigen in animals are possible but only when a deacetylated poly-N-acetyl-d-glucosamine (dPNAG; <30% acetate) glycoform is administered as a conjugate to a carrier protein. Unfortunately, conventional methods for natural extraction or chemical synthesis of dPNAG and its subsequent conjugation to protein carriers can be technically demanding and expensive. Here, we describe an alternative strategy for creating broadly protective vaccine candidates that involved coordinating recombinant poly-N-acetyl-d-glucosamine (rPNAG) biosynthesis with outer membrane vesicle (OMV) formation in laboratory strains of Escherichia coli The glycosylated outer membrane vesicles (glycOMVs) released by these engineered bacteria were decorated with the PNAG glycopolymer and induced high titers of PNAG-specific IgG antibodies after immunization in mice. When a Staphylococcus aureus enzyme responsible for PNAG deacetylation was additionally expressed in these cells, glycOMVs were generated that elicited antibodies to both highly acetylated PNAG (∼95-100% acetate) and a chemically deacetylated dPNAG derivative (∼15% acetate). These antibodies mediated efficient in vitro killing of two distinct PNAG-positive bacterial species, namely S. aureus and Francisella tularensis subsp. holarctica, and mice immunized with PNAG-containing glycOMVs developed protective immunity against these unrelated pathogens. Collectively, our results reveal the potential of glycOMVs for targeting this conserved polysaccharide antigen and engendering protective immunity against the broad range of pathogens that produce surface PNAG.

Intrahost milieu modulates production of outer membrane vesicles, vesicle-associated Shiga toxin 2a and cytotoxicity in Escherichia coli O157:H7 and O104:H4

Outer membrane vesicles (OMVs) are important virulence tools of enterohaemorrhagic Escherichia coli (EHEC), but other biological functions of these nanostructures are unknown. We tested the hypothesis that modulation of OMV production enables EHEC to resist the intrahost environment during infection by investigating if simulated human gastrointestinal conditions affect OMV production in EHEC O157:H7 and O104:H4. All the conditions tested including a low pH, simulated ileal and colonic media, presence of mucin, intestinal epithelial cell lysate or antimicrobial peptides, as well as iron limitation, significantly increased OMV production by these pathogens. Accordingly, a maximum vesiculation in EHEC O104:H4 was observed immediately after its isolation from a patient's intestine, and rapidly decreased during passages in vitro. Most of the simulated intrahost conditions also upregulated the OMV-associated Shiga toxin 2a (Stx2a), the major EHEC virulence factor, and, as a result, OMV cytotoxicity. The data indicates that upregulation of OMV production by the human gastrointestinal milieu contributes to EHEC survival and adaptation within the host during infection. Moreover, the intrahost increase of vesiculation and OMV-associated Stx2a may augment EHEC virulence.

In vitro study of virulence potential of Acinetobacter baumannii outer membrane vesicles

Acinetobacter baumannii has emerged as an important opportunistic pathogen mostly causing nosocomial infections. The virulence factors of this important pathogen are largely unknown. Outer membrane vesicles (OMV) are naturally secreted by many gram negative and gram positive bacteria. These vesicles contain outer membrane proteins, lipids, periplasmic proteins, DNA and RNA. Their role in intracellular and intercellular signaling, transfer of virulence factors and eliciting immune response in host cells has been established in many pathogens. In this study, we investigated OMVs from three multi-drug resistant (MDR) clinical isolates and a non-MDR reference strain of A. baumannii for virulence potential. A. baumannii OMVs showed phospholipase C, hemolytic and leukotoxic activities. We found large variations in virulence potential between OMVs of MDR clinical isolates and non-MDR reference strain. These effector molecules were concentrated in OMVs than whole cell bacterial culture and cell-free supernatant. OMV-mediated phospholipase, hemolytic and leucotoxic activities may have a key role in pathogenicity of A. baumannii infection and may be future targets for therapeutic and preventive strategies. This is, to the best of our knowledge, the first report showing virulence potential of A. baumannii OMVs.

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

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

Biofilms: an emergent form of bacterial life

Bacterial biofilms are formed by communities that are embedded in a self-produced matrix of extracellular polymeric substances (EPS). Importantly, bacteria in biofilms exhibit a set of 'emergent properties' that differ substantially from free-living bacterial cells. In this Review, we consider the fundamental role of the biofilm matrix in establishing the emergent properties of biofilms, describing how the characteristic features of biofilms - such as social cooperation, resource capture and enhanced survival of exposure to antimicrobials - all rely on the structural and functional properties of the matrix. Finally, we highlight the value of an ecological perspective in the study of the emergent properties of biofilms, which enables an appreciation of the ecological success of biofilms as habitat formers and, more generally, as a bacterial lifestyle.

A Novel Role of Listeria monocytogenes Membrane Vesicles in Inhibition of Autophagy and Cell Death

Bacterial membrane vesicle (MV) production has been mainly studied in Gram-negative species. In this study, we show that Listeria monocytogenes, a Gram-positive pathogen that causes the food-borne illness listeriosis, produces MVs both in vitro and in vivo. We found that a major virulence factor, the pore-forming hemolysin listeriolysin O (LLO), is tightly associated with the MVs, where it resides in an oxidized, inactive state. Previous studies have shown that LLO may induce cell death and autophagy. To monitor possible effects of LLO and MVs on autophagy, we performed assays for LC3 lipidation and LDH sequestration as well as analysis by confocal microscopy of HEK293 cells expressing GFP-LC3. The results revealed that MVs alone did not affect autophagy whereas they effectively abrogated autophagy induced by pure LLO or by another pore-forming toxin from Vibrio cholerae, VCC. Moreover, Listeria monocytogenes MVs significantly decreased Torin1-stimulated macroautophagy. In addition, MVs protected against necrosis of HEK293 cells caused by the lytic action of LLO. We explored the mechanisms of LLO-induced autophagy and cell death and demonstrated that the protective effect of MVs involves an inhibition of LLO-induced pore formation resulting in inhibition of autophagy and the lytic action on eukaryotic cells. Further, we determined that these MVs help bacteria to survive inside eukaryotic cells (mouse embryonic fibroblasts). Taken together, these findings suggest that intracellular release of MVs from L. monocytogenes may represent a bacterial strategy to survive inside host cells, by its control of LLO activity and by avoidance of destruction from the autophagy system during infection.

Gre factors-mediated control of hilD transcription is essential for the invasion of epithelial cells by Salmonella enterica serovar Typhimurium

The invasion of epithelial cells by Salmonella enterica serovar Typhimurium is a very tightly regulated process. Signaling cascades triggered by different environmental and physiological signals converge to control HilD, an AraC regulator that coordinates the expression of several virulence factors. The expression of hilD is modulated at several steps of the expression process. Here, we report that the invasion of epithelial cells by S. Typhimurium strains lacking the Gre factors, GreA and GreB, is impaired. By interacting with the RNA polymerase secondary channel, the Gre factors prevent backtracking of paused complexes to avoid arrest during transcriptional elongation. Our results indicate that the Gre factors are required for the expression of the bacterial factors needed for epithelial cell invasion by modulating expression of HilD. This regulation does not occur at transcription initiation and depends on the capacity of the Gre factors to prevent backtracking of the RNA polymerase. Remarkably, genetic analyses indicate that the 3’-untranslated region (UTR) of hilD is required for Gre-mediated regulation of hilD expression. Our data provide new insight into the complex regulation of S. Typhimurium virulence and highlight the role of the hilD 3’-UTR as a regulatory motif.

Salmonella enterica serovar Typhimurium is a foodborne pathogen that causes gastroenteritis in humans. To successfully trigger infection, S. Typhimurium invades epithelial cells, a process that requires the coordinated expression of a set of genes. HilD is a pivotal regulator of S. Typhimurium pathogenicity, as it activates the expression of the genes required for invasion of intestinal epithelium. Expression and activity of HilD are tightly regulated and respond to several environmental and physiological conditions. In this report, we introduce the transcription elongation as a novel level of regulation of hilD. We describe that the Gre factors, proteins that prevent backtracking of paused RNA polymerase complexes during transcription elongation, are required for the expression of HilD and the subsequent expression of genes involved in the invasion of epithelial cells.

Uropathogenic Escherichia coli Releases Extracellular Vesicles That Are Associated with RNA

BACKGROUND

Bacterium-to-host signalling during infection is a complex process involving proteins, lipids and other diffusible signals that manipulate host cell biology for pathogen survival. Bacteria also release membrane vesicles (MV) that can carry a cargo of effector molecules directly into host cells. Supported by recent publications, we hypothesised that these MVs also associate with RNA, which may be directly involved in the modulation of the host response to infection.

METHODS AND RESULTS

Using the uropathogenic Escherichia coli (UPEC) strain 536, we have isolated MVs and found they carry a range of RNA species. Density gradient centrifugation further fractionated and characterised the MV preparation and confirmed that the isolated RNA was associated with the highest particle and protein containing fractions. Using a new approach, RNA-sequencing of libraries derived from three different 'size' RNA populations (<50nt, 50-200nt and 200nt+) isolated from MVs has enabled us to now report the first example of a complete bacterial MV-RNA profile. These data show that MVs carry rRNA, tRNAs, other small RNAs as well as full-length protein coding mRNAs. Confocal microscopy visualised the delivery of lipid labelled MVs into cultured bladder epithelial cells and showed their RNA cargo labelled with 5-EU (5-ethynyl uridine), was transported into the host cell cytoplasm and nucleus. MV RNA uptake by the cells was confirmed by droplet digital RT-PCR of csrC. It was estimated that 1% of MV RNA cargo is delivered into cultured cells.

CONCLUSIONS

These data add to the growing evidence of pathogenic bacterial MV being associated a wide range of RNAs. It further raises the plausibility for MV-RNA-mediated cross-kingdom communication whereby they influence host cell function during the infection process.

Outer membrane vesicles displaying engineered glycotopes elicit protective antibodies

The O-antigen polysaccharide (O-PS) component of lipopolysaccharides on the surface of gram-negative bacteria is both a virulence factor and a B-cell antigen. Antibodies elicited by O-PS often confer protection against infection; therefore, O-PS glycoconjugate vaccines have proven useful against a number of different pathogenic bacteria. However, conventional methods for natural extraction or chemical synthesis of O-PS are technically demanding, inefficient, and expensive. Here, we describe an alternative methodology for producing glycoconjugate vaccines whereby recombinant O-PS biosynthesis is coordinated with vesiculation in laboratory strains of Escherichia coli to yield glycosylated outer membrane vesicles (glycOMVs) decorated with pathogen-mimetic glycotopes. Using this approach, glycOMVs corresponding to eight different pathogenic bacteria were generated. For example, expression of a 17-kb O-PS gene cluster from the highly virulent Francisella tularensis subsp. tularensis (type A) strain Schu S4 in hypervesiculating E. coli cells yielded glycOMVs that displayed F. tularensis O-PS. Immunization of BALB/c mice with glycOMVs elicited significant titers of O-PS-specific serum IgG antibodies as well as vaginal and bronchoalveolar IgA antibodies. Importantly, glycOMVs significantly prolonged survival upon subsequent challenge with F. tularensis Schu S4 and provided complete protection against challenge with two different F. tularensis subsp. holarctica (type B) live vaccine strains, thereby demonstrating the vaccine potential of glycOMVs. Given the ease with which recombinant glycotopes can be expressed on OMVs, the strategy described here could be readily adapted for developing vaccines against many other bacterial pathogens.

Cytotoxic and Inflammatory Responses Induced by Outer Membrane Vesicle-Associated Biologically Active Proteases from Vibrio cholerae

Proteases in Vibrio cholerae have been shown to play a role in its pathogenesis. V. cholerae secretes Zn-dependent hemagglutinin protease (HAP) and calcium-dependent trypsin-like serine protease (VesC) by using the type II secretion system (TIISS). Our present studies demonstrated that these proteases are also secreted in association with outer membrane vesicles (OMVs) and transported to human intestinal epithelial cells in an active form. OMV-associated HAP induces dose-dependent apoptosis in Int407 cells and an enterotoxic response in the mouse ileal loop (MIL) assay, whereas OMV-associated VesC showed a hemorrhagic fluid response in the MIL assay, necrosis in Int407 cells, and an increased interleukin-8 (IL-8) response in T84 cells, which were significantly reduced in OMVs from VesC mutant strain. Our results also showed that serine protease VesC plays a role in intestinal colonization of V. cholerae strains in adult mice. In conclusion, our study shows that V. cholerae OMVs secrete biologically active proteases which may play a role in cytotoxic and inflammatory responses.

Development of a cost-effective vaccine candidate with outer membrane vesicles of a tolA-disrupted Shigella boydii strain

Our previous studies on outer membrane vesicles based vaccine development against shigellosis, revealed the inability of Shigella to release significant amount of vesicles naturally, during growth. Disruption of tolA, one of the genes of the Tol-Pal system of Gram negative bacterial membrane, has increased the vesicle release rate of a Shigella boydii type 4 strain to approximately 60% higher. We also noticed the vesicles, released from tolA-disrupted strain captured more OmpA protein and lipopolysaccharide, compared to the vesicles released from its wild type prototype. Six to seven weeks old BALB/c mice, immunized with 25 μg of three oral doses of the vesicles, released by tolA mutant, conferred 100% protection against lethal homologous challenge through nasal route, compared to only 60% protection after the same dose of wild type immunogen. Mice, immunized with the vesicles from tolA-mutant, manifested significant secretion of mucosal IgG and IgA. A sharp and significant response of pro-inflammatory cytokines (TNF-α, IL-6, IFN-γ) were also observed in the lung lavage of these groups of mice, within 6h post challenge; but at 24h, these inflammatory cytokines showed the sign of subsidence and the system was taken over by the release of anti-inflammatory cytokines (IL-4 and IL-10). Studies with naïve peritoneal macrophages, proved further, the potency of these vesicles to stimulate nitric oxide and TNF-α, IL-12p70, IL-6 and IL-10 productions in-vitro. The ability of these vesicles to trigger polarization of CD4(+) T cells toward Th1 adaptive immune response, had also been observed along with the presence of anti-inflammatory cytokines in the system. Our study demonstrated, the vesicles from tolA-disrupted Shigella were able to suppress Shigella-mediated inflammation in the host and could balance between inflammation and anti-inflammation, promoting better survival and health of the infected mice. Outer membrane vesicles from tolA-mutant, could be a potential cost-effective vaccine candidate against shigellosis.

Comparative proteomics of uropathogenic Escherichia coli during growth in human urine identify UCA-like (UCL) fimbriae as an adherence factor involved in biofilm formation and binding to uroepithelial cells

Uropathogenic Escherichia coli (UPEC) are the primary cause of urinary tract infection (UTI) in humans. For the successful colonisation of the human urinary tract, UPEC employ a diverse collection of secreted or surface-exposed virulence factors including toxins, iron acquisition systems and adhesins. In this study, a comparative proteomic approach was utilised to define the UPEC pan and core surface proteome following growth in pooled human urine. Identified proteins were investigated for subcellular origin, prevalence and homology to characterised virulence factors. Fourteen core surface proteins were identified, as well as eleven iron uptake receptor proteins and four distinct fimbrial types, including type 1, P, F1C/S and a previously uncharacterised fimbrial type, designated UCA-like (UCL) fimbriae in this study. These pathogenicity island (PAI)-associated fimbriae are related to UCA fimbriae of Proteus mirabilis, associated with UPEC and exclusively found in members of the E. coli B2 and D phylogroup. We further demonstrated that UCL fimbriae promote significant biofilm formation on abiotic surfaces and mediate specific attachment to exfoliated human uroepithelial cells. Combined, this study has defined the surface proteomic profiles and core surface proteome of UPEC during growth in human urine and identified a new type of fimbriae that may contribute to UTI.

Outer Membrane Vesicle-Mediated Export of Processed PrtV Protease from Vibrio cholerae

Background

Outer membrane vesicles (OMVs) are known to release from almost all Gram-negative bacteria during normal growth. OMVs carry different biologically active toxins and enzymes into the surrounding environment. We suggest that OMVs may therefore be able to transport bacterial proteases into the target host cells. We present here an analysis of the Vibrio cholerae OMV-associated protease PrtV.

Methodology/Principal Findings

In this study, we demonstrated that PrtV was secreted from the wild type V. cholerae strain C6706 via the type II secretion system in association with OMVs. By immunoblotting and electron microscopic analysis using immunogold labeling, the association of PrtV with OMVs was examined. We demonstrated that OMV-associated PrtV was biologically active by showing altered morphology and detachment of cells when the human ileocecum carcinoma (HCT8) cells were treated with OMVs from the wild type V. cholerae strain C6706 whereas cells treated with OMVs from the prtV isogenic mutant showed no morphological changes. Furthermore, OMV-associated PrtV protease showed a contribution to bacterial resistance towards the antimicrobial peptide LL-37.

Conclusion/Significance

Our findings suggest that OMVs released from V. cholerae can deliver a processed, biologically active form of PrtV that contributes to bacterial interactions with target host cells.