Export of virulence genes and Shiga toxin by membrane vesicles of Escherichia coli O157:H7

Outer membrane vesicles of Acinetobacter baumannii DS002 carry circular DNA similar to bovine meat and milk factors (BMMFs) and SPHINX 2.36 and probably play a role in interdomain lateral gene transfer

The discovery of Replication Competent Circular DNA molecules in mammalian cells and tissues is being linked to debilitating diseases, such as multiple sclerosis (MS), bovine spongiform encephalopathy (BSE), and colorectal cancer (CRC). These circular DNA molecules, otherwise known as bovine meat and milk factors (BMMFs) and Slow Progressive Hidden INfections of variable (X) latency (SPHINX), bear significant (80%) sequence similarity with the plasmids of Acinetobacter baumannii strains. Nanostructures, such as bacterial outer membrane vesicles (OMVs) serve as vehicles for transporting biomolecular cargo and have the potential to facilitate interkingdom lateral mobility of DNA. Strengthening the proposed hypothesis, this study demonstrates that OMVs derived from A. baumannii DS002 carrying four plasmids and genome (pTS236) of phage, AbDs1, successfully reached different parts of the body, including the central nervous system, following the injection of fluorescein isothiocyanate (FITC)-labeled OMVs into experimental mice. Out of the four OMV-associated plasmids, three (pTS4586, pTS9900, and pTS134338) were identified within the lumen, and the fourth one (pTS11291) was found on the surface of OMVs. In addition to the indigenous plasmids, the phage-encoded protein, Orf96, anchored on the surface of the OMVs by establishing a strong interaction with the OMV-associated porin, OmpA. Intriguingly, a subset of labeled OMVs, when incubated with Neuro2A cells, translocated across the membrane and reached to the cytoplasmic space of the cells. Collectively, the experimental evidence presented herein underscores the promising potential of OMVs as vehicles for delivering molecular cargo containing plasmids and phage genomes to diverse mammalian tissues and cells.

IMPORTANCE

Several independent studies have demonstrated the existence of replication competent circular DNA molecules of bacterial and viral origin in mammalian cells and tissues. However, studies about their origin and lateral mobility to mammalian cells are scarce. Our work describes the existence of circular DNA, similar to that of DNA molecules identified in mammalian cells, OMVs derived from soil isolate of A. baumannii DS002. Furthermore, the work also provides visual evidence that demonstrates the passage of labeled OMVs to different organs of experimental mice within hours after intravenously administering OMVs into experimental mice. Some of the labeled OMVs have even crossed the membrane of Neuro2A, suggesting the existence of interkingdom horizontal mobility between bacteria and mammals.

An in-depth exploration of the multifaceted roles of EVs in the context of pathogenic single-cell microorganisms

SUMMARYExtracellular vesicles (EVs) have been recognized throughout scientific communities as potential vehicles of intercellular communication in both eukaryotes and prokaryotes, thereby influencing various physiological and pathological functions of both parent and recipient cells. This review provides an in-depth exploration of the multifaceted roles of EVs in the context of bacteria and protozoan parasite EVs, shedding light on their contributions to physiological processes and disease pathogenesis. These studies highlight EVs as a conserved mechanism of cellular communication, which may lead us to important breakthroughs in our understanding of infection, mechanisms of pathogenesis, and as indicators of disease. Furthermore, EVs are involved in host-microbe interactions, offering insights into the strategies employed by bacteria and protozoan parasites to modulate host responses, evade the immune system, and establish infections.

The effect of altered pH growth conditions on the production, composition, and proteomes of Helicobacter pylori outer membrane vesicles

Gram-negative bacteria release outer membrane vesicles (OMVs) that contain cargo derived from their parent bacteria. Helicobacter pylori is a Gram-negative human pathogen that produces urease to increase the pH of the surrounding environment to facilitate colonization of the gastric mucosa. However, the effect of acidic growth conditions on the production and composition of H. pylori OMVs is unknown. In this study, we examined the production, composition, and proteome of H. pylori OMVs produced during acidic and neutral pH growth conditions. H. pylori growth in acidic conditions reduced the quantity and size of OMVs produced. Additionally, OMVs produced during acidic growth conditions had increased protein, DNA, and RNA cargo compared to OMVs produced during neutral conditions. Proteomic analysis comparing the proteomes of OMVs to their parent bacteria demonstrated significant differences in the enrichment of beta-lactamases and outer membrane proteins between bacteria and OMVs, supporting that differing growth conditions impacts OMV composition. We also identified differences in the enrichment of proteins between OMVs produced during different pH growth conditions. Overall, our findings reveal that growth of H. pylori at different pH levels is a factor that alters OMV proteomes, which may affect their subsequent functions.

ESBL-Escherichia coli extracellular vesicles mediate bacterial resistance to β-lactam and mediate horizontal transfer of blaCTX-M-55

OBJECTIVES

Extracellular vesicles (EVs) have become the focus of research as an emerging method of horizontal gene transfer. In recent years, studies on the association between EVs and the spread of bacterial resistance have emerged, but there is a lack of research on the role of EVs secreted by extended-spectrum β-lactamase (ESBL)-producing Escherichia coli in the spread of β-lactam resistance. Therefore, the aim of this study was to investigate the role of EVs in the transmission of β-lactam resistance.

METHODS

In this study, the role of EVs in the transmission of β-lactam resistance in E. coli was evaluated by the EVs-mediated bacterial resistance to β-lactam antibiotics test and the EVs-mediated blaCTX-M-55 transfer experiments using EVs secreted by ESBL-E. coli.

RESULTS

The results showed that ESBL-EVs were protective against β-lactam antibiotic-susceptible bacteria, and this protective effect was dependent on the integrity of the EVs and showed dose- and time-dependent effects. At the same time, ESBL-EVs can also mediate the horizontal transmission of blaCTX-M-55, and EVs-mediated gene transfer is selective, preferring to transfer in more closely related species.

CONCLUSIONS

In this study, we demonstrated the important role of EVs in the transmission of β-lactam resistance in chicken ESBL-E. coli, and evaluated the risk of EVs-mediated horizontal gene transfer, which provided a theoretical basis for elucidating the mechanism of EVs-mediated resistance transmission.

Membrane vesicles derived from Enterococcus faecalis promote the co-transfer of important antibiotic resistance genes located on both plasmids and chromosomes

BACKGROUND

Bacterial membrane vesicles (BMVs) are novel vehicles of antibiotic resistance gene (ARG) transfer in Gram-negative bacteria, but their role in the spread of ARGs in Gram-positive bacteria has not been defined. The purpose of this study was to evaluate the role of MVs in the transmission of antimicrobial resistance in Gram-positive bacteria.

METHODS

A linezolid-resistant Enterococcus faecalis CQ20 of swine origin was selected as the donor strain. Linezolid-susceptible E. faecalis SC032 of human origin, Enterococcus faecium BM4105 and Escherichia coli were selected as recipient strains. The presence of plasmids (pCQ20-1 and pCQ20-2) and an optrA-carrying transposon Tn6674 in CQ20, MVs and vesiculants was verified by WGS or PCR. MVs were isolated with density gradient centrifugation, and MV-mediated transformation was performed to assess the horizontal transferability of MVs. The MICs for CQ20 and its vesiculants were determined by the broth microdilution method.

RESULTS

CQ20-derived MVs (CQ20-MV) were isolated, and PCR identified the presence of two plasmids and the optrA gene in the CQ20-MVs. MV-mediated transformation to E. faecalis SC032 and E. faecium BM4105 was successfully performed, and the WGS data also showed that both plasmids pCQ20-1 and pCQ20-2 and optrA-carrying transposon Tn6674 were transferred to E. faecalis SC032 and E. faecium BM4105, but failed for E. coli. Additionally, vesiculants that had acquired ARGs still had the ability to spread these genes via MVs.

CONCLUSIONS

To our knowledge, this is the first report of MV-mediated co-transfer of ARG-carrying plasmids and transposons in the Gram-positive bacterium E. faecium.

Bacterial outer membrane vesicles provide an alternative pathway for trafficking of Escherichia coli O157 type III secreted effectors to epithelial cells

IMPORTANCE

Bacteria can package protein cargo into nanosized membrane blebs that are shed from the bacterial membrane and released into the environment. Here, we report that a type of pathogenic bacteria called enterohemorrhagic Escherichia coli O157 (EHEC) uses their membrane blebs (outer membrane vesicles) to package components of their type 3 secretion system and send them into host cells, where they can manipulate host signaling pathways including those involved in infection response, such as immunity. Usually, EHEC use a needle-like apparatus to inject these components into host cells, but packaging them into membrane blebs that get taken up by host cells is another way of delivery that can bypass the need for a functioning injection system.

Characterizing the Pathogenic Potential of Crohn's Disease-Associated Adherent-Invasive Escherichia coli

The microbiome of Crohn's disease (CD) patients is composed of a microbial community that is considered dysbiotic and proinflammatory in nature. The overrepresentation of Enterobacteriaceae species is a common feature of the CD microbiome, and much attention has been given to understanding the pathogenic role this feature plays in disease activity. Over 2 decades ago, a new Escherichia coli subtype called adherent-invasive E. coli (AIEC) was isolated and linked to ileal Crohn's disease. Since the isolation of the first AIEC strain, additional AIEC strains have been isolated from both inflammatory bowel disease (IBD) patients and non-IBD individuals using the original in vitro phenotypic characterization methods. Identification of a definitive molecular marker of the AIEC pathotype has been elusive; however, significant advancements have been made in understanding the genetic, metabolic, and virulence determinants of AIEC infection biology. Here, we review the current knowledge of AIEC pathogenesis to provide additional, objective measures that could be considered in defining AIEC and their pathogenic potential.

OprF functions as a latch to direct Outer Membrane Vesicle release in Pseudomonas aeruginosa

Bacterial Outer Membrane Vesicles (OMVs) contribute to virulence, competition, immune avoidance and communication. This has led to great interest in how they are formed. To date, investigation has focused almost exclusively on what controls the initiation of OMV biogenesis. Regardless of the mechanism of initiation, all species face a similar challenge before an OMV can be released: How does the OM detach from the underlying peptidoglycan (PG) in regions that will ultimately bulge and then vesiculate? The OmpA family of OM proteins (OprF in P. aeruginosa) is widely conserved and unusually abundant in OMVs across species considering their major role in PG attachment. OmpA homologs also have the interesting ability to adopt both PG-bound (two-domain) and PG-released (one-domain) conformations. Using targeted deletion of the PG-binding domain we showed that loss of cell wall association, and not general membrane destabilization, is responsible for hypervesiculation in OprF-modified strains. We therefore propose that OprF functions as a 'latch', capable of releasing PG in regions destined to become OMVs. To test this hypothesis, we developed a protocol to assess OprF conformation in live cells and purified OMVs. While >90% of OprF proteins exist in the two-domain conformation in the OM of cells, we show that the majority of OprF in OMVs is present in the one-domain conformation. With this work, we take some of the first steps in characterizing late-stage OMV biogenesis and identify a family of proteins whose critical role can be explained by their unique ability to fold into two distinct conformations.

Immunomodulatory Potential of Fungal Extracellular Vesicles: Insights for Therapeutic Applications

Extracellular vesicles (EVs) are membranous vesicular organelles that perform a variety of biological functions including cell communication across different biological kingdoms. EVs of mammals and, to a lesser extent, bacteria have been deeply studied over the years, whereas investigations of fungal EVs are still in their infancy. Fungi, encompassing both yeast and filamentous forms, are increasingly recognized for their production of extracellular vesicles (EVs) containing a wealth of proteins, lipids, and nucleic acids. These EVs play pivotal roles in orchestrating fungal communities, bolstering pathogenicity, and mediating interactions with the environment. Fungal EVs have emerged as promising candidates for innovative applications, not only in the management of mycoses but also as carriers for therapeutic molecules. Yet, numerous questions persist regarding fungal EVs, including their mechanisms of generation, release, cargo regulation, and discharge. This comprehensive review delves into the present state of knowledge regarding fungal EVs and provides fresh insights into the most recent hypotheses on the mechanisms driving their immunomodulatory properties. Furthermore, we explore the considerable potential of fungal EVs in the realms of medicine and biotechnology. In the foreseeable future, engineered fungal cells may serve as vehicles for tailoring cargo- and antigen-specific EVs, positioning them as invaluable biotechnological tools for diverse medical applications, such as vaccines and drug delivery.

Role of Legionella pneumophila outer membrane vesicles in host-pathogen interaction

Legionella pneumophila is an opportunistic intracellular pathogen that inhabits artificial water systems and can be transmitted to human hosts by contaminated aerosols. Upon inhalation, it colonizes and grows inside the alveolar macrophages and causes Legionnaires' disease. To effectively control and manage Legionnaires' disease, a deep understanding of the host-pathogen interaction is crucial. Bacterial extracellular vesicles, particularly outer membrane vesicles (OMVs) have emerged as mediators of intercellular communication between bacteria and host cells. These OMVs carry a diverse cargo, including proteins, toxins, virulence factors, and nucleic acids. OMVs play a pivotal role in disease pathogenesis by helping bacteria in colonization, delivering virulence factors into host cells, and modulating host immune responses. This review highlights the role of OMVs in the context of host-pathogen interaction shedding light on the pathogenesis of L. pneumophila. Understanding the functions of OMVs and their cargo provides valuable insights into potential therapeutic targets and interventions for combating Legionnaires' disease.

Bacterial Outer Membrane Vesicles and Immune Modulation of the Host

This article reviews the role of outer membrane vesicles (OMVs) in mediating the interaction between Gram-negative bacteria and their human hosts. OMVs are produced by a diverse range of Gram-negative bacteria during infection and play a critical role in facilitating host-pathogen interactions without requiring direct cell-to-cell contact. This article describes the mechanisms by which OMVs are formed and subsequently interact with host cells, leading to the transport of microbial protein virulence factors and short interfering RNAs (sRNA) to their host targets, exerting their immunomodulatory effects by targeting specific host signaling pathways. Specifically, this review highlights mechanisms by which OMVs facilitate chronic infection through epigenetic modification of the host immune response. Finally, this review identifies critical knowledge gaps in the field and offers potential avenues for future OMV research, specifically regarding rigor and reproducibility in OMV isolation and characterization methods.

A chimeric protein-based vaccine elicits a strong IgG antibody response and confers partial protection against Shiga toxin-producing Escherichia coli in mice

Background

Shiga toxin-producing Escherichia coli (STEC) is a foodborne pathogen that causes gastrointestinal infections, ranging from acute diarrhea and dysentery to life-threatening diseases such as Hemolytic Uremic Syndrome. Currently, a vaccine to prevent STEC infection is an unmet medical need.

Results

We developed a chimeric protein-based vaccine targeting seven virulence factors of STEC, including the Stx2B subunit, Tir, Intimin, EspA, Cah, OmpT, and AggA proteins. Immunization of mice with this vaccine candidate elicited significant humoral and cellular immune responses against STEC. High levels of specific IgG antibodies were found in the serum and feces of immunized mice. However, specific IgA antibodies were not detected in either serum or feces. Furthermore, a significantly higher percentage of antigen-specific CD4+ T cells producing IFN-γ, IL-4, and IL-17 was observed in the spleens of immunized mice. Notably, the immunized mice showed decreased shedding of STEC O157:H7 and STEC O91:H21 strains and were protected against weight loss during experimental infection. Additionally, infection with the STEC O91:H21 strain resulted in kidney damage in control unimmunized mice; however, the extent of damage was slightly lower in immunized mice. Our findings suggest that IgG antibodies induced by this vaccine candidate may have a role in inhibiting bacterial adhesion and complement-mediated killing.

Conclusion

This study provides evidence that IgG responses are involved in the host defense against STEC. However, our results do not rule out that other classes of antibodies also participate in the protection against this pathogen. Additional work is needed to improve the protection conferred by our vaccine candidate and to elucidate the relevant immune responses that lead to complete protection against this pathogen.

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.

The role of PhoP/PhoQ system in regulating stress adaptation response in Escherichia coli O157:H7

The development of acid tolerance response (ATR) as a result of low pH in Escherichia coli O157:H7 (E. coli O157:H7) contaminating beef during processing is considered a major food safety concern. Thus, in order to explore the formation and molecular mechanisms of the tolerance response of E. coli O157:H7 in a simulated beef processing environment, the resistance of a wild-type (WT) strain and its corresponding ΔphoP mutant to acid, heat, and osmotic pressure was evaluated. Strains were pre-adapted under different conditions of pH (5.4 and 7.0), temperature (37 °C and 10 °C), and culture medium (meat extract and Luria-Bertani broth media). In addition, the expression of genes related to stress response and virulence was also investigated among WT and ΔphoP strains under the tested conditions. Pre-acid adaptation increased the resistance of E. coli O157:H7 to acid and heat treatment while resistance to osmotic pressure decreased. Moreover, acid adaptation in meat extract medium simulating slaughter environment increased ATR, whereas pre-adaptation at 10 °C reduced the ATR. Furthermore, it was shown that mildly acidic conditions (pH = 5.4) and the PhoP/PhoQ two-component system (TCS) acted synergistically to enhance acid and heat tolerance in E. coli O157:H7. Additionally, the expression of genes related to arginine and lysine metabolism, heat shock, and invasiveness was up-regulated, which revealed that the mechanism of acid resistance and cross-protection under mildly acidic conditions was mediated by the PhoP/PhoQ TCS. Both acid adaptation and phoP gene knockout reduced the relative expression of stx1 and stx2 genes which were considered as critical pathogenic factors. Collectively, the current findings indicated that ATR could occur in E. coli O157:H7 during beef processing. Thus, there is an increased food safety risk due to the persistence of tolerance response in the following processing conditions. The present study provides a more comprehensive basis for the effective application of hurdle technology in beef processing.

Translocation of outer membrane vesicles from enterohemorrhagic Escherichia coli O157 across the intestinal epithelial barrier

Outer membrane vesicles (OMVs) carrying virulence factors of enterohemorrhagic Escherichia coli (EHEC) are assumed to play a role in the pathogenesis of life-threatening hemolytic uremic syndrome (HUS). However, it is unknown if and how OMVs, which are produced in the intestinal lumen, cross the intestinal epithelial barrier (IEB) to reach the renal glomerular endothelium, the major target in HUS. We investigated the ability of EHEC O157 OMVs to translocate across the IEB using a model of polarized Caco-2 cells grown on Transwell inserts and characterized important aspects of this process. Using unlabeled or fluorescently labeled OMVs, tests of the intestinal barrier integrity, inhibitors of endocytosis, cell viability assay, and microscopic techniques, we demonstrated that EHEC O157 OMVs translocated across the IEB. OMV translocation involved both paracellular and transcellular pathways and was significantly increased under simulated inflammatory conditions. In addition, translocation was not dependent on OMV-associated virulence factors and did not affect viability of intestinal epithelial cells. Importantly, translocation of EHEC O157 OMVs was confirmed in human colonoids thereby supporting physiological relevance of OMVs in the pathogenesis of HUS.

Outer Membrane Vesicles Transmitting blaNDM-1 Mediate the Emergence of Carbapenem-Resistant Hypervirulent Klebsiella pneumoniae

Dissemination of hypervirulent and carbapenem-resistant Klebsiella pneumoniae (CRKP) has been reported worldwide, posing a serious threat to antimicrobial therapy and public health. Outer membrane vesicles (OMVs) act as vectors for the horizontal transfer of virulence and resistance genes. However, K. pneumoniae OMVs that transfer carbapenem resistance genes into hypervirulent K. pneumoniae (hvKP) have been insufficiently investigated. Therefore, this study investigates the transmission of the bla_NDM-1 gene encoding resistance via OMVs released from CRKP and the potential mechanism responsible for the carbapenem-resistant hypervirulent K. pneumoniae (CR-hvKP) emergence. OMVs were isolated via ultracentrifugation from CRKP with or without meropenem selective pressure. OMVs were then used to transform classical K. pneumoniae (ckp) ATCC 10031, extended-spectrum β-lactamase (ESBL)-producing K. pneumoniae ATCC 700603, and hvKP NTUH-K2044. Our results showed that meropenem treatment resulted in changes in the number and diameter of OMVs secreted by CRKP. OMVs derived from CRKP mediated the transfer of bla_NDM-1 to ckp and hvKP, thereby increasing the carbapenem MIC of transformants. Further experiments confirmed that NTUH-K2044 transformants exhibited hypervirulence. Our study demonstrates, for the first time, that OMVs derived from CRKP can carry bla_NDM-1 and deliver resistance genes to other K. pneumoniae strains, even hvKP. The transfer of carbapenem genes into hypervirulent strains may promote the emergence and dissemination of CR-hvKP. This study elucidates a new mechanism underlying the formation of CR-hvKP.

Planktonic and Biofilm-Derived Pseudomonas aeruginosa Outer Membrane Vesicles Facilitate Horizontal Gene Transfer of Plasmid DNA

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria package various cargo, including DNA that can be transferred to other bacteria or to host cells. OMV-associated DNA has been implicated in mediating horizontal gene transfer (HGT) between bacteria, which includes the dissemination of antibiotic resistance genes within and between bacterial species. Despite the known ability of OMVs to mediate HGT, the mechanisms of DNA packaging into OMVs remain poorly characterized, as does the effect of bacterial growth conditions on the DNA cargo composition of OMVs and their subsequent abilities to mediate HGT. In this study, we examined the DNA content of OMVs produced by the opportunistic pathogen Pseudomonas aeruginosa grown in either planktonic or biofilm conditions. Analysis of planktonic growth-derived OMVs revealed their ability to package and protect plasmid DNA from DNase degradation and to transfer plasmid-encoded antibiotic resistance genes to recipient, antibiotic-sensitive P. aeruginosa bacteria at a greater efficiency than transformation with plasmid alone. Comparisons of planktonic and biofilm-derived P. aeruginosa OMVs demonstrated that biofilm-derived OMVs were smaller but were associated with more plasmid DNA than planktonic-derived OMVs. Additionally, biofilm-derived P. aeruginosa OMVs were more efficient in the transformation of competent P. aeruginosa bacteria, compared to transformations with an equivalent number of planktonic-derived OMVs. The findings of this study highlight the importance of bacterial growth conditions for the packaging of DNA within P. aeruginosa OMVs and their ability to facilitate HGT, thus contributing to the spread of antibiotic resistance genes between P. aeruginosa bacteria. IMPORTANCE Bacterial membrane vesicles (BMVs) mediate interbacterial communication, and their ability to package DNA specifically contributes to biofilm formation, antibiotic resistance, and HGT between bacteria. However, the ability of P. aeruginosa OMVs to mediate HGT has not yet been demonstrated. Here, we reveal that P. aeruginosa planktonic and biofilm-derived OMVs can deliver plasmid-encoded antibiotic resistance to recipient P. aeruginosa. Additionally, we demonstrated that P. aeruginosa biofilm-derived OMVs were associated with more plasmid DNA compared to planktonic-derived OMVs and were more efficient in the transfer of plasmid DNA to recipient bacteria. Overall, this demonstrated the ability of P. aeruginosa OMVs to facilitate the dissemination of antibiotic resistance genes, thereby enabling the survival of susceptible bacteria during antibiotic treatment. Investigating the roles of biofilm-derived BMVs may contribute to furthering our understanding of the role of BMVs in HGT and the spread of antibiotic resistance in the environment.

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.

The Discovery of the Role of Outer Membrane Vesicles against Bacteria

Gram-negative bacteria are intrinsically resistant to many commercialized antibiotics. The outer membrane (OM) of Gram-negative bacteria prevents the entry of such antibiotics. Outer membrane vesicles (OMV) are naturally released from the OM of Gram-negative bacteria for a range of purposes, including competition with other bacteria. OMV may carry, as part of the membrane or lumen, molecules with antibacterial activity. Such OMV can be exposed to and can fuse with the cell surface of different bacterial species. In this review we consider how OMV can be used as tools to deliver antimicrobial agents. This includes the characteristics of OMV production and how this process can be used to create the desired antibacterial activity of OMV.

Outer Membrane Vesicles as Molecular Biomarkers for Gram-negative Sepsis: Taking Advantage of Nature's Perfect Packages

Sepsis is an often life-threatening response to infection, occurring when host pro-inflammatory immune responses become abnormally elevated and dysregulated. To diagnose sepsis, the patient must have a confirmed or predicted infection, as well as other symptoms associated with the pathophysiology of sepsis. However, a recent study found that a specific causal organism could not be determined in the majority (70.1%) of sepsis cases, likely due to aggressive antibiotics or localized infections. The timing of a patient's sepsis diagnosis is often predictive of their clinical outcome, underlining the need for a more definitive molecular diagnostic test. Here, we outline the advantages and challenges to using bacterial outer membrane vesicles (OMVs), nanoscale spherical buds derived from the outer membrane of Gram-negative bacteria, as a diagnostic biomarker for Gram-negative sepsis. Advantages include OMV abundance, their robustness in the presence of antibiotics, and their unique features derived from their parent cell that could allow for differentiation between bacterial species. Challenges include the rigorous purification methods required to isolate OMVs from complex biofluids and the additional need to separate OMVs from similarly-sized extracellular vesicles, which can share physical properties with OMVs.

Enterohemorrhagic Escherichia coli and a Fresh View on Shiga Toxin-Binding Glycosphingolipids of Primary Human Kidney and Colon Epithelial Cells and Their Toxin Susceptibility

Enterohemorrhagic Escherichia coli (EHEC) are the human pathogenic subset of Shiga toxin (Stx)-producing E. coli (STEC). EHEC are responsible for severe colon infections associated with life-threatening extraintestinal complications such as the hemolytic-uremic syndrome (HUS) and neurological disturbances. Endothelial cells in various human organs are renowned targets of Stx, whereas the role of epithelial cells of colon and kidneys in the infection process has been and is still a matter of debate. This review shortly addresses the clinical impact of EHEC infections, novel aspects of vesicular package of Stx in the intestine and the blood stream as well as Stx-mediated extraintestinal complications and therapeutic options. Here follows a compilation of the Stx-binding glycosphingolipids (GSLs), globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer) and their various lipoforms present in primary human kidney and colon epithelial cells and their distribution in lipid raft-analog membrane preparations. The last issues are the high and extremely low susceptibility of primary renal and colonic epithelial cells, respectively, suggesting a large resilience of the intestinal epithelium against the human-pathogenic Stx1a- and Stx2a-subtypes due to the low content of the high-affinity Stx-receptor Gb3Cer in colon epithelial cells. The review closes with a brief outlook on future challenges of Stx research.

Outer Membrane Vesicles as Mediators of Plant-Bacterial Interactions

Plants have co-evolved with diverse microorganisms that have developed different mechanisms of direct and indirect interactions with their host. Recently, greater attention has been paid to a direct “message” delivery pathway from bacteria to plants, mediated by the outer membrane vesicles (OMVs). OMVs produced by Gram-negative bacteria play significant roles in multiple interactions with other bacteria within the same community, the environment, and colonized hosts. The combined forces of innovative technologies and experience in the area of plant–bacterial interactions have put pressure on a detailed examination of the OMVs composition, the routes of their delivery to plant cells, and their significance in pathogenesis, protection, and plant growth promotion. This review synthesizes the available knowledge on OMVs in the context of possible mechanisms of interactions between OMVs, bacteria, and plant cells. OMVs are considered to be potential stimulators of the plant immune system, holding potential for application in plant bioprotection.

The gut microbiota prime systemic antiviral immunity via the cGAS-STING-IFN-I axis

The microbiota are vital for immune homeostasis and provide a competitive barrier to bacterial and fungal pathogens. Here, we investigated how gut commensals modulate systemic immunity and response to viral infection. Antibiotic suppression of the gut microbiota reduced systemic tonic type I interferon (IFN-I) and antiviral priming. The microbiota-driven tonic IFN-I-response was dependent on cGAS-STING but not on TLR signaling or direct host-bacteria interactions. Instead, membrane vesicles (MVs) from extracellular bacteria activated the cGAS-STING-IFN-I axis by delivering bacterial DNA into distal host cells. DNA-containing MVs from the gut microbiota were found in circulation and promoted the clearance of both DNA (herpes simplex virus type 1) and RNA (vesicular stomatitis virus) viruses in a cGAS-dependent manner. In summary, this study establishes an important role for the microbiota in peripheral cGAS-STING activation, which promotes host resistance to systemic viral infections. Moreover, it uncovers an underappreciated risk of antibiotic use during viral infections.

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.

Citrobacter rodentium(ϕStx2dact), a murine infection model for enterohemorrhagic Escherichia coli

The formation of attaching and effacing (A/E) lesions on intestinal epithelium, combined with Shiga toxin production, are hallmarks of enterohemorrhagic Escherichia coli (EHEC) infection that can lead to lethal hemolytic uremic syndrome. Although an animal infection model that fully recapitulates human disease remains elusive, mice orally infected with Citrobacter rodentium(ϕStx2dact), a natural murine pathogen lysogenized with an EHEC-derived Shiga toxin 2-producing bacteriophage, develop intestinal A/E lesions and toxin-dependent systemic disease. This model has facilitated investigation of how: (A) phage gene expression and prophage induction contribute to disease and are potentially triggered by antibiotic treatment; (B) virulence gene expression is altered by microbiota and the colonic metabolomic milieu; and (C) innate immune signaling is affected by Stx. Thus, the model provides a unique tool for accessing diverse aspects of EHEC pathogenesis.

Incorporation of DNA to Bacterial Vesicles by in Escherichia coli

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

The extracellular vesicle generation paradox: a bacterial point of view

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

Bacterial Extracellular Vesicles in Gastrointestinal Tract Cancer: An Unexplored Territory

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

Bacterial Outer Membrane Vesicles as Antibiotic Delivery Vehicles

Bacterial outer membrane vesicles (OMVs) are nanometer-scale, spherical vehicles released by Gram-negative bacteria into their surroundings throughout growth. These OMVs have been demonstrated to play key roles in pathogenesis by delivering certain biomolecules to host cells, including toxins and other virulence factors. In addition, this biomolecular delivery function enables OMVs to facilitate intra-bacterial communication processes, such as quorum sensing and horizontal gene transfer. The unique ability of OMVs to deliver large biomolecules across the complex Gram-negative cell envelope has inspired the use of OMVs as antibiotic delivery vehicles to overcome transport limitations. In this review, we describe the advantages, applications, and biotechnological challenges of using OMVs as antibiotic delivery vehicles, studying both natural and engineered antibiotic applications of OMVs. We argue that OMVs hold great promise as antibiotic delivery vehicles, an urgently needed application to combat the growing threat of antibiotic resistance.

Outer Membrane Vesicles Derived from Klebsiella pneumoniae Are a Driving Force for Horizontal Gene Transfer

Gram-negative bacteria release Outer Membrane Vesicles (OMVs) into the extracellular environment. Recent studies recognized these vesicles as vectors to horizontal gene transfer; however, the parameters that mediate OMVs transfer within bacterial communities remain unclear. The present study highlights for the first time the transfer of plasmids containing resistance genes via OMVs derived from Klebsiella pneumoniae (K. pneumoniae). This mechanism confers DNA protection, it is plasmid copy number dependent with a ratio of 3.6 times among high copy number plasmid (pGR) versus low copy number plasmid (PRM), and the transformation efficiency was 3.6 times greater. Therefore, the DNA amount in the vesicular lumen and the efficacy of horizontal gene transfer was strictly dependent on the identity of the plasmid. Moreover, the role of K. pneumoniae-OMVs in interspecies transfer was described. The transfer ability was not related to the phylogenetic characteristics between the donor and the recipient species. K. pneumoniae-OMVs transferred plasmid to Escherichia coli, Salmonella enterica, Pseudomonas aeruginosa and Burkholderia cepacia. These findings address the pivotal role of K. pneumoniae-OMVs as vectors for antimicrobial resistance genes spread, contributing to the development of antibiotic resistance in the microbial communities.

S. Typhi derived vaccines and a proposal for outer membrane vesicles (OMVs) as potential vaccine for typhoid fever

Typhoid fever is a serious systemic infection caused by Salmonella Typhi (S. Typhi), spread by the feco-oral route and closely associated with poor food hygiene and inadequate sanitation. Nearly 93% of S. Typhi strains have acquired antibiotic resistance against most antibiotics. Vaccination is the only promising way to prevent typhoid fever. This review covers the nature and composition of S. Typhi, pathogenecity and mode of infection, epidemiology, and nature of drug resistance. Several components (Vi-polysaccharides, O-antigens, flagellar antigens, full length OMPs, and short peptides from OMPs) of S. Typhi have been utilized for vaccine design for protection against typhoid fever. Vaccine delivery systems also contribute to efficacy of the vaccines. In this study, we propose to develop S. Typhi derived OMVs as vaccine for protection against typhoid fevers.

Pneumococcal Extracellular Vesicles Modulate Host Immunity

Extracellular vesicles (EVs) have recently garnered attention for their participation in host-microbe interactions in pneumococcal infections. However, the effect of EVs on the host immune system remain poorly understood. Our studies focus on EVs produced by Streptococcus pneumoniae (pEVs), and reveal that pEVs are internalized by macrophages, T cells, and epithelial cells. In vitro, pEVs induce NF-κB activation in a dosage-dependent manner and polarize human macrophages to an alternative (M2) phenotype. In addition, pEV pretreatment conditions macrophages to increase bacteria uptake and such macrophages may act as a reservoir for pneumococcal cells by increasing survival of the phagocytosed bacteria. When administered systemically in mice, pEVs induce cytokine release; when immobilized locally, they recruit lymphocytes and macrophages. Taken together, pEVs emerge as critical contributors to inflammatory responses and tissue damage in mammalian hosts.

Escherichia coli O80 hybrid pathotype strains producing Shiga toxin and ESBL: molecular characterization and potential therapeutic options

OBJECTIVES

Enterohaemorrhagic Escherichia coli (EHEC) infections may be complicated by haemolytic uraemic syndrome (HUS). The emerging worldwide EHEC serogroup O80 has acquired a mosaic plasmid combining extraintestinal virulence and antibiotic resistance. This hybrid pathotype is associated with invasive infections that require antibiotic therapy, classically not recommended in EHEC infections, increasing the risk of HUS. We characterized two ESBL-producing O80 EHEC strains, which is an unusual resistance mechanism among EHECs, and determined the safest therapy to be used for invasive infections.

METHODS

WGS of two strains isolated from the stools of an asymptomatic carrier and a patient with HUS was performed using Illumina and Nanopore technologies. Generated reads were combined to assemble genomes. We determined the safest therapy by comparing Shiga toxin (Stx) production by the two strains in the presence of several antibiotics.

RESULTS

The strains were genetically close to the O80 EHEC clone, belonging to ST301 and harbouring stx2d, eae-ξ, ehxA and genes characteristic of the extraintestinal virulence plasmid pS88. Long-read sequencing identified the acquisition of an additional plasmid harbouring CTX-M-type genes (blaCTX-M-14 and blaCTX-M-1). Azithromycin decreased Stx production at subinhibitory concentrations, ciprofloxacin increased it and imipenem had no major effect. The combination of azithromycin and imipenem overall reduced Stx production.

CONCLUSIONS

Acquisition of an additional plasmid harbouring ESBL genes is a step towards increasing the risk of O80 EHEC dissemination and represents a serious public health concern. The combination of azithromycin and imipenem reduced Stx production and suggests that this combination could be tested in clinical trials.

Gene Transfer Potential of Outer Membrane Vesicles of Gram-Negative Bacteria

The increasing spread of multidrug-resistant pathogenic bacteria is one of the major threats to public health worldwide. Bacteria can acquire antibiotic resistance and virulence genes through horizontal gene transfer (HGT). A novel horizontal gene transfer mechanism mediated by outer membrane vesicles (OMVs) has been recently identified. OMVs are rounded nanostructures released during their growth by Gram-negative bacteria. Biologically active toxins and virulence factors are often entrapped within these vesicles that behave as molecular carriers. Recently, OMVs have been reported to contain DNA molecules, but little is known about the vesicle packaging, release, and transfer mechanisms. The present review highlights the role of OMVs in HGT processes in Gram-negative bacteria.

Characterization of Feces-Derived Bacterial Membrane Vesicles and the Impact of Their Origin on the Inflammatory Response

The human gastrointestinal tract harbors a diverse and complex microbiome, which interacts in a variety of ways with the host. There is compelling evidence that gut microbial dysbiosis, defined as an alteration of diversity and abundance in intestinal microbes, is an etiological factor in inflammatory bowel disease (IBD). Membrane vesicles (MVs), which are nano-sized particles released by bacteria, have been found to interact with the host and modulate the development and function of the immune system. As a result MVs have been suggested to play a critical role in both health and disease. In this study we developed a method to isolate, characterize and assess the immunoreactivity of heterogeneous populations of MVs from fecal samples (fMVs) of healthy volunteers. We successfully isolated 2*10⁹-2*10¹⁰ particles/ml from 0.5 gram of feces by using a combination of ultrafiltration and size exclusion chromatography (SEC) from 10 fecal samples. Bead-based flowcytometry in combination with tunable resistive pulse sensing (TRPS) provided a reliable method for (semi-)quantitative determination of fMVs originating from both Gram-positive and Gram-negative bacteria, while transmission electron microscopy confirmed the presence of fMVs. Real time 16s PCR on bacterial cell fractions or isolated fMVs DNA of the most common phyla (Firmicutes, Bacteroidetes, Actinobacteria and Proteobacteria) revealed differences in the relative abundance between bacteria and the fMVs. Moreover, fMVs evoke the release of TNF- by THP-1 cells in a dose-dependent matter. Also, a significant positive correlation was found between Actinobacteria/-Proteobacteria derived vesicles and the release of TNF-. It has become increasingly clear that fMVs could provide an additional layer to the definition of homeostasis or dysbiosis of the microbiota. The current study supports their potential involvement in the intestinal homeostasis or inflammatory disorders and provides putative interesting incentives for future research.

Cytotoxic Effects of Recombinant StxA2-His in the Absence of Its Corresponding B-Subunit

AB₅ protein toxins are produced by certain bacterial pathogens and are composed of an enzymatically active A-subunit and a B-subunit pentamer, the latter being responsible for cell receptor recognition, cellular uptake, and transport of the A-subunit into the cytosol of eukaryotic target cells. Two members of the AB₅ toxin family were described in Shiga toxin-producing Escherichia coli (STEC), namely Shiga toxin (Stx) and subtilase cytotoxin (SubAB). The functional paradigm of AB toxins includes the B-subunit being mandatory for the uptake of the toxin into its target cells. Recent studies have shown that this paradigm cannot be maintained for SubAB, since SubA alone was demonstrated to intoxicate human epithelial cells in vitro. In the current study, we raised the hypothesis that this may also be true for the A-subunit of the most clinically relevant Stx-variant, Stx2a. After separate expression and purification, the recombinant Stx2a subunits StxA2a-His and StxB2a-His were applied either alone or in combination in a 1:5 molar ratio to Vero B4, HeLa, and HCT-116 cells. For all cell lines, a cytotoxic effect of StxA2a-His alone was detected. Competition experiments with Stx and SubAB subunits in combination revealed that the intoxication of StxA2a-His was reduced by addition of SubB1-His. This study showed that the enzymatic subunit StxA2a alone was active on different cells and might therefore play a yet unknown role in STEC disease development.

Bacteriophages of Shiga Toxin-Producing Escherichia coli and Their Contribution to Pathogenicity

Shiga toxins (Stx) of Shiga toxin-producing Escherichia coli (STEC) are generally encoded in the genome of lambdoid bacteriophages, which spend the most time of their life cycle integrated as prophages in specific sites of the bacterial chromosome. Upon spontaneous induction or induction by chemical or physical stimuli, the stx genes are co-transcribed together with the late phase genes of the prophages. After being assembled in the cytoplasm, and after host cell lysis, mature bacteriophage particles are released into the environment, together with Stx. As members of the group of lambdoid phages, Stx phages share many genetic features with the archetypical temperate phage Lambda, but are heterogeneous in their DNA sequences due to frequent recombination events. In addition to Stx phages, the genome of pathogenic STEC bacteria may contain numerous prophages, which are either cryptic or functional. These prophages may carry foreign genes, some of them related to virulence, besides those necessary for the phage life cycle. Since the production of one or more Stx is considered the major pathogenicity factor of STEC, we aim to highlight the new insights on the contribution of Stx phages and other STEC phages to pathogenicity.

Outer membrane vesicles derived from hypervirulent Klebsiella pneumoniae stimulate the inflammatory response

Hypervirulent Klebsiella pneumoniae (hvKP), an increasing important pathotype, was initially recognized as a cause of severe liver abscesses and subsequently as a cause of other complications posing a clinical threat. Outer membrane vesicles (OMVs) secreted from abundant gram-negative bacteria are considered an important vehicle for delivery of effector molecules to target cells. However, the products and role in bacterial pathogenesis of OMVs secreted from hvKP, have not yet been determined. In order to examine the production of OMVs from hvKP and to determine their effects on the stimulation of the host innate immune response, we used ultracentrifugation to obtain homogeneous OMVs from hvKP ATCC 1706 cultured in vitro. Proteomic analysis was performed and hvKP OMVs were found to contain diverse proteins. Furthermore, hvKP OMVs exhibited discrepant cytotoxic effects on different cell types, in vitro. The vesicles induced the expression of proinflammatory cytokines including interleukin (IL)-6 and IL-8 in host cells. In addition, transtracheal injection of hvKP OMVs in wild-type mice led to an inflammatory response manifested by elevated levels of pro-inflammatory mediators including IL-6, IL-8 and TNF-α in bronchoalveolar lavage fluid (BAL), in accord with in vitro experiments. However, hvKP OMVs were insufficient to kill mice. In summary, OMVs originating from hvKP may serve to provoke the inflammatory response.

Outer Membrane Vesicle Induction and Isolation for Vaccine Development

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

Virulence Factor Cargo and Host Cell Interactions of Shiga Toxin-Producing Escherichia coli Outer Membrane Vesicles

Outer membrane vesicles (OMVs), nanoparticles released by Shiga toxin-producing Escherichia coli (STEC), have been identified as novel efficient virulence tools of these pathogens. STEC O157 OMVs carry a cocktail of virulence factors including Shiga toxin 2a (Stx2a), cytolethal distending toxin V (CdtV), EHEC hemolysin, flagellin, and lipopolysaccharide. OMVs are taken up by human intestinal epithelial and microvascular endothelial cells, the major targets during STEC infection, and deliver the virulence factors into host cells. There the toxins separate from OMVs and are trafficked via different pathways to their target compartments, i.e., the cytosol (Stx2a-A subunit), nucleus (CdtV-B subunit), and mitochondria (EHEC hemolysin). This leads to a toxin-specific host cell injury and ultimately apoptotic cell death. Besides their cytotoxic effects, STEC OMVs trigger an inflammatory response via their lipopolysaccharide and flagellin components. In this chapter, we describe methods for the isolation and purification of STEC OMVs, for the detection of OMV-associated virulence factors, and for the analysis of OMV interactions with host cells including OMV cellular uptake and intracellular trafficking of OMVs and OMV-delivered toxins.

Pathogenesis Mediated by Bacterial Membrane Vesicles

The release of extracellular vesicles (EVs) is a process conserved across the three domains of life. Amongst prokaryotes, EVs produced by Gram-negative bacteria, termed outer membrane vesicles (OMVs), were identified more than 50 years ago and a wealth of literature exists regarding their biogenesis, composition and functions. OMVs have been implicated in benefiting numerous metabolic functions of their parent bacterium. Additionally, OMVs produced by pathogenic bacteria have been reported to contribute to pathology within the disease setting. By contrast, the release of EVs from Gram-positive bacteria, known as membrane vesicles (MVs), has only been widely accepted within the last decade. As such, there is a significant disproportion in knowledge regarding MVs compared to OMVs. Here we provide an overview of the literature regarding bacterial membrane vesicles (BMVs) produced by pathogenic and commensal bacteria. We highlight the mechanisms of BMV biogenesis and their roles in assisting bacterial survival, in addition to discussing their functions in promoting disease pathologies and their potential use as novel therapeutic strategies.

Effects of antimicrobials on Shiga toxin production in high-virulent Shiga toxin-producing Escherichia coli

PURPOSE

Antimicrobial treatment of Shiga toxin-producing Escherichia coli (STEC) infections is controversial because antimicrobials may stimulate Shiga toxin (Stx) production, and thereby increase the risk of developing haemolytic uremic syndrome (HUS). Previous in vitro studies have shown this mainly in infections caused by STEC serotype O157:H7. The aim of this study was to investigate induction of Stx transcription and production in different serotypes of STEC isolated from severely ill patients, following their exposure in vitro to six different classes of antimicrobials.

METHODS

We investigated Stx transcription and production in 12 high-virulent STEC strains, all carrying the stx2a gene, of six different serotypes following their exposure to six classes of antimicrobials. Liquid cultures of the STEC strains were incubated with sub-inhibitory concentrations of the antimicrobials. We used reverse-transcription quantitative PCR to measure the relative expression of Stx2a mRNA and an enzyme-linked immunosorbent assay to quantify Stx production.

RESULTS

In general the antibiotics tested showed only minor effects on transcriptional levels of Stx2a. Ciprofloxacin caused an increase of Stx production in all but two strains, while gentamicin, meropenem and azithromycin did not induce Stx production in any of the STEC strains examined. STEC O104:H4 was the serotype that in greatest extent responded to antimicrobial exposure with an increase of stx2a transcription and Stx production.

CONCLUSION

Gentamicin, meropenem and azithromycin exposure did not result in elevated Stx production. We recommend that this finding is investigated further in the search for candidates for future antimicrobial treatment of STEC.

Proteomic Studies of the Biofilm Matrix including Outer Membrane Vesicles of Burkholderia multivorans C1576, a Strain of Clinical Importance for Cystic Fibrosis

Biofilms are aggregates of microbial cells encased in a highly hydrated matrix made up of self-produced extracellular polymeric substances (EPS) which consist of polysaccharides, proteins, nucleic acids, and lipids. While biofilm matrix polysaccharides are unraveled, there is still poor knowledge about the identity and function of matrix-associated proteins. With this work, we performed a comprehensive proteomic approach to disclose the identity of proteins associated with the matrix of biofilm-growing Burkholderia multivorans C1576 reference strain, a cystic fibrosis clinical isolate. Transmission electron microscopy showed that B. multivorans C1576 also releases outer membrane vesicles (OMVs) in the biofilm matrix, as already demonstrated for other Gram-negative species. The proteomic analysis revealed that cytoplasmic and membrane-bound proteins are widely represented in the matrix, while OMVs are highly enriched in outer membrane proteins and siderophores. Our data suggest that cell lysis and OMVs production are the most important sources of proteins for the B. multivorans C1576 biofilm matrix. Of note, some of the identified proteins are lytic enzymes, siderophores, and proteins involved in reactive oxygen species (ROS) scavenging. These proteins might help B. multivorans C1576 in host tissue invasion and defense towards immune system assaults.

Extracellular Vesicle Biogenesis and Functions in Gram-Positive Bacteria

Extracellular vesicles (EVs) are membrane-derived lipid bilayers secreted by bacteria and eukaryotic cells. Bacterial membrane vesicles were discovered over 60 years ago and have been extensively studied in Gram-negative bacteria. During their production, EVs are loaded with proteins, nucleic acids, and various compounds that are subsequently released into the environment. Depending on the packaged cargo, EVs have a broad spectrum of action and are involved in pathogenesis, antibiotic resistance, nutrient uptake, and nucleic acid transfer. Due to differences in cell wall structure, EVs in Gram-positive bacteria have been disregarded for decades, and our understanding of their biogenesis and host cell interaction is incomplete. Recently, studies on bacteria such as Staphylococcus aureus, Streptococcus spp., Bacillus subtilis, and Mycobacterium spp. have demonstrated EV production in Gram-positive bacteria and shown the great importance EVs have in Gram-positive bacterial physiology and disease progression. Here, we review the latest findings on the biogenesis and functions of EVs from Gram-positive bacteria and identify key areas for future research.

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.

Induced fusion and aggregation of bacterial outer membrane vesicles: Experimental and theoretical analysis

Recombinantly engineered bacterial outer membrane vesicles (OMVs) are promising vaccine delivery vehicles. The diversity of exogenous antigens delivered by OMVs can be enhanced by induced fusion of OMV populations. To date there are no reports of induced fusion of bacterial OMVs. Here we measure the pH and salt-induced aggregation and fusion of OMVs and analyze the processes against the Derjaguin-Landau-Verwey-Overbeek (DLVO) colloidal stability model. Vesicle aggregation and fusion kinetics were investigated for OMVs isolated from native E. coli (Nissle 1917) and lipopolysaccharide (LPS) modified E. coli (ClearColi) strains to evaluate the effect of lipid type on vesicle aggregation and fusion. Electrolytes and low pHs induced OMV aggregation for both native and modified LPS constructs, approaching a calculated fusion efficiency of ~25% (i.e. ~1/4 of collision events lead to fusion). However, high fusion efficiency was achieved for Nissle OMVs solely with decreased pH as opposed to a combination of low pH and increased divalent counterion concentration for ClearColi OMVs. The lipid composition of the OMVs from Nissle negatively impacted fusion in the presence of electrolytes, causing higher deviations from DLVO-predicted critical coagulation concentrations with monovalent counterions. The outcome of the work is a defined set of conditions under which investigators can induce OMVs to fuse and make various combinations of vesicle compositions.

Extracellular Vesicles Influence the Growth and Adhesion of Staphylococcus epidermidis Under Antimicrobial Selective Pressure

Staphylococcus epidermidis causes infections associated with orthopedic implants due to its ability to establish persistent biofilms, making infections chronic and hard to treat. Extracellular vesicles (EVs) are part of the bacterial communication system, but the role of S. epidermidis-derived EVs in biofilm formation processes and survival is completely unknown. The aims of this study were (i) to investigate the effect of subinhibitory concentrations of antibiotics on vesiculation in S. epidermidis and evaluate the role of EVs in bacterial survival and adhesion under antimicrobial selective pressure and (ii) to evaluate whether EVs derived from a gentamicin-resistant S. epidermidis strain influence the susceptibility and adhesion of a gentamicin-susceptible strain. A gentamicin-susceptible (GEN^S) strain isolated from implant-associated osteomyelitis was cultured with EVs previously isolated from the same strain growing with subinhibitory concentrations of GEN (0, 0.03, and 0.06 μg × mL^–1) or with EVs from a gentamicin-resistant (GEN^R) strain. EVs were characterized regarding their size, number and protein content. The growth of S. epidermidis cultured with increasing concentrations of GEN (<=> MIC of 0.12 μg × mL^–1) was recorded, viability was determined by quantitative culturing and fluorescence staining, and biofilm biomass on polystyrene was quantified by crystal violet staining. Cells grown in subinhibitory concentrations of GEN produced a larger number of EVs of similar size but with greater protein content than cells grown in control (Ctrl) conditions (0 GEN). Under antimicrobial pressure, EVs promoted different mechanisms of antimicrobial tolerance depending on the EV and GEN concentrations. Cell adhesion to polystyrene decreased in the presence of 0 and 0.03 μg × mL^–1 GEN upon EV stimulation. Compared with Ctrl cells, cells treated with EVs from a GEN^R strain showed increased cell division during the exponential growth phase, faster maximal growth rate, shorter doubling time (8–33 min), and dramatically inhibited cell adhesion. These findings suggest that vesiculation in S. epidermidis is a survival response to subinhibitory concentrations of gentamicin. EVs may contribute to bacterial survival through their involvement (1) in the modulation of the growth rate, affecting cell division, and (2) in cell adhesion, decreasing cell attachment to polystyrene and glass.

Modeling Native EHEC Outer Membrane Vesicles by Creating Synthetic Surrogates

Enterohemorrhagic Escherichia coli (EHEC) is a zoonotic pathogen responsible for life-threating diseases such as hemolytic uremic syndrome. While its major virulence factor, the Shiga toxin (Stx), is known to exert its cytotoxic effect on various endothelial and epithelial cells when in its free, soluble form, Stx was also recently found to be associated with EHEC outer membrane vesicles (OMVs). However, depending on the strain background, other toxins can also be associated with native OMVs (nOMVs), and nOMVs are also made up of immunomodulatory agents such as lipopolysaccharides and flagellin. Thus, it is difficult to determine to which extent a single virulence factor in nOMVs, such as Stx, contributes to the molecular pathogenesis of EHEC. To reduce this complexity, we successfully developed a protocol for the preparation of synthetic OMVs (sOMVs) with a defined lipid composition resembling the E. coli outer membrane and loaded with specific proteins, i.e., bovine serum albumin (BSA) as a proxy for functional Stx2a. Using BSA for parameter evaluation, we found that (1) functional sOMVs can be prepared at room temperature instead of potentially detrimental higher temperatures (e.g., 45 °C), (2) a 1:10 ratio of protein to lipid, i.e., 100 µg protein with 1 mg of lipid mixture, yields homogenously sized sOMVs, and (3) long-term storage for up to one year at 4 °C is possible without losing structural integrity. Accordingly, we reproducibly generated Stx2a-loaded sOMVs with an average diameter of 132.4 ± 9.6 nm that preserve Stx2a’s injuring activity, as determined by cytotoxicity assays with Vero cells. Overall, we successfully created sOMVs and loaded them with an EHEC toxin, which opens the door for future studies on the degree of virulence associated with individual toxins from EHEC and other bacterial pathogens.

The Role of Bacterial Membrane Vesicles in the Dissemination of Antibiotic Resistance and as Promising Carriers for Therapeutic Agent Delivery

The rapid emergence and spread of antibiotic-resistant bacteria continues to be an issue difficult to deal with, especially in the clinical, animal husbandry, and food fields. The occurrence of multidrug-resistant bacteria renders treatment with antibiotics ineffective. Therefore, the development of new therapeutic methods is a worthwhile research endeavor in treating infections caused by antibiotic-resistant bacteria. Recently, bacterial membrane vesicles (BMVs) have been investigated as a possible approach to drug delivery and vaccine development. The BMVs are released by both pathogenic and non-pathogenic Gram-positive and Gram-negative bacteria, containing various components originating from the cytoplasm and the cell envelope. The BMVs are able to transform bacteria with genes that encode enzymes such as proteases, glycosidases, and peptidases, resulting in the enhanced antibiotic resistance in bacteria. The BMVs can increase the resistance of bacteria to antibiotics. However, the biogenesis and functions of BMVs are not fully understood in association with the bacterial pathogenesis. Therefore, this review aims to discuss BMV-associated antibiotic resistance and BMV-based therapeutic interventions.