Gene transfer potential of outer membrane vesicles of Acinetobacter baylyi and effects of stress on vesiculation

Outer membrane vesicles as biomimetic vaccine carriers against infections and cancers

In the last decade, nanoparticle-based therapeutic modalities have emerged as promising treatment options for cancer and infectious diseases. To improve prognosis, chemotherapeutic and antimicrobial drugs must be delivered selectively to the target sites. Researchers have increasingly focused their efforts on improving drug delivery, with a particular emphasis on cancer and infectious diseases. When drugs are administered systemically, they become diluted and can diffuse to all tissues but only until the immune system intervenes and quickly removes them from circulation. To enhance and prolong the systemic circulation of drugs, nanocarriers have been explored and used; however, nanocarriers have a major drawback in that they can trigger immune responses. Numerous nanocarriers for optimal drug delivery have been developed using innovative and effective biointerface technologies. Autologous cell-derived drug carriers, such as outer membrane vesicles (OMVs), have demonstrated improved bioavailability and reduced toxicity. Thus, this study investigates the use of biomimetic OMVs as biomimetic vaccine carriers against infections and cancers to improve our understanding in the field of nanotechnology. In addition, discussion on the advantages, disadvantages, and future prospects of OMVs will also be explored. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Therapeutic Approaches and Drug Discovery > Nanomedicine for Infectious Disease Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Extracellular Vesicles: Recent Insights Into the Interaction Between Host and Pathogenic Bacteria

Extracellular vesicles (EVs) are nanosized lipid particles released by virtually every living cell. EVs carry bioactive molecules, shuttle from cells to cells and transduce signals, regulating cell growth and metabolism. Pathogenic bacteria can cause serious infections via a wide range of strategies, and host immune systems also develop extremely complex adaptations to counteract bacterial infections. As notable carriers, EVs take part in the interaction between the host and bacteria in several approaches. For host cells, several strategies have been developed to resist bacteria via EVs, including expelling damaged membranes and bacteria, neutralizing toxins, triggering innate immune responses and provoking adaptive immune responses in nearly the whole body. For bacteria, EVs function as vehicles to deliver toxins and contribute to immune escape. Due to their crucial functions, EVs have great application potential in vaccines, diagnosis and treatments. In the present review, we highlight the most recent advances, application potential and remaining challenges in understanding EVs in the interaction between the host and bacteria.

Outer Membrane Vesicles of Avian PathogenicEscherichia coli Mediate the Horizontal Transmission of blaCTX-M-55

The CTX-M-55 type extended-spectrum β-lactamase (ESBL) producing Enterobacteriaceae is increasing in prevalence worldwide without the transmission mechanism being fully clarified, which threatens public and livestock health. Outer membrane vesicles (OMVs) have been shown to mediate the gene horizontal transmission in some species. However, whether bla_CTX-M-55 can be transmitted horizontally through OMVs in avian pathogenic Escherichia coli (APEC) has not been reported yet. To test this hypothesis, an ESBL-producing APEC was isolated and whole-genome sequencing (WGS) was performed to analyze the location of bla_CTX-M-55. Ultracentrifugation and size exclusion chromatography was used to isolate and purify OMVs, and the transfer experiment of bla_CTX-M-55 via OMVs was performed finally. Our results showed that the bla_CTX-M-55 was located on an IncI2 plasmid. The number and diameter of OMVs secreted by ESBL-producing APEC treated with different antibiotics were significantly varied. The transfer experiment showed that the OMVs could mediate the horizontal transfer of bla_CTX-M-55, and the frequency of gene transfer ranged from 10⁻⁵ to 10⁻⁶ CFU/mL with the highest frequency observed in the Enrofloxacin treatment group. These findings contribute to a better understanding of the antibiotics in promoting and disseminating resistance in the poultry industry and support the restrictions on the use of antibiotics in the poultry industry.

Bacteriophages in sewage: abundance, roles, and applications

The raw sewage that flows through sewage systems contains a complex microbial community whose main source is the human gut microbiome, with bacteriophages being as abundant as bacteria or even more so. Phages that infect common strains of the human gut bacteriome and transient bacterial pathogens have been isolated in raw sewage, as have other phages corresponding to non-sewage inputs. Although human gut phages do not seem to replicate during their transit through the sewers, they predominate at the entrance of wastewater treatment plants, inside which the dominant populations of bacteria and phages undergo a swift change. The sheer abundance of phages in the sewage virome prompts several questions, some of which are addressed in this review. There is growing concern about their potential role in the horizontal transfer of genes, including those related with bacterial pathogenicity and antibiotic resistance. On the other hand, some phages that infect human gut bacteria are being used as indicators of fecal/viral water pollution and as source tracking markers and have been introduced in water quality legislation. Other potential applications of enteric phages to control bacterial pathogens in sewage or undesirable bacteria that impede the efficacy of wastewater treatments, including biofilm formation on membranes, are still being researched.

Outer Membrane Vesicles of Acinetobacter baumannii DS002 Are Selectively Enriched with TonB-Dependent Transporters and Play a Key Role in Iron Acquisition

Outer membrane vesicles (OMVs) of Acinetobacter baumannii DS002 carry proteins which perform selective biological functions. The proteins involved in cell wall/membrane biogenesis and inorganic ion transport and metabolism occupied a significant portion of the 302 proteins associated with OMVs. Interestingly, the TonB-dependent transporters (TonRs), linked to the active transport of nutrients across the energy-deprived outer membrane, are predominant among proteins involved in inorganic ion transport and metabolism. The OMVs of DS002 contain TonRs capable of transporting iron complexed to catecholate, hydroximate, and mixed types of siderophores. Consistent with this observation, the OMVs were firmly bound to ferric-enterobactin (⁵⁵Fe-Ent) and successfully transported iron into A. baumannii DS002 cells grown under iron-limiting conditions. In addition to the TonRs, OMVs also carry proteins known to promote pathogenesis, immune evasion, and biofilm formation. Our findings provide conclusive evidence for the role of OMVs in the transport of nutrients such as iron and show the presence of proteins with proven roles in pathogenicity and immune response.

IMPORTANCE TonB-dependent transporters (TonRs) play a crucial role in transporting nutrients such as iron, nickel, copper, and complex carbohydrates across the energy-deprived outer membrane. Due to their unique structural features, TonRs capture nutrients in an energy-independent manner and transport them across the outer membrane by harvesting energy derived from the inner membrane-localized Ton-complex. In this study, we report the presence of TonRs capable of transporting various nutrients in OMVs and demonstrate their role in capturing and transporting ferric iron complexed with enterobactin into A. baumannii DS002 cells. The OMV-associated TonRs appear to play a critical role in the survival of A. baumannii, listed as a priority pathogen, under nutrient-deprived conditions.

Bacterial outer membrane vesicles as potential biological nanomaterials for antibacterial therapy

Antibiotic therapy is one of the most important approaches against bacterial infections. However, the improper use of antibiotics and the emergence of drug resistance have compromised the efficacy of traditional antibiotic therapy. In this regard, it is of great importance and significance to develop more potent antimicrobial therapies, including the development of functionalized antibiotics delivery systems and antibiotics-independent antimicrobial agents. Outer membrane vesicles (OMVs), secreted by Gram-negative bacteria and with similar structure to cell-derived exosomes, are natural functional nanomaterials and known to play important roles in many bacterial life events, such as communication, biofilm formation and pathogenesis. Recently, more and more reports have demonstrated the use of OMVs as either active antibacterial agents or antibiotics delivery carriers, implying the great potentials of OMVs in antibacterial therapy. Herein, we aim to provide a comprehensive understanding of OMV and its antibacterial applications, including its biogenesis, biofunctions, isolation, purification and its potentials in killing bacteria, delivering antibiotics and developing vaccine or immunoadjuvants. In addition, the concerns in clinical use of OMVs and the possible solutions are discussed. STATEMENT OF SIGNIFICANCE: The emergence of antibiotic-resistant bacteria has led to the failure of traditional antibiotic therapy, and thus become a big threat to human beings. In this regard, developing more potent antibacterial approaches is of great importance and significance. Recently, bacterial outer membrane vesicles (OMVs), which are natural functional nanomaterials secreted by Gram-negative bacteria, have been used as active agents, drug carriers and vaccine adjuvant for antibacterial therapy. This review provides a comprehensive understanding of OMVs and summarizes the recent progress of OMVs in antibacterial applications. The concerns of OMVs in clinical use and the possible solutions are also discussed. As such, this review may guide the future works in antibacterial OMVs and appeal to both scientists and clinicians.

T6SS secretes an LPS-binding effector to recruit OMVs for exploitative competition and horizontal gene transfer

Outer membrane vesicles (OMVs) can function as nanoscale vectors that mediate bacterial interactions in microbial communities. How bacteria recognize and recruit OMVs inter-specifically remains largely unknown, thus limiting our understanding of the complex physiological and ecological roles of OMVs. Here, we report a ligand-receptor interaction-based OMV recruitment mechanism, consisting of a type VI secretion system (T6SS)-secreted lipopolysaccharide (LPS)-binding effector TeoL and the outer membrane receptors CubA and CstR. We demonstrated that Cupriavidus necator T6SS1 secretes TeoL to preferentially associate with OMVs in the extracellular milieu through interactions with LPS, one of the most abundant components of OMVs. TeoL associated with OMVs can further bind outer membrane receptors CubA and CstR, which tethers OMVs to the recipient cells and allows cargo to be delivered. The LPS-mediated mechanism enables bacterial cells to recruit OMVs derived from different species, and confers advantages to bacterial cells in iron acquisition, interbacterial competition, and horizontal gene transfer (HGT). Moreover, our findings provide multiple new perspectives on T6SS functionality in the context of bacterial competition and HGT, through the recruitment of OMVs.

Membrane vesicles (MVs) from antibiotic-resistant Staphylococcus aureus transfer antibiotic-resistance to antibiotic-susceptible Escherichia coli

AIM

Bacteria naturally produce membrane vesicles (MVs), which have been shown to contribute to the spread of multi-drug resistant bacteria (MDR) by delivering antibiotic-resistant substances to antibiotic-susceptible bacteria. Here, we aim to show that MVs from Gram-positive bacteria are capable of transferring β-lactam antibiotic-resistant substances to antibiotic-sensitive Gram-negative bacteria.

MATERIALS AND METHODS

MVs were collected from a methicillin-resistant strain of Staphylococcus aureus (MRSA) and vesicle-mediated fusion with antimicrobial-sensitive Escherichia coli (RC85). It was performed by exposing the bacteria to the MVs to develop antimicrobial-resistant E. coli (RC85-T).

RESULTS

The RC85-T exhibited a higher resistance to β-lactam antibiotics compared to the parent strain. Although the secretion rates of the MVs from RC85-T and the parent strain were nearly equal, the β-lactamase activity of the MVs from RC85-T was 12-times higher than that of MVs from the parent strain, based on equivalent protein concentrations. Moreover, MVs secreted by RC85-T were able to protect β-lactam-susceptible E. coli from β-lactam antibiotic-induced growth inhibition in a dose-dependent manner.

CONCLUSION

MVs play a role in transferring substances from Gram-positive to Gram-negative bacteria, shown by the release of MVs from RC85-T that were able to protect β-lactam-susceptible bacteria from β-lactam antibiotics.

SIGNIFICANCE AND IMPACT OF STUDY

MVs are involved in the emergence of antibiotic resistant strains in a mixed bacterial culture, helping us to understand how the spread of multidrug resistant bacteria could be reduced.

Considerations for the Analysis of Bacterial Membrane Vesicles: Methods of Vesicle Production and Quantification Can Influence Biological and Experimental Outcomes

Bacterial membrane vesicles (BMVs) are produced by all bacteria and facilitate a range of functions in host-microbe interactions and pathogenesis. Quantification of BMVs is a critical first step in the analysis of their biological and immunological functions. Historically, BMVs have been quantified by protein assay, which remains the preferred method of BMV quantification. However, recent studies have shown that BMV protein content can vary significantly between bacterial strains, growth conditions, and stages of bacterial growth, suggesting that protein concentration may not correlate directly with BMV quantity. Here, we show that the method used to quantify BMVs can alter experimental outcomes. We compared the enumeration of BMVs using different protein assays and nanoparticle tracking analysis (NTA). We show that different protein assays vary significantly in their quantification of BMVs and that their sensitivity varies when quantifying BMVs produced by different species. Moreover, stimulation of epithelial cells with an equivalent amount of BMV protein quantified using different protein assays resulted in significant differences in interleukin 8 (IL-8) responses. Quantification of Helicobacter pylori, Pseudomonas aeruginosa, and Staphylococcus aureus BMVs by NTA and normalization of BMV cargo to particle number revealed that BMV protein, DNA, and RNA contents were variable between strains and species and throughout bacterial growth. Differences in BMV-mediated activation of Toll-like receptors, NF-κB, and IL-8 responses were observed when stimulations were performed with equivalent BMV particle number but not equivalent protein amount. These findings reveal that the method of BMV quantification can significantly affect experimental outcomes, thereby potentially altering the observed biological functions of BMVs.

IMPORTANCE Recent years have seen a surge in interest in the roles of BMVs in host-microbe interactions and interbacterial communication. As a result of such rapid growth in the field, there is a lack of uniformity in BMV enumeration. Here, we reveal that the method used to enumerate BMVs can significantly alter experimental outcomes. Specifically, standardization of BMVs by protein amount reduced the ability to distinguish strain differences in the immunological functions of BMVs. In contrast, species-, strain-, and growth stage-dependent differences in BMV cargo content were evident when BMVs were enumerated by particle number, and this was reflected in differences in their ability to induce immune responses. These findings indicate that parameters critical to BMV function, including bacterial species, strain, growth conditions, and sample purity, should form the basis of standard reporting in BMV studies. This will ultimately bring uniformity to the field to advance our understanding of BMV functions.

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.

Antimicrobial-induced horizontal transfer of antimicrobial resistance genes in bacteria: a mini-review

The emergence and spread of antimicrobial resistance (AMR) among pathogenic bacteria constitute an accelerating crisis for public health. The selective pressures caused by increased use and misuse of antimicrobials in medicine and livestock production have accelerated the overall selection of resistant bacteria. In addition, horizontal gene transfer (HGT) plays an important role in the spread of resistance genes, for example mobilizing reservoirs of AMR from commensal bacteria into pathogenic ones. Antimicrobials, besides antibacterial function, also result in undesirable effects in the microbial populations, including the stimulation of HGT. The main aim of this narrative review was to present an overview of the current knowledge of the impact of antimicrobials on HGT in bacteria, including the effects of transformation, transduction and conjugation, as well as other less well-studied mechanisms of HGT. It is widely accepted that conjugation plays a major role in the spread of AMR in bacteria, and the focus of this review is therefore mainly on the evidence provided that antimicrobial treatment affects this process. Other mechanisms of HGT have so far been deemed less important in this respect; however, recent discoveries suggest their role may be larger than previously thought, and the review provides an update on the rather limited knowledge currently available regarding the impact of antimicrobial treatment on these processes as well. A conclusion from the review is that there is an urgent need to investigate the mechanisms of antimicrobial-induced HGT, since this will be critical for developing new strategies to combat the spread of AMR.

Bacterial membrane vesicle functions, laboratory methods, and applications

Bacterial membrane vesicles are cupped-shaped structures formed by bacteria in response to environmental stress, genetic alteration, antibiotic exposure, and others. Due to the structural similarities shared with the producer organism, they can retain certain characteristics like stimulating immune responses. They are also able to carry molecules for long distances, without changes in the concentration and integrity of the molecule. Bacteria originally secrete membrane vesicles for gene transfer, excretion, cell to cell interaction, pathogenesis, and protection against phages. These functions are unique and have several innovative applications in the pharmaceutical industry that have attracted both scientific and commercial interest.This led to the development of efficient methods to artificially stimulate vesicle production, purification, and manipulation in the lab at nanoscales. Also, for specific applications, engineering methods to impart pathogen antigens against specific diseases or customization as cargo vehicles to deliver payloads to specific cells have been reported. Many applications of bacteria membrane vesicles are in cancer drugs, vaccines, and adjuvant development with several candidates in clinical trials showing promising results. Despite this, applications in therapy and commercialization stay timid probably due to some challenges one of which is the poor understanding of biogenesis mechanisms. Nevertheless, so far, bacterial membrane vesicles seem to be a reliable and cost-efficient technology with several therapeutic applications. Research toward characterizing more membrane vesicles, genetic engineering, and nanotechnology will enable the scope of applications to widen. This might include solutions to other currently faced medical and healthcare-related challenges.

Engineered Remolding and Application of Bacterial Membrane Vesicles

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

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.

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

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

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.

Extracellular vesicles derived from gut microbiota in inflammatory bowel disease and colorectal cancer: new players?

The human gut microbiome encompasses inter alia, the myriad bacterial species that create the optimal host-microorganism balance essential for normal metabolic and immune function. Various lines of evidence suggest that dysregulation of the microbiota-host interaction is linked to pathologies such as inflammatory bowel disease (IBD) and colorectal cancer (CRC). Extracellular vesicles (EVs), found in virtually all body fluids and produced by both eukaryotic cells and bacteria are involved in cell-cell communication and crosstalk mechanisms, such as the immune response, barrier function and intestinal flora. This review highlights advancements in knowledge of the functional role that EVs may have in IBD and CRC, and discusses the possible use of EVs derived from intestinal microbiota in therapeutic strategies for treating these conditions.

Examining horizontal gene transfer in microbial communities

Bacteria acquire novel DNA through horizontal gene transfer (HGT), a process that enables an organism to rapidly adapt to changing environmental conditions, provides a competitive edge and potentially alters its relationship with its host. Although the HGT process is routinely exploited in laboratories, there is a surprising disconnect between what we know from laboratory experiments and what we know from natural environments, such as the human gut microbiome. Owing to a suite of newly available computational algorithms and experimental approaches, we have a broader understanding of the genes that are being transferred and are starting to understand the ecology of HGT in natural microbial communities. This Review focuses on these new technologies, the questions they can address and their limitations. As these methods are applied more broadly, we are beginning to recognize the full extent of HGT possible within a microbiome and the punctuated dynamics of HGT, specifically in response to external stimuli. Furthermore, we are better characterizing the complex selective pressures on mobile genetic elements and the mechanisms by which they interact with the bacterial host genome.

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.

Microbial vesicle-mediated communication: convergence to understand interactions within and between domains of life

All cells produce extracellular vesicles (EVs). These biological packages contain complex mixtures of molecular cargo and have a variety of functions, including interkingdom communication. Recent discoveries highlight the roles microbial EVs may play in the environment with respect to interactions with plants as well as nutrient cycling. These studies have also identified molecules present within EVs and associated with EV surfaces that contribute to these functions. In parallel, studies of engineered nanomaterials have developed methods to track and model small particle behavior in complex systems and measure the relative importance of various surface features on transport and function. While studies of EV behavior in complex environmental conditions have not yet employed transdisciplinary approaches, it is increasingly clear that expertise from disparate fields will be critical to understand the role of EVs in these systems. Here, we outline how the convergence of biology, soil geochemistry, and colloid science can both develop and address questions surrounding the basic principles governing EV-mediated interkingdom interactions.

Outer membrane vesicles mediated horizontal transfer of an aerobic denitrification gene between Escherichia coli

Bacterial genetic material can be horizontally transferred between microorganisms via outer membrane vesicles (OMVs) released by bacteria. Up to now, the application of vesicle-mediated horizontal transfer of "degrading genes" in environmental remediation has not been reported. In this study, the nirS gene from an aerobic denitrification bacterium, Pseudomonas stutzeri, was enclosed in a pET28a plasmid, transformed into Escherichia coli (E. coli) DH5α and expressed in E. coli BL21. The E. coli DH5α released OMVs containing the recombination plasmid pET28a-nirS-EGFP. When compared with the free pET28a-nirS-EGFP plasmid's inability to transform, nirS in OMVs could be transferred into E. coli BL21 with the transformation frequency of 2.76 × 10⁶ CFU/g when the dosage of OMVs was 200 µg under natural conditions, and nirS could express successfully in recipient bacteria. Furthermore, the recipient bacteria that received OMVs containing pET28a-nirS-EGFP could produce 18.16 U/mL activity of nitrite reductase.

Outer Membrane Protein-Coated Nanoparticles as Antibacterial Vaccine Candidates

Nanoformulations are novel therapeutic strategies as compared to traditional treatments. The development of biomimetic nanoparticles by combining the natural cellular material with synthetic nanoparticles has inspired innovative vaccine strategies for modifying the antibacterial immunity. A lot of work has been done in which synthetic nanoparticles are coated with biomimetic cellular membranes for enhancement of biological functions and treatments. Outer membrane protein of bacteria not only act as adjuvant but also contain a large number of immunogenic antigens that play an important role in motivating the native immunity and stimulating the immune responses of the body. Outer membrane protein coating onto the surfaces of synthetic nanoparticles has synergistic effects to produce antibacterial responses. This article reviews the recent improvements related to the bacterial membrane-coated nanoparticles for antibacterial immunization.

Staphylococcus aureus membrane vesicles contain immunostimulatory DNA, RNA and peptidoglycan that activate innate immune receptors and induce autophagy

Gram‐positive bacteria ubiquitously produce membrane vesicles (MVs), and although they contribute to biological functions, our knowledge regarding their composition and immunogenicity remains limited. Here we examine the morphology, contents and immunostimulatory functions of MVs produced by three Staphylococcus aureus strains; a methicillin resistant clinical isolate, a methicillin sensitive clinical isolate and a laboratory‐adapted strain. We observed differences in the number and morphology of MVs produced by each strain and showed that they contain microbe‐associated molecular patterns (MAMPs) including protein, nucleic acids and peptidoglycan. Analysis of MV‐derived RNA indicated the presence of small RNA (sRNA). Furthermore, we detected variability in the amount and composition of protein, nucleic acid and peptidoglycan cargo carried by MVs from each S. aureus strain. S. aureus MVs activated Toll‐like receptor (TLR) 2, 7, 8, 9 and nucleotide‐binding oligomerization domain containing protein 2 (NOD2) signalling and promoted cytokine and chemokine release by epithelial cells, thus identifying that MV‐associated MAMPs including DNA, RNA and peptidoglycan are detected by pattern recognition receptors (PRRs). Moreover, S. aureus MVs induced the formation of and colocalized with autophagosomes in epithelial cells, while inhibition of lysosomal acidification using bafilomycin A1 resulted in accumulation of autophagosomal puncta that colocalized with MVs, revealing the ability of the host to degrade MVs via autophagy. This study reveals the ability of DNA, RNA and peptidoglycan associated with MVs to activate PRRs in host epithelial cells, and their intracellular degradation via autophagy. These findings advance our understanding of the immunostimulatory roles of Gram‐positive bacterial MVs in mediating pathogenesis, and their intracellular fate within the host.

Outer Membrane Vesicles (OMVs) Produced by Gram-Negative Bacteria: Structure, Functions, Biogenesis, and Vaccine Application

Gram-negative bacteria produce outer membrane vesicles (OMVs) with 10 to 300 nm of diameter. The contribution of OMVs to bacterial pathogenesis is a topic of great interest, and their capacity to be combined with antigens impact in the future to the development of vaccines.

Immunomodulatory roles and novel applications of bacterial membrane vesicles

Bacteria release extracellular vesicles (EVs) known as bacterial membrane vesicles (BMVs) during their normal growth. Gram-negative bacteria produce BMVs termed outer membrane vesicles (OMVs) that are composed of a range of biological cargo and facilitate numerous bacterial functions, including promoting pathogenesis and mediating disease in the host. By contrast, less is understood about BMVs produced by Gram-positive bacteria, which are referred to as membrane vesicles (MVs), however their contribution to mediating bacterial pathogenesis has recently become evident. In this review, we summarise the mechanisms whereby BMVs released by Gram-negative and Gram-positive bacteria are produced, in addition to discussing their key functions in promoting bacterial survival, mediating pathogenesis and modulating host immune responses. Furthermore, we discuss the mechanisms whereby BMVs produced by both commensal and pathogenic organisms can enter host cells and interact with innate immune receptors, in addition to how they modulate host innate and adaptive immunity to promote immunotolerance or drive the onset and progression of disease. Finally, we highlight current and emerging applications of BMVs in vaccine design, biotechnology and cancer therapeutics.

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

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

Ancient Evolutionary Origin and Properties of Universally Produced Natural Exosomes Contribute to Their Therapeutic Superiority Compared to Artificial Nanoparticles

Extracellular vesicles (EVs), such as exosomes, are newly recognized fundamental, universally produced natural nanoparticles of life that are seemingly involved in all biologic processes and clinical diseases. Due to their universal involvements, understanding the nature and also the potential therapeutic uses of these nanovesicles requires innovative experimental approaches in virtually every field. Of the EV group, exosome nanovesicles and larger companion micro vesicles can mediate completely new biologic and clinical processes dependent on the intercellular transfer of proteins and most importantly selected RNAs, particularly miRNAs between donor and targeted cells to elicit epigenetic alterations inducing functional cellular changes. These recipient acceptor cells are nearby (paracrine transfers) or far away after distribution via the circulation (endocrine transfers). The major properties of such vesicles seem to have been conserved over eons, suggesting that they may have ancient evolutionary origins arising perhaps even before cells in the primordial soup from which life evolved. Their potential ancient evolutionary attributes may be responsible for the ability of some modern-day exosomes to withstand unusually harsh conditions, perhaps due to unique membrane lipid compositions. This is exemplified by ability of the maternal milk exosomes to survive passing the neonatal acid/enzyme rich stomach. It is postulated that this resistance also applies to their durable presence in phagolysosomes, thus suggesting a unique intracellular release of their contained miRNAs. A major discussed issue is the generally poorly realized superiority of these naturally evolved nanovesicles for therapies when compared to human-engineered artificial nanoparticles, e.g., for the treatment of diseases like cancers.

Mycolic acid-containing bacteria trigger distinct types of membrane vesicles through different routes

Bacterial membrane vesicles (MVs) are attracting considerable attention in diverse fields of life science and biotechnology due to their potential for various applications. Although there has been progress in determining the mechanisms of MV formation in Gram-negative and Gram-positive bacteria, the mechanisms in mycolic acid-containing bacteria remain an unsolved question due to its complex cell envelope structure. Here, by adapting super-resolution live-cell imaging and biochemical analysis, we show that Corynebacterium glutamicum form distinct types of MVs via different routes in response to environmental conditions. DNA-damaging stress induced MV formation through prophage-triggered cell lysis, whereas envelope stress induced MV formation through mycomembrane blebbing. The MV formation routes were conserved in other mycolic acid-containing bacteria. Our results show how the complex cell envelope structure intrinsically generates various types of MVs and will advance our knowledge on how different types of MVs can be generated from a single cell organism.

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

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

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.

Proteomic Analysis of Vesicle-Producing Pseudomonas aeruginosa PAO1 Exposed to X-Ray Irradiation

Ionizing irradiation kills pathogens by destroying nucleic acids without protein structure destruction. However, how pathogens respond to irradiation stress has not yet been fully elucidated. Here, we observed that Pseudomonas aeruginosa PAO1 could release nucleic acids into the extracellular environment under X-ray irradiation. Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), X-ray irradiation was observed to induce outer membrane vesicle (OMV) formation in P. aeruginosa PAO1. The size distribution of the OMVs of the irradiated PAO1 was similar to that of the OMVs of the non-irradiated PAO1 according to nanoparticle tracking analysis (NTA). The pyocin-related proteins are involved in OMV production in P. aeruginosa PAO1 under X-ray irradiation conditions, and that this is regulated by the key SOS gene recA. The OMV production was significantly impaired in the irradiated PAO1 Δlys mutant, suggesting that Lys endolysin is associated with OMV production in P. aeruginosa PAO1 upon irradiation stress. Meanwhile, no significant difference in OMV production was observed between PAO1 lacking the pqsR, lasR, or rhlR genes and the parent strain, demonstrating that the irradiation-induced OMV biosynthesis of P. aeruginosa was independent of the Pseudomonas quinolone signal (PQS).

Outer membrane vesicle vaccines

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

Membrane Vesicle Production as a Bacterial Defense Against Stress

Membrane vesicles are the nano-sized vesicles originating from membranes. The production of membrane vesicles is a common feature among bacteria. Depending on the bacterial growth phase and environmental conditions, membrane vesicles show diverse characteristics. Various physiological and ecological roles have been attributed to membrane vesicles under both homeostatic and stressful conditions. Pathogens encounter several stressors during colonization in the hostile environment of host tissues. Nutrient deficiency, the presence of antibiotics as well as elements of the host’s immune system are examples of stressors threatening pathogens inside their host. To combat stressors and survive, pathogens have established various defensive mechanisms, one of them is production of membrane vesicles. Pathogens produce membrane vesicles to alleviate the destructive effects of antibiotics or other types of antibacterial treatments. Additionally, membrane vesicles can also provide benefits for the wider bacterial community during infections, through the transfer of resistance or virulence factors. Hence, given that membrane vesicle production may affect the activities of antibacterial agents, their production should be considered when administering antibacterial treatments. Besides, regarding that membrane vesicles play vital roles in bacteria, disrupting their production may suggest an alternative strategy for battling against pathogens. Here, we aim to review the stressors encountered by pathogens and shed light on the roles of membrane vesicles in increasing pathogen adaptabilities in the presence of stress-inducing factors.

Significant increase in the secretion of extracellular vesicles and antibiotics resistance from methicillin-resistant Staphylococcus aureus induced by ampicillin stress

Extracellular vesicles (EVs) containing specific cargo molecules from the cell of origin are naturally secreted from bacteria. EVs play significant roles in protecting the bacterium, which can contribute to their survival in the presence of antibiotics. Herein, we isolated EVs from methicillin-resistant Staphylococcus aureus (MRSA) in an environment with or without stressor by adding ampicillin at a lower concentration than the minimum inhibitory concentration (MIC). We investigated whether EVs from MRSA under stress condition or normal condition could defend susceptible bacteria in the presence of several β-lactam antibiotics, and directly degrade the antibiotics. A comparative proteomic approach was carried out in both types of EVs to investigate β-lactam resistant determinants. The secretion of EVs from MRSA under antibiotic stressed conditions was increased by 22.4-fold compared with that of EVs without stress. Proteins related to the degradation of β-lactam antibiotics were abundant in EVs released from the stressed condition. Taken together, the present data reveal that EVs from MRSA play a crucial role in the survival of β-lactam susceptible bacteria by acting as the first line of defense against β-lactam antibiotics, and antibiotic stress leads to release EVs with high defense activity.

Biofilms: hot spots of horizontal gene transfer (HGT) in aquatic environments, with a focus on a new HGT mechanism

Biofilms in water environments are thought to be hot spots for horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs). ARGs can be spread via HGT, though mechanisms are known and have been shown to depend on the environment, bacterial communities and mobile genetic elements. Classically, HGT mechanisms include conjugation, transformation and transduction; more recently, membrane vesicles (MVs) have been reported as DNA reservoirs implicated in interspecies HGT. Here, we review the current knowledge on the HGT mechanisms with a focus on the role of MVs and the methodological innovations in the HGT research.

Transductomics: sequencing-based detection and analysis of transduced DNA in pure cultures and microbial communities

BACKGROUND

Horizontal gene transfer (HGT) plays a central role in microbial evolution. Our understanding of the mechanisms, frequency, and taxonomic range of HGT in polymicrobial environments is limited, as we currently rely on historical HGT events inferred from genome sequencing and studies involving cultured microorganisms. We lack approaches to observe ongoing HGT in microbial communities.

RESULTS

To address this knowledge gap, we developed a DNA sequencing-based "transductomics" approach that detects and characterizes microbial DNA transferred via transduction. We validated our approach using model systems representing a range of transduction modes and show that we can detect numerous classes of transducing DNA. Additionally, we show that we can use this methodology to obtain insights into DNA transduction among all major taxonomic groups of the intestinal microbiome.

CONCLUSIONS

The transductomics approach that we present here allows for the detection and characterization of genes that are potentially transferred between microbes in complex microbial communities at the time of measurement and thus provides insights into real-time ongoing horizontal gene transfer. This work extends the genomic toolkit for the broader study of mobile DNA within microbial communities and could be used to understand how phenotypes spread within microbiomes. Video Abstract.

Staphylococcus aureus secretes immunomodulatory RNA and DNA via membrane vesicles

Bacterial-derived RNA and DNA can function as ligands for intracellular receptor activation and induce downstream signaling to modulate the host response to bacterial infection. The mechanisms underlying the secretion of immunomodulatory RNA and DNA by pathogens such as Staphylococcus aureus and their delivery to intracellular host cell receptors are not well understood. Recently, extracellular membrane vesicle (MV) production has been proposed as a general secretion mechanism that could facilitate the delivery of functional bacterial nucleic acids into host cells. S. aureus produce membrane-bound, spherical, nano-sized, MVs packaged with a select array of bioactive macromolecules and they have been shown to play important roles in bacterial virulence and in immune modulation through the transmission of biologic signals to host cells. Here we show that S. aureus secretes RNA and DNA molecules that are mostly protected from degradation by their association with MVs. Importantly, we demonstrate that MVs can be delivered into cultured macrophage cells and subsequently stimulate a potent IFN-β response in recipient cells via activation of endosomal Toll-like receptors. These findings advance our understanding of the mechanisms by which bacterial nucleic acids traffic extracellularly to trigger the modulation of host immune responses.

Membrane vesicles from a Dietzia bacterium containing multiple cargoes and their roles in iron delivery

Membrane vesicles (MVs) released from bacteria act as extracellular vehicles carrying various functional cargoes between cells. MVs with different cargoes play multiple roles in stress adaptation, nutrient acquisition and microbial interactions. However, previous studies have primarily focused on MVs from Gram-negative bacteria, while the characteristics of cargoes in MVs from Gram-positive bacteria and their involvement in microbial interactions remain to be elucidated. Here, we used a Gram-positive strain, Dietzia sp. DQ12-45-1b from Corynebacteriales, to analyse the characteristics and functions of MVs. We identified the 'antioxidant' canthaxanthin is stored within MVs by LC-MS/MS. In addition, nearly the entire genomic content of strain DQ12-45-1b are evenly distributed in MVs, suggesting that MVs from DQ12-45-1b might involve in horizontal gene transfer. Finally, the mycobactin-type siderophores were detected in MVs. The iron-loaded MVs effectively mediate iron binding and delivery to homologous bacteria from the order Corynebacteriales, but not to more distantly related species from the orders Pseudomonadales, Bacillales and Enterobacterales. These results revealed that the iron-loaded MVs are shared between homologous species. Together, we report the Gram-positive bacterium Dietzia sp. DQ12-45-1b released MVs that contain canthaxanthin, DNA and siderophores and prove that MVs act as public goods between closely related species.

Horizontal 'gene drives' harness indigenous bacteria for bioremediation

Engineering bacteria to clean-up oil spills is rapidly advancing but faces regulatory hurdles and environmental concerns. Here, we develop a new technology to harness indigenous soil microbial communities for bioremediation by flooding local populations with catabolic genes for petroleum hydrocarbon degradation. Overexpressing three enzymes (almA, xylE, p450cam) in Escherichia coli led to degradation of 60-99% of target hydrocarbon substrates. Mating experiments, fluorescence microscopy and TEM revealed indigenous bacteria could obtain these vectors from E. coli through several mechanisms of horizontal gene transfer (HGT), including conjugation and cytoplasmic exchange through nanotubes. Inoculating petroleum-polluted sediments with E. coli carrying the vector pSF-OXB15-p450camfusion showed that the E. coli cells died after five days but a variety of bacteria received and carried the vector for over 60 days after inoculation. Within 60 days, the total petroleum hydrocarbon content of the polluted soil was reduced by 46%. Pilot experiments show that vectors only persist in indigenous populations when under selection pressure, disappearing when this carbon source is removed. This approach to remediation could prime indigenous bacteria for degrading pollutants while providing minimal ecosystem disturbance.

Antibiotic Resistance: Moving From Individual Health Norms to Social Norms in One Health and Global Health

Antibiotic resistance is a problem for human health, and consequently, its study had been traditionally focused toward its impact for the success of treating human infections in individual patients (individual health). Nevertheless, antibiotic-resistant bacteria and antibiotic resistance genes are not confined only to the infected patients. It is now generally accepted that the problem goes beyond humans, hospitals, or long-term facility settings and that it should be considered simultaneously in human-connected animals, farms, food, water, and natural ecosystems. In this regard, the health of humans, animals, and local antibiotic-resistance-polluted environments should influence the health of the whole interconnected local ecosystem (One Health). In addition, antibiotic resistance is also a global problem; any resistant microorganism (and its antibiotic resistance genes) could be distributed worldwide. Consequently, antibiotic resistance is a pandemic that requires Global Health solutions. Social norms, imposing individual and group behavior that favor global human health and in accordance with the increasingly collective awareness of the lack of human alienation from nature, will positively influence these solutions. In this regard, the problem of antibiotic resistance should be understood within the framework of socioeconomic and ecological efforts to ensure the sustainability of human development and the associated human-natural ecosystem interactions.

The Serratia grimesii outer membrane vesicles-associated grimelysin triggers bacterial invasion of eukaryotic cells

Serratia grimesii are facultative pathogenic bacteria that can penetrate a wide range of host cells and cause infection, especially in immunocompromised patients. Previously, we have found that bacterial metalloprotease grimelysin is a potential virulence determinant of S. grimesii invasion (E. S. Bozhokina et al., (2011). Cell Biology International, 35(2), 111-118). Protease is characterized as an actin-hydrolyzing enzyme with a narrow specificity toward other cell proteins. It is not known, however, whether grimelysin is transported into eukaryotic cells. Here, we show, for the first time, that S. grimesii can generate outer membrane vesicles (OMVs) displayed specific proteolytic activity against actin, characteristic of grimelysin. The presence of grimelysin was also confirmed by the Western blot analysis of S. grimesii OMVs lysate. Furthermore, confocal microscopy analysis revealed that the S. grimesii grimelysin-containing OMVs attached to the host cell membrane. Finally, pretreatment of HeLa cells with S. grimesii OMVs before the cells were infected with bacteria increased the bacterial penetration several times. These data strongly suggest that protease grimelysin promotes S. grimesii internalization by modifying bacterial and/or host molecule(s) when it is delivered as a component of OMVs.

What do we know about plasmids carried by members of the Acinetobacter genus?

Several Acinetobacter spp. act as opportunistic pathogens causing healthcare-associated infections worldwide, and in this respect their ability to resist antimicrobial compounds has certainly boosted up their global propagation. Acinetobacter clinical strains have demonstrated a remarkable ability to evolve and become resistant to almost all available drugs in the antimicrobial arsenal, including the last-resort carbapenem β-lactams. The dissemination of antimicrobial resistant genes (ARG), heavy metals-detoxification systems and other traits such as virulence factors is facilitated by mobile genetic elements (MGE) through horizontal gene transfer. Among them, plasmids have been shown to play a critical role in this genus. Despite the continuous increase of Acinetobacter plasmid sequences present in databases, there are no reports describing the basic traits carried by these MGE. To fill this gap, a broad analysis of the Acinetobacter plasmidome was performed. A search for Acinetobacter complete plasmids indicated that 905 sequences have been deposited in the NCBI-GenBank public database, of which 492 are harbored by Acinetobacter baumannii strains. Plasmid-classification schemes based on Rep proteins homology have so far described 23 different groups for A. baumannii (GR1-23), and 16 Acinetobacter Rep3 Groups (AR3G1-16) for the complete genus. Acinetobacter plasmids size ranges from 1.3 to 400 kb. Interestingly, widespread plasmids which are < 20 kb make up 56% of the total present in members of this genus. This led to the proposal of Acinetobacter plasmid assignation to two groups according to their size (< 20 kb and > 20 kb). Usually, smaller plasmids are not self-transmissible, and thereby employ alternative mechanisms of dissemination. For instance, a subgroup of < 20 kb-plasmids belonging to the pRAY-family, lack a rep gene, but encode a relaxase enabling their mobilization by conjugative plasmids. Other subgroup, including small GR2 Acinetobacter plasmids, does not encode a relaxase gene. However, they could still be mobilized by conjugative plasmids which recognize an oriT region carried by these small plasmids. Also, these < 20 kb-plasmids usually carry accessory genes bordered by XerC/D-recombinases recognition sites which have been hypothesized to mediate plasmid plasticity. Conversely, many cases of larger plasmids are self-transmissible and might encode virulence factors and their regulators, thus controlling strain pathogenicity. The ARGs carried by the > 20 kb-plasmids are usually encoded within other MGEs such as transposons, or as part of integrons. It has been recently noted that some of the > 20 kb-plasmids are derived from excised phages, and thus dubbed as phage-like plasmids. All in all, the plethora of plasmids found in strains of this genus and the multiple strategies promoting their evolution and dissemination have certainly contributed to survival of the Acinetobacter members in different habitats, including the clinical environment.

OMV Vaccines and the Role of TLR Agonists in Immune Response

Outer Membrane Vesicles (OMVs) are bacterial nanoparticles that are spontaneously released during growth both in vitro and in vivo by Gram-negative bacteria. They are spherical, bilayered membrane nanostructures that contain many components found within the external surface of the parent bacterium. Naturally, OMVs serve the bacteria as a mechanism to deliver DNA, RNA, proteins, and toxins, as well as to promote biofilm formation and remodel the outer membrane during growth. On the other hand, as OMVs possess the optimal size to be uptaken by immune cells, and present a range of surface-exposed antigens in native conformation and Toll-like receptor (TLR) activating components, they represent an attractive and powerful vaccine platform able to induce both humoral and cell-mediated immune responses. This work reviews the TLR-agonists expressed on OMVs and their capability to trigger individual TLRs expressed on different cell types of the immune system, and then focuses on their impact on the immune responses elicited by OMVs compared to traditional vaccines.

Dissemination of the blaCTX-M-15 gene among Enterobacteriaceae via outer membrane vesicles

Background

Bacterial outer membrane vesicles (OMVs) are an emerging source of antibiotic resistance transfer but their role in the spread of the blaCTX-M-15 gene encoding the most frequent CTX-M ESBL in Enterobacteriaceae is unknown.

Objectives

To determine the presence of blaCTX-M-15 and other antibiotic resistance genes in OMVs of the CTX-M-15-producing MDR Escherichia coli O104:H4 outbreak strain and the ability of these OMVs to spread these genes among Enterobacteriaceae under different conditions.

Methods

OMV-borne antibiotic resistance genes were detected by PCR; OMV-mediated transfer of blaCTX-M-15 and the associated blaTEM-1 was quantified under laboratory conditions, simulated intraintestinal conditions and under ciprofloxacin stress; resistance to antibiotics and the ESBL phenotype were determined by the CLSI disc diffusion methods and the presence of pESBL by plasmid profiling and Southern blot hybridization.

Results

E. coli O104:H4 OMVs carried blaCTX-M-15 and blaTEM-1 located on the pESBL plasmid, but not chromosomal antibiotic resistance genes. The OMVs transferred blaCTX-M-15, blaTEM-1 and the associated pESBL into Enterobacteriaceae of different species. The frequencies of the OMV-mediated transfer were significantly increased under simulated intraintestinal conditions and under ciprofloxacin stress when compared with laboratory conditions. The ‘vesiculants’ (i.e. recipients that received the blaCTX-M-15- and blaTEM-1-harbouring pESBL via OMVs) acquired resistance to cefotaxime, ceftazidime and cefpodoxime and expressed the ESBL phenotype. They were able to further spread pESBL and the blaCTX-M-15 and blaTEM-1 genes via OMVs.

Conclusions

OMVs are efficient vehicles for dissemination of the blaCTX-M-15 gene among Enterobacteriaceae and may contribute to blaCTX-M-15 transfer in the human intestine.

Green Synthesis of AuNPs by Acinetobacter sp. GWRVA25: Optimization, Characterization, and Its Antioxidant Activity

Bacteriogenic synthesis of metal nanoparticles is ecofriendly and greatly influenced by physico-chemical reaction parameters with respect to shape and size. Thus, present work aimed to synthesize and optimization of bacteriogenic gold nanoparticles (AuNPs) and study their antioxidant activity. Acinetobacter sp. cells were able to synthesize AuNPs, when challenged with tetra-chloroauric acid (HAuCl₄). By physicochemical optimization, maximum synthesis was obtained with 72 h old culture using 2.1 × 10⁹ CFU/ml cell density. Whereas, pH-7 is suitable for AuNPs synthesis. HAuCl₄ concentration (0.5 mM) enhanced the formation of monodispersed and spherical nanoparticles (15 ± 10 nm). At 37°C temperature, Acinetobacter sp. released nanoparticles in supernatant. From characterization, AuNPs were found to be crystalline in nature with negative surface charge. AuNPs showed up to 86% different radical scavenging ability, exhibiting antioxidant activity. In conclusion, spherical AuNPs can be synthesized using Acinetobacter sp. through physicochemical optimization. This is the first report of antioxidant activity exhibited by monodispersed bacteriogenic AuNPs synthesized using Acinetobacter sp.

Agrobacterium tumefaciens Small Lipoprotein Atu8019 Is Involved in Selective Outer Membrane Vesicle (OMV) Docking to Bacterial Cells

Outer membrane vesicles (OMVs), released from Gram-negative bacteria, have been attributed to intra- and interspecies communication and pathogenicity in diverse bacteria. OMVs carry various components including genetic material, toxins, signaling molecules, or proteins. Although the molecular mechanism(s) of cargo delivery is not fully understood, recent studies showed that transfer of the OMV content to surrounding cells is mediated by selective interactions. Here, we show that the phytopathogen Agrobacterium tumefaciens, the causative agent of crown gall disease, releases OMVs, which attach to the cell surface of various Gram-negative bacteria. The OMVs contain the conserved small lipoprotein Atu8019. An atu8019-deletion mutant produced wildtype-like amounts of OMVs with a subtle but reproducible reduction in cell-attachment. Otherwise, loss of atu8019 did not alter growth, susceptibility against cations or antibiotics, attachment to plant cells, virulence, motility, or biofilm formation. In contrast, overproduction of Atu8019 in A. tumefaciens triggered cell aggregation and biofilm formation. Localization studies revealed that Atu8019 is surface exposed in Agrobacterium cells and in OMVs supporting a role in cell adhesion. Purified Atu8019 protein reconstituted into liposomes interacted with model membranes and with the surface of several Gram-negative bacteria. Collectively, our data suggest that the small lipoprotein Atu8019 is involved in OMV docking to specific bacteria.

Horizontal Gene Transfer of Antibiotic Resistance from Acinetobacter baylyi to Escherichia coli on Lettuce and Subsequent Antibiotic Resistance Transmission to the Gut Microbiome

Agricultural use of antibiotics is recognized by the U.S. Centers for Disease Control and Prevention as a major contributor to antibiotic-resistant infections. While most One Health attention has been on the potential for antibiotic resistance transmission from livestock and contaminated meat products to people, plant foods are fundamental to the food chain for meat eaters and vegetarians alike. We hypothesized that environmental bacteria that colonize plant foods may serve as platforms for the persistence of antibiotic-resistant bacteria and for horizontal gene transfer of antibiotic-resistant genes. Donor Acinetobacter baylyi and recipient Escherichia coli were cocultured in vitro, in planta on lettuce, and in vivo in BALB/c mice. We showed that nonpathogenic, environmental A. baylyi is capable of transferring plasmids conferring antibiotic resistance to E. coli clinical isolates on lettuce leaf discs. Furthermore, transformant E. coli from the in planta assay could then colonize the mouse gut microbiome. The target antibiotic resistance plasmid was identified in mouse feces up to 5 days postinfection. We specifically identified in vivo transfer of the plasmid to resident Klebsiella pneumoniae in the mouse gut. Our findings highlight the potential for environmental bacteria exposed to antibiotics to transmit resistance genes to mammalian pathogens during ingestion of leafy greens.IMPORTANCE Previous efforts have correlated antibiotic-fed livestock and meat products with respective antibiotic resistance genes, but virtually no research has been conducted on the transmission of antibiotic resistance from plant foods to the mammalian gut (C. S. Hölzel, J. L. Tetens, and K. Schwaiger, Pathog Dis 15:671-688, 2018, https://doi.org/10.1089/fpd.2018.2501; C. M. Liu et al., mBio 9:e00470-19, 2018, https://doi.org/10.1128/mBio.00470-18; B. Spellberg et al., NAM Perspectives, 2016, https://doi.org/10.31478/201606d; J. O'Neill, Antimicrobials in agriculture and the environment, 2015; Centers for Disease Control and Prevention, Antibiotic resistance threats in the United States, 2019). Here, we sought to determine if horizontal transmission of antibiotic resistance genes can occur between lettuce and the mammalian gut microbiome, using a mouse model. Furthermore, we have created a new model to study horizontal gene transfer on lettuce leaves using an antibiotic-resistant transformant of A. baylyi (AbzeoR).