Pseudomonas aeruginosa Outer Membrane Vesicles Triggered by Human Mucosal Fluid and Lysozyme Can Prime Host Tissue Surfaces for Bacterial Adhesion

In vitro and ex vivo models of microbial keratitis: Present and future

Microbial keratitis (MK) is an infection of the cornea, caused by bacteria, fungi, parasites, or viruses. MK leads to significant morbidity, being the fifth leading cause of blindness worldwide. There is an urgent requirement to better understand pathogenesis in order to develop novel diagnostic and therapeutic approaches to improve patient outcomes. Many in vitro, ex vivo and in vivo MK models have been developed and implemented to meet this aim. Here, we present current in vitro and ex vivo MK model systems, examining their varied design, outputs, reporting standards, and strengths and limitations. Major limitations include their relative simplicity and the perceived inability to study the immune response in these MK models, an aspect widely accepted to play a significant role in MK pathogenesis. Consequently, there remains a dependence on in vivo models to study this aspect of MK. However, looking to the future, we draw from the broader field of corneal disease modelling, which utilises, for example, three-dimensional co-culture models and dynamic environments observed in bioreactors and organ-on-a-chip scenarios. These remain unexplored in MK research, but incorporation of these approaches will offer further advances in the field of MK corneal modelling, in particular with the focus of incorporation of immune components which we anticipate will better recapitulate pathogenesis and yield novel findings, therefore contributing to the enhancement of MK outcomes.

Extracellular vesicles released by host epithelial cells during Pseudomonas aeruginosa infection function as homing beacons for neutrophils

BACKGROUND

Pseudomonas aeruginosa (PA) is an opportunistic pathogen that can cause sight threatening infections in the eye and fatal infections in the cystic fibrosis airway. Extracellular vesicles (EVs) are released by host cells during infection and by the bacteria themselves; however, there are no studies on the composition and functional role of host-derived EVs during PA infection of the eye or lung. Here we investigated the composition and capacity of EVs released by PA infected epithelial cells to modulate innate immune responses in host cells.

METHODS

Human telomerase immortalized corneal epithelial cells (hTCEpi) cells and human telomerase immortalized bronchial epithelial cells (HBECs) were treated with a standard invasive test strain of Pseudomonas aeruginosa, PAO1, for 6 h. Host derived EVs were isolated by qEV size exclusion chromatography. EV proteomic profiles during infection were compared using mass spectrometry and functional studies were carried out using hTCEpi cells, HBECs, differentiated neutrophil-like HL-60 cells, and primary human neutrophils isolated from peripheral blood.

RESULTS

EVs released from PA infected corneal epithelial cells increased pro-inflammatory cytokine production in naïve corneal epithelial cells and induced neutrophil chemotaxis independent of cytokine production. The EVs released from PA infected bronchial epithelial cells were also chemotactic although they failed to induce cytokine secretion from naïve HBECs. At the proteomic level, EVs derived from PA infected corneal epithelial cells exhibited lower complexity compared to bronchial epithelial cells, with the latter having reduced protein expression compared to the non-infected control.

CONCLUSIONS

This is the first study to comprehensively profile EVs released by corneal and bronchial epithelial cells during Pseudomonas infection. Together, these findings show that EVs released by PA infected corneal and bronchial epithelial cells function as potent mediators of neutrophil migration, contributing to the exuberant neutrophil response that occurs during infection in these tissues.

P. aeruginosa tRNA-fMet halves secreted in outer membrane vesicles suppress lung inflammation in cystic fibrosis

Although tobramycin increases lung function in people with cystic fibrosis (pwCF), the density of Pseudomonas aeruginosa (P. aeruginosa) in the lungs is only modestly reduced by tobramycin; hence, the mechanism whereby tobramycin improves lung function is not completely understood. Here, we demonstrate that tobramycin increases 5' tRNA-fMet halves in outer membrane vesicles (OMVs) secreted by laboratory and CF clinical isolates of P. aeruginosa. The 5' tRNA-fMet halves are transferred from OMVs into primary CF human bronchial epithelial cells (CF-HBEC), decreasing OMV-induced IL-8 and IP-10 secretion. In mouse lungs, increased expression of the 5' tRNA-fMet halves in OMVs attenuated KC (murine homolog of IL-8) secretion and neutrophil recruitment. Furthermore, there was less IL-8 and neutrophils in bronchoalveolar lavage fluid isolated from pwCF during the period of exposure to tobramycin versus the period off tobramycin. In conclusion, we have shown in mice and in vitro studies on CF-HBEC that tobramycin reduces inflammation by increasing 5' tRNA-fMet halves in OMVs that are delivered to CF-HBEC and reduce IL-8 and neutrophilic airway inflammation. This effect is predicted to improve lung function in pwCF receiving tobramycin for P. aeruginosa infection.NEW & NOTEWORTHY The experiments in this report identify a novel mechanism, whereby tobramycin reduces inflammation in two models of CF. Tobramycin increased the secretion of tRNA-fMet halves in OMVs secreted by P. aeruginosa, which reduced the OMV-LPS-induced inflammatory response in primary cultures of CF-HBEC and in mouse lung, an effect predicted to reduce lung damage in pwCF.

Extracellular vesicles from Candida albicans modulate immune cells function and play a protective role in fungal keratitis

Fungal keratitis (FK) is a highly blinding infectious corneal disease caused by pathogenic fungi. Candida albicans (C. albicans) is one of the main pathogens of fungal keratitis. Extracellular vesicles (EVs), lipid bilayer compartments released by almost all living cells, including fungi, have garnered attention for their role in pathogenic microbial infection and host immune responses in recent years. Studies have reported that pretreating the host with fungal EVs can reduce the inflammatory response of the host when attacked by fungi and reduce the lethality of fungal infection. However, there are no studies that have evaluated whether C. albicans EVs can modulate the inflammatory response associated with C. albicans keratitis. Our study revealed that C. albicans EVs could activate the polymorphonuclear cells (PMNs) and promote their secretion of proinflammatory cytokines and nitric oxide (NO), enhance their phagocytic and fungicidal abilities against C. albicans. C. albicans EVs also induced a proinflammatory response in RAW264.7 cells, which was characterized by increased production of inflammatory cytokines and elevated expression of the chemokine CCL2. Similarly, stimulation of C. albicans EVs to RAW264.7 cells also enhanced the phagocytosis and killing ability of cells against C. albicans. Besides, in our in vivo experiments, after receiving subconjunctival injection of C. albicans EVs, C57BL/6 mice were infected with C. albicans. The results demonstrated that pre-exposure to C. albicans EVs could effectively diminish the severity of keratitis, reduce fungal load and improve prognosis. Overall, we conclude that C. albicans EVs can modulate the function of immune cells and play a protective role in C. albicans keratitis.

Cytotoxic rhamnolipid micelles drive acute virulence in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that has developed multi- or even pan-drug resistance toward most frontline and last resort antibiotics, leading to increasing frequency of infections and deaths among hospitalized patients, especially those with compromised immune systems. Further complicating treatment, P. aeruginosa produces numerous virulence factors that contribute to host tissue damage and immune evasion, promoting bacterial colonization and pathogenesis. In this study, we demonstrate the importance of rhamnolipid production in host-pathogen interactions. Secreted rhamnolipids form micelles that exhibited highly acute toxicity toward murine macrophages, rupturing the plasma membrane and causing organellar membrane damage within minutes of exposure. While rhamnolipid micelles (RMs) were particularly toxic to macrophages, they also caused membrane damage in human lung epithelial cells, red blood cells, Gram-positive bacteria, and even noncellular models like giant plasma membrane vesicles. Most importantly, rhamnolipid production strongly correlated with P. aeruginosa virulence against murine macrophages in various panels of clinical isolates. Altogether, our findings suggest that rhamnolipid micelles are highly cytotoxic virulence factors that drive acute cellular damage and immune evasion during P. aeruginosa infections.

P. aeruginosa tRNA-fMet halves secreted in outer membrane vesicles suppress lung inflammation in Cystic Fibrosis

Although tobramycin increases lung function in people with cystic fibrosis (pwCF), the density of Pseudomonas aeruginosa (P. aeruginosa) in the lungs is only modestly reduced by tobramycin; hence, the mechanism whereby tobramycin improves lung function is not completely understood. Here, we demonstrate that tobramycin increases 5' tRNA-fMet halves in outer membrane vesicles (OMVs) secreted by laboratory and CF clinical isolates of P. aeruginosa . The 5' tRNA-fMet halves are transferred from OMVs into primary CF human bronchial epithelial cells (CF-HBEC), decreasing OMV-induced IL-8 and IP-10 secretion. In mouse lung, increased expression of the 5' tRNA-fMet halves in OMVs attenuated KC secretion and neutrophil recruitment. Furthermore, there was less IL-8 and neutrophils in bronchoalveolar lavage fluid isolated from pwCF during the period of exposure to tobramycin versus the period off tobramycin. In conclusion, we have shown in mice and in vitro studies on CF-HBEC that tobramycin reduces inflammation by increasing 5' tRNA-fMet halves in OMVs that are delivered to CF-HBEC and reduce IL-8 and neutrophilic airway inflammation. This effect is predicted to improve lung function in pwCF receiving tobramycin for P. aeruginosa infection.

New and noteworthy

The experiments in this report identify a novel mechanim whereby tobramycin reduces inflammation in two models of CF. Tobramycin increased the secretion of tRNA-fMet haves in OMVs secreted by P. aeruginiosa , which reduced the OMV-LPS induced inflammatory response in primary cultures of CF-HBEC and in mouse lung, an effect predicted to reduce lung damage in pwCF.

Graphical abstract

The anti-inflammatory effect of tobramycin mediated by 5' tRNA-fMet halves secreted in P. aeruginosa OMVs. (A) P. aeruginosa colonizes the CF lungs and secrets OMVs. OMVs diffuse through the mucus layer overlying bronchial epithelial cells and induce IL-8 secretion, which recruits neutrophils that causes lung damage. ( B ) Tobramycin increases 5' tRNA-fMet halves in OMVs secreted by P. aeruginosa . 5' tRNA-fMet halves are delivered into host cells after OMVs fuse with lipid rafts in CF-HBEC and down-regulate protein expression of MAPK10, IKBKG, and EP300, which suppresses IL-8 secretion and neutrophils in the lungs. A reduction in neutrophils in CF BALF is predicted to improve lung function and decrease lung damage.

From trash to treasure: the role of bacterial extracellular vesicles in gut health and disease

Bacterial extracellular vesicles (BEVs) have emerged as critical factors involved in gut health regulation, transcending their traditional roles as byproducts of bacterial metabolism. These vesicles function as cargo carriers and contribute to various aspects of intestinal homeostasis, including microbial balance, antimicrobial peptide secretion, physical barrier integrity, and immune system activation. Therefore, any imbalance in BEV production can cause several gut-related issues including intestinal infection, inflammatory bowel disease, metabolic dysregulation, and even cancer. BEVs derived from beneficial or commensal bacteria can act as potent immune regulators and have been implicated in maintaining gut health. They also show promise for future clinical applications in vaccine development and tumor immunotherapy. This review examines the multifaceted role of BEVs in gut health and disease, and also delves into future research directions and potential applications.

Pseudomonas aeruginosa-Derived Extracellular Vesicles Modulate Corneal Inflammation: Role in Microbial Keratitis?

Pseudomonas aeruginosa keratitis occurs following trauma, in immunocompromised patients, and in otherwise healthy contact lens wearers. Characterized by a light-blocking infiltrate, P. aeruginosa keratitis is the most serious complication associated with contact lens wear and, in severe cases, can lead to vision loss. Bacterial extracellular vesicles (B EVs) are membrane-enclosed nanometer-scale particles secreted from bacteria and are packed with bioactive molecules. B EVs have been shown to mediate biological functions that regulate host pathogenic responses. In the present study, we isolated P. aeruginosa-derived EVs using size exclusion chromatography and compared the proteomic compositions and functional activities of P. aeruginosa-derived EVs and P. aeruginosa-derived free protein (FP) on corneal epithelial cells and neutrophils. Importantly, P. aeruginosa-derived EVs and FP exhibited unique protein profiles, with EVs being enriched in P. aeruginosa virulence proteins. P. aeruginosa-derived EVs promoted corneal epithelial cell secretion of interleukin-6 (IL-6) and IL-8, whereas these cytokines were not upregulated following treatment with FP. In contrast, FP had a negative effect on the host inflammatory response and impaired neutrophil killing. Both P. aeruginosa-derived EVs and FP promoted intracellular bacterial survival in corneal epithelial cells. Collectively, these data suggest that P. aeruginosa-derived EVs and FP may play a critical role in the pathogenesis of corneal infection by interfering with host innate immune defense mechanisms.

Intact O-antigen is critical structure for the exceptional tubular shape of outer membrane vesicles in Francisella tularensis

Francisella tularensis is a highly infectious Gram-negative coccobacillus which causes the disease tularemia. The potential for its misuse as a biological weapon has led disease control and prevention centers to classify this bacterium as a category A agent. Bacterial outer membrane vesicles (OMVs) are spherical particles 20-250 nm in size produced by all Gram-negative bacteria and constitute one of the major secretory pathways. Bacteria use them in interacting with both other bacterial cells and eukaryotic (host) cells. OMVs of Francisella contain number of its so far described virulence factors and immunomodulatory proteins. Their role in host-pathogen interactions can therefore be presumed, and the possibility exists also for their potential use in a subunit vaccine. Moreover, Francisella microbes produce both usual spherical and unusual tubular OMVs. Because OMVs emerge from the outermost surface of the bacterial cell, we focused on the secretion of OMVs in several mutant Francisella strains with disrupted surface structures (namely the O-antigen). O-antigen in Francisella is not only the structural component of LPS but also forms another important virulence factor: the O-antigen polysaccharide capsule. Mutant strain phenotypes were evaluated by growth curves, vesiculation rates, their sensitivity to the complement contained in serum, and proliferation inside murine bone marrow macrophages. Morphologies of both OMVs and the bacteria were visualized by electron microscopy. The O-antigen mutant strains were considerably attenuated in serum resistance and intracellular proliferation. All the strains showed lower ability to form the tubular OMVs. Some strains formed tubular protrusions from their outer membrane but their stability was weak. Some hypervesiculating strains were revealed that will serve as source of OMVs for further studies of their protective potential. Our results suggest the presence of LPS and the O-antigen capsule on the surface of Francisella to be critical not only for its virulence but also for the exceptional tubular shape of its OMVs.

Bordetella pertussis outer membrane vesicles as virulence factor vehicles that influence bacterial interaction with macrophages

Gram-negative pathogenic bacteria constitutively shed outer membrane vesicles (OMVs) which play a significant role in the host-pathogen interaction, eventually determining the outcome of the infection. We previously found that Bordetella pertussis, the etiological agent of whooping cough, survives the innate interaction with human macrophages remaining alive inside these immune cells. Adenylate cyclase (CyaA), one of the main toxins of this pathogen, was found involved in the modulation of the macrophage defense response, eventually promoting bacterial survival within the cells. We here investigated whether B. pertussis OMVs, loaded with most of the bacterial toxins and CyaA among them, modulate the macrophage response to the bacterial infection. We observed that the pre-incubation of macrophages with OMVs led to a decreased macrophage defense response to the encounter with the bacteria, in a CyaA dependent way. Our results suggest that CyaA delivered by B. pertussis OMVs dampens macrophages protective function by decreasing phagocytosis and the bactericidal capability of these host cells. By increasing the chances of bacterial survival to the innate encounter with the macrophages, B. pertussis OMVs might play a relevant role in the course of infection, promoting bacterial persistence within the host and eventually, shaping the whole infection process.

Role of Host and Bacterial Lipids in Pseudomonas aeruginosa Respiratory Infections

The opportunistic pathogen Pseudomonas aeruginosa is one of the most common agents of respiratory infections and has been associated with high morbidity and mortality rates. The ability of P. aeruginosa to cause severe respiratory infections results from the coordinated action of a variety of virulence factors that promote bacterial persistence in the lungs. Several of these P. aeruginosa virulence mechanisms are mediated by bacterial lipids, mainly lipopolysaccharide, rhamnolipid, and outer membrane vesicles. Other mechanisms arise from the activity of P. aeruginosa enzymes, particularly ExoU, phospholipase C, and lipoxygenase A, which modulate host lipid signaling pathways. Moreover, host phospholipases, such as cPLA2α and sPLA2, are also activated during the infectious process and play important roles in P. aeruginosa pathogenesis. These mechanisms affect key points of the P. aeruginosa-host interaction, such as: i) biofilm formation that contributes to bacterial colonization and survival, ii) invasion of tissue barriers that allows bacterial dissemination, iii) modulation of inflammatory responses, and iv) escape from host defenses. In this mini-review, we present the lipid-based mechanism that interferes with the establishment of P. aeruginosa in the lungs and discuss how bacterial and host lipids can impact the outcome of P. aeruginosa respiratory infections.

Outer Membrane Vesicles (OMVs) of Pseudomonas aeruginosa Provide Passive Resistance but Not Sensitization to LPS-Specific Phages

Outer membrane vesicles (OMVs) released from gram-negative bacteria are key elements in bacterial physiology, pathogenesis, and defence. In this study, we investigated the role of Pseudomonas aeruginosa OMVs in the anti-phage defence as well as in the potential sensitization to LPS-specific phages. Using transmission electron microscopy, virion infectivity, and neutralization assays, we have shown that both phages efficiently absorb on free vesicles and are unable to infect P. aeruginosa host. Nevertheless, the accompanying decrease in PFU titre (neutralization) was only observed for myovirus KT28 but not podovirus LUZ7. Next, we verified whether OMVs derived from wild-type PAO1 strain can sensitize the LPS-deficient mutant (Δwbpl PAO1) resistant to tested phages. The flow cytometry experiments proved a quite effective and comparable association of OMVs to Δwbpl PAO1 and wild-type PAO1; however, the growth kinetic curves and one-step growth assay revealed no sensitization event of the OMV-associated phage-resistant P. aeruginosa deletant to LPS-specific phages. Our findings for the first time identify naturally formed OMVs as important players in passive resistance (protection) of P. aeruginosa population to phages, but we disproved the hypothesis of transferring phage receptors to make resistant strains susceptible to LPS-dependent phages.

Potential Therapeutic Targets for Combination Antibody Therapy against Pseudomonas aeruginosa Infections

Despite advances in antimicrobial therapy and even the advent of some effective vaccines, Pseudomonas aeruginosa (P. aeruginosa) remains a significant cause of infectious disease, primarily due to antibiotic resistance. Although P. aeruginosa is commonly treatable with readily available therapeutics, these therapies are not always efficacious, particularly for certain classes of patients (e.g., cystic fibrosis (CF)) and for drug-resistant strains. Multi-drug resistant P. aeruginosa infections are listed on both the CDC’s and WHO’s list of serious worldwide threats. This increasing emergence of drug resistance and prevalence of P. aeruginosa highlights the need to identify new therapeutic strategies. Combinations of monoclonal antibodies against different targets and epitopes have demonstrated synergistic efficacy with each other as well as in combination with antimicrobial agents typically used to treat these infections. Such a strategy has reduced the ability of infectious agents to develop resistance. This manuscript details the development of potential therapeutic targets for polyclonal antibody therapies to combat the emergence of multidrug-resistant P. aeruginosa infections. In particular, potential drug targets for combinational immunotherapy against P. aeruginosa are identified to combat current and future drug resistance.

Francisella tularensis Outer Membrane Vesicles Participate in the Early Phase of Interaction With Macrophages

Francisella tularensis is known to release unusually shaped tubular outer membrane vesicles (OMV) containing a number of previously identified virulence factors and immunomodulatory proteins. In this study, we present that OMV isolated from the F. tularensis subsp. holarctica strain FSC200 enter readily into primary bone marrow-derived macrophages (BMDM) and seem to reside in structures resembling late endosomes in the later intervals. The isolated OMV enter BMDM generally via macropinocytosis and clathrin-dependent endocytosis, with a minor role played by lipid raft-dependent endocytosis. OMVs proved to be non-toxic and had no negative impact on the viability of BMDM. Unlike the parent bacterium itself, isolated OMV induced massive and dose-dependent proinflammatory responses in BMDM. Using transmission electron microscopy, we also evaluated OMV release from the bacterial surface during several stages of the interaction of Francisella with BMDM. During adherence and the early phase of the uptake of bacteria, we observed numerous tubular OMV-like protrusions bulging from the bacteria in close proximity to the macrophage plasma membrane. This suggests a possible role of OMV in the entry of bacteria into host cells. On the contrary, the OMV release from the bacterial surface during its cytosolic phase was negligible. We propose that OMV play some role in the extracellular phase of the interaction of Francisella with the host and that they are involved in the entry mechanism of the bacteria into macrophages.

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).

Corneal Infection Models: Tools to Investigate the Role of Biofilms in Bacterial Keratitis

Bacterial keratitis is a corneal infection which may cause visual impairment or even loss of the infected eye. It remains a major cause of blindness in the developing world. Staphylococcus aureus and Pseudomonas aeruginosa are common causative agents and these bacterial species are known to colonise the corneal surface as biofilm populations. Biofilms are complex bacterial communities encased in an extracellular polymeric matrix and are notoriously difficult to eradicate once established. Biofilm bacteria exhibit different phenotypic characteristics from their planktonic counterparts, including an increased resistance to antibiotics and the host immune response. Therefore, understanding the role of biofilms will be essential in the development of new ophthalmic antimicrobials. A brief overview of biofilm-specific resistance mechanisms is provided, but this is a highly multifactorial and rapidly expanding field that warrants further research. Progression in this field is dependent on the development of suitable biofilm models that acknowledge the complexity of the ocular environment. Abiotic models of biofilm formation (where biofilms are studied on non-living surfaces) currently dominate the literature, but co-culture infection models are beginning to emerge. In vitro, ex vivo and in vivo corneal infection models have now been reported which use a variety of different experimental techniques and animal models. In this review, we will discuss existing corneal infection models and their application in the study of biofilms and host-pathogen interactions at the corneal surface.

Contact lens-related corneal infection: Intrinsic resistance and its compromise

Contact lenses represent a widely utilized form of vision correction with more than 140 million wearers worldwide. Although generally well-tolerated, contact lenses can cause corneal infection (microbial keratitis), with an approximate annualized incidence ranging from ~2 to ~20 cases per 10,000 wearers, and sometimes resulting in permanent vision loss. Research suggests that the pathogenesis of contact lens-associated microbial keratitis is complex and multifactorial, likely requiring multiple conspiring factors that compromise the intrinsic resistance of a healthy cornea to infection. Here, we outline our perspective of the mechanisms by which contact lens wear sometimes renders the cornea susceptible to infection, focusing primarily on our own research efforts during the past three decades. This has included studies of host factors underlying the constitutive barrier function of the healthy cornea, its response to bacterial challenge when intrinsic resistance is not compromised, pathogen virulence mechanisms, and the effects of contact lens wear that alter the outcome of host-microbe interactions. For almost all of this work, we have utilized the bacterium Pseudomonas aeruginosa because it is the leading cause of lens-related microbial keratitis. While not yet common among corneal isolates, clinical isolates of P. aeruginosa have emerged that are resistant to virtually all currently available antibiotics, leading the United States CDC (Centers for Disease Control) to add P. aeruginosa to its list of most serious threats. Compounding this concern, the development of advanced contact lenses for biosensing and augmented reality, together with the escalating incidence of myopia, could portent an epidemic of vision-threatening corneal infections in the future. Thankfully, technological advances in genomics, proteomics, metabolomics and imaging combined with emerging models of contact lens-associated P. aeruginosa infection hold promise for solving the problem - and possibly life-threatening infections impacting other tissues.

Cystic Fibrosis and Pseudomonas aeruginosa: the Host-Microbe Interface

In human pathophysiology, the clash between microbial infection and host immunity contributes to multiple diseases. Cystic fibrosis (CF) is a classical example of this phenomenon, wherein a dysfunctional, hyperinflammatory immune response combined with chronic pulmonary infections wreak havoc upon the airway, leading to a disease course of substantial morbidity and shortened life span. Pseudomonas aeruginosa is an opportunistic pathogen that commonly infects the CF lung, promoting an accelerated decline of pulmonary function. Importantly, P. aeruginosa exhibits significant resistance to innate immune effectors and to antibiotics, in part, by expressing specific virulence factors (e.g., antioxidants and exopolysaccharides) and by acquiring adaptive mutations during chronic infection. In an effort to review our current understanding of the host-pathogen interface driving CF pulmonary disease, we discuss (i) the progression of disease within the primitive CF lung, specifically focusing on the role of host versus bacterial factors; (ii) critical, neutrophil-derived innate immune effectors that are implicated in CF pulmonary disease, including reactive oxygen species (ROS) and antimicrobial peptides (e.g., LL-37); (iii) P. aeruginosa virulence factors and adaptive mutations that enable evasion of the host response; and (iv) ongoing work examining the distribution and colocalization of host and bacterial factors within distinct anatomical niches of the CF lung.

Chemoorganotrophic Bacteria From Lake Fryxell, Antarctica, Including Pseudomonas Strain LFY10, a Cold-Adapted, Halotolerant Bacterium Useful in Teaching Labs

Lake Fryxell, situated in the McMurdo Dry Valleys of Antarctica, is an intriguing aquatic ecosystem because of its perennial ice cover, highly stratified water column, and extreme physicochemical conditions, which collectively restrict lake biodiversity to solely microbial forms. To expand our current understanding of the cultivable biodiversity of Lake Fryxell, water samples were collected from depths of 10 and 17 m, and pure cultures of eight diverse strains of aerobic, chemoorganotrophic bacteria were obtained. Despite having high 16S rRNA gene sequence similarity to mesophilic bacteria inhabiting various temperate environments, all Lake Fryxell isolates were psychrotolerant, with growth occurring at 0°C and optimal growth from 18–24°C for all isolates. Phylogenetic analyses showed the isolates to be members of six taxonomic groups, including the genera Brevundimonas, Arthrobacter, Sphingobium, Leifsonia, and Pseudomonas, as well as the family Microbacteriaceae (one strain could not reliably be assigned to a specific genus based on our analysis). Pseudomonas strain LFY10 stood out as a useful tool for teaching laboratory activities because of its substantial cold adaptation (visible growth is evident in 1–2 days at 4°C), beta-hemolytic activity, and halotolerance to 8.5% (w/v) NaCl. These cold-adapted bacteria likely play a role in carbon mineralization and other nutrient cycling in Lake Fryxell, and their characterization broadens our understanding of microbial biodiversity in aquatic polar ecosystems.

Bacterial outer membrane vesicles, a potential vaccine candidate in interactions with host cells based

Both Gram-Positive and Gram-Negative bacteria can secrete outer membrane vesicles (OMVs) in their growth and metabolism process. Originally, OMVs were considered as a by-product of bacterial merisis. However, many scientists have reported the important role of OMVs in many fields recently. In this review, we briefly introduce OMVs biological functions and then summarize the findings about the OMVs interactions with host cells. At last, we will make an expectation about the prospects of the application of OMVs as vaccines.

A novel murine model for contact lens wear reveals clandestine IL-1R dependent corneal parainflammation and susceptibility to microbial keratitis upon inoculation with Pseudomonas aeruginosa

PURPOSE

Contact lens wear carries a risk of complications, including corneal infection. Solving these complications has been hindered by limitations of existing animal models. Here, we report development of a new murine model of contact lens wear.

METHODS

C57BL/6 mice were fitted with custom-made silicone-hydrogel contact lenses with or without prior inoculation with Pseudomonas aeruginosa (PAO1-GFP). Contralateral eyes served as controls. Corneas were monitored for pathology, and examined ex vivo using high-magnification, time-lapse imaging. Fluorescent reporter mice allowed visualization of host cell membranes and immune cells. Lens-colonizing bacteria were detected by viable counts and FISH. Direct-colony PCR was used for bacterial identification.

RESULTS

Without deliberate inoculation, lens-wearing corneas remained free of visible pathology, and retained a clarity similar to non-lens wearing controls. CD11c-YFP reporter mice revealed altered numbers, and distribution, of CD11c-positive cells in lens-wearing corneas after 24 h. Worn lenses showed bacterial colonization, primarily by known conjunctival or skin commensals. Corneal epithelial cells showed vacuolization during lens wear, and after 5 days, cells with phagocyte morphology appeared in the stroma that actively migrated over resident keratocytes that showed altered morphology. Immunofluorescence confirmed stromal Ly6G-positive cells after 5 days of lens wear, but not in MyD88 or IL-1R gene-knockout mice. P. aeruginosa-contaminated lenses caused infectious pathology in most mice from 1 to 13 days.

CONCLUSIONS

This murine model of contact lens wear appears to faithfully mimic events occurring during human lens wear, and could be valuable for experiments, not possible in humans, that help solve the pathogenesis of lens-related complications.

Vesicular Delivery of the Antifungal Antibiotics of Lysobacter enzymogenes C3

Lysobacter enzymogenes C3 is a predatory strain of Gram-negative gliding bacteria that produces antifungal antibiotics by the polyketide synthetic pathway. Outer membrane vesicles (OMV) are formed as a stress response and can deliver virulence factors to host cells. The production of OMV by C3 and their role in antifungal activity are reported here. Vesicles in the range of 130 to 150 nm in diameter were discovered in the cell-free supernatants of C3 cultures. These OMV contain molecules characteristic of bacterial outer membranes, such as lipopolysaccharide and phospholipids. In addition, they contain chitinase activity and essentially all of the heat-stable antifungal activity in cell supernatants. We show here that C3 OMV can directly inhibit growth of the yeast Saccharomyces cerevisiae as well as that of the filamentous fungus Fusarium subglutinans The activity is dependent on physical contact between OMV and the cells. Furthermore, fluorescent lipid labeling of C3 OMV demonstrated transfer of the membrane-associated probe to yeast cells, suggesting the existence of a mechanism of delivery for membrane-associated molecules. Mass spectrometric analysis of C3 OMV extracts indicates the presence of molecules with molecular weights identical to some of the previously identified antifungal products of C3. These data together suggest that OMV act as an important remote mobile component of predation by Lysobacter IMPORTANCE The data presented here suggest a newly discovered function of outer membrane vesicles (OMV) that are produced from the outer membrane of the bacterial species Lysobacter enzymogenes strain C3. We show that these OMV can be released from the surface of the cells to deliver antibiotics to target fungal organisms as a mechanism of killing or growth inhibition. Understanding the role of OMV in antibiotic delivery can generally lead to improved strategies for dealing with antibiotic-resistant organisms. These results also add to the evidence that some bacterially produced antibiotics can be discovered and purified using methods designed for isolation of nanoscale vesicles. Information on these systems can lead to better identification of active molecules or design of delivery vehicles for these molecules.

Immunization with Pseudomonas aeruginosa outer membrane vesicles stimulates protective immunity in mice

Pseudomonas aeruginosa is an opportunistic pathogen responsible for a wide range of severe nosocomial and community acquired infections, these infections are major health problems for cystic fibrosis patients and immune-compromised individuals. The emergence of multidrug-resistant isolates highlights the need to develop alternative strategies for treatment of P. aeruginosa infections. Outer membrane vesicles (OMVs) are spherical nanometer-sized proteolipids that are secreted from numerous of pathogenic Gram-negative bacteria, and a number of studies have confirmed the protective efficacy for use of OMVs as candidate vaccines. In this study, OMVs from P. aeruginosa (PA_OMVs) were isolated, formulated with aluminum phosphate adjuvant and used as a vaccine in a mouse model of acute lung infection. The results confirmed that active immunization with PA_OMVs was able to reduce bacterial colonization, cytokine secretion and tissue damage in the lung tissue, thus protecting mice from lethal challenge of P. aeruginosa. Cytokines assay validated that immunization with PA_OMVs was efficient to induce a mixed cellular immune response in mice. Further, high level of specific antibodies was detected in mice immunized with PA_OMVs, and results from opsonophagocytic killing assay and passive immunization suggested that humoral immune response may be critical for PA_OMVs mediated protection. These findings demonstrated that PA_OMVs may be served as a novel candidate vaccine for the prevention of P. aeruginosa infection.

The bile salt sodium taurocholate induces Campylobacter jejuni outer membrane vesicle production and increases OMV-associated proteolytic activity

Campylobacter jejuni, the leading cause of bacterial acute gastroenteritis worldwide, secretes an arsenal of virulence-associated proteins within outer membrane vesicles (OMVs). C. jejuni OMVs contain three serine proteases (HtrA, Cj0511, and Cj1365c) that cleave the intestinal epithelial cell (IEC) tight and adherens junction proteins occludin and E-cadherin, promoting enhanced C. jejuni adhesion to and invasion of IECs. C. jejuni OMVs also induce IECs innate immune responses. The bile salt sodium taurocholate (ST) is sensed as a host signal to coordinate the activation of virulence-associated genes in the enteric pathogen Vibrio cholerae. In this study, the effect of ST on C. jejuni OMVs was investigated. Physiological concentrations of ST do not have an inhibitory effect on C. jejuni growth until the early stationary phase. Coculture of C. jejuni with 0.1% or 0.2% (w/v) ST stimulates OMV production, increasing both lipid and protein concentrations. C. jejuni ST-OMVs possess increased proteolytic activity and exhibit a different protein profile compared to OMVs isolated in the absence of ST. ST-OMVs exhibit enhanced cytotoxicity and immunogenicity to T84 IECs and enhanced killing of Galleria mellonella larvae. ST increases the level of mRNA transcripts of the OMVs-associated serine protease genes and the cdtABC operon that encodes the cytolethal distending toxin. Coculture with ST significantly enhances the OMVs-induced cleavage of E-cadherin and occludin. C. jejuni OMVs also cleave the major endoplasmic reticulum chaperone protein BiP/GRP78 and this activity is associated with the Cj1365c protease. These data suggest that C. jejuni responds to the presence of physiological concentrations of the bile salt ST that increases OMV production and the synthesis of virulence-associated factors that are secreted within the OMVs. We propose that these events contribute to pathogenesis.

Bacterial outer membrane vesicles: New insights and applications

Outer membrane vesicles (OMVs) (∼50-250 nm in diameter) are produced by both pathogenic and nonpathogenic bacteria as a canonical end product of secretion. In this review, we focus on the OMVs produced by gram-negative bacteria. We provide an overview of the OMV structure, various factors regulating their production, and their role in modulating host immune response using a few representative examples. In light of the importance of the diverse cargoes carried by OMVs, we discuss the different modes of their entry into the host cell and advances in the high-throughput detection of these OMVs. A conspicuous application of OMVs lies in the field of vaccination; we discuss its success in immunization against human diseases such as pertussis, meningitis, shigellosis and aqua-farming endangering diseases like edwardsiellosis.

Contributions of MyD88-dependent receptors and CD11c-positive cells to corneal epithelial barrier function against Pseudomonas aeruginosa

Previously we reported that corneal epithelial barrier function against Pseudomonas aeruginosa was MyD88-dependent. Here, we explored contributions of MyD88-dependent receptors using vital mouse eyes and confocal imaging. Uninjured IL-1R (−/−) or TLR4 (−/−) corneas, but not TLR2 (−/−), TLR5 (−/−), TLR7 (−/−), or TLR9 (−/−), were more susceptible to P. aeruginosa adhesion than wild-type (3.8-fold, 3.6-fold respectively). Bacteria adherent to the corneas of IL-1R (−/−) or TLR5 (−/−) mice penetrated beyond the epithelial surface only if the cornea was superficially-injured. Bone marrow chimeras showed that bone marrow-derived cells contributed to IL-1R-dependent barrier function. In vivo, but not ex vivo, stromal CD11c+ cells responded to bacterial challenge even when corneas were uninjured. These cells extended processes toward the epithelial surface, and co-localized with adherent bacteria in superficially-injured corneas. While CD11c+ cell depletion reduced IL-6, IL-1β, CXCL1, CXCL2 and CXCL10 transcriptional responses to bacteria, and increased susceptibility to bacterial adhesion (>3-fold), the epithelium remained resistant to bacterial penetration. IL-1R (−/−) corneas also showed down-regulation of IL-6 and CXCL1 genes with and without bacterial challenge. These data show complex roles for TLR4, TLR5, IL-1R and CD11c+ cells in constitutive epithelial barrier function against P. aeruginosa, with details dependent upon in vivo conditions.

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

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

Outer membrane vesicle-associated lipase FtlA enhances cellular invasion and virulence in Francisella tularensis LVS

Francisella tularensis is a highly infectious intracellular pathogen that infects a wide range of host species and causes fatal pneumonic tularemia in humans. ftlA was identified as a potential virulence determinant of the F. tularensis live vaccine strain (LVS) in our previous transposon screen, but its function remained undefined. Here, we show that an unmarked deletion mutant of ftlA was avirulent in a pneumonia mouse model with a severely impaired capacity to infect host cells. Consistent with its sequence homology with GDSL lipase/esterase family proteins, the FtlA protein displayed lipolytic activity in both E. coli and F. tularensis with a preference for relatively short carbon-chain substrates. FtlA thus represents the first F. tularensis lipase to promote bacterial infection of host cells and in vivo fitness. As a cytoplasmic protein, we found that FtlA was secreted into the extracellular environment as a component of outer membrane vesicles (OMVs). Further confocal microscopy analysis revealed that the FtlA-containing OMVs isolated from F. tularensis LVS attached to the host cell membrane. Finally, the OMV-associated FtlA protein complemented the genetic deficiency of the ΔftlA mutant in terms of host cell infection when OMVs purified from the parent strain were co-incubated with the mutant bacteria. These lines of evidence strongly suggest that the FtlA lipase promotes F. tularensis adhesion and internalization by modifying bacterial and/or host molecule(s) when it is secreted as a component of OMVs.

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

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

Environmentally controlled bacterial vesicle-mediated export

Over the past two decades, researchers studying both microbial and host cell communities have gained an appreciation for the ability of bacteria to produce, regulate, and functionally utilize outer membrane vesicles (OMVs) as a means to survive and interact with their cellular and acellular environments. Common ground has emerged, as it appears that vesicle production is an environmentally controlled and specific secretion process; however, it has been challenging to discover the principles that govern fundamentals of vesicle-mediated transport. Namely, there does not appear to be a single mechanism modulating OMV export, nor universal "markers" for OMV cargo incorporation, nor particular host cell responses common to treatment with all OMVs. Given the diversity of species studied, their differences in envelope architecture and composition, the diversity of environmentally regulated bacterial processes, and the variety of interactions between bacteria and their abiotic and biotic environments, this is hardly surprising. Nevertheless, the ability of bacteria to control exported material in the context of a packaged insoluble particle, a vesicle, is emerging as a significant contribution to bacterial viability, biofilm communities, and bacterial-host interactions. In this review, we focus on detailing important, recent findings regarding the content and functional differences in bacterially secreted vesicles that are influenced by growth conditions.

A Two-Component Regulatory System Impacts Extracellular Membrane-Derived Vesicle Production in Group A Streptococcus

Export of macromolecules via extracellular membrane-derived vesicles (MVs) plays an important role in the biology of Gram-negative bacteria. Gram-positive bacteria have also recently been reported to produce MVs; however, the composition and mechanisms governing vesiculogenesis in Gram-positive bacteria remain undefined. Here, we describe MV production in the Gram-positive human pathogen group A streptococcus (GAS), the etiological agent of necrotizing fasciitis and streptococcal toxic shock syndrome. M1 serotype GAS isolates in culture exhibit MV structures both on the cell wall surface and in the near vicinity of bacterial cells. A comprehensive analysis of MV proteins identified both virulence-associated protein substrates of the general secretory pathway in addition to "anchorless surface proteins." Characteristic differences in the contents, distributions, and fatty acid compositions of specific lipids between MVs and GAS cell membrane were also observed. Furthermore, deep RNA sequencing of vesicular RNAs revealed that GAS MVs contained differentially abundant RNA species relative to bacterial cellular RNA. MV production by GAS strains varied in a manner dependent on an intact two-component system, CovRS, with MV production negatively regulated by the system. Modulation of MV production through CovRS was found to be independent of both GAS cysteine protease SpeB and capsule biosynthesis. Our data provide an explanation for GAS secretion of macromolecules, including RNAs, lipids, and proteins, and illustrate a regulatory mechanism coordinating this secretory response.

IMPORTANCE

Group A streptococcus (GAS) is a Gram-positive bacterial pathogen responsible for more than 500,000 deaths annually. Establishment of GAS infection is dependent on a suite of proteins exported via the general secretory pathway. Here, we show that GAS naturally produces extracellular vesicles with a unique lipid composition that are laden with proteins and RNAs. Interestingly, both virulence-associated proteins and RNA species were found to be differentially abundant in vesicles relative to the bacteria. Furthermore, we show that genetic disruption of the virulence-associated two-component regulator CovRS leads to an increase in vesicle production. This study comprehensively describes the protein, RNA, and lipid composition of GAS-secreted MVs and alludes to a regulatory system impacting this process.

Human Tear Fluid Reduces Culturability of Contact Lens-Associated Pseudomonas aeruginosa Biofilms but Induces Expression of the Virulence-Associated Type III Secretion System

PURPOSE

The type III secretion system (T3SS) is a significant virulence determinant for Pseudomonas aeruginosa. Using a rodent model, we found that contact lens (CL)-related corneal infections were associated with lens surface biofilms. Here, we studied the impact of human tear fluid on CL-associated biofilm growth and T3SS expression.

METHODS

P. aeruginosa biofilms were formed on contact lenses for up to 7 days with or without human tear fluid, then exposed to tear fluid for 5 or 24 h. Biofilms were imaged using confocal microscopy. Bacterial culturability was quantified by viable counts, and T3SS gene expression measured by RT-qPCR. Controls included trypticase soy broth, PBS and planktonic bacteria.

RESULTS

With or without tear fluid, biofilms grew to ∼10⁸ CFU viable bacteria by 24 h. Exposing biofilms to tear fluid after they had formed without it on lenses reduced bacterial culturability ∼180-fold (P<.001). CL growth increased T3SS gene expression versus planktonic bacteria [5.46 ± 0.24-fold for T3SS transcriptional activitor exsA (P=.02), and 3.76 ± 0.36-fold for T3SS effector toxin exoS (P=.01)]. Tear fluid further enhanced exsA and exoS expression in CL-grown biofilms, but not planktonic bacteria, by 2.09 ± 0.38-fold (P=.04) and 1.89 ± 0.26-fold (P<.001), respectively.

CONCLUSIONS

Considering the pivitol role of the T3SS in P. aeruginosa infections, its induction in CL-grown P. aeruginosa biofilms by tear fluid might contribute to the pathogenesis of CL-related P. aeruginosa keratitis.