The ADP-ribosylation domain of Pseudomonas aeruginosa ExoS is required for membrane bleb niche formation and bacterial survival within epithelial cells

Pseudomonas aeruginosa infection increases palmitoyl carnitine release by host-derived extracellular vesicles

Pseudomonas aeruginosa (PA), an opportunistic gram-negative pathogen, is the most common pathogen identified in all culture positive cases of infectious keratitis. Extracellular vesicles (EVs) are released by most cells in the body and function in intercellular communication. We have previously reported a change in the proteome of host-derived EVs from corneal epithelial cells during PA infection. In the present study, we investigated changes in the metabolome of host-derived EVs from PA infected (PA-C EVs) and non-infected cells (C EVs). We found that one metabolite, palmitoyl carnitine (PAMC), was significantly upregulated in PA-C EVs. To determine the significance of PAMC release, we investigated the effect of PAMC treatment on corneal epithelial cells and neutrophils. EVs were isolated from culture media using size exclusion chromatography. EVs were then characterized using nanoparticle tracking analysis, transmission electron microscopy, and western blot. Metabolomics was performed using an untargeted approach. We found that palmitoyl carnitine (PAMC) was the most abundant metabolite present in PA-C EVs and was increased more than 3 fold compared to C EVs. Treatment of corneal epithelial cells with increasing levels of PAMC increased nuclear translocation of the NF-κB subunit p65. This was associated with an increase in IL-8 production and neutrophil migration. PAMC also increased levels of mitochondrial calcium. Upon inoculation of corneal epithelial cells with PA, 50 μM PAMC completely eradicated intracellular PA, but stimulated growth of extracellular PA. Taken together, these findings suggest that PA exploits EV release by host cells to deplete PAMC from the intracellular environment.

Pseudomonas aeruginosa impairs mitochondrial function and metabolism during infection of corneal epithelial cells

Pseudomonas aeruginosa (PA) is a gram-negative opportunistic pathogen that can infect the cornea as a result of trauma or contact lens wear. In addition to their known energy producing role, mitochondria are important mediators of immune signaling and host defense. While certain pathogens have developed strategies to evade host defenses by modulating host mitochondrial dynamics and metabolism, the ability of PA to harness host cell mitochondria during corneal infection is unknown. Using a combination of biochemical and imaging techniques, we show that PA infection of corneal epithelial cells induced mitochondrial fission in a DRP1-dependent manner that preceded PINK1/Parkin and FUNDC1-mediated mitophagy. PA also impaired NADH-linked respiration through a reduction in complex 1. This corresponded to a decrease in metabolic pathways related to glycolysis and the TCA cycle. Metabolomics analysis further demonstrated an upregulation of the pentose phosphate pathway, arginine, purine, and pyrimidine metabolism in PA infected cells. These pathways may provide a key source of nucleotides, amino acids, and nitrogen for both the host cell and PA, in addition to antioxidant functions. Following treatment with gentamicin to kill all extracellular bacteria, metabolic flux analysis showed that corneal epithelial cells were able to restore mitochondrial function despite the continued presence of intracellular PA. Taken together, these data demonstrate that mitochondrial dysfunction and metabolic rewiring in host cells is triggered by extracellular PA, but once inside, PA requires healthy mitochondria to ensure host cell survival.

Evidence for intracellular Pseudomonas aeruginosa

Pseudomonas aeruginosa is a significant cause of global morbidity and mortality. Although it is often regarded as an extracellular pathogen toward human cells, numerous investigations report its ability to survive and replicate within host cells, and additional studies demonstrate specific mechanisms enabling it to adopt an intracellular lifestyle. This ability of P. aeruginosa remains less well-investigated than that of other intracellular bacteria, although it is currently gaining attention. If intracellular bacteria are not killed after entering host cells, they may instead receive protection from immune recognition and experience reduced exposure to antibiotic therapy, among additional potential advantages shared with other facultative intracellular pathogens. For this review, we compiled studies that observe intracellular P. aeruginosa across strains, cell types, and experimental systems in vitro, as well as contextualize these findings with the few studies that report similar observations in vivo. We also seek to address key findings that drove the perception that P. aeruginosa remains extracellular in order to reconcile what is currently understood about intracellular pathogenesis and highlight open questions regarding its contribution to disease.

Intracellular replication of Pseudomonas aeruginosa in epithelial cells requires suppression of the caspase-4 inflammasome

Pathogenesis of Pseudomonas aeruginosa infections can include bacterial survival inside epithelial cells. Previously, we showed that this involves multiple roles played by the type three secretion system (T3SS), and specifically the effector ExoS. This includes ExoS-dependent inhibition of a lytic host cell response that subsequently enables intracellular replication. Here, we studied the underlying cell death response to intracellular P. aeruginosa, comparing wild-type to T3SS mutants varying in capacity to induce cell death and that localize to different intracellular compartments. Results showed that corneal epithelial cell death induced by intracellular P. aeruginosa lacking the T3SS, which remains in vacuoles, correlated with the activation of nuclear factor-κB as measured by p65 relocalization and tumor necrosis factor alpha transcription and secretion. Deletion of caspase-4 through CRISPR-Cas9 mutagenesis delayed cell death caused by these intracellular T3SS mutants. Caspase-4 deletion also countered more rapid cell death caused by T3SS effector-null mutants still expressing the T3SS apparatus that traffic to the host cell cytoplasm, and in doing so rescued intracellular replication normally dependent on ExoS. While HeLa cells lacked a lytic death response to T3SS mutants, it was found to be enabled by interferon gamma treatment. Together, these results show that epithelial cells can activate the noncanonical inflammasome pathway to limit proliferation of intracellular P. aeruginosa, not fully dependent on bacterially driven vacuole escape. Since ExoS inhibits the lytic response, the data implicate targeting of caspase-4, an intracellular pattern recognition receptor, as another contributor to the role of ExoS in the intracellular lifestyle of P. aeruginosa. IMPORTANCE Pseudomonas aeruginosa can exhibit an intracellular lifestyle within epithelial cells in vivo and in vitro. The type three secretion system (T3SS) effector ExoS contributes via multiple mechanisms, including extending the life of invaded host cells. Here, we aimed to understand the underlying cell death inhibited by ExoS when P. aeruginosa is intracellular. Results showed that intracellular P. aeruginosa lacking T3SS effectors could elicit rapid cell lysis via the noncanonical inflammasome pathway. Caspase-4 contributed to cell lysis even when the intracellular bacteria lacked the entire T33S and were consequently unable to escape vacuoles, representing a naturally occurring subpopulation during wild-type infection. Together, the data show the caspase-4 inflammasome as an epithelial cell defense against intracellular P. aeruginosa, and implicate its targeting as another mechanism by which ExoS preserves the host cell replicative niche.

Intracellular replication of Pseudomonas aeruginosa in epithelial cells requires suppression of the caspase-4 inflammasome

Pathogenesis of Pseudomonas aeruginosa infections can include bacterial survival inside epithelial cells. Previously, we showed this involves multiple roles played by the type three-secretion system (T3SS), and specifically the effector ExoS. This includes ExoS-dependent inhibition of a lytic host cell response that subsequently enables intracellular replication. Here, we studied the underlying cell death response to intracellular P. aeruginosa, comparing wild-type to T3SS mutants varying in capacity to induce cell death and that localize to different intracellular compartments. Results showed that corneal epithelial cell death induced by intracellular P. aeruginosa lacking the T3SS, which remains in vacuoles, correlated with activation of NF-κB as measured by p65 relocalization and TNFα transcription and secretion. Deletion of caspase-4 through CRISPR-Cas9 mutagenesis delayed cell death caused by these intracellular T3SS mutants. Caspase-4 deletion also countered more rapid cell death caused by T3SS effector-null mutants still expressing the TSSS apparatus that traffic to the host cell cytoplasm, and in doing so rescued intracellular replication normally dependent on ExoS. While HeLa cells lacked a lytic death response to T3SS mutants, it was found to be enabled by interferon gamma treatment. Together, these results show that epithelial cells can activate the noncanonical inflammasome pathway to limit proliferation of intracellular P. aeruginosa, not fully dependent on bacterially-driven vacuole escape. Since ExoS inhibits the lytic response, the data implicate targeting of caspase-4, an intracellular pattern recognition receptor, as another contributor to the role of ExoS in the intracellular lifestyle of P. aeruginosa.

Host-microbe interactions in cornea

Corneal infections result through interaction between microbes and host innate immune receptors. Damage to the cornea occurs as a result of microbial virulence factors and is often exacerbated by lack of a controlled host immune response; the latter contributing to bystander damage to corneal structure. Understanding mechanisms involved in host microbial interactions is critical to development of novel therapeutic targets, ultimate control of microbial pathogenesis, and restoration of tissue homeostasis. Studies on these interactions continue to provide exciting findings directly related to this ultimate goal.

Pseudomonas aeruginosa Can Diversify after Host Cell Invasion to Establish Multiple Intracellular Niches

Within epithelial cells, Pseudomonas aeruginosa depends on its type III secretion system (T3SS) to escape vacuoles and replicate rapidly in the cytosol. Previously, it was assumed that intracellular subpopulations remaining T3SS-negative (and therefore in vacuoles) were destined for degradation in lysosomes, supported by data showing vacuole acidification. Here, we report in both corneal and bronchial human epithelial cells that vacuole-associated bacteria can persist, sometimes in the same cells as cytosolic bacteria. Using a combination of phase-contrast, confocal, and correlative light-electron microscopy (CLEM), we also found they can demonstrate biofilm-associated markers: cdrA and cyclic-di-GMP (c-di-GMP). Vacuolar-associated bacteria, but not their cytosolic counterparts, tolerated the cell-permeable antibiotic ofloxacin. Surprisingly, use of mutants showed that both persistence in vacuoles and ofloxacin tolerance were independent of the biofilm-associated protein CdrA or exopolysaccharides (Psl, Pel, alginate). A T3SS mutant (ΔexsA) unable to escape vacuoles phenocopied vacuole-associated subpopulations in wild-type PAO1-infected cells, with results revealing that epithelial cell death depended upon bacterial viability. Intravital confocal imaging of infected mouse corneas confirmed that P. aeruginosa formed similar intracellular subpopulations within epithelial cells in vivo. Together, these results show that P. aeruginosa differs from other pathogens by diversifying intracellularly into vacuolar and cytosolic subpopulations that both contribute to pathogenesis. Their different gene expression and behavior (e.g., rapid replication versus slow replication/persistence) suggest cooperation favoring both short- and long-term interests and another potential pathway to treatment failure. How this intracellular diversification relates to previously described "acute versus chronic" virulence gene-expression phenotypes of P. aeruginosa remains to be determined. IMPORTANCE Pseudomonas aeruginosa can cause sight- and life-threatening opportunistic infections, and its evolving antibiotic resistance is a growing concern. Most P. aeruginosa strains can invade host cells, presenting a challenge to therapies that do not penetrate host cell membranes. Previously, we showed that the P. aeruginosa type III secretion system (T3SS) plays a pivotal role in survival within epithelial cells, allowing escape from vacuoles, rapid replication in the cytoplasm, and suppression of host cell death. Here, we report the discovery of a novel T3SS-negative subpopulation of intracellular P. aeruginosa within epithelial cells that persist in vacuoles rather than the cytoplasm and that tolerate a cell-permeable antibiotic (ofloxacin) that is able to kill cytosolic bacteria. Classical biofilm-associated markers, although demonstrated by this subpopulation, are not required for vacuolar persistence or antibiotic tolerance. These findings advance our understanding of how P. aeruginosa hijacks host cells, showing that it diversifies into multiple populations with T3SS-negative members enabling persistence while rapid replication is accomplished by more vulnerable T3SS-positive siblings. Intracellular P. aeruginosa persisting and tolerating antibiotics independently of the T3SS or biofilm-associated factors could present additional challenges to development of more effective therapeutics.

Autophagy in the normal and diseased cornea

The cornea and covering tear film are together the 'objective lens' of the eye through which 80% of light is refracted. Despite exposure to a physically harsh and at times infectious or toxic environment, transparency essential for sight is in most cases maintained. Such resiliency makes the avascular cornea a superb model for the exploration of autophagy in the regulation of homeostasis with relevancy to all organs. Nonetheless, missense mutations and inflammation respectively clog or apparently overwhelm autophagic flux to create dystrophies much like in neurodegenerative diseases or further exacerbate inflammation. Here there is opportunity to generate novel topical therapies towards the restoration of homeostasis with potential broad application.

Modulation of the immune response by the Pseudomonas aeruginosa type-III secretion system

Pseudomonas aeruginosa is an opportunistic pathogen that can cause critical cellular damage and subvert the immune response to promote its survival. Among the numerous virulence factors of P. aeruginosa, the type III secretion system (T3SS) is involved in host cell pathogenicity. Using a needle-like structure, T3SS detects eukaryotic cells and injects toxins directly into their cytosol, thus highlighting its ability to interfere with the host immune response. In this mini-review, we discuss how the T3SS and bacterial effectors secreted by this pathway not only activate the immune response but can also manipulate it to promote the establishment of P. aeruginosa infections.

A Model of Intracellular Persistence of Pseudomonas aeruginosa in Airway Epithelial Cells

Pseudomonas aeruginosa (P.a.) is a major human pathogen capable of causing chronic infections in hosts with weakened barrier functions and host defenses, most notably airway infections commonly observed in individuals with the genetic disorder cystic fibrosis (CF). While mainly described as an extracellular pathogen, previous in vitro studies have described the molecular events leading to P.a. internalization in diverse epithelial cell types. However, the long-term fate of intracellular P.a. remains largely unknown. Here, we developed a model allowing for a better understanding of long-term (up to 120 h) intracellular bacterial survival in the airway epithelial cell line BEAS-2B. Using a tobramycin protection assay, we characterized the internalization, long-term intracellular survival, and cytotoxicity of the lab strain PAO1, as well as clinical CF isolates, and conducted analyses at the single-cell level using confocal microscopy and flow cytometry techniques. We observed that infection at low multiplicity of infection allows for intracellular survival up to 120 h post-infection without causing significant host cytotoxicity. Finally, infection with clinical isolates revealed significant strain-to-strain heterogeneity in intracellular survival, including a high persistence phenotype associated with bacterial replication within host cells. Future studies using this model will further elucidate the host and bacterial mechanisms that promote P. aeruginosa intracellular persistence in airway epithelial cells, a potentially unrecognized bacterial reservoir during chronic infections.

Epidemiology, Microbiology, and Genetics of Contact Lens-Related and Non-Contact Lens-Related Infectious Keratitis

ABSTRACT

Infectious keratitis is a rare but severe condition associated with a range of ocular and systemic predisposing conditions, including ocular trauma, prior surgery, surface disease, and contact lens (CL) wear. This review explores the epidemiology of infectious keratitis, specifically the differences in disease incidence and risk factors, causative organism profile and virulence characteristics and host microbiome, genetics, gene expression, proteomics, and metabolomic characteristics in CL-related and non-CL-related diseases. Differences exist in the epidemiology, demographics, causative organisms, and their virulence characteristics in CL-related and non-CL-related diseases, and there is less evidence to support differences between these groups of individuals in the ocular surface microbiome, genetics, and pathways of disease. Genetic variations, however, in the host immune profile are implicated in both the onset and severity of infectious keratitis in CL and non-CL wearers. As technologies in metabolomics, proteomics, and genomics improved to be better able to process small-volume samples from the ocular surface, there will be improved understanding of the interplay between the CL, ocular surface, host immune profile, and the microbial environment. This may result in a more personalized approach in the management of disease to reduce disease severity.

Non-Canonical Host Intracellular Niche Links to New Antimicrobial Resistance Mechanism

Globally, infectious diseases are one of the leading causes of death among people of all ages. The development of antimicrobials to treat infectious diseases has been one of the most significant advances in medical history. Alarmingly, antimicrobial resistance is a widespread phenomenon that will, without intervention, make currently treatable infections once again deadly. In an era of widespread antimicrobial resistance, there is a constant and pressing need to develop new antibacterial drugs. Unraveling the underlying resistance mechanisms is critical to fight this crisis. In this review, we summarize some emerging evidence of the non-canonical intracellular life cycle of two priority antimicrobial-resistant bacterial pathogens: Pseudomonas aeruginosa and Staphylococcus aureus. The bacterial factors that modulate this unique intracellular niche and its implications in contributing to resistance are discussed. We then briefly discuss some recent research that focused on the promises of boosting host immunity as a combination therapy with antimicrobials to eradicate these two particular pathogens. Finally, we summarize the importance of various strategies, including surveillance and vaccines, in mitigating the impacts of antimicrobial resistance in general.

Exotoxin S secreted by internalized Pseudomonas aeruginosa delays lytic host cell death

The Pseudomonas aeruginosa toxin ExoS, secreted by the type III secretion system (T3SS), supports intracellular persistence via its ADP-ribosyltransferase (ADPr) activity. For epithelial cells, this involves inhibiting vacuole acidification, promoting vacuolar escape, countering autophagy, and niche construction in the cytoplasm and within plasma membrane blebs. Paradoxically, ExoS and other P. aeruginosa T3SS effectors can also have antiphagocytic and cytotoxic activities. Here, we sought to reconcile these apparently contradictory activities of ExoS by studying the relationships between intracellular persistence and host epithelial cell death. Methods involved quantitative imaging and the use of antibiotics that vary in host cell membrane permeability to selectively kill intracellular and extracellular populations after invasion. Results showed that intracellular P. aeruginosa mutants lacking T3SS effector toxins could kill (permeabilize) cells when extracellular bacteria were eliminated. Surprisingly, wild-type strain PAO1 (encoding ExoS, ExoT and ExoY) caused cell death more slowly, the time extended from 5.2 to 9.5 h for corneal epithelial cells and from 10.2 to 13.0 h for HeLa cells. Use of specific mutants/complementation and controls for initial invasion showed that ExoS ADPr activity delayed cell death. Triggering T3SS expression only after bacteria invaded cells using rhamnose-induction in T3SS mutants rescued the ExoS-dependent intracellular phenotype, showing that injected effectors from extracellular bacteria were not required. The ADPr activity of ExoS was further found to support internalization by countering the antiphagocytic activity of both the ExoS and ExoT RhoGAP domains. Together, these results show two additional roles for ExoS ADPr activity in supporting the intracellular lifestyle of P. aeruginosa; suppression of host cell death to preserve a replicative niche and inhibition of T3SS effector antiphagocytic activities to allow invasion. These findings add to the growing body of evidence that ExoS-encoding (invasive) P. aeruginosa strains can be facultative intracellular pathogens, and that intracellularly secreted T3SS effectors contribute to pathogenesis.

While the ADPr domain of the T3SS effector ExoS plays multiple roles in the intracellular lifestyle of P. aeruginosa, ExoS can also be cytotoxic and/or antiphagocytic. Here, we show that when P. aeruginosa enters the cytosol of epithelial cells, cell death is triggered independently of T3SS effector toxins, but ExoS ADPr activity delays this to enable continued intracellular survival and replication. Using rhamnose induction to express the T3SS only after invasion restored this ExoS-dependent phenotype, showing that intracellularly secreted effectors can enable intracellular pathogenesis. ExoS ADPr activity also countered antiphagocytic activity of ExoS and ExoT RhoGAP domains. These results show two additional roles for ExoS ADPr activity in promoting internalization of P. aeruginosa and protecting the intracellular niche, continuing to challenge the notions that P. aeruginosa is exclusively an extracellular pathogen, that it needs to inject T3SS effectors across plasma membranes, and that ExoS is necessarily cytotoxic to host cells.

Pseudomonas aeruginosa survives in epithelia by ExoS-mediated inhibition of autophagy and mTOR

One major factor that contributes to the virulence of Pseudomonas aeruginosa is its ability to reside and replicate unchallenged inside airway epithelial cells. The mechanism by which P. aeruginosa escapes destruction by intracellular host defense mechanisms, such as autophagy, is not known. Here, we show that the type III secretion system effector protein ExoS facilitates P. aeruginosa survival in airway epithelial cells by inhibiting autophagy in host cells. Autophagy inhibition is independent of mTOR activity, as the latter is also inhibited by ExoS, albeit by a different mechanism. Deficiency of the critical autophagy gene Atg7 in airway epithelial cells, both in vitro and in mouse models, greatly enhances the survival of ExoS-deficient P. aeruginosa but does not affect the survival of ExoS-containing bacteria. The inhibitory effect of ExoS on autophagy and mTOR depends on the activity of its ADP-ribosyltransferase domain. Inhibition of mTOR is caused by ExoS-mediated ADP ribosylation of RAS, whereas autophagy inhibition is due to the suppression of autophagic Vps34 kinase activity.

Exoenzyme Y Contributes to End-Organ Dysfunction Caused by Pseudomonas aeruginosa Pneumonia in Critically Ill Patients: An Exploratory Study

Pseudomonas aeruginosa is an opportunistic pathogen that causes pneumonia in immunocompromised and intensive care unit (ICU) patients. During host infection, P. aeruginosa upregulates the type III secretion system (T3SS), which is used to intoxicate host cells with exoenzyme (Exo) virulence factors. Of the four known Exo virulence factors (U, S, T and Y), ExoU has been shown in prior studies to associate with high mortality rates. Preclinical studies have shown that ExoY is an important edema factor in lung infection caused by P. aeruginosa, although its importance in clinical isolates of P. aeruginosa is unknown. We hypothesized that expression of ExoY would be highly prevalent in clinical isolates and would significantly contribute to patient morbidity secondary to P. aeruginosa pneumonia. A single-center, prospective observational study was conducted at the University of Alabama at Birmingham Hospital. Mechanically ventilated ICU patients with a bronchoalveolar lavage fluid culture positive for P. aeruginosa were included. Enrolled patients were followed from ICU admission to discharge and clinical P. aeruginosa isolates were genotyped for the presence of exoenzyme genes. Ninety-nine patients were enrolled in the study. ExoY was present in 93% of P. aeruginosa clinical isolates. Moreover, ExoY alone (ExoY⁺/ExoU⁻) was present in 75% of P. aeruginosa isolates, compared to 2% ExoU alone (ExoY⁻/ExoU⁺). We found that bacteria isolated from human samples expressed active ExoY and ExoU, and the presence of ExoY in clinical isolates was associated with end-organ dysfunction. This is the first study we are aware of that demonstrates that ExoY is important in clinical outcomes secondary to nosocomial pneumonia.

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.

Epithelial cell lysates induce ExoS expression and secretion by Pseudomonas aeruginosa

The type three secretion system (T3SS) is important for the intracellular survival of Pseudomonas aeruginosa. Known T3SS inducers include low Ca2+, serum or host cell contact. Here, we used corneal epithelial cell lysates to test if host cytosolic factors could also induce the T3SS. Invasive P. aeruginosa strain PAO1 was exposed to cell lysates for 16 h, and expression of T3SS effectors determined by q-PCR and Western immunoblot. Lysate exposure reduced PAO1 growth (∼5-fold) versus trypticase soy broth (TSB), but also resulted in appearance of a protein in culture supernatants, but not bacterial cell pellets, which reacted with antibody raised against ExoS. T3SS-inducing media (TSBi) caused the expression and secretion of ExoS and ExoT. Heat-treated lysates induced the protein; 1:3 diluted lysates did not. The protein that bound anti-ExoS antibody was found in supernatants of lysate-exposed exoT mutants, but not exoS or pscC mutants, suggesting a secreted form of ExoS, albeit slightly larger than that induced by TSBi. Lysate-exposed strain PAK expressed the same protein. Lysates caused PAO1 exoS and exoT gene expression, but only ∼20% and ∼6% of TSBi, respectively. T3SS induction by epithelial cell lysates could help explain T3SS expression by internalized P. aeruginosa.

Tanshinones: First-in-Class Inhibitors of the Biogenesis of the Type 3 Secretion System Needle of Pseudomonas aeruginosa for Antibiotic Therapy

The type 3 secretion system (T3SS) found as cell-surface appendages of many pathogenic Gram-negative bacteria, although nonessential for bacterial survival, is an important therapeutic target for drug discovery and development aimed at inhibiting bacterial virulence without inducing antibiotic resistance. We designed a fluorescence-polarization-based assay for high-throughput screening as a mechanistically well-defined general strategy for antibiotic discovery targeting the T3SS and made a serendipitous discovery of a subset of tanshinones-natural herbal compounds in traditional Chinese medicine widely used for the treatment of cardiovascular and cerebrovascular diseases-as effective inhibitors of the biogenesis of the T3SS needle of multi-drug-resistant Pseudomonas aeruginosa. By inhibiting the T3SS needle assembly and, thus, cytotoxicity and pathogenicity, selected tanshinones reduced the secretion of bacterial virulence factors toxic to macrophages in vitro, and rescued experimental animals challenged with lethal doses of Pseudomonas aeruginosa in a murine model of acute pneumonia. As first-in-class inhibitors with a demonstrable safety profile in humans, tanshinones may be used directly to alleviate Pseudomonas-aeruginosa-associated pulmonary infections without inducing antibiotic resistance. Since the T3SS is highly conserved among Gram-negative bacteria, this antivirulence strategy may be applicable to the discovery and development of novel classes of antibiotics refractory to existing resistance mechanisms for the treatment of many bacterial infections.

Killing from the inside: Intracellular role of T3SS in the fate of Pseudomonas aeruginosa within macrophages revealed by mgtC and oprF mutants

While considered solely an extracellular pathogen, increasing evidence indicates that Pseudomonas aeruginosa encounters intracellular environment in diverse mammalian cell types, including macrophages. In the present study, we have deciphered the intramacrophage fate of wild-type P. aeruginosa PAO1 strain by live and electron microscopy. P. aeruginosa first resided in phagosomal vacuoles and subsequently could be detected in the cytoplasm, indicating phagosomal escape of the pathogen, a finding also supported by vacuolar rupture assay. The intracellular bacteria could eventually induce cell lysis, both in a macrophage cell line and primary human macrophages. Two bacterial factors, MgtC and OprF, recently identified to be important for survival of P. aeruginosa in macrophages, were found to be involved in bacterial escape from the phagosome as well as in cell lysis caused by intracellular bacteria. Strikingly, type III secretion system (T3SS) genes of P. aeruginosa were down-regulated within macrophages in both mgtC and oprF mutants. Concordantly, cyclic di-GMP (c-di-GMP) level was increased in both mutants, providing a clue for negative regulation of T3SS inside macrophages. Consistent with the phenotypes and gene expression pattern of mgtC and oprF mutants, a T3SS mutant (ΔpscN) exhibited defect in phagosomal escape and macrophage lysis driven by internalized bacteria. Importantly, these effects appeared to be largely dependent on the ExoS effector, in contrast with the known T3SS-dependent, but ExoS independent, cytotoxicity caused by extracellular P. aeruginosa towards macrophages. Moreover, this macrophage damage caused by intracellular P. aeruginosa was found to be dependent on GTPase Activating Protein (GAP) domain of ExoS. Hence, our work highlights T3SS and ExoS, whose expression is modulated by MgtC and OprF, as key players in the intramacrophage life of P. aeruginosa which allow internalized bacteria to lyse macrophages.

The ability of professional phagocytes to ingest and kill microorganisms is central to host defense and Pseudomonas aeruginosa has developed mechanisms to avoid being killed by phagocytes. While considered an extracellular pathogen, P. aeruginosa has been reported to be engulfed by macrophages in animal models. Here, we visualized the fate of P. aeruginosa within cultured macrophages, revealing macrophage lysis driven by intracellular P. aeruginosa. Two bacterial factors, MgtC and OprF, recently discovered to be involved in the intramacrophage survival of P. aeruginosa, appeared to play a role in this cytotoxicity caused by intracellular bacteria. We provided evidence that type III secretion system (T3SS) gene expression is lowered intracellularly in mgtC and oprF mutants. We further showed that intramacrophage P. aeruginosa uses its T3SS, specifically the ExoS effector, to promote phagosomal escape and cell lysis. We thus describe a transient intramacrophage stage of P. aeruginosa that could contribute to bacterial dissemination.

Blowing epithelial cell bubbles with GumB: ShlA-family pore-forming toxins induce blebbing and rapid cellular death in corneal epithelial cells

Medical devices, such as contact lenses, bring bacteria in direct contact with human cells. Consequences of these host-pathogen interactions include the alteration of mammalian cell surface architecture and induction of cellular death that renders tissues more susceptible to infection. Gram-negative bacteria known to induce cellular blebbing by mammalian cells, Pseudomonas and Vibrio species, do so through a type III secretion system-dependent mechanism. This study demonstrates that a subset of bacteria from the Enterobacteriaceae bacterial family induce cellular death and membrane blebs in a variety of cell types via a type V secretion-system dependent mechanism. Here, we report that ShlA-family cytolysins from Proteus mirabilis and Serratia marcescens were required to induce membrane blebbling and cell death. Blebbing and cellular death were blocked by an antioxidant and RIP-1 and MLKL inhibitors, implicating necroptosis in the observed phenotypes. Additional genetic studies determined that an IgaA family stress-response protein, GumB, was necessary to induce blebs. Data supported a model where GumB and shlBA are in a regulatory circuit through the Rcs stress response phosphorelay system required for bleb formation and pathogenesis in an invertebrate model of infection and proliferation in a phagocytic cell line. This study introduces GumB as a regulator of S. marcescens host-pathogen interactions and demonstrates a common type V secretion system-dependent mechanism by which bacteria elicit surface morphological changes on mammalian cells. This type V secretion-system mechanism likely contributes bacterial damage to the corneal epithelial layer, and enables access to deeper parts of the tissue that are more susceptible to infection.

Neutrophil Extracellular Traps Confine Pseudomonas aeruginosa Ocular Biofilms and Restrict Brain Invasion

Bacterial biofilm infections are difficult to eradicate because of antibiotic insusceptibility and high recurrence rates. Biofilm formation by Pseudomonas aeruginosa, a leading cause of bacterial keratitis, is facilitated by the bacterial Psl exopolysaccharide and associated with heightened virulence. Using intravital microscopy, we observed that neutrophilic recruitment to corneal infections limits P. aeruginosa biofilms to the outer eye surface, preventing bacterial dissemination. Neutrophils moved to the base of forming biofilms, where they underwent neutrophil extracellular trap formation (NETosis) in response to high expression of the bacterial type-3 secretion system (T3SS). NETs formed a barrier "dead zone," confining bacteria to the external corneal environment and inhibiting bacterial dissemination into the brain. Once formed, ocular biofilms were resistant to antibiotics and neutrophil killing, advancing eye pathology. However, blocking both Psl and T3SS together with antibiotic treatment broke down the biofilm and reversed keratitis, suggesting future therapeutic strategies for this intractable infection.

Pseudomonas aeruginosa ExoS Induces Intrinsic Apoptosis in Target Host Cells in a Manner That is Dependent on its GAP Domain Activity

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that causes serious infections in immunocompromised individuals and cystic fibrosis patients. ExoS and ExoT are two homologous bifunctional Type III Secretion System (T3SS) virulence factors that induce apoptosis in target host cells. They possess a GTPase Activating Protein (GAP) domain at their N-termini, which share ~76% homology, and an ADP-ribosyltransferase (ADPRT) domain at their C-termini, which target non-overlapping substrates. Both the GAP and the ADPRT domains contribute to ExoT's cytotoxicity in target epithelial cells, whereas, ExoS-induced apoptosis is reported to be primarily due to its ADPRT domain. In this report, we demonstrate that ExoS/GAP domain is both necessary and sufficient to induce mitochondrial apoptosis. Our data demonstrate that intoxication with ExoS/GAP domain leads to enrichment of Bax and Bim into the mitochondrial outer-membrane, disruption of mitochondrial membrane and release of and cytochrome c into the cytosol, which activates initiator caspase-9 and effector caspase-3, that executes cellular death. We posit that the contribution of the GAP domain in ExoS-induced apoptosis was overlooked in prior studies due to its slower kinetics of cytotoxicity as compared to ADPRT. Our data clarify the field and reveal a novel virulence function for ExoS/GAP as an inducer of apoptosis.

The Impact of ExoS on Pseudomonas aeruginosa Internalization by Epithelial Cells Is Independent of fleQ and Correlates with Bistability of Type Three Secretion System Gene Expression

Pseudomonas aeruginosa is internalized into multiple types of epithelial cell in vitro and in vivo and yet is often regarded as an exclusively extracellular pathogen. Paradoxically, ExoS, a type three secretion system (T3SS) effector, has antiphagocytic activities but is required for intracellular survival of P. aeruginosa and its occupation of bleb niches in epithelial cells. Here, we addressed mechanisms for this dichotomy using invasive (ExoS-expressing) P. aeruginosa and corresponding effector-null isogenic T3SS mutants, effector-null mutants of cytotoxic P. aeruginosa with and without ExoS transformation, antibiotic exclusion assays, and imaging using a T3SS-GFP reporter. Except for effector-null PA103, all strains were internalized while encoding ExoS. Intracellular bacteria showed T3SS activation that continued in replicating daughter cells. Correcting the fleQ mutation in effector-null PA103 promoted internalization by >10-fold with or without ExoS. Conversely, mutating fleQ in PAO1 reduced internalization by >10-fold, also with or without ExoS. Effector-null PA103 remained less well internalized than PAO1 matched for fleQ status, but only with ExoS expression, suggesting additional differences between these strains. Quantifying T3SS activation using GFP fluorescence and quantitative reverse transcription-PCR (qRT-PCR) showed that T3SS expression was hyperinducible for strain PA103ΔexoUT versus other isolates and was unrelated to fleQ status. These findings support the principle that P. aeruginosa is not exclusively an extracellular pathogen, with internalization influenced by the relative proportions of T3SS-positive and T3SS-negative bacteria in the population during host cell interaction. These data also challenge current thinking about T3SS effector delivery into host cells and suggest that T3SS bistability is an important consideration in studying P. aeruginosa pathogenesis.

P. aeruginosa is often referred to as an extracellular pathogen, despite its demonstrated capacity to invade and survive within host cells. Fueling the confusion, P. aeruginosa encodes T3SS effectors with anti-internalization activity that, paradoxically, play critical roles in intracellular survival. Here, we sought to address why ExoS does not prevent internalization of the P. aeruginosa strains that natively encode it. Results showed that ExoS exerted unusually strong anti-internalization activity under conditions of expression in the effector-null background of strain PA103, often used to study T3SS effector activity. Inhibition of internalization was associated with T3SS hyperinducibility and ExoS delivery. PA103 fleQ mutation, preventing flagellar assembly, further reduced internalization but did so independently of ExoS. The results revealed intracellular T3SS expression by all strains and suggested that T3SS bistability influences P. aeruginosa internalization. These findings reconcile controversies in the literature surrounding P. aeruginosa internalization and support the principle that P. aeruginosa is not exclusively an extracellular pathogen.

Ndk, a novel host-responsive regulator, negatively regulates bacterial virulence through quorum sensing in Pseudomonas aeruginosa

Pathogenic bacteria could adjust gene expression to enable their survival in the distinct host environment. However, the mechanism by which bacteria adapt to the host environment is not well described. In this study, we demonstrated that nucleoside diphosphate kinase (Ndk) of Pseudomonas aeruginosa is critical for adjusting the bacterial virulence determinants during infection. Ndk expression was down-regulated in the pulmonary alveoli of a mouse model of acute pneumonia. Knockout of ndk up-regulated transcription factor ExsA-mediated T3S regulon expression and decreased exoproduct-related gene expression through the inhibition of the quorum sensing hierarchy. Moreover, in vitro and in vivo studies demonstrated that the ndk mutant exhibits enhanced cytotoxicity and host pathogenicity by increasing T3SS proteins. Taken together, our data reveal that ndk is a critical novel host-responsive gene required for coordinating P. aeruginosa virulence upon acute infection.

Rin1 restores host phagocytic activity during invasion by Pseudomonas aeruginosa

Pseudomonas aeruginosa uses a type III secretion system to deliver toxic effector proteins directly into host cells and alter host protein functions. Exoenzyme S (ExoS), a type III effector protein, ADP-ribosylates Rab5 GTPase and impairs early phagocytic events in macrophage cells. In this study, we tested the hypothesis that Rin1, a Ras effector protein and Rab5 guanine nucleotide exchange factor, generates an intrinsic Rab5 activity cycle during phagocytosis of live P. aeruginosa; thus, allowing proper phagocytic killing. We found that Rab5 activity was attenuated at a very early time point (2.5 min) of the phagocytic process of live but not of heat-inactivated P. aeruginosa. However, upon overexpressing Rin1 in macrophages, the Rab5 activity sustained for a prolonged time (∼20 min) counteracting the negative effects during phagocytosis of live P. aeruginosa. Ras, also a substrate of the ADP-ribosyltransferase activity of ExoS, remained active during the early events of phagocytosis of live as well as heat-inactivated P. aeruginosa. Further examinations revealed that the Rin1 : Vps9 domain (the Rab5 nucleotide catalytic domain) and the Rin1 : RA domain (the Ras association domain of Rin1) are both required for optimal Rin1 function. Finally, the time-based analysis of the ADP-ribosylation status of Rab5 and Ras obtained from this study was consistent in the context of the regulation of (i) Rab5 activity by Rin1 : Vps9 domain and (ii) Ras interaction with Rin1 via the Rin1 : RA domain. These observations highlight a novel crosstalk between Rin1-Rab5 and Rin1-Ras complexes that offsets the anti-phagocytic effects of ExoS in macrophages.

The Pseudomonas aeruginosa Exoenzyme Y: A Promiscuous Nucleotidyl Cyclase Edema Factor and Virulence Determinant

Exoenzyme Y (ExoY) was identified as a component of the Pseudomonas aeruginosa type 3 secretion system secretome in 1998. It is a common contributor to the arsenal of type 3 secretion system effectors, as it is present in approximately 90% of Pseudomonas isolates. ExoY has adenylyl cyclase activity that is dependent upon its association with a host cell cofactor. However, recent evidence indicates that ExoY is not just an adenylyl cyclase; rather, it is a promiscuous cyclase capable of generating purine and pyrimidine cyclic nucleotide monophosphates. ExoY's enzymatic activity causes a characteristic rounding of mammalian cells, due to microtubule breakdown. In endothelium, this cell rounding disrupts cell-to-cell junctions, leading to loss of barrier integrity and an increase in tissue edema. Microtubule breakdown seems to depend upon tau phosphorylation, where the elevation of cyclic nucleotide monophosphates activates protein kinases A and G and causes phosphorylation of endothelial microtubule associated protein tau. Phosphorylation is a stimulus for tau release from microtubules, leading to microtubule instability. Phosphorylated tau accumulates inside endothelium as a high molecular weight, oligomeric form, and is then released from the cell. Extracellular high molecular weight tau causes a transmissible cytotoxicity that significantly hinders cellular repair following infection. Thus, ExoY may contribute to bacterial virulence in at least two ways; first, by microtubule breakdown leading to loss of endothelial cell barrier integrity, and second, by promoting release of a high molecular weight tau cytotoxin that impairs cellular recovery following infection.

Involvement of Membrane Blebbing in Immunological Disorders and Cancer

Cellular blebbing is a unique form of dynamic protrusion emanating from the plasma membrane which can be either apoptotic or nonapoptotic in nature. Blebs have been observed in a wide variety of cell types and in response to multiple mechanical and chemical stimuli. They have been linked to various physiological and pathological processes including tumor motility and invasion, as well as to various immunological disorders. They can form and retract extremely rapidly in seconds or minutes, or slowly over hours or days. This review focuses on recent evidence regarding the role of blebbing in cell locomotion with particular emphasis on its role in tumor metastasis, indicating the role of specific causative molecules. The phenomenon of blebbing has been observed in endocrine-resistant breast cancer cells in response to brief exposure to extracellular alkaline pH, which leads to enhanced invasive capacity. Genetic or pharmacological targeting of cellular blebs could serve as a potential therapeutic option to control tumor metastasis.

Structural Analysis of the Regulatory Domain of ExsA, a Key Transcriptional Regulator of the Type Three Secretion System in Pseudomonas aeruginosa

Pseudomonas aeruginosa employs a type three secretion system to facilitate infections in mammalian hosts. The operons encoding genes of structural components of the secretion machinery and associated virulence factors are all under the control of the AraC-type transcriptional activator protein, ExsA. ExsA belongs to a unique subfamily of AraC-proteins that is regulated through protein-protein contacts rather than small molecule ligands. Prior to infection, ExsA is inhibited through a direct interaction with the anti-activator ExsD. To activate ExsA upon host cell contact this interaction is disrupted by the anti-antiactivator protein ExsC. Here we report the crystal structure of the regulatory domain of ExsA, which is known to mediate ExsA dimerization as well as ExsD binding. The crystal structure suggests two models for the ExsA dimer. Both models confirmed the previously shown involvement of helix α-3 in ExsA dimerization but one also suggest a role for helix α-2. These structural data are supported by the observation that a mutation in α-2 greatly diminished the ability of ExsA to activate transcription in vitro. Additional in vitro transcription studies revealed that a conserved pocket, used by AraC and the related ToxT protein for the binding of small molecule regulators, although present in ExsA is not involved in binding of ExsD.

The Role of ExoS in Dissemination of Pseudomonas aeruginosa during Pneumonia

Hospital-acquired pneumonia is associated with high rates of morbidity and mortality, and dissemination to the bloodstream is a recognized risk factor for particularly poor outcomes. Yet the mechanism by which bacteria in the lungs gain access to the bloodstream remains poorly understood. In this study, we used a mouse model of Pseudomonas aeruginosa pneumonia to examine this mechanism. P. aeruginosa uses a type III secretion system to deliver effector proteins such as ExoS directly into the cytosol of eukaryotic cells. ExoS, a bi-functional GTPase activating protein (GAP) and ADP-ribosyltransferase (ADPRT), inhibits phagocytosis during pneumonia but has also been linked to a higher incidence of dissemination to the bloodstream. We used a novel imaging methodology to identify ExoS intoxicated cells during pneumonia and found that ExoS is injected into not only leukocytes but also epithelial cells. Phagocytic cells, primarily neutrophils, were targeted for injection with ExoS early during infection, but type I pneumocytes became increasingly injected at later time points. Interestingly, injection of these pneumocytes did not occur randomly but rather in discrete regions, which we designate ““fields of cell injection” (FOCI). These FOCI increased in size as the infection progressed and contained dead type I pneumocytes. Both of these phenotypes were attenuated in infections caused by bacteria secreting ADPRT-deficient ExoS, indicating that FOCI growth and type I pneumocyte death were dependent on the ADPRT activity of ExoS. During the course of infection, increased FOCI size was associated with enhanced disruption of the pulmonary-vascular barrier and increased bacterial dissemination into the blood, both of which were also dependent on the ADPRT activity of ExoS. We conclude that the ADPRT activity of ExoS acts upon type I pneumocytes to disrupt the pulmonary-vascular barrier during P. aeruginosa pneumonia, leading to bacterial dissemination.

Dissemination to the bloodstream is a poor prognostic sign in patients with hospital-acquired pneumonia, yet the mechanism by which this occurs is poorly understood. To begin to address this issue, we have used a mouse model of P. aeruginosa pneumonia to study the mechanism by which the type-III-secreted effector protein ExoS enhances bacterial dissemination. We show that intoxication of type I pneumocytes by ExoS leads to cell death and disruption of the pulmonary-vascular barrier, allowing bacterial dissemination into the bloodstream. These effects required the ADP-ribosyltransferase activity of ExoS, as strains secreting an ExoS variant lacking this activity demonstrated reduced type I pneumocytes death and pulmonary-vascular breakdown. This study indicates that inhibitors of the ADP-ribosyltransferase activity of ExoS could serve as novel therapeutics for the prevention of bacteremic pneumonia.

Pseudomonas aeruginosa-induced bleb-niche formation in epithelial cells is independent of actinomyosin contraction and enhanced by loss of cystic fibrosis transmembrane-conductance regulator osmoregulatory function

The opportunistic pathogen Pseudomonas aeruginosa can infect almost any site in the body but most often targets epithelial cell-lined tissues such as the airways, skin, and the cornea of the eye. A common predisposing factor is cystic fibrosis (CF), caused by defects in the cystic fibrosis transmembrane-conductance regulator (CFTR). Previously, we showed that when P. aeruginosa enters epithelial cells it replicates intracellularly and occupies plasma membrane blebs. This phenotype is dependent on the type 3 secretion system (T3SS) effector ExoS, shown by others to induce host cell apoptosis. Here, we examined mechanisms for P. aeruginosa-induced bleb formation, focusing on its relationship to apoptosis and the CFTR. The data showed that P. aeruginosa-induced blebbing in epithelial cells is independent of actin contraction and is inhibited by hyperosmotic media (400 to 600 mOsM), distinguishing bacterially induced blebs from apoptotic blebs. Cells with defective CFTR displayed enhanced bleb formation upon infection, as demonstrated using bronchial epithelial cells from a patient with cystic fibrosis and a CFTR inhibitor, CFTR(Inh)-172. The defect was found to be correctable either by incubation in hyperosmotic media or by complementation with CFTR (pGFP-CFTR), suggesting that the osmoregulatory function of CFTR counters P. aeruginosa-induced bleb-niche formation. Accordingly, and despite their reduced capacity for bacterial internalization, CFTR-deficient cells showed greater bacterial occupation of blebs and enhanced intracellular replication. Together, these data suggest that P. aeruginosa bleb niches are distinct from apoptotic blebs, are driven by osmotic forces countered by CFTR, and could provide a novel mechanism for bacterial persistence in the host.

Pseudomonas aeruginosa is an opportunistic pathogen problematic in hospitalized patients and those with cystic fibrosis (CF). Previously, we showed that P. aeruginosa can enter epithelial cells and replicate within them and traffics to the membrane blebs that it induces. This “bleb-niche” formation requires ExoS, previously shown to cause apoptosis. Here, we show that the driving force for bleb-niche formation is osmotic pressure, differentiating P. aeruginosa-induced blebs from apoptotic blebs. Either CFTR inhibition or CFTR mutation (as seen in people with CF) causes P. aeruginosa to make more bleb niches and provides an osmotic driving force for blebbing. CFTR inhibition also enhances bacterial occupation of blebs and intracellular replication. Since CFTR is targeted for removal from the plasma membrane when P. aeruginosa invades a healthy cell, these findings could relate to pathogenesis in both CF and healthy patient populations.

The importance of the Pseudomonas aeruginosa type III secretion system in epithelium traversal depends upon conditions of host susceptibility

Pseudomonas aeruginosa is invasive or cytotoxic to host cells, depending on the type III secretion system (T3SS) effectors encoded. While the T3SS is known to be involved in disease in vivo, how it participates remains to be clarified. Here, mouse models of superficial epithelial injury (tissue paper blotting with EGTA treatment) and immunocompromise (MyD88 deficiency) were used to study the contribution of the T3SS transcriptional activator ExsA to epithelial traversal. Corneas of excised eyeballs were inoculated with green fluorescent protein (GFP)-expressing PAO1 or isogenic exsA mutants for 6 h ex vivo before bacterial traversal and epithelial thickness were quantified by using imaging. In the blotting-EGTA model, exsA mutants were defective in capacity for traversal. Accordingly, an ∼16-fold variability in exsA expression among PAO1 isolates from three sources correlated with epithelial loss. In contrast, MyD88-/- epithelia remained susceptible to P. aeruginosa traversal despite exsA mutation. Epithelial lysates from MyD88-/- mice had reduced antimicrobial activity compared to those from wild-type mice with and without prior antigen challenge, particularly 30- to 100-kDa fractions, for which mass spectrometry revealed multiple differences, including (i) lower baseline levels of histones, tubulin, and lumican and (ii) reduced glutathione S-transferase, annexin, and dermatopontin, after antigen challenge. Thus, the importance of ExsA in epithelial traversal by invasive P. aeruginosa depends on the compromise enabling susceptibility, suggesting that strategies for preventing infection will need to extend beyond targeting the T3SS. The data also highlight the importance of mimicking conditions allowing susceptibility in animal models and the need to monitor variability among bacterial isolates from different sources, even for the same strain.

The ADP-ribosyltransferase domain of the effector protein ExoS inhibits phagocytosis of Pseudomonas aeruginosa during pneumonia

Pseudomonas aeruginosa is a Gram-negative pathogen commonly associated with nosocomial infections such as hospital-acquired pneumonia. It uses a type III secretion system to deliver effector proteins directly into the cytosol of host cells. Type III secretion in P. aeruginosa has been linked to severe disease and worse clinical outcomes in animal and human studies. The majority of P. aeruginosa strains secrete ExoS, a bifunctional toxin with GTPase-activating protein and ADP-ribosyltransferase activities. Numerous in vitro studies have investigated the targets and cellular effects of ExoS, linking both its enzymatic activities with inhibition of bacterial internalization. However, little is known about how this toxin facilitates the progression of infection in vivo. In this study, we used a mouse model to investigate the role of ExoS in inhibiting phagocytosis during pneumonia. We first confirmed previous findings that the ADP-ribosyltransferase activity of ExoS, but not the GTPase-activating protein activity, was responsible for bacterial persistence and decreased host survival in this model. We then used two distinct assays to demonstrate that ExoS inhibited phagocytosis during pneumonia. In contrast to the findings of several in vitro studies, this in vivo inhibition was also dependent on the ADP-ribosyltransferase activity, but not the GTPase-activating protein activity, of ExoS. These results demonstrate for the first time the antiphagocytic function of ExoS in the context of an actual infection and indicate that blocking the ADP-ribosyltransferase activity of ExoS may have potential therapeutic benefit.

Pseudomonas aeruginosa is a major cause of hospital-acquired infections. To cause severe disease, this bacterium uses a type III secretion system that delivers four effector proteins, ExoS, ExoT, ExoU, and ExoY, into host cells. The majority of P. aeruginosa strains secrete ExoS, a bifunctional toxin with GTPase-activating protein and ADP-ribosyltransferase activities. In cell culture models, both enzymatic activities have been associated with decreased bacterial internalization. However, our study is the first to examine a role for ExoS in blocking phagocytosis in an animal model. We report that ExoS does inhibit phagocytosis during pneumonia. The ADP-ribosyltransferase activity, but not the GTPase-activating protein activity, of ExoS is necessary for this effect. Our findings highlight the ability of P. aeruginosa to manipulate the inflammatory response during pneumonia to facilitate bacterial survival.

Who's really in control: microbial regulation of protein trafficking in the epithelium

Due to evolutionary pressure, there are many complex interactions at the interface between pathogens and eukaryotic host cells wherein host cells attempt to clear invading microorganisms and pathogens counter these mechanisms to colonize and invade host tissues. One striking observation from studies focused on this interface is that pathogens have multiple mechanisms to modulate and disrupt normal cellular physiology to establish replication niches and avoid clearance. The precision by which pathogens exert their effects on host cells makes them excellent tools to answer questions about cell physiology of eukaryotic cells. Furthermore, an understanding of these mechanisms at the host-pathogen interface will benefit our understanding of how pathogens cause disease. In this review, we describe a few examples of how pathogens disrupt normal cellular physiology and protein trafficking at epithelial cell barriers to underscore how pathogens modulate cellular processes to cause disease and how this knowledge has been utilized to learn about cellular physiology.

Pseudomonas aeruginosa utilizes the type III secreted toxin ExoS to avoid acidified compartments within epithelial cells

Invasive Pseudomonas aeruginosa (PA) can enter epithelial cells wherein they mediate formation of plasma membrane bleb-niches for intracellular compartmentalization. This phenotype, and capacity for intracellular replication, requires the ADP-ribosyltransferase (ADPr) activity of ExoS, a PA type III secretion system (T3SS) effector protein. Thus, PA T3SS mutants lack these capacities and instead traffic to perinuclear vacuoles. Here, we tested the hypothesis that the T3SS, via the ADPr activity of ExoS, allows PA to evade acidic vacuoles that otherwise suppress its intracellular viability. The acidification state of bacteria-occupied vacuoles within infected corneal epithelial cells was studied using LysoTracker to visualize acidic, lysosomal vacuoles. Steady state analysis showed that within cells wild-type PAO1 localized to both membrane bleb-niches and vacuoles, while both exsA (transcriptional activator) and popB (effector translocation) T3SS mutants were only found in vacuoles. The acidification state of occupied vacuoles suggested a relationship with ExoS expression, i.e. vacuoles occupied by the exsA mutant (unable to express ExoS) were more often acidified than either popB mutant or wild-type PAO1 occupied vacuoles (p < 0.001). An exoS-gfp reporter construct pJNE05 confirmed that high exoS transcriptional output coincided with low occupation of acidified vacuoles, and vice versa, for both popB mutants and wild-type bacteria. Complementation of a triple effector null mutant of PAO1 with exoS (pUCPexoS) reduced the number of acidified bacteria-occupied vacuoles per cell; pUCPexoSE381D which lacks ADPr activity did not. The H⁺-ATPase inhibitor bafilomycin rescued intracellular replication to wild-type levels for exsA mutants, showing its viability is suppressed by vacuolar acidification. Taken together, the data show that the mechanism by which ExoS ADPr activity allows intracellular replication by PA involves suppression of vacuolar acidification. They also show that variability in ExoS expression by wild-type PA inside cells can differentially influence the fate of individual intracellular bacteria, even within the same cell.

Host defense at the ocular surface

Microbial infections of the cornea frequently cause painful, blinding and debilitating disease that is often difficult to treat and may require corneal transplantation. In addition, sterile corneal infiltrates that are associated with contact lens wear cause pain, visual impairment and photophobia. In this article, we review the role of Toll-Like Receptors (TLR) in bacterial keratitis and sterile corneal infiltrates, and describe the role of MD-2 regulation in LPS responsiveness by corneal epithelial cells. We conclude that both live bacteria and bacterial products activate Toll-Like Receptors in the cornea, which leads to chemokine production and neutrophil recruitment to the corneal stroma. While neutrophils are essential for bacterial killing, they also cause tissue damage that results in loss of corneal clarity. These disparate outcomes, therefore, represent a spectrum of disease severity based on this pathway, and further indicate that targeting the TLR pathway is a feasible approach to treating inflammation caused by live bacteria and microbial products. Further, as the P. aeruginosa type III secretion system (T3SS) also plays a critical role in disease pathogenesis by inducing neutrophil apoptosis and facilitating bacterial growth in the cornea, T3SS exotoxins are additional targets for therapy for P. aeruginosa keratitis.

Metastatic MTLn3 and non-metastatic MTC adenocarcinoma cells can be differentiated by Pseudomonas aeruginosa

Cancer patients are known to be highly susceptible to Pseudomonas aeruginosa (Pa) infection, but it remains unknown whether alterations at the tumor cell level can contribute to infection. This study explored how cellular changes associated with tumor metastasis influence Pa infection using highly metastatic MTLn3 cells and non-metastatic MTC cells as cell culture models. MTLn3 cells were found to be more sensitive to Pa infection than MTC cells based on increased translocation of the type III secretion effector, ExoS, into MTLn3 cells. Subsequent studies found that higher levels of ExoS translocation into MTLn3 cells related to Pa entry and secretion of ExoS within MTLn3 cells, rather than conventional ExoS translocation by external Pa. ExoS includes both Rho GTPase activating protein (GAP) and ADP-ribosyltransferase (ADPRT) enzyme activities, and differences in MTLn3 and MTC cell responsiveness to ExoS were found to relate to the targeting of ExoS-GAP activity to Rho GTPases. MTLn3 cell migration is mediated by RhoA activation at the leading edge, and inhibition of RhoA activity decreased ExoS translocation into MTLn3 cells to levels similar to those of MTC cells. The ability of Pa to be internalized and transfer ExoS more efficiently in association with Rho activation during tumor metastasis confirms that alterations in cell migration that occur in conjunction with tumor metastasis contribute to Pa infection in cancer patients. This study also raises the possibility that Pa might serve as a biological tool for dissecting or detecting cellular alterations associated with tumor metastasis.

Host response and bacterial virulence factor expression in Pseudomonas aeruginosa and Streptococcus pneumoniae corneal ulcers

P. aeruginosa and S. pneumoniae are major bacterial causes of corneal ulcers in industrialized and in developing countries. The current study examined host innate immune responses at the site of infection, and also expression of bacterial virulence factors in clinical isolates from patients in south India. Corneal ulcer material was obtained from 49 patients with confirmed P. aeruginosa and 27 patients with S. pneumoniae, and gene expression of Toll Like Receptors (TLR), cytokines and inflammasome proteins was measured by quantitative PCR. Expression of P. aeruginosa type III secretion exotoxins and S. pneumoniae pneumolysin was detected by western blot analysis. We found that neutrophils comprised >90% cells in corneal ulcers, and that there was elevated expression of TLR2, TLR4, TLR5 and TLR9, the NLRP3 and NLRC4 inflammasomes and the ASC adaptor molecule. IL-1α IL-1β and IFN-γ expression was also elevated; however, there was no significant difference in expression of any of these genes between corneal ulcers from P. aeruginosa and S. pneumoniae infected patients. We also show that 41/49 (84%) of P. aeruginosa clinical isolates expressed ExoS and ExoT, whereas 5/49 (10%) of isolates expressed ExoS, ExoT and ExoU with only 2/49 isolates expressing ExoT and ExoU. In contrast, all 27 S. pneumoniae clinical isolates produced pneumolysin. Taken together, these findings demonstrate that ExoS/T expressing P. aeruginosa and pneumolysin expressing S. pneumoniae predominate in bacterial keratitis. While P. aeruginosa strains expressing both ExoU and ExoS are usually rare, these strains actually outnumbered strains expressing only ExoU in the current study. Further, as neutrophils are the predominant cell type in these corneal ulcers, they are the likely source of cytokines and of the increased TLR and inflammasome expression.

Why does the healthy cornea resist Pseudomonas aeruginosa infection?

PURPOSE

To provide our perspective on why the cornea is resistant to infection based on our research results with Pseudomonas (P) aeruginosa. We focus on our current understanding of the interplay between bacteria, tear fluid, and the corneal epithelium that determines health as the usual outcome, and propose a theoretical model for how contact lens wear might change those interactions to enable susceptibility to P aeruginosa infection.

METHODS

Use of "null-infection" in vivo models, cultured human corneal epithelial cells, contact lens-wearing animal models, and bacterial genetics help to elucidate mechanisms by which P aeruginosa survives at the ocular surface, adheres, and traverses multilayered corneal epithelia. These models also help elucidate the molecular mechanisms of corneal epithelial innate defense.

RESULTS

Tear fluid and the corneal epithelium combine to make a formidable defense against P aeruginosa infection of the cornea. Part of that defense involves the expression of antimicrobials such as β-defensins, the cathelicidin LL-37, cytokeratin-derived antimicrobial peptides, and RNase7. Immunomodulators such as SP-D and ST2 also contribute. Innate defenses of the cornea depend in part on MyD88, a key adaptor protein of TLR and IL-1R signaling, but the basal lamina represents the final barrier to bacterial penetration. Overcoming these defenses involves P aeruginosa adaptation, expression of the type III secretion system, proteases, and P aeruginosa biofilm formation on contact lenses.

CONCLUSION

After more than 2 decades of research focused on understanding how contact lens wear predisposes to P aeruginosa infection, our working hypothesis places blame for microbial keratitis on bacterial adaptation to ocular surface defenses, combined with changes to the biochemistry of the corneal surface caused by trapping bacteria and tear fluid against the cornea under the lens.

Regulation of Rab5 function during phagocytosis of live Pseudomonas aeruginosa in macrophages

Pseudomonas aeruginosa, a Gram-negative opportunistic human pathogen, is a frequent cause of severe hospital-acquired infections. Effectors produced by the type III secretion system disrupt mammalian cell membrane trafficking and signaling and are integral to the establishment of P. aeruginosa infection. One of these effectors, ExoS, ADP-ribosylates several host cell proteins, including Ras and Rab GTPases. In this study, we demonstrated that Rab5 plays a critical role during early stages of P. aeruginosa invasion of J774-Eclone macrophages. We showed that live, but not heat-inactivated, P. aeruginosa inhibited phagocytosis and that this occurred in conjunction with downregulation of Rab5 activity. Inactivation of Rab5 was dependent on ExoS ADP-ribosyltransferase activity, and in J744-Eclone cells, ExoS ADP-ribosyltransferase activity caused a more severe inhibition of phagocytosis than ExoS Rho GTPase activity. Furthermore, we found that expression of Rin1, a Rab5 guanine exchange factor, but not Rabex5 and Rap6, partially reversed the inactivation of Rab5 during invasion of live P. aeruginosa. These studies provide evidence that live P. aeruginosa cells are able to influence their rate of phagocytosis in macrophages by directly regulating activation of Rab5.

Regulation of small GTPases by GEFs, GAPs, and GDIs

Small GTPases use GDP/GTP alternation to actuate a variety of functional switches that are pivotal for cell dynamics. The GTPase switch is turned on by GEFs, which stimulate dissociation of the tightly bound GDP, and turned off by GAPs, which accelerate the intrinsically sluggish hydrolysis of GTP. For Ras, Rho, and Rab GTPases, this switch incorporates a membrane/cytosol alternation regulated by GDIs and GDI-like proteins. The structures and core mechanisms of representative members of small GTPase regulators from most families have now been elucidated, illuminating their general traits combined with scores of unique features. Recent studies reveal that small GTPase regulators have themselves unexpectedly sophisticated regulatory mechanisms, by which they process cellular signals and build up specific cell responses. These mechanisms include multilayered autoinhibition with stepwise release, feedback loops mediated by the activated GTPase, feed-forward signaling flow between regulators and effectors, and a phosphorylation code for RhoGDIs. The flipside of these highly integrated functions is that they make small GTPase regulators susceptible to biochemical abnormalities that are directly correlated with diseases, notably a striking number of missense mutations in congenital diseases, and susceptible to bacterial mimics of GEFs, GAPs, and GDIs that take command of small GTPases in infections. This review presents an overview of the current knowledge of these many facets of small GTPase regulation.

Self-trimerization of ExsD limits inhibition of the Pseudomonas aeruginosa transcriptional activator ExsA in vitro

The opportunistic pathogen Pseudomonas aeruginosa ranks among the leading causes of nosocomial infection. The type III secretion system (T3SS) aids acute Pseudomonas aeruginosa infection by injecting potent cytotoxins into host cells to suppress the host's innate immune response. Expression of all T3SS-related genes is strictly dependent on the transcription factor ExsA. Consequently, ExsA and the biological processes that regulate ExsA function are of great biomedical interest. The present study focused on the ExsA-ExsC-ExsD-ExsE signaling cascade, which ties host cell contact to the upregulation of T3SS gene expression. Prior to T3SS induction, the antiactivator protein ExsD binds to ExsA and blocks ExsA-dependent transcription by interfering with ExsA dimerization and promoter interactions. Upon host cell contact, ExsD is sequestered by the T3SS chaperone ExsC, resulting in the release of ExsA and upregulation of the T3SS. Previous studies have shown that the ExsD-ExsA interactions are not freely reversible. Because independently folded ExsD and ExsA were not found to interact, it has been hypothesized that folding intermediates of the two proteins form the complex. Here, we demonstrate, for the first time, that ExsD alone is sufficient to inhibit ExsA-dependent transcription in vitro and that no other cellular factors are required. More significantly, we show that independently folded ExsD and ExsA are capable of interacting, but only at 37 °C and not at 30 °C. Guided by the crystal structure of ExsD, we designed a monomeric variant of the protein, and demonstrated that ExsD trimerization prevents ExsD from inhibiting ExsA-dependent transcription at 30 °C. We propose that this unique mechanism plays an important role in T3SS regulation.

Translocon-independent intracellular replication by Pseudomonas aeruginosa requires the ADP-ribosylation domain of ExoS

Pseudomonas aeruginosa, a significant cause of human morbidity and mortality, uses a type 3 secretion system (T3SS) to inject effector toxins into host cells. We previously reported that P. aeruginosa uses ADP-ribosyltransferase (ADPr) activity of the T3SS effector ExoS for intracellular replication. T3SS translocon (ΔpopB)-mutants, which can export, but not translocate effectors across host membranes, retained intracellular replication. We hypothesized that secreted effectors mediate translocon-independent intracellular replication. Translocon mutants of PAO1 lacking one or more of its three known effectors (ExoS, ExoT and ExoY) were used. All translocon mutants, irrespective of effectors expressed, localized to intracellular vacuoles. Translocon-effector null mutants and translocon-exoS mutants showed defective intracellular replication. Mutants in exoT, exoY or both replicated as efficiently as translocon mutants expressing all effectors. Complementation of translocon-effector null mutants with native exoS or a membrane localization domain mutant of exoS, but not the ADPr mutant exoS (pUCPexoSE381D), restored intracellular replication, correlating with increased bacteria per vacuole. Thus, P. aeruginosa is capable of intravacuolar replication that requires ExoS ADPr activity, but not the translocon. These data suggest that T3SS effectors can participate in pathogenesis without translocon-mediated translocation across host membranes, and that intracellular bacteria can contribute to P. aeruginosa pathogenesis within epithelial cells.

Bacterial pathogens commandeer Rab GTPases to establish intracellular niches

Intracellular bacterial pathogens deploy virulence factors termed effectors to inhibit degradation by host cells and to establish intracellular niches where growth and differentiation take place. Here, we describe mechanisms by which human bacterial pathogens (including Chlamydiae; Coxiella burnetii; Helicobacter pylori; Legionella pneumophila; Listeria monocytogenes; Mycobacteria; Pseudomonas aeruginosa, Salmonella enterica) modulate endocytic and exocytic Rab GTPases in order to thrive in host cells. Host cell Rab GTPases are critical for intracellular transport following pathogen phagocytosis or endocytosis. At the molecular level bacterial effectors hijack Rab protein function to: evade degradation, direct transport to particular intracellular locations and monopolize host vesicles carrying molecules that are needed for a stable niche and/or bacterial growth and differentiation. Bacterial effectors may serve as specific receptors for Rab GTPases or as enzymes that post-translationally modify Rab proteins or endosomal membrane lipids required for Rab function. Emerging data indicate that bacterial effector expression is temporally and spatially regulated and multiple virulence factors may act concertedly to usurp Rab GTPase function, alter signaling and ensure niche establishment and intracellular bacterial growth, making this field an exciting area for further study.

Examining the role of actin-plasma membrane association in Pseudomonas aeruginosa infection and type III secretion translocation in migratory T24 epithelial cells

The opportunistic pathogen Pseudomonas aeruginosa targets wounded epithelial barriers, but the cellular alteration that increases susceptibility to P. aeruginosa infection remains unclear. This study examined how cell migration contributes to the establishment of P. aeruginosa infections using (i) highly migratory T24 epithelial cells as a cell culture model, (ii) mutations in the type III secretion (T3S) effector ExoS to manipulate P. aeruginosa infection, and (iii) high-resolution immunofluorescent microscopy to monitor ExoS translocation. ExoS includes both GTPase-activating (GAP) and ADP-ribosyltransferase (ADPRT) activities, and P. aeruginosa cells expressing wild-type ExoS preferentially bound to the leading edge of T24 cells, where ExoS altered leading-edge architecture and actin anchoring in conjunction with interrupting T3S translocation. Inactivation of ExoS GAP activity allowed P. aeruginosa to be internalized and secrete ExoS within T24 cells, but as with wild-type ExoS, translocation was limited in association with disruption of actin anchoring. Inactivation of ExoS ADPRT activity resulted in significantly enhanced T3S translocation by P. aeruginosa cells that remained extracellular and in conjunction with maintenance of actin-plasma membrane association. Infection with P. aeruginosa expressing ExoS lacking both GAP and ADPRT activities resulted in the highest level of T3S translocation, and this occurred in conjunction with the entry and alignment of P. aeruginosa and ExoS along actin filaments. Collectively, in using ExoS mutants to modulate and visualize T3S translocation, we were able to (i) confirm effector secretion by internalized P. aeruginosa, (ii) differentiate the mechanisms underlying the effects of ExoS GAP and ADPRT activities on P. aeruginosa internalization and T3S translocation, (iii) confirm that ExoS ADPRT activity targeted a cellular substrate that interrupted T3S translocation, (iv) visualize the ability of P. aeruginosa and ExoS to align with actin filaments, and (v) demonstrate an association between actin anchoring at the leading edge of T24 cells and the establishment of P. aeruginosa infection. Our studies also highlight the contribution of ExoS to the opportunistic nature of P. aeruginosa infection through its ability to exert cytotoxic effects that interrupt T3S translocation and P. aeruginosa internalization, which in turn limit the P. aeruginosa infectious process.

ExoS and ExoT ADP ribosyltransferase activities mediate Pseudomonas aeruginosa keratitis by promoting neutrophil apoptosis and bacterial survival

Pseudomonas aeruginosa is a leading cause of blinding corneal ulcers worldwide. To determine the role of type III secretion in the pathogenesis of P. aeruginosa keratitis, corneas of C57BL/6 mice were infected with P. aeruginosa strain PAO1 or PAK, which expresses ExoS, ExoT, and ExoY, but not ExoU. PAO1- and PAK-infected corneas developed severe disease with pronounced opacification and rapid bacterial growth. In contrast, corneas infected with ΔpscD or ΔpscJ mutants that cannot assemble a type III secretion system, or with mutants lacking the translocator proteins, do not develop clinical disease, and bacteria are rapidly killed by infiltrating neutrophils. Furthermore, survival of PAO1 and PAK strains in the cornea and development of corneal disease was impaired in ΔexoS, ΔexoT, and ΔexoST mutants of both strains, but not in a ΔexoY mutant. ΔexoST mutants were also rapidly killed in neutrophils in vitro and were impaired in their ability to promote neutrophil apoptosis in vivo compared with PAO1. Point mutations in the ADP ribosyltransferase (ADPR) regions of ExoS or ExoT also impaired proapoptotic activity in infected neutrophils, and exoST(ADPR-) mutants replicated the ΔexoST phenotype in vitro and in vivo, whereas mutations in rho-GTPase-activating protein showed the same phenotype as PAO1. Together, these findings demonstrate that the pathogenesis of P. aeruginosa keratitis in ExoS- and ExoT-producing strains is almost entirely due to their ADPR activities, which subvert the host response by targeting the antibacterial activity of infiltrating neutrophils.

3D quantitative imaging of unprocessed live tissue reveals epithelial defense against bacterial adhesion and subsequent traversal requires MyD88

While a plethora of in vivo models exist for studying infectious disease and its resolution, few enable factors involved in the maintenance of health to be studied in situ. This is due in part to a paucity of tools for studying subtleties of bacterial-host interactions at a cellular level within live organs or tissues, requiring investigators to rely on overt outcomes (e.g. pathology) in their research. Here, a suite of imaging technologies were combined to enable 3D and temporal subcellular localization and quantification of bacterial distribution within the murine cornea without the need for tissue processing or dissection. These methods were then used to demonstrate the importance of MyD88, a central adaptor protein for Toll-Like Receptor (TLR) mediated signaling, in protecting a multilayered epithelium against both adhesion and traversal by the opportunistic bacterial pathogen Pseudomonas aeruginosa ex vivo and in vivo.

Adenylate cyclase activity of Pseudomonas aeruginosa ExoY can mediate bleb-niche formation in epithelial cells and contributes to virulence

We previously showed that ADP-ribosylation (ADP-r) activity of ExoS, a type III secreted toxin of Pseudomonas aeruginosa, enables bacterial replication in corneal and respiratory epithelial cells and correlates with bacterial trafficking to plasma membrane blebs (bleb-niche formation). Here, we explored another type III secreted toxin, ExoY, for its impact on intracellular trafficking and survival, and for virulence in vivo using a murine corneal infection model. Chromosomal or plasmid-mediated expression of exoY in invasive P. aeruginosa (strain PAO1) enabled bacteria to form and traffic to epithelial membrane blebs in the absence of other known effectors. In contrast, plasmid expression of any of four adenylate cyclase mutant forms of exoY did not enable bleb-niche formation, and bacteria localized to perinuclear vacuoles as for effector-null mutant controls. None of the plasmid-complemented bacteria used in this study showed ADP-r activity in the absence of ExoS and ExoT. In contrast to ADP-r activity of ExoS, bleb-niche formation induced by ExoY's adenylate cyclase activity was not accompanied by enhanced intracellular replication. In vivo results showed that ExoY-adenylate cyclase activity promoted P. aeruginosa corneal virulence in susceptible mice. Together the data show that adenylate cyclase activity of P. aeruginosa ExoY, similarly to the ADP-r activity of ExoS, can mediate bleb-niche formation in epithelial cells. While this activity did not promote intracellular replication in vitro, ExoY conferred increased virulence in vivo in susceptible mice. Mechanisms for bleb-niche formation and relationships to intracellular replication and virulence in vivo require further investigation for both ExoS and ExoY.