Recognition of intestinal epithelial HIF-1alpha activation by Pseudomonas aeruginosa

Immunometabolic Regulation of Bacterial Infection, Biofilms, and Antibiotic Susceptibility

BACKGROUND

Upon infection, mucosal tissues activate a brisk inflammatory response to clear the pathogen, i.e., resistance to disease. Resistance to disease is orchestrated by tissue-resident macrophages, which undergo profound metabolic reprogramming after sensing the pathogen. These metabolically activated macrophages release many inflammatory factors, which promote their bactericidal function. However, in immunocompetent individuals, pathogens like Pseudomonas aeruginosa, Staphylococcus aureus, and Salmonella evade this type of immunity, generating communities that thrive for the long term.

SUMMARY

These organisms develop features that render them less susceptible to eradication, such as biofilms and increased tolerance to antibiotics. Furthermore, after antibiotic therapy withdrawal, "persister" cells rapidly upsurge, triggering inflammatory relapses that worsen host health. How these pathogens persisted in inflamed tissues replete with activated macrophages remains poorly understood.

KEY MESSAGES

In this review, we discuss recent findings indicating that the ability of P. aeruginosa, S. aureus, and Salmonella to evolve biofilms and antibiotic tolerance is promoted by the similar metabolic routes that regulate macrophage metabolic reprogramming.

Targeting hypoxia-inducible factor-1 alpha suppresses Helicobacter pylori-induced gastric injury via attenuation of both cag-mediated microbial virulence and proinflammatory host responses

Helicobacter pylori-induced inflammation is the strongest known risk factor for gastric adenocarcinoma. Hypoxia-inducible factor-1 (HIF-1α) is a key transcriptional regulator of immunity and carcinogenesis. To examine the role of this mediator within the context of H. pylori-induced injury, we first demonstrated that HIF-1α levels were significantly increased in parallel with the severity of gastric lesions in humans. In interventional studies targeting HIF-1α, H. pylori-infected mice were treated ± dimethyloxalylglycine (DMOG), a prolyl hydroxylase inhibitor that stabilizes HIF-1α. H. pylori significantly increased proinflammatory chemokines/cytokines and inflammation in vehicle-treated mice; however, this was significantly attenuated in DMOG-treated mice. DMOG treatment also significantly decreased function of the H. pylori type IV secretion system (T4SS) in vivo and significantly reduced T4SS-mediated NF-κB activation and IL-8 induction in vitro. These results suggest that prolyl hydroxylase inhibition protects against H. pylori-mediated pathologic responses, and is mediated, in part, via attenuation of H. pylori cag-mediated virulence and suppression of host proinflammatory responses.

New understanding of gut microbiota and colorectal anastomosis leak: A collaborative review of the current concepts

Anastomotic leak (AL) is a life-threatening postoperative complication following colorectal surgery, which has not decreased over time. Until now, no specific risk factors or surgical technique could be targeted to improve anastomotic healing. In the past decade, gut microbiota dysbiosis has been recognized to contribute to AL, but the exact effects are still vague. In this context, interpretation of the mechanisms underlying how the gut microbiota contributes to AL is significant for improving patients' outcomes. This review concentrates on novel findings to explain how the gut microbiota of patients with AL are altered, how the AL-specific pathogen colonizes and is enriched on the anastomosis site, and how these pathogens conduct their tissue breakdown effects. We build up a framework between the gut microbiota and AL on three levels. Firstly, factors that shape the gut microbiota profiles in patients who developed AL after colorectal surgery include preoperative intervention and surgical factors. Secondly, AL-specific pathogenic or collagenase bacteria adhere to the intestinal mucosa and defend against host clearance, including the interaction between bacterial adhesion and host extracellular matrix (ECM), the biofilm formation, and the weakened host commercial bacterial resistance. Thirdly, we interpret the potential mechanisms of pathogen-induced poor anastomotic healing.

Systematic Review on the Influence of Tissue Oxygenation on Gut Microbiota and Anastomotic Healing

BACKGROUND

Anastomotic leak rates have not improved over several decades despite improvements in surgical techniques and patient care. The gut microbiome has been implicated in the development of leaks. The exact mechanisms by which tissue oxygenation affects gut microbial composition and anastomotic healing physiology are unclear. Also, commonly used carbon dioxide (CO₂) is a known vasodilator that improves tissue oxygen tension. We performed a systematic review to determine the influence of hyperoxia, hypoxia, and hypercapnia on the gut microbiome and anastomotic healing.

METHODS

A literature search was performed in MEDLINE, EMBASE, and COCHRANE to identify studies investigating the effects of hyperoxia, hypoxia, and hypercapnia on anastomotic healing and gut microbiota published between 1998 and 2018. Two reviewers screened the articles for eligibility and quality. Fifty-three articles underwent full text review, and a narrative synthesis was undertaken.

RESULTS

Hyperoxia is associated with better anastomotic healing, increased gastrointestinal oxygen tension, and may reduce gut anaerobes. Hypoxia is associated with poor healing and increased gut anaerobes. However, it is unclear if hypoxia is the most important predictor of anastomotic leaks. Low pressure CO₂ pneumoperitoneum and mild systemic hypercapnia are both associated with increased gastrointestinal oxygen tension and may improve anastomotic healing. We found no studies which investigated the effect of hypercapnia on gut microbiota in the context of anastomotic healing.

CONCLUSIONS

Tissue oxygenation influences gut anastomotic healing, but little evidence exists to demonstrate the influence on the gut microbiome in the context of healing. Further studies are needed to determine if anastomotic microbiome changes with altered tissue oxygenation and if this affects healing and leak rates. If confirmed, altering tissue oxygenation through hyperoxia or hypercapnia could be feasible means of altering the microbiome such that anastomotic leak rates reduce.

Platelet activating factor receptor regulates colitis-induced pulmonary inflammation through the NLRP3 inflammasome

Extra-intestinal manifestations (EIM) are common in inflammatory bowel disease (IBD). One such EIM is sub-clinical pulmonary inflammation, which occurs in up to 50% of IBD patients. In animal models of colitis, pulmonary inflammation is driven by neutrophilic infiltrations, primarily in response to the systemic bacteraemia and increased bacterial load in the lungs. Platelet activating factor receptor (PAFR) plays a critical role in regulating pulmonary responses to infection in conditions, such as chronic obstructive pulmonary disease and asthma. We investigated the role of PAFR in pulmonary EIMs of IBD, using dextran sulfate sodium (DSS) and anti-CD40 murine models of colitis. Both models induced neutrophilic inflammation, with increased TNF and IL-1β levels, bacterial load and PAFR protein expression in mouse lungs. Antagonism of PAFR decreased lung neutrophilia, TNF, and IL-1β in an NLRP3 inflammasome-dependent manner. Lipopolysaccharide from phosphorylcholine (ChoP)-positive bacteria induced NLRP3 and caspase-1 proteins in human alveolar epithelial cells, however antagonism of PAFR prevented NLRP3 activation by ChoP. Amoxicillin reduced bacterial populations in the lungs and reduced NLRP3 inflammasome protein levels, but did not reduce PAFR. These data suggest a role for PAFR in microbial pattern recognition and NLRP3 inflammasome signaling in the lung.

Effect of Hypoxia on the Pathogenesis of Acinetobacter baumannii and Pseudomonas aeruginosa In Vitro and in Murine Experimental Models of Infection

Hypoxia modulates bacterial virulence and the inflammation response through hypoxia-inducible factor 1α (HIF-1α). Here we study the influence of hypoxia on Acinetobacter baumannii and Pseudomonas aeruginosa infections. In vitro, hypoxia increases the bactericidal activities of epithelial cells against A. baumannii and P. aeruginosa, reducing extracellular bacterial concentrations to 50.5% ± 7.5% and 90.8% ± 13.9%, respectively, at 2 h postinfection. The same phenomenon occurs in macrophages (67.6% ± 18.2% for A. baumannii at 2 h and 50.3% ± 10.9% for P. aeruginosa at 24 h). Hypoxia decreases the adherence of A. baumannii to epithelial cells (42.87% ± 8.16% at 2 h) and macrophages (52.0% ± 18.7% at 24 h), as well as that of P. aeruginosa (24.9% ± 4.5% in epithelial cells and 65.7% ± 5.5% in macrophages at 2 h). Moreover, hypoxia decreases the invasion of epithelial cells (48.6% ± 3.8%) and macrophages (8.7% ± 6.9%) by A. baumannii at 24 h postinfection and by P. aeruginosa at 2 h postinfection (75.0% ± 16.3% and 63.4% ± 5.4%, respectively). In vivo, hypoxia diminishes bacterial loads in fluids and tissues in animal models of infection by both pathogens. In contrast, mouse survival time was shorter under hypoxia (23.92 versus 36.42 h) with A. baumannii infection. No differences in the production of cytokines or HIF-1α were found between hypoxia and normoxia in vitro or in vivo We conclude that hypoxia increases the bactericidal activities of host cells against both pathogens and reduces the interaction of pathogens with host cells. Moreover, hypoxia accelerates the rate at which animals die despite the lower bacterial concentrations in vivo.

Ventilator-Associated Pneumonia in Critically Ill Children: A New Paradigm

Ventilator-associated pneumonia (VAP) is a serious complication of critical illness. Surveillance definitions have undergone revisions for more objective and consistent reporting. The 1 organism-1 disease paradigm for microbial involvement may not adequately apply to many cases of VAP, in which pathogens are introduced to a pre-existing and often complex microbial community that facilitates or hinders the potential pathogen, consequently determining whether progression to VAP occurs. As omics technology is applied to VAP, a paradigm is emerging incorporating simultaneous assessments of microbial populations and their activity, as well as the host response, to personalize prevention and treatment.

Intestinal Epithelial Ecto-5'-Nucleotidase (CD73) Regulates Intestinal Colonization and Infection by Nontyphoidal Salmonella

Ecto-5'-nucleotidase (CD73) is expressed abundantly on the apical surface of intestinal epithelial cells (IECs) and functions as the terminal enzyme in the generation of extracellular adenosine. Previous work demonstrated that adenosine signaling in IECs results in a number of tissue-protective effects during inflammation; however, a rationale for its apical expression has been lacking. We hypothesized that the highly polarized expression of CD73 is indicative of an important role for extracellular adenosine as a mediator of host-microbe interactions. We show that adenosine harbors bacteriostatic activity against Salmonella enterica serovar Typhimurium that is not shared by the related purine metabolite 5'-AMP, inosine, or hypoxanthine. Analysis of Salmonella colonization in IEC-specific CD73 knockout mice (CD73^f/fVillin^Cre ) revealed a nearly 10-fold increase in colonization compared to that in controls. Despite the increased luminal colonization by Salmonella, CD73^f/fVillin^Cre mice were protected against Salmonella colitis and showed reduced Salmonella burdens in viscera, suggesting that adenosine promotes dissemination. The knockdown of CD73 expression in cultured IECs resulted in dramatic defects in intraepithelial localization and replication as well as defective transepithelial translocation by Salmonella In conclusion, we define a novel antimicrobial activity of adenosine in the gastrointestinal tract and unveil an important role for adenosine as a regulator of host-microbe interactions. These findings have broad implications for the development of new therapeutic agents for infectious disease.

Modeling Acinetobacter baumannii wound infections: The critical role of iron

BACKGROUND

Acinetobacter baumannii has emerged as an increasingly important and successful opportunistic human pathogen due to its ability to withstand harsh environmental conditions, its characteristic virulence factors, and quick adaptability to stress.

METHODS

We developed a clinically relevant murine model of A. baumannii traumatic wound infection to determine the effect of local wound environment on A. baumannii virulence. Mice underwent rectus muscle crush injury combined with ischemia created by epigastric vessel ligation, followed by A. baumannii inoculation. Reiterative experiments were performed using (1) a mutant deficient in the production of the siderophore acinetobactin, or (2) iron supplementation of the wound milieu. Mice were euthanized 7 days later, and rectus muscle analyzed for signs of clinical infection, HIF1α accumulation, bacterial abundance, and colony morphotype. To determine the effect of wound milieu on bacterial virulence, Galleria mellonella infection model was used.

RESULTS

The combination of rectus muscle injury with ischemia and A. baumannii inoculation resulted in 100% incidence of clinical wound infection that was significantly higher compared with other groups (n = 15/group, p < 0.0001). The highest level of wound infection was accompanied by the highest level of A. baumannii colonization (p < 0.0001) and the highest degree of HIF1α accumulation (p < 0.05). A. baumannii strains isolated from injured/ischemic muscle with clinical infection displayed a rough morphotype and a higher degree of virulence as judged by G. mellonella killing assay as compared with smooth morphotype colonies isolated from injured muscle without clinical infection (100% vs. 60%, n = 30 Log-Rank test, p = 0.0422). Iron supplementation prevented wound infection (n = 30, p < 0.0001) and decreased HIF1α (p = 0.039643). Similar results of decrease in wound infection and HIF1α were obtained when A. baumannii wild type was replaced with its derivative mutant [INCREMENT]BasD deficient in acinetobactin production.

CONCLUSION

The ability of A. baumannii to cause infections in traumatized wound relies on its ability to scavenge iron and can be prevented by iron supplementation to the wound milieu.

Collapse of the Microbiome, Emergence of the Pathobiome, and the Immunopathology of Sepsis

The definition of sepsis has been recently modified to accommodate emerging knowledge in the field, while at the same time being recognized as challenging, if not impossible, to define. Here, we seek to clarify the current understanding of sepsis as one that has been typically framed as a disorder of inflammation to one in which the competing interests of the microbiota, pathobiota, and host immune cells lead to loss of resilience and nonresolving organ dysfunction. Here, we challenge the existence of the idea of noninfectious sepsis given that critically ill humans never exist in a germ-free state. Finally, we propose a new vision of the pathophysiology of sepsis that includes the invariable loss of the host's microbiome with the emergence of a pathobiome consisting of both "healthcare-acquired and healthcare-adapted pathobiota." Under this framework, the critically ill patient is viewed as a host colonized by pathobiota dynamically expressing emergent properties which drive, and are driven by, a pathoadaptive immune response.

Why Quorum Sensing Controls Private Goods

Cell-cell communication, also termed quorum sensing (QS), is a widespread process that coordinates gene expression in bacterial populations. The generally accepted view is that QS optimizes the cell density-dependent benefit attained from cooperative behaviors, often in the form of secreted products referred to as "public goods." This view is challenged by an increasing number of cell-associated products or "private goods" reported to be under QS-control for which a collective benefit is not apparent. A prominent example is nucleoside hydrolase from Pseudomonas aeruginosa, a periplasmic enzyme that catabolizes adenosine. Several recent studies have shown that private goods can function to stabilize cooperation by co-regulated public goods, seemingly explaining their control by QS. Here we argue that this property is a by-product of selection for other benefits rather than an adaptation. Emphasizing ecophysiological context, we propose alternative explanations for the QS control of private goods. We suggest that the benefit attained from private goods is associated with high cell density, either because a relevant ecological condition correlates with density, or because the private good is, directly or indirectly, involved in cooperative behavior. Our analysis helps guide a systems approach to QS, with implications for antivirulence drug design and synthetic biology.

Lack of evidence for tissue hypoxia as a contributing factor in anastomotic leak following colon anastomosis and segmental devascularization in rats

PURPOSE

Current surgical dogma dictates that tissue ischemia and hypoxia are major contributing factors in anastomotic leak despite scant evidence. The aim of this study was to determine if tissue hypoxia is a feature of anastomotic leakage in rats following colon resection and segmental devascularization.

METHODS

Rats were randomly assigned to undergo sham operation, segmental colon devascularization alone, colectomy alone, or segmental devascularization plus colectomy. Tissue hypoxia present at the colon anastomosis site across the various treatment groups was determined at sacrifice on postoperative day 6. Pimonidazole HCl was injected 30 min prior to sacrifice. Anastomotic tissues were examined and scored for healing versus leakage using an anastomotic healing score (AHS). Collagen content, hypoxia, enteric smooth muscle and periendothelial stromal patterning, and apoptosis were evaluated histologically.

RESULTS

No differences in tissue hypoxia were noted in the 16% of anastomotic tissues with poor healing compared to the remaining 84% of rats whose anastomoses healed well. No significant changes were found in cell death in the submucosa of any group. Consistent with previous findings, poor healing was associated with lower collagen content. Submucosal thickness correlated with increased arteriole diameter (R ² = 0.25, p < 0.005).

CONCLUSIONS

These results demonstrate that tissue hypoxia is not a distinctive feature of anastomotic tissues that fail to heal and leak, even when their blood supply is interrupted. These findings suggest that compensatory factors may mitigate the effects of ischemia and hypoxia during healing of anastomotic tissues and that the process of leakage involves factors beyond their acute effects.

The Influence of Host Stress on the Mechanism of Infection: Lost Microbiomes, Emergent Pathobiomes, and the Role of Interkingdom Signaling

Mammals constantly face stressful situations, be it extended periods of starvation, sleep deprivation from fear of predation, changing environmental conditions, or loss of habitat. Today, mammals are increasingly exposed to xenobiotics such as pesticides, pollutants, and antibiotics. Crowding conditions such as those created for the purposes of meat production from animals or those imposed upon humans living in urban environments or during world travel create new levels of physiologic stress. As such, human progress has led to an unprecedented exposure of both animals and humans to accidental pathogens (i.e., those that have not co-evolved with their hosts). Strikingly missing in models of infection pathogenesis are the various elements of these conditions, in particular host physiologic stress. The compensatory factors released in the gut during host stress have profound and direct effects on the metabolism and virulence of the colonizing microbiota and the emerging pathobiota. Here, we address unanswered questions to highlight the relevance and importance of incorporating host stress to the field of microbial pathogenesis.

The Shift of an Intestinal "Microbiome" to a "Pathobiome" Governs the Course and Outcome of Sepsis Following Surgical Injury

Sepsis following surgical injury remains a growing and worrisome problem following both emergent and elective surgery. Although early resuscitation efforts and prompt antibiotic therapy have improved outcomes in the first 24 to 48  h, late onset sepsis is now the most common cause of death in modern intensive care units. This time shift may be, in part, a result of prolonged exposure of the host to the stressors of critical illness which, over time, erode the health promoting intestinal microbiota and allow for virulent pathogens to predominate. Colonizing pathogens can then subvert the immune system and contribute to the deterioration of the host response. Here, we posit that novel approaches integrating the molecular, ecological, and evolutionary dynamics of the evolving gut microbiome/pathobiome during critical illness are needed to understand and prevent the late onset sepsis that develops following prolonged critical illness.

Hypoxia and HIF-1 activation in bacterial infections

For most of the living beings, oxygen is one of the essential elements required to sustain life. Deprivation of oxygen causes tissue hypoxia and this severely affects host cell and organ functions. Tissue hypoxia is a prominent microenvironmental condition occurring in infections and there is a body of evidence that hypoxia and inflammation are interconnected with each other. The primary key factor mediating the mammalian hypoxic response is hypoxia inducible factor (HIF)-1, which regulates oxygen homeostasis on cellular, tissue and organism level. Recent studies show that HIF-1 plays a central role in angiogenesis, cancer and cardiovascular disease but also in bacterial infections. Activation of HIF-1 depends on the nature of the pathogen and the characteristics of infections in certain hosts. Up to date, it is not completely clear whether the phenomenon of HIF-1 activation in infections has a protective or detrimental effect on the host. In this review, we give an overview of whether and how hypoxia and HIF-1 affect the course of infections.

Is Quorum Sensing Interference a Viable Alternative to Treat Pseudomonas aeruginosa Infections?

Quorum sensing (QS) coordinates the expression of multiple virulence factors in Pseudomonas aeruginosa; hence its inhibition has been postulated as a new alternative to treat its infections. In particular, QS interference approaches claim that they attenuate bacterial virulence without directly decreasing bacterial growth and suggest that in vivo the immune system would control the infections. Moreover, since in vitro experiments performed in rich medium demonstrate that interfering with QS decreases the production of virulence factors without affecting bacterial growth it was assumed than in vivo therapies will minimize the selection of resistant strains. Therefore, the underlying assumptions toward an effective implementation of a successful Quorum sensing interference (QSI) therapy for treating P. aeruginosa infections are that (i) QS only exerts important effects in the regulation of virulence genes but it does not affect metabolic processes linked to growth, (ii) the expression of virulence factors is only positively regulated by QS, (iii) inhibition of virulence factors in vivo do not affect bacterial growth, (iv) the immune system of the infected patients will be able to get rid of the infections, and (v) the therapy will be effective in the strains that are actively producing the infections. Nevertheless, for QSI in P. aeruginosa, substantial experimental evidence against the validity of most of these assumptions has accumulated during the past years, suggesting that a far better understanding of its virulence and its behavior during infections is needed in order to design truly solid QSI therapeutic alternatives to combat this remarkable pathogen.

Pseudomonas aeruginosa wound infection involves activation of its iron acquisition system in response to fascial contact

BACKGROUND

Wound infections are traditionally thought to occur when microbial burden exceeds the innate clearance capacity of host immune system. Here, we introduce the idea that the wound environment itself plays a significant contributory role to wound infection.

METHODS

We developed a clinically relevant murine model of soft tissue infection to explore the role of activation of microbial virulence in response to tissue factors as a mechanism by which pathogenic bacteria cause wound infections. Mice underwent abdominal skin incision and light muscle injury with a crushing forceps versus skin incision alone followed by topical inoculation of Pseudomonas aeruginosa. Mice were sacrificed on postoperative Day 6, and abdominal tissues were analyzed for clinical signs of wound infection. To determine if specific wound tissue components induce bacterial virulence, P. aeruginosa was exposed to the skin, fascia, and muscle.

RESULTS

Gross wound infection caused by P. aeruginosa was observed to be significantly increased in injured tissues versus noninjured (80% vs.10%) tissues (n = 20 per group, p < 0.0001). Exposure of P. aeruginosa to individual tissue components demonstrated that fascia significantly induced bacterial virulence as judged by the production of pyocyanin, a redox-active phenazine compound known to kill immune cells. Whole-genome transcriptional profiling of P. aeruginosa exposed to the fascia demonstrated activation of multiple genes responsible for the synthesis of the iron scavenging molecule pyochelin.

CONCLUSION

We conclude that wound elements, in particular fascia, may play a significant role in enhancing the virulence of P. aeruginosa and may contribute to the pathogenesis of clinical wound infection.

Myeloid HIFs are dispensable for resolution of inflammation during skeletal muscle regeneration

Besides their role in cellular responses to hypoxia, hypoxia-inducible factors (HIFs) are involved in innate immunity and also have anti-inflammatory (M2) functions, such as resolution of inflammation preceding healing. Whereas the first steps of the inflammatory response are associated with proinflammatory (M1) macrophages (MPs), resolution of inflammation is associated with anti-inflammatory MPs exhibiting an M2 phenotype. This M1 to M2 sequence is observed during postinjury muscle regeneration, which provides an excellent paradigm to study the resolution of sterile inflammation. In this study, using in vitro and in vivo approaches in murine models, we demonstrated that deletion of hif1a or hif2a in MPs has no impact on the acquisition of an M2 phenotype. Furthermore, using a multiscale methodological approach, we showed that muscles did not require macrophagic hif1a or hif2a to regenerate. These results indicate that macrophagic HIFs do not play a crucial role during skeletal muscle regeneration induced by sterile tissue damage.

Myeloid HIF-1 is protective in Helicobacter pylori-mediated gastritis

Helicobacter pylori infection triggers chronic inflammation of the gastric mucosa that may progress to gastric cancer. The hypoxia-inducible factors (HIFs) are the central mediators of cellular adaptation to low oxygen levels (hypoxia), but they have emerged recently as major transcriptional regulators of immunity and inflammation. No studies have investigated whether H. pylori affects HIF signaling in immune cells and a potential role for HIF in H. pylori-mediated gastritis. HIF-1 and HIF-2 expression was examined in human H. pylori-positive gastritis biopsies. Subsequent experiments were performed in naive and polarized bone marrow-derived macrophages from wild-type (WT) and myeloid HIF-1α-null mice (HIF-1(Δmyel)). WT and HIF-1(Δmyel) mice were inoculated with H. pylori by oral gavage and sacrificed 6 mo postinfection. HIF-1 was specifically expressed in macrophages of human H. pylori-positive gastritis biopsies. Macrophage HIF-1 strongly contributed to the induction of proinflammatory genes (IL-6, IL-1β) and inducible NO synthase in response to H. pylori. HIF-2 expression and markers of M2 macrophage differentiation were decreased in response to H. pylori. HIF-1(Δmyel) mice inoculated with H. pylori for 6 mo presented with a similar bacterial colonization than WT mice but, surprisingly, a global increase of inflammation, leading to a worsening of the gastritis, measured by an increased epithelial cell proliferation. In conclusion, myeloid HIF-1 is protective in H. pylori-mediated gastritis, pointing to the complex counterbalancing roles of innate immune and inflammatory phenotypes in driving this pathology.

Amelioration of hypoxia and LPS-induced intestinal epithelial barrier dysfunction by emodin through the suppression of the NF-κB and HIF-1α signaling pathways

Intestinal barrier dysfunction occurs in critical illnesses and involves the inflammatory and hypoxic injury of intestinal epithelial cells. Researchers are still defining the underlying mechanisms and evaluating therapeutic strategies for restoring intestinal barrier function. The anti-inflammatory drug, emodin, has been shown to exert a protective effect on intestinal barrier function; however, its mechanisms of action remain unknown. In this study, we investigated the protective effects of emodin on intestinal barrier function and the underlying mechanisms in intestinal epithelial cells challenged with lipopolysaccharide (LPS) and hypoxia/reoxygenation (HR). To induce barrier dysfunction, Caco-2 monolayers were subjected to HR with or without LPS treatment. Transepithelial electrical resistance and paracellular permeability were measured to evaluate barrier function. The expression of the tight junction (TJ) proteins, zonula occludens (ZO)-1, occludin, and claudin-1, as well as that of hypoxia-inducible factor (HIF)-1α, phosphor-IκB-α, phosphor-nuclear factor (NF)-κB p65 and cyclooxygenase (COX)-2 was determined by western blot analysis. The results revealed that emodin markedly attenuated the decrease in transepithelial electrical resistance and the increase in paracellular permeability in the Caco-2 monolayers treated with LPS and subjected to HR. Emodin also markedly alleviated the damage caused by LPS and HR (manifested by a decrease in the expression of the TJ protein, ZO-1), and inhibited the expression of HIF-1α, IκB-α, NF-κB and COX-2 in a dose-dependent manner. In conclusion, our data suggest that emodin attenuates LPS- and HR-induced intestinal epithelial barrier dysfunction by inhibiting the HIF-1α and NF-κB signaling pathways and preventing the damage caused to the TJ barrier (shown by the decrease in the expression of ZO-1).

Redefining the gut as the motor of critical illness

The gut is hypothesized to play a central role in the progression of sepsis and multiple organ dysfunction syndrome. Critical illness alters gut integrity by increasing epithelial apoptosis and permeability and by decreasing epithelial proliferation and mucus integrity. Additionally, toxic gut-derived lymph induces distant organ injury. Although the endogenous microflora ordinarily exist in a symbiotic relationship with the gut epithelium, severe physiological insults alter this relationship, leading to induction of virulence factors in the microbiome, which, in turn, can perpetuate or worsen critical illness. This review highlights newly discovered ways in which the gut acts as the motor that perpetuates the systemic inflammatory response in critical illness.

Human intestinal epithelial cell-derived molecule(s) increase enterohemorrhagic Escherichia coli virulence

To better understand the role of host cell-derived molecules on enterohemorrhagic Escherichia coli (EHEC) infection, we studied EHEC virulence gene expression when exposed to cell-free spent (conditioned) medium (CM) from HCT-8 intestinal epithelial cells. Exposure to HCT-8 CM for 1 h and 3 h increased the expression of 32 of 41 EHEC locus of enterocyte effacement (LEE) virulence genes compared with fresh medium (FM). Expression of the Shiga toxin 1 (stx1B) gene was up-regulated at 1 h of exposure. Seventeen genes encoded by prophage 933W, including those for Stx2, were also up-regulated at both time-points. The increase in 933W prophage expression was mirrored by a 2.7-fold increase in phage titers. Consistent with the increase in virulence gene expression, we observed a fivefold increase in EHEC attachment to epithelial cells when exposed to CM. The increase in EHEC attachment was abolished when CM was heated to 95 °C or treated with proteinase K to degrade the proteins. The host cell-derived molecule(s) were larger than 3 kDa, which suggests that the molecule(s) that increase EHEC virulence and attachment are protein-based.

Acyl-homoserine lactone quorum sensing: from evolution to application

Quorum sensing (QS) is a widespread process in bacteria that employs autoinducing chemical signals to coordinate diverse, often cooperative activities such as bioluminescence, biofilm formation, and exoenzyme secretion. Signaling via acyl-homoserine lactones is the paradigm for QS in Proteobacteria and is particularly well understood in the opportunistic pathogen Pseudomonas aeruginosa. Despite thirty years of mechanistic research, empirical studies have only recently addressed the benefits of QS and provided support for the traditional assumptions regarding its social nature and its role in optimizing cell-density-dependent group behaviors. QS-controlled public-goods production has served to investigate principles that explain the evolution and stability of cooperation, including kin selection, pleiotropic constraints, and metabolic prudence. With respect to medical application, appreciating social dynamics is pertinent to understanding the efficacy of QS-inhibiting drugs and the evolution of resistance. Future work will provide additional insight into the foundational assumptions of QS and relate laboratory discoveries to natural ecosystems.

Pseudomonas aeruginosa Alkyl quinolones repress hypoxia-inducible factor 1 (HIF-1) signaling through HIF-1α degradation

The transcription factor hypoxia-inducible factor 1 (HIF-1) has recently emerged to be a crucial regulator of the immune response following pathogen perception, including the response to the important human pathogen Pseudomonas aeruginosa. However, as mechanisms involved in HIF-1 activation by bacterial pathogens are not fully characterized, understanding how bacteria and bacterial compounds impact on HIF-1α stabilization remains a major challenge. In this context, we have focused on the effect of secreted factors of P. aeruginosa on HIF-1 regulation. Surprisingly, we found that P. aeruginosa cell-free supernatant significantly repressed HIF-1α protein levels. Further characterization revealed that HIF-1α downregulation was dependent on a subset of key secreted factors involved in P. aeruginosa pathogenesis, the 2-alkyl-4-quinolone (AQ) quorum sensing (QS) signaling molecules, and in particular the pseudomonas quinolone signal (PQS). Under hypoxic conditions, the AQ-dependent downregulation of HIF-1α was linked to the suppressed induction of the important HIF-1 target gene hexokinase II. Furthermore, we demonstrated that AQ molecules directly target HIF-1α protein degradation through the 26S-proteasome proteolytic pathway but independently of the prolyl hydroxylase domain (PHD). In conclusion, this is the first report showing that bacterial molecules can repress HIF-1α protein levels. Manipulation of HIF-1 signaling by P. aeruginosa AQs could have major consequences for the host response to infection and may facilitate the infective properties of this pathogen.

Recent advances in small bowel diseases: Part I

As is the case in all parts of gastroenterology and hepatology, there have been many advances in our knowledge and understanding of small intestinal diseases. Over 1000 publications were reviewed for 2008 and 2009, and the important advances in basic science as well as clinical applications were considered. In Part I of this Editorial Review, seven topics are considered: intestinal development; proliferation and repair; intestinal permeability; microbiotica, infectious diarrhea and probiotics; diarrhea; salt and water absorption; necrotizing enterocolitis; and immunology/allergy. These topics were chosen because of their importance to the practicing physician.

Pseudomonas aeruginosa overrides the virulence inducing effect of opioids when it senses an abundance of phosphate

The gut during critical illness represents a complex ecology dominated by the presence of healthcare associated pathogens, nutrient scarce conditions, and compensatory host stress signals. We have previously identified key environmental cues, opioids and phosphate depletion that independently activate the virulence of Pseudomonas aeruginosa. Opioids induce quinolone signal production (PQS), whereas phosphate depletion leads to a triangulated response between MvfR-PQS, pyoverdin, and phosphosensory/phosphoregulatory systems (PstS-PhoB). Yet how P. aeruginosa manages its response to opioids during nutrient scarce conditions when growth is limited and a quorum is unlikely to be achieved is important in the context of pathogenesis in gut during stress. To mimic this environment, we created nutrient poor conditions and exposed P. aeruginosa PAO1 to the specific k-opioid receptor agonist U-50,488. Bacterial cells exposed to the k-opioid expressed a striking increase in virulence- and multi-drug resistance-related genes that correlated to a lethal phenotype in C. elegans killing assays. Under these conditions, HHQ, a precursor of PQS, rather than PQS itself, became the main inducer for pqsABCDE operon expression. P. aeruginosa virulence expression in response to k-opioids required PqsE since ΔPqsE was attenuated in its ability to activate virulence- and efflux pumps-related genes. Extracellular inorganic phosphate completely changed the transcriptional response of PAO1 to the k- opioid preventing pqsABCDE expression, the activation of multiple virulence- and efflux pumps-related genes, and the ability of P. aeruginosa to kill C. elegans. These results indicate that when P. aeruginosa senses resource abundance in the form of phosphate, it overrides its response to compensatory host signals such as opioids to express a virulent and lethal phenotype. These studies confirm a central role for phosphate in P. aeruginosa virulence that might be exploited to design novel anti- virulence strategies.

Interkingdom adenosine signal reduces Pseudomonas aeruginosa pathogenicity

Pseudomonas aeruginosa is becoming recognized as an important pathogen in the gastrointestinal (GI) tract. Here we demonstrate that adenosine, derived from hydrolysis of ATP from the eucaryotic host, is a potent interkingdom signal in the GI tract for this pathogen. The addition of adenosine nearly abolished P. aeruginosa biofilm formation and abolished swarming by preventing production of rhamnolipids. Since the adenosine metabolite inosine did not affect biofilm formation and since a mutant unable to metabolize adenosine behaved like the wild-type strain, adenosine metabolism is not required to reduce pathogenicity. Adenosine also reduces production of the virulence factors pyocyanin, elastase, extracellular polysaccharide, siderophores and the Pseudomonas quinolone signal which led to reduced virulence with Caenorhabditis elegans. To provide insights into how adenosine reduces the virulence of P. aeruginosa, a whole-transcriptome analysis was conducted which revealed that adenosine addition represses genes similar to an iron-replete condition; however, adenosine did not directly bind Fur. Therefore, adenosine decreases P. aeruginosa pathogenicity as an interkingdom signal by causing genes related to iron acquisition to be repressed.

Pseudomonas aeruginosa potentiates the lethal effect of intestinal ischemia-reperfusion injury: the role of in vivo virulence activation

BACKGROUND

Experimental models of intestinal ischemia-reperfusion (IIR) injury are invariably performed in mice harboring their normal commensal flora, even though multiple IIR events occur in humans during prolonged intensive care confinement when they are colonized by a highly pathogenic hospital flora. The aims of this study were to determine whether the presence of the human pathogen Pseudomonas aeruginosa in the distal intestine potentiates the lethality of mice exposed to IIR and to determine what role any in vivo virulence activation plays in the observed mortality.

METHODS

Seven- to 9-week-old C57/BL6 mice were exposed to 15 minutes of superior mesenteric artery occlusion (SMAO) followed by direct intestinal inoculation of 1.0 × 10(6) colony-forming unit of P. aeruginosa PAO1 into the ileum and observed for mortality. Reiterative studies were performed in separate groups of mice to evaluate both the migration/dissemination pattern and in vivo virulence activation of intestinally inoculated strains using live photon camera imaging of both a constitutive bioluminescent P. aeruginosa PAO1 derivative XEN41 and an inducible reporter derivative of PAO1, the PAO1/lecA:luxCDABE that conditionally expresses the quorum sensing-dependent epithelial disrupting virulence protein PA 1 Lectin (PA-IL).

RESULTS

Mice exposed to 15 minutes of SMAO and reperfusion with intestinal inoculation of P. aeruginosa had a significantly increased mortality rate (p < 0.001) of 100% compared with <10% for sham-operated mice intestinally inoculated with P. aeruginosa without SMAO and IIR alone (<50%). Migration/dissemination patterns of P. aeruginosa in mice subjected to IIR demonstrated proximal migration of distally injected strains and translocation to mesenteric lymph nodes, liver, spleen, lung, and kidney. A key role for in vivo virulence expression of the barrier disrupting adhesin PA-IL during IIR was established since its expression was enhanced during IR and mutant strains lacking PA-IL displayed attenuated mortality.

CONCLUSIONS

The presence of intestinal P. aeruginosa potentiates the lethal effect of IIR in mice in part due to in vivo virulence activation of its epithelial barrier disrupting protein PA-IL.

Candida albicans isolates from the gut of critically ill patients respond to phosphate limitation by expressing filaments and a lethal phenotype

Candida albicans is an opportunistic pathogen that proliferates in the intestinal tract of critically ill patients where it continues to be a major cause of infectious-related mortality. The precise cues that shift intestinal C. albicans from its ubiquitous indolent colonizing yeast form to an invasive and lethal filamentous form remain unknown. We have previously shown that severe phosphate depletion develops in the intestinal tract during extreme physiologic stress and plays a major role in shifting intestinal Pseudomonas aeruginosa to express a lethal phenotype via conserved phosphosensory-phosphoregulatory systems. Here we studied whether phosphate dependent virulence expression could be similarly demonstrated for C. albicans. C. albicans isolates from the stool of critically ill patients and laboratory prototype strains (SC5314, BWP17, SN152) were evaluated for morphotype transformation and lethality against C. elegans and mice during exposure to phosphate limitation. Isolates ICU1 and ICU12 were able to filament and kill C. elegans in a phosphate dependent manner. In a mouse model of intestinal phosphate depletion (30% hepatectomy), direct intestinal inoculation of C. albicans caused mortality that was prevented by oral phosphate supplementation. Prototype strains displayed limited responses to phosphate limitation; however, the pho4Δ mutant displayed extensive filamentation during low phosphate conditions compared to its isogenic parent strain SN152, suggesting that mutation in the transcriptional factor Pho4p may sensitize C. albicans to phosphate limitation. Extensive filamentation was also observed in strain ICU12 suggesting that this strain is also sensitized to phosphate limitation. Analysis of the sequence of PHO4 in strain ICU12, its transcriptional response to phosphate limitation, and phosphatase assays confirmed that ICU12 demonstrates a profound response to phosphate limitation. The emergence of strains of C. albicans with marked responsiveness to phosphate limitation may represent a fitness adaptation to the complex and nutrient scarce environment typical of the gut of a critically ill patient.

Proteus mirabilis interkingdom swarming signals attract blow flies

Flies transport specific bacteria with their larvae that provide a wider range of nutrients for those bacteria. Our hypothesis was that this symbiotic interaction may depend on interkingdom signaling. We obtained Proteus mirabilis from the salivary glands of the blow fly Lucilia sericata; this strain swarmed significantly and produced a strong odor that attracts blow flies. To identify the putative interkingdom signals for the bacterium and flies, we reasoned that as swarming is used by this bacterium to cover the food resource and requires bacterial signaling, the same bacterial signals used for swarming may be used to communicate with blow flies. Using transposon mutagenesis, we identified six novel genes for swarming (ureR, fis, hybG, zapB, fadE and PROSTU_03490), then, confirming our hypothesis, we discovered that fly attractants, lactic acid, phenol, NaOH, KOH and ammonia, restore swarming for cells with the swarming mutations. Hence, compounds produced by the bacterium that attract flies also are utilized for swarming. In addition, bacteria with the swarming mutation rfaL attracted fewer blow flies and reduced the number of eggs laid by the flies. Therefore, we have identified several interkingdom signals between P. mirabilis and blow flies.

Host stress and virulence expression in intestinal pathogens: development of therapeutic strategies using mice and C. elegans

The intestinal tract of a host exposed to extreme physiologic stress and modern medical intervention represents a relatively unexplored yet important area of infection research, given the frequency with which this site becomes colonized by highly pathogenic microorganisms that cause subsequent sepsis. Our laboratory has focused on the host tissue derived environmental cues that are released into the intestinal tract during extreme physiologic stress that induce the expression of virulence in colonizing pathogens with the goal of developing novel gut directed therapies that maintain host pathogen neutrality through the course of host stress. Here we demonstrate that maintenance of phosphate sufficiency/ abundance within the intestinal microenvironment may be considered as a universal strategy to prevent virulence activation across a broad range of pathogens that colonize the gut and cause sepsis, given that phosphate depletion occurs following stress and is a universal cue that activates the virulence of a wide variety of organisms. Using small animal models (Caenorhabditis elegans and mice) to create local phosphate depletion at sites of colonization of Pseudomonas aeruginosa, a common cause of lethal gut-derived sepsis, we demonstrate the importance of maintaining phosphate sufficiency to suppress the expression of a lethal phenotype during extreme physiologic stress. The molecular details and potential therapeutic implications are reviewed.

Agent-based dynamic knowledge representation of Pseudomonas aeruginosa virulence activation in the stressed gut: Towards characterizing host-pathogen interactions in gut-derived sepsis

Background

There is a growing realization that alterations in host-pathogen interactions (HPI) can generate disease phenotypes without pathogen invasion. The gut represents a prime region where such HPI can arise and manifest. Under normal conditions intestinal microbial communities maintain a stable, mutually beneficial ecosystem. However, host stress can lead to changes in environmental conditions that shift the nature of the host-microbe dialogue, resulting in escalation of virulence expression, immune activation and ultimately systemic disease. Effective modulation of these dynamics requires the ability to characterize the complexity of the HPI, and dynamic computational modeling can aid in this task. Agent-based modeling is a computational method that is suited to representing spatially diverse, dynamical systems. We propose that dynamic knowledge representation of gut HPI with agent-based modeling will aid in the investigation of the pathogenesis of gut-derived sepsis.

Methodology/Principal Findings

An agent-based model (ABM) of virulence regulation in Pseudomonas aeruginosa was developed by translating bacterial and host cell sense-and-response mechanisms into behavioral rules for computational agents and integrated into a virtual environment representing the host-microbe interface in the gut. The resulting gut milieu ABM (GMABM) was used to: 1) investigate a potential clinically relevant laboratory experimental condition not yet developed - i.e. non-lethal transient segmental intestinal ischemia, 2) examine the sufficiency of existing hypotheses to explain experimental data - i.e. lethality in a model of major surgical insult and stress, and 3) produce behavior to potentially guide future experimental design - i.e. suggested sample points for a potential laboratory model of non-lethal transient intestinal ischemia. Furthermore, hypotheses were generated to explain certain discrepancies between the behaviors of the GMABM and biological experiments, and new investigatory avenues proposed to test those hypotheses.

Conclusions/Significance

Agent-based modeling can account for the spatio-temporal dynamics of an HPI, and, even when carried out with a relatively high degree of abstraction, can be useful in the investigation of system-level consequences of putative mechanisms operating at the individual agent level. We suggest that an integrated and iterative heuristic relationship between computational modeling and more traditional laboratory and clinical investigations, with a focus on identifying useful and sufficient degrees of abstraction, will enhance the efficiency and translational productivity of biomedical research.

Quorum quenching quandary: resistance to antivirulence compounds

Quorum sensing (QS) is the regulation of gene expression in response to the concentration of small signal molecules, and its inactivation has been suggested to have great potential to attenuate microbial virulence. It is assumed that unlike antimicrobials, inhibition of QS should cause less Darwinian selection pressure for bacterial resistance. Using the opportunistic pathogen Pseudomonas aeruginosa, we demonstrate here that bacterial resistance arises rapidly to the best-characterized compound that inhibits QS (brominated furanone C-30) due to mutations that increase the efflux of C-30. Critically, the C-30-resistant mutant mexR was more pathogenic to Caenorhabditis elegans in the presence of C-30, and the same mutation arises in bacteria responsible for chronic cystic fibrosis infections. Therefore, bacteria may evolve resistance to many new pharmaceuticals thought impervious to resistance.

The human microbiome and surgical disease

OBJECTIVE

The purpose of this review article is to summarize what is currently known about microbes associated with the human body and to provide examples of how this knowledge impacts the care of surgical patients.

BACKGROUND

Pioneering research over the past decade has demonstrated that human beings live in close, constant contact with dynamic communities of microbial organisms. This new reality has wide-ranging implications for the care of surgical patients.

METHODS AND RESULTS

Recent advances in the culture-independent study of the human microbiome are reviewed. To illustrate the translational relevance of these studies to surgical disease, we discuss in detail what is known about the role of microbes in the pathogenesis of obesity, gastrointestinal malignancies, Crohn disease, and perioperative complications including surgical site infections and sepsis. The topics of mechanical bowel preparation and perioperative antibiotics are also discussed.

CONCLUSIONS

Heightened understanding of the microbiome in coming years will likely offer opportunities to refine the prevention and treatment of a wide variety of surgical conditions.

Hypoxia-inducible factor plays a gut-injurious role in intestinal ischemia reperfusion injury

Gut injury and loss of normal intestinal barrier function are key elements in the paradigm of gut-origin systemic inflammatory response syndrome, acute lung injury, and multiple organ dysfunction syndrome (MODS). As hypoxia-inducible factor (HIF-1) is a critical determinant of the physiological and pathophysiological response to hypoxia and ischemia, we asked whether HIF-1 plays a proximal role in the induction of gut injury and subsequent lung injury. Using partially HIF-1α-deficient mice in an isolated superior mesenteric artery occlusion (SMAO) intestinal ischemia reperfusion (I/R) injury model (45 min SMAO followed by 3 h of reperfusion), we showed a direct relationship between HIF-1 activation and intestinal I/R injury. Specifically, partial HIF-1α deficiency attenuated SMAO-induced increases in intestinal permeability, lipid peroxidation, mucosal caspase-3 activity, and IL-1β mRNA levels. Furthermore, partial HIF-1α deficiency prevented the induction of ileal mucosal inducible nitric oxide synthase (iNOS) protein levels after SMAO and iNOS deficiency ameliorated SMAO-induced villus injury. Resistance to SMAO-induced gut injury was also associated with resistance to lung injury, as reflected by decreased levels of myeloperoxidase, IL-6 and IL-10 in the lungs of HIF-1α(+/-) mice. In contrast, a short duration of SMAO (15 min) followed by 3 h of reperfusion neither induced mucosal HIF-1α protein levels nor caused significant gut and lung injury in wild-type or HIF-1α(+/-) mice. This study indicates that intestinal HIF-1 activation is a proximal regulator of I/R-induced gut mucosal injury and gut-induced lung injury. However, the duration and severity of the gut I/R insult dictate whether HIF-1 plays a gut-protective or deleterious role.

Impaired expression of hypoxia-inducible factor-1α in cystic fibrosis airway epithelial cells - a role for HIF-1 in the pathophysiology of CF?

The continuous infection-inflammation cycle plays a crucial role in the progression of cystic fibrosis (CF) disease. This noxious loop can be aggravated by a reduced partial pressure of oxygen in the blood, hypoxemia, present in CF patients. These interconnected factors, hypoxia, inflammation and infection, by stabilizing the hypoxia-inducible factor-1α (HIF-1α) protein subunit, are able to activate the transcription factor HIF-1. To date, data investigating the potential role of HIF-1 in CF are scarce. Our results demonstrated that HIF-1α protein expression was altered in CF-affected compared to CFTR-corrected airway epithelial cells in unsimulated and simulated hypoxic conditions. In contrast, when CF-affected cells were infected with Pseudomonas aeruginosa, HIF-1α was more stabilized compared to CFTR-corrected cells. As HIF-1 is linked with an efficient immune response and pulmonary complications in cystic fibrosis, this difference in HIF-1α protein levels could have an impact in the CF pathology and the persistence of P. aeruginosa infection.

HIF-1 mediates pathogenic inflammatory responses to intestinal ischemia-reperfusion injury

Acute lung injury (ALI) and the development of the multiple organ dysfunction syndrome (MODS) are major causes of death in trauma patients. Gut inflammation and loss of gut barrier function as a consequence of splanchnic ischemia-reperfusion (I/R) have been implicated as the initial triggering events that contribute to the development of the systemic inflammatory response, ALI, and MODS. Since hypoxia-inducible factor (HIF-1) is a key regulator of the physiological and pathophysiological response to hypoxia, we asked whether HIF-1 plays a proximal role in the induction of gut injury and subsequent lung injury. Utilizing partially HIF-1α-deficient mice in a global trauma hemorrhagic shock (T/HS) model, we found that HIF-1 activation was necessary for the development of gut injury and that the prevention of gut injury was associated with an abrogation of lung injury. Specifically, in vivo studies demonstrated that partial HIF-1α deficiency ameliorated T/HS-induced increases in intestinal permeability, bacterial translocation, and caspase-3 activation. Lastly, partial HIF-1α deficiency reduced TNF-α, IL-1β, cyclooxygenase-2, and inducible nitric oxide synthase levels in the ileal mucosa after T/HS whereas IL-1β mRNA levels were reduced in the lung after T/HS. This study indicates that prolonged intestinal HIF-1 activation is a proximal regulator of I/R-induced gut mucosal injury and gut-induced lung injury. Consequently, these results provide unique information on the initiating events in trauma-hemorrhagic shock-induced ALI and MODS as well as potential therapeutic insights.

Oral PEG 15-20 protects the intestine against radiation: role of lipid rafts

Intestinal injury following abdominal radiation therapy or accidental exposure remains a significant clinical problem that can result in varying degrees of mucosal destruction such as ulceration, vascular sclerosis, intestinal wall fibrosis, loss of barrier function, and even lethal gut-derived sepsis. We determined the ability of a high-molecular-weight polyethylene glycol-based copolymer, PEG 15-20, to protect the intestine against the early and late effects of radiation in mice and rats and to determine its mechanism of action by examining cultured rat intestinal epithelia. Rats were exposed to fractionated radiation in an established model of intestinal injury, whereby an intestinal segment is surgically placed into the scrotum and radiated daily. Radiation injury score was decreased in a dose-dependent manner in rats gavaged with 0.5 or 2.0 g/kg per day of PEG 15-20 (n = 9-13/group, P < 0.005). Complementary studies were performed in a novel mouse model of abdominal radiation followed by intestinal inoculation with Pseudomonas aeruginosa (P. aeruginosa), a common pathogen that causes lethal gut-derived sepsis following radiation. Mice mortality was decreased by 40% in mice drinking 1% PEG 15-20 (n = 10/group, P < 0.001). Parallel studies were performed in cultured rat intestinal epithelial cells treated with PEG 15-20 before radiation. Results demonstrated that PEG 15-20 prevented radiation-induced intestinal injury in rats, prevented apoptosis and lethal sepsis attributable to P. aeruginosa in mice, and protected cultured intestinal epithelial cells from apoptosis and microbial adherence and possible invasion. PEG 15-20 appeared to exert its protective effect via its binding to lipid rafts by preventing their coalescence, a hallmark feature in intestinal epithelial cells exposed to radiation.

Gram-negative bacterial sensors for eukaryotic signal molecules

Ample evidence exists showing that eukaryotic signal molecules synthesized and released by the host can activate the virulence of opportunistic pathogens. The sensitivity of prokaryotes to host signal molecules requires the presence of bacterial sensors. These prokaryotic sensors, or receptors, have a double function: stereospecific recognition in a complex environment and transduction of the message in order to initiate bacterial physiological modifications. As messengers are generally unable to freely cross the bacterial membrane, they require either the presence of sensors anchored in the membrane or transporters allowing direct recognition inside the bacterial cytoplasm. Since the discovery of quorum sensing, it was established that the production of virulence factors by bacteria is tightly growth-phase regulated. It is now obvious that expression of bacterial virulence is also controlled by detection of the eukaryotic messengers released in the micro-environment as endocrine or neuro-endocrine modulators. In the presence of host physiological stress many eukaryotic factors are released and detected by Gram-negative bacteria which in return rapidly adapt their physiology. For instance, Pseudomonas aeruginosa can bind elements of the host immune system such as interferon-γ and dynorphin and then through quorum sensing circuitry enhance its virulence. Escherichia coli sensitivity to the neurohormones of the catecholamines family appears relayed by a recently identified bacterial adrenergic receptor. In the present review, we will describe the mechanisms by which various eukaryotic signal molecules produced by host may activate Gram-negative bacteria virulence. Particular attention will be paid to Pseudomonas, a genus whose representative species, P. aeruginosa, is a common opportunistic pathogen. The discussion will be particularly focused on the pivotal role played by these new types of pathogen sensors from the sensing to the transduction mechanism involved in virulence factors regulation. Finally, we will discuss the consequence of the impact of host signal molecules on commensally or opportunistic pathogens associated with different human tissue.

Feedback effects of host-derived adenosine on enteropathogenic Escherichia coli

Enteropathogenic E. coli (EPEC) is a common cause of diarrhea in children in developing countries. After adhering to intestinal cells, EPEC secretes effector proteins into host cells, causing cell damage and eventually death. We previously showed that EPEC infection triggers the release of ATP from host cells and that ATP is broken down to ADP, AMP, and adenosine. Adenosine produced from the breakdown of extracellular ATP triggers fluid secretion in intestinal monolayers and may be an important mediator of EPEC-induced diarrhea. Here we examined whether adenosine has any effects on EPEC bacteria. Adenosine stimulated EPEC growth in several types of media in vitro. Adenosine also altered the pattern of EPEC adherence to cultured cells from a localized adherence pattern to a more diffuse pattern. Adenosine changed the expression of virulence factors in EPEC, inhibiting the expression of the bundle-forming pilus (BFP) and enhancing expression of the EPEC secreted proteins (Esps). In vivo, experimental manipulations of adenosine levels had strong effects on the outcome of EPEC infection in rabbit intestinal loops. In addition to its previously reported effects on host tissues, adenosine has strong effects on EPEC bacteria, stimulating EPEC growth, altering its adherence pattern, and changing the expression of several important virulence genes. Adenosine, like noradrenaline, is a small, host-derived molecule that is utilized as a signal by EPEC.

Dangers, and benefits of the cytokine mediated response to injury and infection

The inflammatory response is essential for survival in an environment where continuous exposure to noxious events threaten the integrity of the organism. However, the beneficial effects of the response are influenced by factors, which disadvantage individuals within a population. These factors include malnutrition, infection, genotype, gender, pre-existing inflammation, and chronic intoxication. Although the inflammatory response is generally successful in dealing with noxious events, life-long exposure to these events takes its toll on the integrity of the body and becomes apparent as chronic disease, atherosclerosis, organ failure, and frailty. Progress in ameliorating the consequences of lifetime exposure to inflammatory events can only occur if a fuller understanding can be obtained of the factors, which influence the persistence and outcome of the inflammatory response at an individual level. A multitude of studies has shown that specific nutrients, diets, and dietary restriction are able to modulate the inflammatory response in the population as a whole. To advance in this area, precise knowledge is needed of how the disadvantageous factors, mentioned above, affect the individual's response to anti-inflammatory nutrients.

Comparative genomic characterization of Francisella tularensis strains belonging to low and high virulence subspecies

Tularemia is a geographically widespread, severely debilitating, and occasionally lethal disease in humans. It is caused by infection by a gram-negative bacterium, Francisella tularensis. In order to better understand its potency as an etiological agent as well as its potential as a biological weapon, we have completed draft assemblies and report the first complete genomic characterization of five strains belonging to the following different Francisella subspecies (subsp.): the F. tularensis subsp. tularensis FSC033, F. tularensis subsp. holarctica FSC257 and FSC022, and F. tularensis subsp. novicida GA99-3548 and GA99-3549 strains. Here, we report the sequencing of these strains and comparative genomic analysis with recently available public Francisella sequences, including the rare F. tularensis subsp. mediasiatica FSC147 strain isolate from the Central Asian Region. We report evidence for the occurrence of large-scale rearrangement events in strains of the holarctica subspecies, supporting previous proposals that further phylogenetic subdivisions of the Type B clade are likely. We also find a significant enrichment of disrupted or absent ORFs proximal to predicted breakpoints in the FSC022 strain, including a genetic component of the Type I restriction-modification defense system. Many of the pseudogenes identified are also disrupted in the closely related rarely human pathogenic F. tularensis subsp. mediasiatica FSC147 strain, including modulator of drug activity B (mdaB) (FTT0961), which encodes a known NADPH quinone reductase involved in oxidative stress resistance. We have also identified genes exhibiting sequence similarity to effectors of the Type III (T3SS) and components of the Type IV secretion systems (T4SS). One of the genes, msrA2 (FTT1797c), is disrupted in F. tularensis subsp. mediasiatica and has recently been shown to mediate bacterial pathogen survival in host organisms. Our findings suggest that in addition to the duplication of the Francisella Pathogenicity Island, and acquisition of individual loci, adaptation by gene loss in the more recently emerged tularensis, holarctica, and mediasiatica subspecies occurred and was distinct from evolutionary events that differentiated these subspecies, and the novicida subspecies, from a common ancestor. Our findings are applicable to future studies focused on variations in Francisella subspecies pathogenesis, and of broader interest to studies of genomic pathoadaptation in bacteria.

Red death in Caenorhabditis elegans caused by Pseudomonas aeruginosa PAO1

During host injury, Pseudomonas aeruginosa can be cued to express a lethal phenotype within the intestinal tract reservoir-a hostile, nutrient scarce environment depleted of inorganic phosphate. Here we determined if phosphate depletion activates a lethal phenotype in P. aeruginosa during intestinal colonization. To test this, we allowed Caenorhabditis elegans to feed on lawns of P. aeruginosa PAO1 grown on high and low phosphate media. Phosphate depletion caused PAO1 to kill 60% of nematodes whereas no worms died on high phosphate media. Unexpectedly, intense redness was observed in digestive tubes of worms before death. Using a combination of transcriptome analyses, mutants, and reporter constructs, we identified 3 global virulence systems that were involved in the "red death" response of P. aeruginosa during phosphate depletion; they included phosphate signaling (PhoB), the MvfR-PQS pathway of quorum sensing, and the pyoverdin iron acquisition system. Activation of all 3 systems was required to form a red colored PQS+Fe(3+) complex which conferred a lethal phenotype in this model. When pyoverdin production was inhibited in P. aeruginosa by providing excess iron, red death was attenuated in C. elegans and mortality was decreased in mice intestinally inoculated with P. aeruginosa. Introduction of the red colored PQS+Fe(3+) complex into the digestive tube of C. elegans or mouse intestine caused mortality associated with epithelial disruption and apoptosis. In summary, red death in C. elegans reveals a triangulated response between PhoB, MvfR-PQS, and pyoverdin in response to phosphate depletion that activates a lethal phenotype in P. aeruginosa.

The immune system in critical illness

The mammalian immune system comprises a complex network of physical and molecular elements that protect the individual from danger in the environment. An evolutionarily ancient innate immune system recognizes danger through pattern-recognition receptors that are encoded in the genome and mobilizes a rapid and potent but nonspecific response. This response is responsible for the clinical syndromes of sepsis and the multiple organ dysfunction syndrome. The adaptive immune system is highly selective in its targets and is endowed with memory but is slow in initial activation. Critical illness results in derangements of all components of the immune response, but the very complexity of the process has frustrated attempts to correct these derangements and to affect significantly the clinical course of sepsis.

Structure-function aspects of PstS in multi-drug-resistant Pseudomonas aeruginosa

The increasing prevalence of multi-drug-resistant (MDR) strains of Pseudomonas aeruginosa among critically ill humans is of significant concern. In the current study, we show that MDR clinical isolates of P. aeruginosa representing three distinct genotypes that display high virulence against intestinal epithelial cells, form novel appendage-like structures on their cell surfaces. These appendages contain PstS, an extracellular phosphate binding protein. Using anti-PstS antibodies, we determined that the PstS-rich appendages in MDR strains are involved in adherence to and disruption of the integrity of cultured intestinal epithelial cell monolayers. The outer surface-expressed PstS protein was also identified to be present in P. aeruginosa MPAO1, although to a lesser degree, and its role in conferring an adhesive and barrier disruptive phenotype against intestinal epithelial cells was confirmed using an isogenic DeltaPstS mutant. Formation of the PstS rich appendages was induced during phosphate limitation and completely suppressed in phosphate-rich media. Injection of MDR strains directly into the intestinal tract of surgically injured mice, a known model of phosphate limitation, caused high mortality rates (60%-100%). Repletion of intestinal phosphate in this model completely prevented mortality. Finally, significantly less outer surface PstS was observed in the MPAO1 mutant DeltaHxcR thus establishing a role for the alternative type II secretion system Hxc in outer surface PstS expression. Gene expression analysis performed by RT-PCR confirmed this finding and further demonstrated abundant expression of pstS analogous to pa5369, pstS analogous to pa0688/pa14-55410, and hxcX in MDR strains. Taken together, these studies provide evidence that outer surface PstS expression confers a highly virulent phenotype of MDR isolates against the intestinal epithelium that alters their adhesive and barrier disrupting properties against the intestinal epithelium.

The re-emerging role of the intestinal microflora in critical illness and inflammation: why the gut hypothesis of sepsis syndrome will not go away

Recent advances in the ability to genetically interrogate microbial communities within the intestinal tract of humans have revealed many striking findings. That there may be as many as 300 unculturable and unclassified microbes within the human intestinal tract opens the possibility that yet-unidentified microbes may play a role in various human diseases [( 1) ]. Technologically, the regional and spatial aspects of intestinal microbial communities can now be better appreciated by emerging genetic and in vivo imaging systems using a bioinformatics approach [( 2) ]. Finally, in situ PCR of tissues and blood now allows the detection of microbes at concentrations that would otherwise remain undetected by culture alone [( 3) ]. In the aggregate, these studies have empowered clinicians to readdress the issue of how our microbial partners are affected by extreme states of physiologic stress and antibiotic use through the course of critical illness. The role of microbes in systemic inflammatory states, such as systemic inflammatory response syndrome, as well as in primary intestinal mucosal diseases, such as necrotizing enterocolitis, inflammatory bowel disease, and ischemia-reperfusion injury, can now be more completely defined, and the microbial genes that mediate the immune activation during these disorders can be identified. The 2008 roadmap initiative at the National Institutes of Health to fully define the human microbiome is further testament to the power of this technology and the importance of understanding how intestinal microbes, their genes, and their gene products affect the course of human disease and inflammation.