The importance of six common bacteria in intestinal strangulation

Dynorphin activates quorum sensing quinolone signaling in Pseudomonas aeruginosa

There is now substantial evidence that compounds released during host stress directly activate the virulence of certain opportunistic pathogens. Here, we considered that endogenous opioids might function as such compounds, given that they are among the first signals to be released at multiple tissue sites during host stress. We tested the ability of various opioid compounds to enhance the virulence of Pseudomonas aeruginosa using pyocyanin production as a biological readout, and demonstrated enhanced virulence when P. aeruginosa was exposed to synthetic (U-50,488) and endogenous (dynorphin) κ-agonists. Using various mutants and reporter strains of P. aeruginosa, we identified involvement of key elements of the quorum sensing circuitry such as the global transcriptional regulator MvfR and the quorum sensing-related quinolone signaling molecules PQS, HHQ, and HQNO that respond to κ-opioids. The in vivo significance of κ-opioid signaling of P. aeruginosa was demonstrated in mice by showing that dynorphin is released from the intestinal mucosa following ischemia/reperfusion injury, activates quinolone signaling in P. aeruginosa, and enhances the virulence of P. aeruginosa against Lactobacillus spp. and Caenorhabditis elegans. Taken together, these data demonstrate that P. aeruginosa can intercept opioid compounds released during host stress and integrate them into core elements of quorum sensing circuitry leading to enhanced virulence.

Precisely how bacterial pathogens such as Pseudomonas aeruginosa cause fatal infections in critically ill humans is unknown. Evidence suggests that a major source of infection may be the patient's own intestinal microflora, which is subjected to unusual environmental conditions during critical illness. Here, we show that intestinal P. aeruginosa can be alerted to the presence of a physiological disturbance in its host by dynorphin, a human morphine-like chemical released during severe stress. Exposure of P. aeruginosa to dynorphin activates its virulence machinery to produce harmful toxins and to suppress the growth of probiotic bacteria, which are known to promote intestinal health. The molecular mechanisms of these events involve the activation of highly regulated virulence machinery in Pseudomonas, called quorum sensing, that allows bacteria to sense host stress and respond with enhanced harmfulness. These observations suggest that opportunistic pathogens like P. aeruginosa are equipped with sophisticated surveillance systems that take advantage of a weakened host by intercepting and responding to naturally occurring host chemicals that are normally used as signaling molecules for immune activation and analgesia. Elucidation of the effect of dynorphin on Pseudomonas exposes a major mechanism by which this organism behaves as a true opportunist.

The struggle for iron: gastrointestinal microbes modulate the host immune response during infection

The gastrointestinal track is one source of potential bacterial entry into the host, and the local immune system at the mucosal border is paramount in establishing host immune tolerance and the immune response to invading organisms. Macrophages use iron for production of hydroxy-radical and superoxide reactions, which are necessary for microbial killing. Presumably, as a survival strategy, bacteria, which also require iron for survival, have adapted the ability to sequester iron from the host, thereby limiting the availability to macrophages. As current modes of antimicrobial therapy are evolving, examination of nontraditional therapies is emerging. One such potential therapy involves altering the bacterial micronutrient iron concentration. Necrotizing enterocolitis is a clinical condition where such a strategy makes intuitive sense. This review will describe the immune response to gastrointestinal infection, the mechanisms that the gastrointestinal system uses to absorb intraluminal iron, and the critical role iron plays in the infectious process.

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

Human intestinal epithelial cell monolayers (Caco-2) subjected to hypoxia and reoxygenation release soluble factors into the apical medium that activate the virulence of the opportunistic pathogen Pseudomonas aeruginosa to express the potent barrier-dysregulating protein PA-I lectin/adhesin. In this study, we defined the role of hypoxia-inducible factor (HIF)-1alpha in this response. We tested the ability of medium from Caco-2 cells with forced expression of HIF-1alpha to increase PA-I expression in P. aeruginosa and found that medium from Caco-2 cells overexpressing HIF-1alpha increased PA-I expression compared with medium from control cells (P < 0.001, ANOVA). To identify the components responsible for this response, medium was fractionated by molecular weight and subjected to mass spectroscopy, which identified adenosine as the possible mediator. Both adenosine and its immediate downstream metabolite inosine induced PA-I expression in P. aeruginosa in a dose-dependent fashion. Because inosine was not detectable in the medium of Caco-2 cells exposed to hypoxia or overexpressing HIF-1alpha, we hypothesized that P. aeruginosa itself might metabolize adenosine to inosine. Using mutant and parental strains of P. aeruginosa, we demonstrated that P. aeruginosa metabolized adenosine to inosine via adenosine deaminase and that the conditioned medium enhanced the extracellular accumulation of inosine. Together, these results provide evidence that P. aeruginosa can recognize and respond to extracellular end products of intestinal hypoxia that are released after activation of HIF-1alpha. The ability of P. aeruginosa to metabolize adenosine to inosine may represent a subversive microbial virulence strategy that deprives the epithelium of the cytoprotective actions of adenosine.

The epidemiology and pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC), a syndrome characterized by crepitant necrosis of the bowel, has emerged as the most common neonatal gastrointestinal emergency in many countries of the world. In the United States, NEC strikes 1 to 8% of patients admitted to neonatal intensive care units, almost all of whom are premature infants. The incidence is low in certain countries wity a low premature birth rate, e.g., Japan. Two theories of pathogenesis are: the Santulli theory, which implicates three factors: ischaemia, bacteria and substrate; and the Lawrence theory, which stresses the injurious role of bacterial toxins on the immature gut of the preterm infant. Clinical and experimental evidence support each of the theories, but neither theory can explain certain clinical phenomena, particularly the resistance to NEC manifested by more than 90% of preterm infants, who never develop the syndrome. A unifying hypothesis of pathogenesis and a mathematical model of NEC are outlined. Because clustering of cases may occur, the design of clinical trials of preventive measures for NEC must include simultaneous control infants.

Impact of the indigenous flora in animal models of shock and sepsis

Septicemia is currently the 10th leading cause of death in the United States, and shock and trauma patients are the source of much of the morbidity and mortality associated with septicemia. There is substantial evidence that the composition of the indigenous flora plays an important role in modulating outcome variables in animal models of shock and sepsis. Germ-free animals that lack an indigenous flora are not as susceptible to shock as their conventionally reared counterparts. And, in conventionally reared animals, the composition of the intestinal flora can also modulate outcome in shock and sepsis. For example, certain bacterial species/strains disseminate from the intestinal tract more easily than others, antibiotic-induced alterations of the flora can modulate the incidence of systemic spread, and a certain threshold number of intestinal bacteria facilitates extraintestinal dissemination. The composition of the intestinal flora can also affect intestinal permeability, the production of inflammatory mediators, and the responses of immune cells in extraintestinal sites. And, there is evidence that prior exposure to endotoxin, via either the oral or systemic route, can influence outcome in animals challenged with parenteral endotoxin, a widely used model of endotoxin shock. The general composition of intestinal flora of experimental animals can be characterized with relative ease. This knowledge can aid data interpretation, either to help explain irreproducible or expected results or to verify that observed differences are likely related to the dependent variable studied rather than the composition of the indigenous flora.

Pneumatosis intestinalis complicating C. difficile pseudomembranous colitis

Pneumatosis intestinalis (PI) is characterized by multiple gas-filled cysts or linear gas within the bowel wall. PI may be idiopathic (15%) or secondary (85%) to a variety of disorders. We report here the first otherwise healthy adult with C. difficile infection complicated by PI and review the possible mechanisms of this previously unrecognized complication of pseudomembranous colitis. With treatment of the underlying infection, the PI resolved within 6 days of presentation.

Neonatal necrotizing enterocolitis. Inflammatory bowel disease of the newborn

Neonatal necrotizing enterocolitis is the most common serious gastrointestinal disorder encountered in neonatal intensive care units. It is a major cause of morbidity and mortality in the newborn, particularly in premature infants. Consistent risk factors are birth weight and prematurity. Polycythemia and hyperviscosity altering blood flow and infectious agents are also implicated. Clinical findings include abdominal distention and diarrhea, and systemic symptoms such as apnea, acidosis, and lethargy. Pneumatosis intestinalis can be demonstrated radiographically. Mucosal ulcerations, hemorrhage, and thrombosis occur early, followed by inflammatory changes. Later still necrosis develops. Ischemia, infection, and enteral feedings are suspected to be involved in the pathophysiology. Eicosanoids, especially thromboxane, platelet-activating factor, and leukotrienes are likely mediators.

Comparative effects of ischemia, bacteria, and substrate on the pathogenesis of intestinal necrosis

This study was undertaken to evaluate the relative contribution of ischemia, bacteria, and luminal substrate, the pathogenetic components of necrotizing enterocolitis (NEC), to the development of intestinal necrosis. Sprague-Dawley rats, either germ-free (No. = 25) or conventionally colonized (No. = 20) underwent laparotomy. Isolated ileal segments were created, two per rat. Ischemia was produced in one segment by application of a microaneurysm clip; the other segment served as a control. Segments were injected with 1 mL of either normal saline, dilute Similac formula, or standard formula. Groups were as follows: Group I (germ-free), received saline; Group II (germ-free), dilute formula; Group III (germ-free), standard formula; Group IV (conventional), saline; Group V (conventional), dilute formula; Group VI (conventional), standard formula. At 48 hours, the rats were evaluated for survival, gross bowel integrity, histologic severity of necrosis (graded 0 to 4+), and bacteriology. Gross analysis of bowel integrity showed no lesions in the ischemic segments of the germ-free rats (Groups I, II, and III) and necrosis in 75% of conventionally colonized animals (Groups IV, V, and VI; P less than 0.001). Microscopic necrosis was more common (P less than 0.001) in ischemic segments of conventional rats than in ischemic segments of germ-free rats. There was no difference in necrosis attributable to ischemic time or to the presence of either standard or dilute formula. Of the three pathogenetic factors evaluated, the presence of bacteria was most crucial to the development of bowel necrosis in this model. Improved treatment and prevention of NEC may depend upon suppression and/or modification of the gut flora.

Clostridial necrotizing enterocolitis

In a bacteriologic investigation of infants with necrotizing enterocolitis (NEC), 16 of 50 infants had clostridia in cultures of blood or of peritoneal fluid obtained by paracentesis. Twenty-eight of the 50 infants had enteric bacteria other than clostridia, and six infants had sterile cultures. Of the 16 infants with clostridia, nine had C. perfringens and seven had other species of clostridia. Compared to infants with nonclostridial NEC, those with clostridial NEC were larger and more mature, had more extensive pneumatosis intestinalis and gangrene and more rapid progression of NEC. The nine infants with C. perfringens had a fulminant form of NEC, analogous to gas-gangrene of the intestine. Mortality in this group was 78% (7/9). The seven infants with clostridial species other than C. perfringens had a mortality comparable to that of infants with nonclostridial NEC (32%). Improved survival from NEC associated with C. perfringens may be possible only by prevention, rather than earlier diagnosis and improved heroic treatment.

A bacteriologic basis for the clinical presentations of necrotizing enterocolitis

A study to identify putative bacterial pathogens in infants with necrotizing enterocolitis (NEC) was begun in 1976. Cultures of blood and of peritoneal fluid obtained by paracentesis were carried out in 25 infants with NEC. Segments of intestine excised at operation were Gram stained. Of the 25 infants, 8 recovered with medical management and 17 required operations. The 8 medically treated infants had sterile peritoneal fluid and, with 2 exceptions, sterile blood cultures. Of the 17 operated infants, 16 had bacteria in their blood and/or peritoneal fluid. The majority of resected bowel specimens from these infants contained a confirmatory morphologic type of bacterium within the wall. The clinical course of 8 infants with clostridia was compared to that of 8 infants with gram-negative enteric bacteria (Klebsiella, E. coli, or Bacteroides fragilis). The infants with clostridia were sicker. They had more extensive pneumatosis intestinalis, a higher incidence of portal venous gas, more rapid progression to gangrene, and more extensive gangrene. Infants with gram-negative rods had lower birth weights and lower platelet counts than the clostridial group. The difference in mortality between the two groups was not significant. The inherent pathogenicity of the gut flora may influence the clinical course of NEC. Among infants who develop intestinal gangrene, the clostridia appear to be more virulent than gram-negative enteric bacteria.

Pneumatosis cystoides intestinalis

Pneumatosis cystoides intestinalis (PCI) is an uncommon disease of previously unknown etiology, characterized by the presence of gas-filled cysts in the walls of some portion of the gastrointestinal tract. This investigation showed that PCI was readily produced in the germfree rat by inoculating its peritoneal cavity with a pure culture of either C. perfringens or C. tertium. Similar inoculation of the germfree animal with any one of eight other clostridia species did not result in the formation of PCI. The bacterial theory for the formation of at least some cases of PCI is now established. This work suggests that part of the treatment of those fulminating diseases known to be frequently associated with PCI should be directed at controlling a possible clostridial infection.