Noninvasive measurement of anatomic structure and intraluminal oxygenation in the gastrointestinal tract of living mice with spatial and spectral EPR imaging

Environmental determinants of transformation efficiency in Helicobacter pylori

Helicobacter pylori uses natural competence and homologous recombination to adapt to the dynamic environment of the stomach mucosa and maintain chronic colonization. Although H. pylori competence is constitutive, its rate of transformation is variable, and little is known about factors that influence it. To examine this, we first determined the transformation efficiency of H. pylori strains under low O2 (5% O2, 7.6% CO2, 7.6% H2) and high O2 (15% O2, 2.9% CO2, 2.9% H2) conditions using DNA containing an antibiotic resistance marker. H. pylori transformation efficiency was 6- to 32-fold greater under high O2 tension, which was robust across different H. pylori strains, genetic loci, and bacterial growth phases. Since changing the O2 concentration for these initial experiments also changed the concentrations of CO2 and H2, transformations were repeated under conditions where O2, CO2, and H2 were each varied individually. The results showed that the increase in transformation efficiency under high O2 was largely due to a decrease in CO2. An increase in pH similar to that caused by low CO2 was also sufficient to increase transformation efficiency. These results have implications for the physiology of H. pylori in the gastric environment, and they provide optimized conditions for the laboratory construction of H. pylori mutants using natural transformation.

Host interactions with Segmented Filamentous Bacteria: an unusual trade-off that drives the post-natal maturation of the gut immune system

Segmented Filamentous Bacteria (SFB) are present in the gut microbiota of a large number of vertebrate species where they are found intimately attached to the intestinal epithelium. SFB has recently attracted considerable attention due to its outstanding capacity to stimulate innate and adaptive host immune responses without causing pathology. Recent genomic analysis placed SFB between obligate and facultative symbionts, unraveled its highly auxotrophic needs, and provided a rationale for the complex SFB life-style in close contact with the epithelium. Herein, we examine how the SFB life-style may underlie its potent immunostimulatory properties and discuss how the trade-off set up between SFB and its hosts can simultaneously help to establish and maintain the ecological niche of SFB in the intestine and drive the post-natal maturation of the host gut immune barrier.

Compressed sensing of spatial electron paramagnetic resonance imaging

PURPOSE

To improve image quality and reduce data requirements for spatial electron paramagnetic resonance imaging (EPRI) by developing a novel reconstruction approach using compressed sensing (CS).

METHODS

EPRI is posed as an optimization problem, which is solved using regularized least-squares with sparsity promoting penalty terms, consisting of the l1 norms of the image itself and the total variation of the image. Pseudo-random sampling was employed to facilitate recovery of the sparse signal. The reconstruction was compared with the traditional filtered back-projection reconstruction for simulations, phantoms, isolated rat hearts, and mouse gastrointestinal (GI) tracts labeled with paramagnetic probes.

RESULTS

A combination of pseudo-random sampling and CS was able to generate high-fidelity EPR images at high acceleration rates. For three-dimensional (3D) phantom imaging, CS-based EPRI showed little visual degradation at nine-fold acceleration. In rat heart datasets, CS-based EPRI produced high quality images with eight-fold acceleration. A high resolution mouse GI tract reconstruction demonstrated a visual improvement in spatial resolution and a doubling in signal-to-noise ratio (SNR).

CONCLUSION

A novel 3D EPRI reconstruction using compressed sensing was developed and offers superior SNR and reduced artifacts from highly undersampled data.

Bile salts affect expression of Escherichia coli O157:H7 genes for virulence and iron acquisition, and promote growth under iron limiting conditions

Bile salts exhibit potent antibacterial properties, acting as detergents to disrupt cell membranes and as DNA-damaging agents. Although bacteria inhabiting the intestinal tract are able to resist bile’s antimicrobial effects, relatively little is known about how bile influences virulence of enteric pathogens. Escherichia coli O157:H7 is an important pathogen of humans, capable of causing severe diarrhea and more serious sequelae. In this study, the transcriptome response of E. coli O157:H7 to bile was determined. Bile exposure induced significant changes in mRNA levels of genes related to virulence potential, including a reduction of mRNA for the 41 genes making up the locus of enterocyte effacement (LEE) pathogenicity island. Bile treatment had an unusual effect on mRNA levels for the entire flagella-chemotaxis regulon, resulting in two- to four-fold increases in mRNA levels for genes associated with the flagella hook-basal body structure, but a two-fold decrease for “late” flagella genes associated with the flagella filament, stator motor, and chemotaxis. Bile salts also caused increased mRNA levels for seventeen genes associated with iron scavenging and metabolism, and counteracted the inhibitory effect of the iron chelating agent 2,2’-dipyridyl on growth of E. coli O157:H7. These findings suggest that E. coli O157:H7 may use bile as an environmental signal to adapt to changing conditions associated with the small intestine, including adaptation to an iron-scarce environment.

Twin-arginine translocation system (tat) mutants of Salmonella are attenuated due to envelope defects, not respiratory defects

The twin-arginine translocation system (Tat) transports folded proteins across the cytoplasmic membrane and is critical to virulence in Salmonella and other pathogens. Experimental and bioinformatic data indicate that 30 proteins are exported via Tat in Salmonella Typhimurium. However, there are no data linking specific Tat substrates with virulence. We inactivated every Tat-exported protein and determined the virulence phenotype of mutant strains. Although a tat mutant is highly attenuated, no single Tat-exported substrate accounts for this virulence phenotype. Rather, the attenuation is due primarily to envelope defects caused by failure to translocate three Tat substrates, the N-acetylmuramoyl-l-alanine amidases, AmiA and AmiC, and the cell division protein, SufI. Strikingly, neither the amiA amiC nor the sufI mutations alone conferred any virulence defect. Although AmiC and SufI have previously been localized to the divisome, the synthetic phenotypes observed are the first to suggest functional overlap. Many Tat substrates are involved in anaerobic respiration, but we show that a mutant completely deficient in anaerobic respiration retains full virulence in both the oral and systemic phases of infection. Similarly, an obligately aerobic mutant is fully virulent. These results suggest that in the classic mouse model of infection, S. Typhimurium is replicating only in aerobic environments.

Dysbiosis in inflammatory bowel diseases: the oxygen hypothesis

The healthy intestine is characterized by a low level of oxygen and by the presence of large bacterial communities of obligate anaerobes. Dysbiosis of the gut microbiota has been reported in patients suffering from inflammatory bowel diseases (IBDs), but the mechanisms causing this imbalance remain unknown. Observations have included a decrease in obligate anaerobes of the phylum Firmicutes and an increase in facultative anaerobes, including members of the family Enterobacteriaceae. The shift of bacterial communities from obligate to facultative anaerobes strongly suggests a disruption in anaerobiosis and points to a role for oxygen in intestinal dysbiosis. Proposals to evaluate this hypothesis of a role for oxygen in IBD dysbiosis are provided. If this hypothesis is confirmed, decreasing oxygen in the intestine could open novel means to rebalance the microbiota and could provide novel preventative or therapeutic strategies for IBD patients in whom current treatments are ineffective.

Colonization resistance and microbial ecophysiology: using gnotobiotic mouse models and single-cell technology to explore the intestinal jungle

The highly diverse intestinal microbiota forms a structured community engaged in constant communication with itself and its host and is characterized by extensive ecological interactions. A key benefit that the microbiota affords its host is its ability to protect against infections in a process termed colonization resistance (CR), which remains insufficiently understood. In this review, we connect basic concepts of CR with new insights from recent years and highlight key technological advances in the field of microbial ecology. We present a selection of statistical and bioinformatics tools used to generate hypotheses about synergistic and antagonistic interactions in microbial ecosystems from metagenomic datasets. We emphasize the importance of experimentally testing these hypotheses and discuss the value of gnotobiotic mouse models for investigating specific aspects related to microbiota-host-pathogen interactions in a well-defined experimental system. We further introduce new developments in the area of single-cell analysis using fluorescence in situ hybridization in combination with metabolic stable isotope labeling technologies for studying the in vivo activities of complex community members. These approaches promise to yield novel insights into the mechanisms of CR and intestinal ecophysiology in general, and give researchers the means to experimentally test hypotheses in vivo at varying levels of biological and ecological complexity.

The redox interplay between nitrite and nitric oxide: From the gut to the brain

The reversible redox conversion of nitrite and nitric oxide ((•)NO) in a physiological setting is now widely accepted. Nitrite has long been identified as a stable intermediate of (•)NO oxidation but several lines of evidence support the reduction of nitrite to nitric oxide in vivo. In the gut, this notion implies that nitrate from dietary sources fuels the longstanding production of nitrite in the oral cavity followed by univalent reduction to (•)NO in the stomach. Once formed, (•)NO boosts a network of reactions, including the production of higher nitrogen oxides that may have a physiological impact via the post-translational modification of proteins and lipids. Dietary compounds, such as polyphenols, and different prandial states (secreting specific gastric mediators) modulate the outcome of these reactions. The gut has unusual characteristics that modulate nitrite and (•)NO redox interplay: (1) wide range of pH (neutral vs acidic) and oxygen tension (c.a. 70 Torr in the stomach and nearly anoxic in the colon), (2) variable lumen content and (3) highly developed enteric nervous system (sensitive to (•)NO and dietary compounds, such as glutamate). The redox interplay of nitrite and (•)NO might also participate in the regulation of brain homeostasis upon neuronal glutamatergic stimulation in a process facilitated by ascorbate and a localized and transient decrease of oxygen tension. In a way reminiscent of that occurring in the stomach, a nitrite/(•)NO/ascorbate redox interplay in the brain at glutamatergic synapses, contributing to local (•)NO increase, may impact on (•)NO-mediated process. We here discuss the implications of the redox conversion of nitrite to (•)NO in the gut, how nitrite-derived (•)NO may signal from the digestive to the central nervous system, influencing brain function, as well as a putative ascorbate-driven nitrite/NO pathway occurring in the brain.

Role of oxygen gradients in shaping redox relationships between the human intestine and its microbiota

The unique anatomy and physiology of the intestine in conjunction with its microbial content create the steepest oxygen gradients in the body, which plunge to near anoxia at the luminal midpoint. Far from static, intestinal oxygen gradients ebb and flow with every meal. This in turn governs the redox effectors nitric oxide, hydrogen sulfide, and reactive oxygen species of both host and bacterial origin. This review illustrates how the intestine and microbes utilize oxygen gradients as a backdrop for mechanistically shaping redox relationships and a functional coexistence.

Assessment of oxidative stress and antioxidant property using electron spin resonance (ESR) spectroscopy

The pathophysiology of hypertension or stroke is associated with an excess of ROS generation in the vascular system, and results in induction of various pathological cascades of cerebrovascular damage. We have demonstrated that electron spin resonance methods using a spin trap or spin probe will be useful for understanding redox status under conditions of oxidative stress in the spontaneously hypertensive rat or stroke-prone spontaneously hypertensive rat brain. We have used electron spin resonance imaging and noninvasive L-band electron spin resonance to characterize the higher degree of brain oxidative stress in the stroke-prone spontaneously hypertensive rat and spontaneously hypertensive rat than in the Wistar-Kyoto rat brain, and the lower extent of oxidative stress in the spontaneously hypertensive rat than in the stroke-prone spontaneously hypertensive rat brain. Indeed, we may be able to confirm propofol medium-chain triglyceride/long-chain triglyceride (MCT/LCT) as neuroprotective anesthesia and crocetin as antioxidant food factor against human stroke after screening for antioxidant properties in stroke models such as stroke-prone spontaneously hypertensive rat. Thus, our electron spin resonance biomedical application suggests that it could be used to assess antioxidant effects on oxidative stress in the brain using spontaneously hypertensive rat and stroke-prone spontaneously hypertensive rat. We hope that further advances in the instrumentation used for electron spin resonance imaging and the development of optimized nontoxic spin probes will make this technology even more promising for novel clinical prediction or noninvasive diagnosis of human stroke. After screening drugs or foods for antioxidant property using in vitro or in vivo electron spin resonance assessment, it will be possible to find and develop novel drugs or food factors with such properties for the prevention of stroke in the near future.

Tetrathiatriarylmethyl radical with a single aromatic hydrogen as a highly sensitive and specific superoxide probe

Superoxide (O(2)(•-)) plays crucial roles in normal physiology and disease; however, its measurement remains challenging because of the limited sensitivity and/or specificity of prior detection methods. We demonstrate that a tetrathiatriarylmethyl (TAM) radical with a single aromatic hydrogen (CT02-H) can serve as a highly sensitive and specific O(2)(•-) probe. CT02-H is an analogue of the fully substituted TAM radical CT-03 (Finland trityl) with an electron paramagnetic resonance (EPR) doublet signal due to its aromatic hydrogen. Owing to the neutral nature and negligible steric hindrance of the hydrogen, O(2)(•-) preferentially reacts with CT02-H at this site with production of the diamagnetic quinone methide via oxidative dehydrogenation. Upon reaction with O(2)(•-), CT02-H loses its EPR signal and this EPR signal decay can be used to quantitatively measure O(2)(•-). This is accompanied by a change in color from green to purple, with the quinone methide product exhibiting a unique UV-Vis absorbance (ε=15,900 M(-1) cm(-1)) at 540 nm, providing an additional O(2)(•-) detection method. More than five-fold higher reactivity of CT02-H for O(2)(•-) relative to CT-03 was demonstrated, with a second-order rate constant of 1.7×10(4) M(-1) s(-1) compared to 3.1×10(3) M(-1) s(-1) for CT-03. CT02-H exhibited high specificity for O(2)(•-) as evidenced by its inertness to other oxidoreductants. The O(2)(•-) generation rates detected by CT02-H from xanthine/xanthine oxidase were consistent with those measured by cytochrome c reduction but detection sensitivity was 10- to 100-fold higher. EPR detection of CT02-H enabled measurement of very low O(2)(•-) flux with a detection limit of 0.34 nM/min over 120 min. HPLC in tandem with electrochemical detection was used to quantitatively detect the stable quinone methide product and is a highly sensitive and specific method for measurement of O(2)(•-), with a sensitivity limit of ~2×10(-13) mol (10 nM with 20-μl injection volume). Based on the O(2)-dependent linewidth broadening of its EPR spectrum, CT02-H also enables simultaneous measurement of O(2) concentration and O(2)(•-) generation and was shown to provide sensitive detection of extracellular O(2)(•-) generation in endothelial cells stimulated either by menadione or with anoxia/reoxygenation. Thus, CT02-H is a unique probe that provides very high sensitivity and specificity for measurement of O(2)(•-) by either EPR or HPLC methods.

Electron paramagnetic resonance oxygen imaging in vivo

This review covers the last 15 years of the development of EPR in vivo oxygen imaging. During this time, a number of major technological and methodological advances have taken place. Narrow line width, long relaxation time, and non-toxic triaryl methyl radicals were introduced in the late 1990s. These not only improved continuous wave (CW) imaging, but also enabled the application of pulse EPR imaging to animals. Recent developments in pulse technology have brought an order of magnitude increase in image acquisition speed, enhancement of sensitivity, and considerable improvement in the precision and accuracy of oxygen measurements. Consequently, pulse methods take up a significant part of this review.

Coordination of hypoxia adaptation and iron homeostasis in human pathogenic fungi

In mammals, hypoxia causes facilitated erythropoiesis that requires increased iron availability with established links between oxygen and iron in regulation of the transcription factor hypoxia-inducible factor. Therefore, cellular responses to hypoxia and iron starvation are linked in mammals and are host conditions that pathogens encounter during infection. In human pathogenic fungi, molecular mechanisms underlying hypoxia adaptation and iron homeostasis have been investigated. However, the interconnected regulation of hypoxia adaptation and iron homeostasis remains to be fully elucidated. This review discusses the potential transcriptional regulatory links between hypoxia adaptation and iron homeostasis in human pathogenic fungi. Transcriptome analyses demonstrate that core regulators of hypoxia adaptation and iron homeostasis are involved in regulation of several common genes responsible for iron acquisition and ergosterol biosynthesis. Importantly, iron starvation increases susceptibility of fungal cells to antifungal drugs and decreased levels of ergosterol, while key hypoxia regulators are also involved in responses to antifungal drugs and mediating ergosterol levels. We suggest that pathogenic fungi have developed a coordinated regulatory system in response to hypoxia and iron starvation through (i) regulation of expression of hypoxia-responsive and iron-responsive genes via cross-linked key regulators, and/or (ii) regulation of factors involved in ergosterol biosynthesis. Thus, both oxygen and iron availability are intimately tied with fungal virulence and responses to existing therapeutics and further elucidation of their interrelationship should have significant clinical implications.

Normal adaptation of Candida albicans to the murine gastrointestinal tract requires Efg1p-dependent regulation of metabolic and host defense genes

Although gastrointestinal colonization by the opportunistic fungal pathogen Candida albicans is generally benign, severe systemic infections are thought to arise due to escape of commensal C. albicans from the gastrointestinal (GI) tract. The C. albicans transcription factor Efg1p is a major regulator of GI colonization, hyphal morphogenesis, and virulence. The goals of this study were to identify the Efg1p regulon during GI tract colonization and to compare C. albicans gene expression during colonization of different organs of the GI tract. Our results identified significant differences in gene expression between cells colonizing the cecum and ileum. During colonization, efg1(-) null mutant cells expressed higher levels of genes involved in lipid catabolism, carnitine biosynthesis, and carnitine utilization than did colonizing wild-type (WT) cells. In addition, during laboratory growth, efg1(-) null mutant cells grew to a higher density than WT cells. The efg1(-) null mutant grew in depleted medium, while WT cells could grow only if the depleted medium was supplemented with carnitine, a compound that promotes the metabolism of fatty acids. Altered gene expression and altered growth capability support the ability of efg1(-) cells to hypercolonize naïve mice. Also, Efg1p was shown to be important for transcriptional responses to the stresses present in the cecum environment. For example, during colonization, SOD5, encoding a superoxide dismutase, was highly upregulated in an Efg1p-dependent manner. Ectopic expression of SOD5 in an efg1(-) null mutant increased the fitness of the efg1(-) null mutant cells during colonization. These data show that EFG1 is an important regulator of GI colonization.

Metabolic regulation of oxygen and redox homeostasis by p53: lessons from evolutionary biology?

The genetic links between p53 and metabolic processes such as oxidative phosphorylation are being studied with increasing interest given that cellular metabolism seems to play an important role in tumorigenesis. This review focuses on how p53 regulation of various metabolic genes may influence redox homeostasis, as the genome is constantly susceptible to oxidative damage, a consequence of living in an aerobic environment. Because p53-like genetic sequences are also found in life forms that may not necessarily benefit from tumor suppression, an evolutionary introduction is given in an attempt to understand why p53 might regulate a basic cellular activity such as metabolism. The presented epidemiologic and experimental data suggest that one reason may be for the homeostatic regulation of oxygen, the essential substrate for reactive oxygen species generation.

Oxidation of cod liver oil during gastrointestinal in vitro digestion

Oxidation of cod liver oil rich in long-chain n-3 polyunsaturated fatty acids (LC n-3 PUFA) was investigated during a gastrointestinal (GI) in vitro digestion. The digestion stimulated TBA-reactive substances (TBARS) formation in both the gastric and intestinal steps, whereas levels of lipid hydroperoxides remained nearly constant. The presence of digestive compounds was decisive for the TBARS development because TBARS did not change when the cod liver oil was subjected only to the temperature and pH gradient of the GI model. Preformed oxidation products in the cod liver oil resulted in further elevated TBARS levels during the digestion. Addition of hemoglobin (11.5 μM) to emulsified cod liver oil dramatically increased TBARS and lipid hydroperoxide levels during GI digestion, whereas 1 mg α-tocopherol/g oil did not show any protection against oxidation. Specific concern thus needs to be taken in the design of foods containing LC n-3 PUFA to preserve these lipids and avoid harmful oxidation, both before and after consumption.

Electron paramagnetic resonance-based pH mapping using spectral-spatial imaging of sequentially scanned spectra

The development of electron paramagnetic resonance (EPR)-based mapping of pH is an important advancement for the field of diagnostic imaging. The ability to accurately quantify pH change in vivo and monitor spatial distribution is desirable for the assessment of a number of pathological conditions in the human body as well as the monitoring of treatment response. In this work we introduce a method for EPR-based pH mapping utilizing a method of spectral-spatial imaging of sequentially scanned spectra to decrease the missing gradient rotation angle, without increasing the spatial field of view. Repeated in vitro measurements of pH phantom tubes demonstrated higher precision measurements of the hyperfine coupling constant (HFC) compared to previous EPR-based methods, resulting in mean pH values accurate to less than 0.1 pH across a range of physiologically observed values.

Hypoxia and fungal pathogenesis: to air or not to air?

Over the last 3 decades, the frequency of life-threatening human fungal infections has increased as advances in medical therapies, solid-organ and hematopoietic stem cell transplantations, an increasing geriatric population, and HIV infections have resulted in significant rises in susceptible patient populations. Although significant advances have been made in understanding how fungi cause disease, the dynamic microenvironments encountered by fungi during infection and the mechanisms by which they adapt to these microenvironments are not fully understood. As inhibiting and preventing in vivo fungal growth are main goals of antifungal therapies, understanding in vivo fungal metabolism in these host microenvironments is critical for the improvement of existing therapies or the design of new approaches. In this minireview, we focus on the emerging appreciation that pathogenic fungi like Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are exposed to oxygen-limited or hypoxic microenvironments during fungal pathogenesis. The implications of these in vivo hypoxic microenvironments for fungal metabolism and pathogenesis are discussed with an aim toward understanding the potential impact of hypoxia on invasive fungal infection outcomes.

Mitochondrial adaptations to utilize hydrogen sulfide for energy and signaling

Sulfur is a versatile molecule with oxidation states ranging from -2 to +6. From the beginning, sulfur has been inexorably entwined with the evolution of organisms. Reduced sulfur, prevalent in the prebiotic Earth and supplied from interstellar sources, was an integral component of early life as it could provide energy through oxidization, even in a weakly oxidizing environment, and it spontaneously reacted with iron to form iron-sulfur clusters that became the earliest biological catalysts and structural components of cells. The ability to cycle sulfur between reduced and oxidized states may have been key in the great endosymbiotic event that incorporated a sulfide-oxidizing α-protobacteria into a host sulfide-reducing Archea, resulting in the eukaryotic cell. As eukaryotes slowly adapted from a sulfidic and anoxic (euxinic) world to one that was highly oxidizing, numerous mechanisms developed to deal with increasing oxidants; namely, oxygen, and decreasing sulfide. Because there is rarely any reduced sulfur in the present-day environment, sulfur was historically ignored by biologists, except for an occasional report of sulfide toxicity. Twenty-five years ago, it became evident that the organisms in sulfide-rich environments could synthesize ATP from sulfide, 10 years later came the realization that animals might use sulfide as a signaling molecule, and only within the last 4 years did it become apparent that even mammals could derive energy from sulfide generated in the gastrointestinal tract. It has also become evident that, even in the present-day oxic environment, cells can exploit the redox chemistry of sulfide, most notably as a physiological transducer of oxygen availability. This review will examine how the legacy of sulfide metabolism has shaped natural selection and how some of these ancient biochemical pathways are still employed by modern-day eukaryotes.

Hydrogen sulfide (H₂S) and hypoxia inhibit salmonid gastrointestinal motility: evidence for H₂S as an oxygen sensor

Hydrogen sulfide (H(2)S) has been shown to affect gastrointestinal (GI) motility and signaling in mammals and O(2)-dependent H(2)S metabolism has been proposed to serve as an O(2) 'sensor' that couples hypoxic stimuli to effector responses in a variety of other O(2)-sensing tissues. The low P(O2) values and high H(2)S concentrations routinely encountered in the GI tract suggest that H(2)S might also be involved in hypoxic responses in these tissues. In the present study we examined the effect of H(2)S on stomach, esophagus, gallbladder and intestinal motility in the rainbow trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch) and we evaluated the potential for H(2)S in oxygen sensing by examining GI responses to hypoxia in the presence of known inhibitors of H(2)S biosynthesis and by adding the sulfide donor cysteine (Cys). We also measured H(2)S production by intestinal tissue in real time and in the presence and absence of oxygen. In tissues exhibiting spontaneous contractions, H(2)S inhibited contraction magnitude (area under the curve and amplitude) and frequency, and in all tissues it reduced baseline tension in a concentration-dependent relationship. Longitudinal intestinal smooth muscle was significantly more sensitive to H(2)S than other tissues, exhibiting significant inhibitory responses at 1-10 μmol l(-1) H(2)S. The effects of hypoxia were essentially identical to those of H(2)S in longitudinal and circular intestinal smooth muscle; of special note was a unique transient stimulatory effect upon application of both hypoxia and H(2)S. Inhibitors of enzymes implicated in H(2)S biosynthesis (cystathionine β-synthase and cystathionine γ-lyase) partially inhibited the effects of hypoxia whereas the hypoxic effects were augmented by the sulfide donor Cys. Furthermore, tissue production of H(2)S was inversely related to O(2); addition of Cys to intestinal tissue homogenate stimulated H(2)S production when the tissue was gassed with 100% nitrogen (~0% O(2)), whereas addition of oxygen (~10% O(2)) reversed this to net H(2)S consumption. This study shows that the inhibitory effects of H(2)S on the GI tract of a non-mammalian vertebrate are identical to those reported in mammals and they provide further evidence that H(2)S is a key mediator of the hypoxic response in a variety of O(2)-sensitive tissues.

Intragastric nitration by dietary nitrite: implications for modulation of protein and lipid signaling

Inorganic nitrite, derived from the reduction of nitrate in saliva, has recently emerged as a protagonist in nitric oxide ((•)NO) biology as it can be univalently reduced to (•)NO, in the healthy human stomach. Important physiological implications have been attributed to nitrite-derived (•)NO in the gastrointestinal tract, namely modulation of host defense, blood flow, mucus formation and motility. At acidic pH, nitrite generates different nitrogen oxides depending on the local microenvironment (redox status, gastric content, pH, inflammatory conditions), including (•)NO, nitrogen dioxide ((•)NO(2)), dinitrogen trioxide (N(2)O(3)), and peroxynitrite. Thus, the gastric environment is a significant source of nitrating and nitrosating agents, especially in individuals consuming a nitrate/nitrite-rich diet on a daily basis. Both, the gastric lumen and mucosa contain putative targets for nitration, not only proteins and lipids from ingested aliments but also endogenous proteins secreted by the oxyntic glands. The physiological and functional consequences of nitration of gastric mediators will impact on local processes including food digestion and ulcerogenesis. Additionally, gastric nitration products (such as nitrated lipids) may be absorbed and affect systemic pathways. Thus, dietary ingestion of nitrate will have direct consequences for endogenous protein nitration, as indicated by our preliminary data.

Anaerobic respiration of Escherichia coli in the mouse intestine

The intestine is inhabited by a large microbial community consisting primarily of anaerobes and, to a lesser extent, facultative anaerobes, such as Escherichia coli, which we have shown requires aerobic respiration to compete successfully in the mouse intestine (S. A. Jones et al., Infect. Immun. 75:4891-4899, 2007). If facultative anaerobes efficiently lower oxygen availability in the intestine, then their sustained growth must also depend on anaerobic metabolism. In support of this idea, mutants lacking nitrate reductase or fumarate reductase have extreme colonization defects. Here, we further explore the role of anaerobic respiration in colonization using the streptomycin-treated mouse model. We found that respiratory electron flow is primarily via the naphthoquinones, which pass electrons to cytochrome bd oxidase and the anaerobic terminal reductases. We found that E. coli uses nitrate and fumarate in the intestine, but not nitrite, dimethyl sulfoxide, or trimethylamine N-oxide. Competitive colonizations revealed that cytochrome bd oxidase is more advantageous than nitrate reductase or fumarate reductase. Strains lacking nitrate reductase outcompeted fumarate reductase mutants once the nitrate concentration in cecal mucus reached submillimolar levels, indicating that fumarate is the more important anaerobic electron acceptor in the intestine because nitrate is limiting. Since nitrate is highest in the absence of E. coli, we conclude that E. coli is the only bacterium in the streptomycin-treated mouse large intestine that respires nitrate. Lastly, we demonstrated that a mutant lacking the NarXL regulator (activator of the NarG system), but not a mutant lacking the NarP-NarQ regulator, has a colonization defect, consistent with the advantage provided by NarG. The emerging picture is one in which gene regulation is tuned to balance expression of the terminal reductases that E. coli uses to maximize its competitiveness and achieve the highest possible population in the intestine.

Standard-based method for proton-electron double resonance imaging of oxygen

Proton-electron double resonance imaging (PEDRI) has been utilized for indirect determination of oxygen concentrations in aqueous samples and living systems. Due to the complexity of the problem, there are seven oxygen related parameters that need to be measured to determine the distribution of oxygen. We present an improved approach in which image intensities from only two PEDRI acquisitions with different EPR irradiation powers are required to determine the distribution of a paramagnetic probe and oxygen in an analyzed sample. This is achieved using three reference samples with known concentrations of a paramagnetic probe and oxygen placed inside the resonator together with the measurement sample. An EPR-off image, which has low signal intensity at low magnetic field (0.02 T) is not required for the calculations, significantly reducing the total time of the experiments and the noise while enhancing the accuracy of these oxygen measurements. The Finland trityl radical was used as the paramagnetic probe and oxygen concentrations could be accurately measured and imaged over the physiological range from 0 to 240 μM.

Trade-off between bile resistance and nutritional competence drives Escherichia coli diversification in the mouse gut

Bacterial diversification is often observed, but underlying mechanisms are difficult to disentangle and remain generally unknown. Moreover, controlled diversification experiments in ecologically relevant environments are lacking. We studied bacterial diversification in the mammalian gut, one of the most complex bacterial environments, where usually hundreds of species and thousands of bacterial strains stably coexist. Herein we show rapid genetic diversification of an Escherichia coli strain upon colonisation of previously germ-free mice. In addition to the previously described mutations in the EnvZ/OmpR operon, we describe the rapid and systematic selection of mutations in the flagellar flhDC operon and in malT, the transcriptional activator of the maltose regulon. Moreover, within each mouse, the three mutant types coexisted at different levels after one month of colonisation. By combining in vivo studies and determination of the fitness advantages of the selected mutations in controlled in vitro experiments, we provide evidence that the selective forces that drive E. coli diversification in the mouse gut are the presence of bile salts and competition for nutrients. Altogether our results indicate that a trade-off between stress resistance and nutritional competence generates sympatric diversification of the gut microbiota. These results illustrate how experimental evolution in natural environments enables identification of both the selective pressures that organisms face in their natural environment and the diversification mechanisms.

Synthesis of trityl radical-conjugated disulfide biradicals for measurement of thiol concentration

Measurement of thiol concentrations is of great importance for characterizing their critical role in normal metabolism and disease. Low-frequency electron paramagnetic resonance (EPR) spectroscopy and imaging, coupled with the use of exogenous paramagnetic probes, have been indispensable techniques for the in vivo measurement of various physiological parameters owing to the specificity, noninvasiveness and good depth of magnetic field penetration in animal tissues. However, in vivo detection of thiol levels by EPR spectroscopy and imaging is limited due to the need for improved probes. We report the first synthesis of trityl radical-conjugated disulfide biradicals (TSSN and TSST) as paramagnetic thiol probes. The use of trityl radicals in the construction of these biradicals greatly facilitates thiol measurement by EPR spectroscopy since trityls have extraordinary stability in living tissues with a single narrow EPR line that enables high sensitivity and resolution for in vivo EPR spectroscopy and imaging. Both biradicals exhibit broad characteristic EPR spectra at room temperature because of their intramolecular spin-spin interaction. Reaction of these biradicals with thiol compounds such as glutathione (GSH) and cysteine results in the formation of trityl monoradicals which exhibit high spectral sensitivity to oxygen. The moderately slow reaction between the biradicals and GSH (k(2) ∼ 0.3 M(-1) s(-1) for TSSN and 0.2 M(-1) s(-1) for TSST) allows for in vivo measurement of GSH concentration without altering the redox environment in biological systems. The GSH concentration in rat liver was determined to be 3.49 ± 0.14 mM by TSSN and 3.67 ± 0.24 mM by TSST, consistent with the value (3.71 ± 0.09 mM) determined by the Ellman's reagent. Thus, these trityl-based thiol probes exhibit unique properties enabling measurement of thiols in biological systems and should be of great value for monitoring redox metabolism.

Anaerobic environment of the intestine primes pathogenic Shigella for infection

Marteyn et al. have investigated the role of oxygen and the regulator FNR in infection by the intracellular enteric pathogen Shigella flexneri. FNR is active under anaerobic conditions like those present in the lumen of the distal intestine. FNR causes elongation of a secretion apparatus required for bacterial entry into cells and represses secretion of proteins that trigger entry. Higher oxygen levels present at the intestinal cell surface are sufficient to inactivate FNR, thereby derepressing secretion. Thus, bacteria are 'primed' in the anaerobic environment of the lumen, and entry is triggered by the aerobic conditions at the intestinal cell surface. FNR is conserved among many enteric pathogens, suggesting that regulation of virulence in response to oxygen may be widely conserved.

Noninvasive biophotonic imaging for studies of infectious disease

According to World Health Organization estimates, infectious organisms are responsible for approximately one in four deaths worldwide. Animal models play an essential role in the development of vaccines and therapeutic agents but large numbers of animals are required to obtain quantitative microbiological data by tissue sampling. Biophotonic imaging (BPI) is a highly sensitive, nontoxic technique based on the detection of visible light, produced by luciferase-catalysed reactions (bioluminescence) or by excitation of fluorescent molecules, using sensitive photon detectors. The development of bioluminescent/fluorescent microorganisms therefore allows the real-time noninvasive detection of microorganisms within intact living animals. Multiple imaging of the same animal throughout an experiment allows disease progression to be followed with extreme accuracy, reducing the number of animals required to yield statistically meaningful data. In the study of infectious disease, the use of BPI is becoming widespread due to the novel insights it can provide into established models, as well as the impact of the technique on two of the guiding principles of using animals in research, namely reduction and refinement. Here, we review the technology of BPI, from the instrumentation through to the generation of a photonic signal, and illustrate how the technique is shedding light on infection dynamics in vivo.

Breathing life into pathogens: the influence of oxygen on bacterial virulence and host responses in the gastrointestinal tract

The gastrointestinal tract provides a variety of environmental challenges to any bacterium seeking to successfully colonize or cause disease in a host. A major obstacle is the varied oxygen concentrations encountered at different sites in the intestine. Here we review the mechanisms bacterial pathogens utilize to sense oxygen within the gastrointestinal tract, and recent insights into how this acts as a signal to trigger virulence and to modulate host responses.

Synthesis of 14N- and 15N-labeled trityl-nitroxide biradicals with strong spin-spin interaction and improved sensitivity to redox status and oxygen

Simultaneous evaluation of redox status and oxygenation in biological systems is of great importance for the understanding of biological functions. Electron paramagnetic resonance (EPR) spectroscopy coupled with the use of the nitroxide radicals have been an indispensable technique for this application but are still limited by low oxygen sensitivity and low EPR resolution in part due to the moderately broad EPR triplet and spin quenching through bioreduction. In this study, we showed that these drawbacks can be overcome through the use of trityl-nitroxide biradicals allowing for the simultaneous measurement of redox status and oxygenation. A new trityl-nitroxide biradical TNN14 composed of a pyrrolidinyl-nitroxide and a trityl and its isotopically labeled (15)N analogue TNN15 were synthesized and characterized. Both biradicals exhibited much stronger spin-spin interaction with J > 400 G compared with that of the previous synthesized trityl-nitroxide biradicals TN1 (∼160 G) and TN2 (∼52 G) with longer linker chain length. The enhanced stability of TNN14 was evaluated using ascorbate as reductant, and the effect of different types of cyclodextrins on its stability in the presence of ascorbate was also investigated. Both biradicals are sensitive to redox status, and their corresponding trityl-hydroxylamines resulting from the reduction of the biradicals by ascorbate share the same oxygen sensitivity. Of note is that the (15)N-labeled TNN15-H with an EPR doublet exhibits improved EPR signal amplitude as compared with that of TNN14-H with an EPR triplet. In addition, cyclic voltammetric studies verify the characteristic electrochemical behaviors of the trityl-nitroxide biradicals.

Different contributions of HtrA protease and chaperone activities to Campylobacter jejuni stress tolerance and physiology

The microaerophilic bacterium Campylobacter jejuni is the most common cause of bacterial food-borne infections in the developed world. Tolerance to environmental stress relies on proteases and chaperones in the cell envelope, such as HtrA and SurA. HtrA displays both chaperone and protease activities, but little is known about how each of these activities contributes to stress tolerance in bacteria. In vitro experiments showed temperature-dependent protease and chaperone activities of C. jejuni HtrA. A C. jejuni mutant lacking only the protease activity of HtrA was used to show that the HtrA chaperone activity is sufficient for growth at high temperature or under oxidative stress, whereas the HtrA protease activity is essential only under conditions close to the growth limit for C. jejuni. However, the protease activity was required to prevent induction of the cytoplasmic heat shock response even under optimal growth conditions. Interestingly, the requirement of HtrA at high temperatures was found to depend on the oxygen level, and our data suggest that HtrA may protect oxidatively damaged proteins. Finally, protease activity stimulates HtrA production and oligomer formation, suggesting that a regulatory role depends on the protease activity of HtrA. Studying a microaerophilic organism encoding only two known periplasmic chaperones (HtrA and SurA) revealed an efficient HtrA chaperone activity and proposed multiple roles of the protease activity, increasing our understanding of HtrA in bacterial physiology.

Stability and instability in the lysogenic state of phage lambda

Complex gene regulatory circuits exhibit emergent properties that are difficult to predict from the behavior of the components. One such property is the stability of regulatory states. Here we analyze the stability of the lysogenic state of phage λ. In this state, the virus maintains a stable association with the host, and the lytic functions of the virus are repressed by the viral CI repressor. This state readily switches to the lytic pathway when the host SOS system is induced. A low level of SOS-dependent switching occurs without an overt stimulus. We found that the intrinsic rate of switching to the lytic pathway, measured in a host lacking the SOS response, was almost undetectably low, probably less than 10(-8)/generation. We surmise that this low rate has not been selected directly during evolution but results from optimizing the rate of switching in a wild-type host over the natural range of SOS-inducing conditions. We also analyzed a mutant, λprm240, in which the promoter controlling CI expression was weakened, rendering lysogens unstable. Strikingly, the intrinsic stability of λprm240 lysogens depended markedly on the growth conditions; lysogens grown in minimal medium were nearly stable but switched at high rates when grown in rich medium. These effects on stability likely reflect corresponding effects on the strength of the prm240 promoter, measured in an uncoupled assay system. Several derivatives of λprm240 with altered stabilities were characterized. This mutant and its derivatives afford a model system for further analysis of stability.

Microaerobic conditions enhance type III secretion and adherence of enterohaemorrhagic Escherichia coli to polarized human intestinal epithelial cells

Advances in the understanding of the pathogenesis of enterohaemorrhagic Escherichia coli (EHEC) have greatly benefited from the use of human epithelial cell lines under aerobic conditions. However, in the target site of EHEC infection, the human intestine, conditions are microaerobic. In our study we used polarized human colon carcinoma cells in a vertical diffusion chamber system to investigate the influence of reduced apical oxygen levels on EHEC colonization. While apical microaerobiosis did not affect cell integrity and barrier function, numbers of adherent bacteria were significantly increased under low compared with high apical oxygen concentrations. In addition, expression and translocation of EHEC type III secreted (T3S) effector proteins was considerably enhanced under microaerobic conditions and dependent on the presence of host cells. Increased colonization was mainly mediated via EspA as adherence levels of an isogenic deletion mutant were not influenced by low oxygen levels. Other potential adherence factors (E. coli common pilus and flagella) were only minimally expressed under high and low oxygen levels. Addition of nitrate and trimethylamine N-oxide as terminal electron acceptors for anaerobic respiration failed to further increase bacterial colonization or T3S under microaerobiosis. This study indicates that EHEC T3S and colonization are enhanced by the microaerobic environment in the gut and therefore might be underestimated in conventional aerobic cell culture systems.

Adaptations of Candida albicans for growth in the mammalian intestinal tract

Although the fungus Candida albicans is a commensal colonizer of humans, the organism is also an important opportunistic pathogen. Most infections caused by C. albicans arise from organisms that were previously colonizing the host as commensals, and therefore successful establishment of colonization is a prerequisite for pathogenicity. To elucidate fungal activities that promote colonization, an analysis of the transcription profile of C. albicans cells recovered from the intestinal tracts of mice was performed. The results showed that within the C. albicans colonizing population, cells expressed genes characteristic of the laboratory-grown exponential phase and genes characteristic of post-exponential-phase cells. Thus, gene expression both promoted the ability to grow rapidly (a characteristic of exponential-phase cells) and enhanced the ability to resist stresses (a characteristic of post-exponential-phase cells). Similarities in gene expression in commensal colonizing cells and cells invading host tissue during disease were found, showing that C. albicans cells adopt a particular cell surface when growing within a host in both situations. In addition, transcription factors Cph2p and Tec1p were shown to regulate C. albicans gene expression during intestinal colonization.

Hydrogenase-3 contributes to anaerobic acid resistance of Escherichia coli

Background

Hydrogen production by fermenting bacteria such as Escherichia coli offers a potential source of hydrogen biofuel. Because H₂ production involves consumption of 2H⁺, hydrogenase expression is likely to involve pH response and regulation. Hydrogenase consumption of protons in E. coli has been implicated in acid resistance, the ability to survive exposure to acid levels (pH 2–2.5) that are three pH units lower than the pH limit of growth (pH 5–6). Enhanced survival in acid enables a larger infective inoculum to pass through the stomach and colonize the intestine. Most acid resistance mechanisms have been defined using aerobic cultures, but the use of anaerobic cultures will reveal novel acid resistance mechanisms.

Methods and Principal Findings

We analyzed the pH regulation of bacterial hydrogenases in live cultures of E. coli K-12 W3110. During anaerobic growth in the range of pH 5 to 6.5, E. coli expresses three hydrogenase isoenzymes that reversibly oxidize H₂ to 2H⁺. Anoxic conditions were used to determine which of the hydrogenase complexes contribute to acid resistance, measured as the survival of cultures grown at pH 5.5 without aeration and exposed for 2 hours at pH 2 or at pH 2.5. Survival of all strains in extreme acid was significantly lower in low oxygen than for aerated cultures. Deletion of hyc (Hyd-3) decreased anoxic acid survival 3-fold at pH 2.5, and 20-fold at pH 2, but had no effect on acid survival with aeration. Deletion of hyb (Hyd-2) did not significantly affect acid survival. The pH-dependence of H₂ production and consumption was tested using a H₂-specific Clark-type electrode. Hyd-3-dependent H₂ production was increased 70-fold from pH 6.5 to 5.5, whereas Hyd-2-dependent H₂ consumption was maximal at alkaline pH. H₂ production, was unaffected by a shift in external or internal pH. H₂ production was associated with hycE expression levels as a function of external pH.

Conclusions

Anaerobic growing cultures of E. coli generate H₂ via Hyd-3 at low external pH, and consume H₂ via Hyd-2 at high external pH. Hyd-3 proton conversion to H₂ is required for acid resistance in anaerobic cultures of E. coli.

Like will to like: abundances of closely related species can predict susceptibility to intestinal colonization by pathogenic and commensal bacteria

The intestinal ecosystem is formed by a complex, yet highly characteristic microbial community. The parameters defining whether this community permits invasion of a new bacterial species are unclear. In particular, inhibition of enteropathogen infection by the gut microbiota ( = colonization resistance) is poorly understood. To analyze the mechanisms of microbiota-mediated protection from Salmonella enterica induced enterocolitis, we used a mouse infection model and large scale high-throughput pyrosequencing. In contrast to conventional mice (CON), mice with a gut microbiota of low complexity (LCM) were highly susceptible to S. enterica induced colonization and enterocolitis. Colonization resistance was partially restored in LCM-animals by co-housing with conventional mice for 21 days (LCM(con21)). 16S rRNA sequence analysis comparing LCM, LCM(con21) and CON gut microbiota revealed that gut microbiota complexity increased upon conventionalization and correlated with increased resistance to S. enterica infection. Comparative microbiota analysis of mice with varying degrees of colonization resistance allowed us to identify intestinal ecosystem characteristics associated with susceptibility to S. enterica infection. Moreover, this system enabled us to gain further insights into the general principles of gut ecosystem invasion by non-pathogenic, commensal bacteria. Mice harboring high commensal E. coli densities were more susceptible to S. enterica induced gut inflammation. Similarly, mice with high titers of Lactobacilli were more efficiently colonized by a commensal Lactobacillus reuteri(RR) strain after oral inoculation. Upon examination of 16S rRNA sequence data from 9 CON mice we found that closely related phylotypes generally display significantly correlated abundances (co-occurrence), more so than distantly related phylotypes. Thus, in essence, the presence of closely related species can increase the chance of invasion of newly incoming species into the gut ecosystem. We provide evidence that this principle might be of general validity for invasion of bacteria in preformed gut ecosystems. This might be of relevance for human enteropathogen infections as well as therapeutic use of probiotic commensal bacteria.

Imaging the Gastrointestinal Tract of Small Animals

Animal models of human diseases are increasingly available and are invaluable for studies of organ pathophysiology. Megacolon, abnormal dilatation of the colon not caused by mechanical obstruction, involves the destruction of the autonomic nervous system innervating the colon. Animal models of megacolon include mouse models of Chagas disease and Hirschprung's disease. Small animal imaging has become an important research tool and recent advances in preclinical imaging modalities have enhanced the information content available from longitudinal studies of animal models of human diseases. While numerous applications of imaging technologies have been reported to study the brain and heart of mouse models, fewer studies of the gastrointestinal system have been undertaken due to technical limitations caused by peristaltic and respiratory motion. Various imaging modalities relevant to study of the gastrointestinal tract of intact live animals are reviewed herein.

oxyR, a LysR-type regulator involved in Klebsiella pneumoniae mucosal and abiotic colonization

Colonization of the gastrointestinal tract is the first event in Klebsiella pneumoniae nosocomial infections, followed by colonization of the bladder or respiratory tract or entry into the bloodstream. To survive in the host, bacteria must harbor specific traits and overcome multiple stresses. OxyR is a conserved bacterial transcription factor with a key role both in the upregulation of defense mechanisms against oxidative stress and in pathogenesis by enhancing biofilm formation, fimbrial expression, and mucosal colonization. A homolog of oxyR was detected in silico in the K. pneumoniae sequenced genome and amplified from the LM21 wild-type strain. To determine the role of oxyR in K. pneumoniae host-interaction processes, an oxyR isogenic mutant was constructed, and its behavior was assessed. At concentrations lower than 10(7) ml(-1), oxyR-deficient organisms were easily killed by micromolar concentrations of H(2)O(2) and exhibited typical aerobic phenotypes. The oxyR mutant was impaired in biofilm formation and types 1 and 3 fimbrial gene expression. In addition, the oxyR mutant was unable to colonize the murine gastrointestinal tract, and in vitro assays showed that it was defective in adhesion to Int-407 and HT-29 intestinal epithelial cells. The behavior of the oxyR mutant was also determined under hostile conditions, reproducing stresses encountered in the gastrointestinal environment: deletion of oxyR resulted in higher sensitivity to bile and acid stresses but not to osmotic stress. These results show the pleiotropic role of oxyR in K. pneumoniae gastrointestinal colonization.

Human gut microbiome adopts an alternative state following small bowel transplantation

Small bowel transplants provide an exceptional opportunity for long-term study of the microbial ecology of the human small bowel. The ileostomy created at time of transplant for ongoing monitoring of the allograft provides access to samples of ileal effluent and mucosal biopsies. In this study, we used qPCR to assay the bacterial population of the small bowel lumen of 17 small bowel transplant patients over time. Surprisingly, the posttransplant microbial community was found to be dominated by Lactobacilli and Enterobacteria, both typically facultative anaerobes. This represents an inversion of the normal community that is dominated instead by the strictly anaerobic Bacteroides and Clostridia. We found this inverted community also in patients with ileostomies who did not receive a transplant, suggesting that the ileostomy itself is the primary ecological determinant shaping the microbiota. After surgical closure of the ileostomy, the community reverted to the normal structure. Therefore, we hypothesized that the ileostomy allows oxygen into the otherwise anaerobic distal ileum, thus driving the transition from one microbial community structure to another. Supporting this hypothesis, metabolomic profiling of both communities demonstrated an enrichment for metabolites associated with aerobic respiration in samples from patients with open ileostomies. Viewed from an ecological perspective, the two communities constitute alternative stable states of the human ileum. That the small bowel appears to function normally despite these dramatic shifts suggests that its ecological resilience is greater than previously realized.

Intestinal ischemia-reperfusion injury: reversible and irreversible damage imaged in vivo

The early events in an intestinal ischemic episode have been difficult to evaluate. Using in vivo microscopy we have analyzed in real-time the effects of short (15 min) and long (40-50 min) ischemia with subsequent reperfusion (IR), evaluating structure, integrity, and functioning of the mouse jejunal mucosa while monitoring blood flow by confocal microscopy. IR was imposed by inflation/deflation of a vascular occluder, and blood flow was monitored and confirmed with scanning confocal imaging. After short ischemia, villus tip cells revealed a rapid increase (23%) in the intracellular NAD(P)H concentration (confocal autofluorescence microscopy), and the pH-sensitive probe BCECF showed a biphasic response of the intracellular pH (pH(i)), quickly alkalinizing from the resting value of 6.8 +/- 0.1 to 7.1 +/- 0.1 but then strongly acidifying to 6.3 +/- 0.1. Upon reperfusion, values returned toward control. In contrast, results were heterogeneous after long IR. During long ischemia, one-third of the epithelial cells remained viable with reversible changes upon reperfusion, but remaining cells lost membrane integrity (Lucifer Yellow uptake, LY) and had membrane blebs during ischemia. These effects became more pronounced as the reperfusion interval progressed when cells exhibited more severely affected NAD(P)H and pH(i) values, larger blebs, and more LY uptake and eventually were shed from the villus. Results from stereo microscopy suggest that these irreversible effects of IR may have occurred as a result of incomplete restorations of local blood flow, especially at the antimesenteric side of the intestine. We conclude that the adverse effects of short ischemia on the jejunum epithelium are fully reversible during the reperfusion interval. However, after long ischemia, reperfusion cannot restore normal structure and functioning of a majority of cells, which deteriorate further. Our results provide a basis for defining the cellular events that cause tissue to transit from reversible to irreversible damage during IR.

Segmented surface coil resonator for in vivo EPR applications at 1.1GHz

A four-loop segmented surface coil resonator (SSCR) with electronic frequency and coupling adjustments was constructed with 18mm aperture and loading capability suitable for in vivo Electron Paramagnetic Resonance (EPR) spectroscopy and imaging applications at L-band. Increased sample volume and loading capability were achieved by employing a multi-loop three-dimensional surface coil structure. Symmetrical design of the resonator with coupling to each loop resulted in high homogeneity of RF magnetic field. Parallel loops were coupled to the feeder cable via balancing circuitry containing varactor diodes for electronic coupling and tuning over a wide range of loading conditions. Manually adjusted high Q trimmer capacitors were used for initial tuning with subsequent tuning electronically controlled using varactor diodes. This design provides transparency and homogeneity of magnetic field modulation in the sample volume, while matching components are shielded to minimize interference with modulation and ambient RF fields. It can accommodate lossy samples up to 90% of its aperture with high homogeneity of RF and modulation magnetic fields and can function as a surface loop or a slice volume resonator. Along with an outer coaxial NMR surface coil, the SSCR enabled EPR/NMR co-imaging of paramagnetic probes in living rats to a depth of 20mm.

Esterified trityl radicals as intracellular oxygen probes

Triarylmethyl (trityl) radicals exhibit high stability and narrow linewidth under physiological conditions which provide high sensitivity and resolution for the measurement of O2 concentrations, making them attractive as EPR oximetry probes. However, the application of previously available compounds has been limited by their poor intracellular permeability. We recently reported the synthesis and characterization of esterified trityl radicals as potential intracellular EPR probes and their oxygen sensitivity, redox properties, and enzyme-mediated hydrolysis were investigated. In this paper, we report the cellular permeability and stability of these trityls in the presence of bovine aortic endothelial cells. Results show that the acetoxymethoxycarbonyl-containing trityl AMT-02 exhibits high stability in the presence of cells and can be effectively internalized. The intracellular hydrolysis of AMT-02 to the carboxylate form of the trityl (CT-03) was also observed. In addition, this internalized trityl probe was applied to measure intracellular O2 concentrations and the effects of menadione and KCN on the rates of O2 consumption in endothelial cells. This study demonstrates that these esterified trityl radicals can function as effective EPR oximetry probes measuring intracellular O2 concentration and consumption.

Simple data acquisition method for multi-dimensional EPR spectral-spatial imaging using a combination of constant-time and projection-reconstruction modalities

A combination of the constant-time spectral-spatial imaging (CTSSI) modality and projection-reconstruction modality was tested to simplify data acquisition for multi-dimensional CW EPR spectral-spatial imaging. In this method, 3D spectral-spatial image data were obtained by simple repetition of conventional 2D CW imaging process, except that the field gradient amplitude was incremented in constant steps in each repetition. The data collection scheme was no different from the conventional CW imaging system for spectral-spatial data acquisition. No special equipment and/or rewriting of existing software were required. The data acquisition process for multi-dimensional spectral-spatial imaging is consequently simplified. There is also no "missing-angle" issue because the CTSSI modality was employed to reconstruct 2D spectral-spatial images. Extra reconstruction processes to obtain higher spatial dimensions were performed using a conventional projection-reconstruction modality. This data acquisition technique can be applied to any conventional CW EPR (spatial) imaging system for multi-dimensional spectral-spatial imaging.

Formation of hydrogen peroxide and reduction of peroxynitrite via dismutation of superoxide at reperfusion enhances myocardial blood flow and oxygen consumption in postischemic mouse heart

Reactive oxygen/nitrogen species suppress myocardial oxygen consumption. In this study, we determined that endogenous hydrogen peroxide through dismutation of superoxide enhances postischemic myocardial blood perfusion and oxygen consumption. Electron paramagnetic resonance oximetry was applied to monitor in vivo tissue Po2 in mouse heart subjected to regional ischemia reperfusion. Heart rate, arterial blood pressure, blood flow, infarction, and activities of mitochondrial NADH dehydrogenase and cytochrome c oxidase were measured in six groups of wild-type (WT) and endothelial nitricoxide synthase knock-out (eNOS(-/-)) mice treated with phosphate-buffered saline (PBS), superoxide dismutase mimetic (SOD(m)) M40403 [a manganese(II)-bis(cyclohexylpyridine)-substituted macrocyclic superoxide dismutase mimetic, C21H35Cl2MnN5], 10006329 EUK 134 [EUK134, manganese 3-methoxy N,N(1)-bis(salicyclidene)ethylenediamine chloride], and SOD(m) plus glibenclamide to study the protective effect of hydrogen peroxide via dismutation of superoxide on the activation of sarcolemmal potassium channels. In the PBS group, there was an overshoot of tissue Po2 after reperfusion. Treatment with SOD(m), EUK134, and SOD(m) + glibenclamide protected mitochondrial enzyme activities, reduced infarct size, and suppressed the postischemic hyperoxygenation. In particular, in the SOD(m)-treated group, there was a transient peak of tissue Po2 at 9 min after reperfusion, which was dependent on endogenous hydrogen peroxide but not nitric oxide formation as it appeared in both WT and eNOS(-/-) mice. Blood flow and rate pressure product were higher in the SOD(m) group than in other groups, which contributed to the transient oxygen peak. Thus, SOD mimetics protected mouse heart from superoxide-induced reperfusion injury. With treatment of different SOD mimetics, it is concluded that endogenous hydrogen peroxide via dismutation of superoxide at reperfusion enhances postischemic myocardial blood perfusion and mitochondrial oxygen consumption, possibly through activation of sarcolemmal ATP-sensitive potassium channels.

Assessment of liver phagocytic activity using EPR spectrometry and imaging

The aim of the present study was to evaluate the usefulness of electron paramagnetic resonance (EPR) spectroscopy and imaging in assessing the phagocytic activity of the liver after administration of India ink. We conducted experiments on livers from control rodents and from rodents in which the Kupffer cell population had been depleted by pretreatment with gadolinium chloride. The EPR signal intensity recorded in liver homogenates was about two times lower in GdCl(3) treated rats than in control rats. EPR imaging carried out on precision-cut liver slices indicated a good correlation between the depletion of Kupffer cells and the EPR signal intensity.

Fast and simple method for the simultaneous evaluation of the capacity and efficiency of food antioxidants in trapping peroxyl radicals in an intestinal model system

A simple oxygraphic method, for which the theoretical and experimental bases have been recently revised, has been successfully applied to evaluate the peroxyl radical chain-breaking characteristics of some typical food antioxidants in micelle systems, among which is a system that reproduces conditions present in the upper part of the digestive tract, where the absorption and digestion of lipids occur. This method permits one to obtain from a single experimental run the peroxyl radical trapping capacity (PRTC, that is, the number of moles of peroxyl radicals trapped by a given amount of food), the peroxyl radical trapping efficiency (PRTE, that is, the reciprocal of the amount of food that reduces to half the steady-state concentration of peroxyl radicals), and the half-life of the antioxidant ( t(1/2)) when only a small fraction of peroxyl radicals reacts with the antioxidants present in foods. Examples of application of the method to various types of foodstuffs have been reported, assessing the general validity of the method in the simple and fast evaluation of the above-reported fundamental antioxidant characteristics of foods.