Expression of the Helicobacter pylori ureI gene is required for acidic pH activation of cytoplasmic urease

Characterization of the ArsRS regulon of Helicobacter pylori, involved in acid adaptation

The human gastric pathogen Helicobacter pylori is extremely well adapted to the highly acidic conditions encountered in the stomach. The pronounced acid resistance of H. pylori relies mainly on the ammonia-producing enzyme urease; however, urease-independent mechanisms are likely to contribute to acid adaptation. Acid-responsive gene regulation is mediated at least in part by the ArsRS two-component system consisting of the essential OmpR-like response regulator ArsR and the nonessential cognate histidine kinase ArsS, whose autophosphorylation is triggered in response to low pH. In this study, by global transcriptional profiling of an ArsS-deficient H. pylori mutant grown at pH 5.0, we define the ArsR approximately P-dependent regulon consisting of 109 genes, including the urease gene cluster, the genes encoding the aliphatic amidases AmiE and AmiF, and the rocF gene encoding arginase. We show that ArsR approximately P controls the acid-induced transcription of amiE and amiF by binding to extended regions located upstream of the -10 box of the respective promoters. In contrast, transcription of rocF is repressed by ArsR approximately P at neutral, acidic, and mildly alkaline pH via high-affinity binding of the response regulator to a site overlapping the promoter of the rocF gene.

Involvement of the HP0165-HP0166 two-component system in expression of some acidic-pH-upregulated genes of Helicobacter pylori

About 200 genes of the gastric pathogen Helicobacter pylori increase expression at medium pHs of 6.2, 5.5, and 4.5, an increase that is abolished or much reduced by the buffering action of urease. Genes up-regulated by a low pH include the two-component system HP0165-HP0166, suggesting a role in the regulation of some of the pH-sensitive genes. To identify targets of HP0165-HP0166, the promoter regions of genes up-regulated by a low pH were grouped based on sequence similarity. Probes for promoter sequences representing each group were subjected to electrophoretic mobility shift assays (EMSA) with recombinant HP0166-His(6) or a mutated response regulator, HP0166-D52N-His(6), that can specifically determine the role of phosphorylation of HP0166 in binding (including a control EMSA with in-vitro-phosphorylated HP0166-His(6)). Nineteen of 45 promoter-regulatory regions were found to interact with HP0166-His(6). Seven promoters for genes encoding alpha-carbonic anhydrase, omp11, fecD, lpp20, hypA, and two with unknown function (pHP1397-1396 and pHP0654-0675) were clustered in gene group A, which may respond to changes in the periplasmic pH at a constant cytoplasmic pH and showed phosphorylation-dependent binding in EMSA with HP0166-D52N-His(6). Twelve promoters were clustered in groups B and C whose up-regulation likely also depends on a reduction of the cytoplasmic pH at a medium pH of 5.5 or 4.5. Most of the target promoters in groups B and C showed phosphorylation-dependent binding with HP0166-D52N-His(6), but promoters for ompR (pHP0166-0162), pHP0682-0681, and pHP1288-1289 showed phosphorylation-independent binding. These findings, combined with DNase I footprinting, suggest that HP0165-0166 is an acid-responsive signaling system affecting the expression of pH-sensitive genes. Regulation of these genes responds either to a decrease in the periplasmic pH alone (HP0165 dependent) or also to a decrease in the cytoplasmic pH (HP0165 independent).

Acid acclimation by Helicobacter pylori

Helicobacter pylori is a Gram-negative neutralophile associated with peptic ulcers and gastric cancer. It has a unique ability to colonize the human stomach by acid acclimation. It uses the pH-gated urea channel, UreI, to enhance urea access to intrabacterial urease and a membrane-anchored periplasmic carbonic anhydrase to regulate periplasmic pH to approximately 6.1 in acidic media, whereas other neutralophiles cannot regulate periplasmic pH and thus only transit the stomach.

Differential regulation of urease activity in Helicobacter hepaticus and Helicobacter pylori

Helicobacter hepaticus is a pathogen of rodents, which causes diverse enteric and hepatic inflammatory diseases and malignancies. The urease enzyme is an important colonization factor of gastric Helicobacter species like Helicobacter pylori, but little is known about the role and regulation of urease in enterohepatic Helicobacter species. Here it is reported that urease activity of H. hepaticus does not contribute to acid resistance, and that it is nickel-responsive at the post-translational level. H. hepaticus strain ATCC 51449 did not grow or survive at pH 3·0, and supplementation with urea or NiCl2 did not abrogate this acid sensitivity. Furthermore, urease enzyme activity of H. hepaticus was acid-independent, which contrasts with the acid-induced urease system of H. pylori. Nickel supplementation of Brucella medium resulted in a tenfold increase in urease activity in both H. hepaticus and H. pylori, but the maximum level of urease activity in H. hepaticus was still three- to fivefold lower when compared to H. pylori in the same conditions. The increase in urease activity of H. hepaticus was not associated with elevation of urease mRNA or protein levels. Inhibition of protein synthesis by chloramphenicol did not affect nickel-responsive induction of urease activity in H. hepaticus, and confirmed that nickel induction occurs at the post-translational level, probably by activation of preformed apo-enzyme. In conclusion, both the role of the urease enzyme and the regulation of urease activity differ between the enterohepatic pathogen H. hepaticus and the gastric pathogen H. pylori.

Acid-induced activation of the urease promoters is mediated directly by the ArsRS two-component system of Helicobacter pylori

The nickel-containing enzyme urease is an essential colonization factor of the human gastric pathogen Helicobacter pylori which enables the bacteria to survive the low-pH conditions of the stomach. Transcription of the urease genes is positively controlled in response to increasing concentrations of nickel ions and acidic pH. Here we demonstrate that acid-induced transcription of the urease genes is mediated directly by the ArsRS two-component system. Footprint analyses identify binding sites of the phosphorylated ArsR response regulator within the ureA and ureI promoters. Furthermore, deletion of a distal upstream ArsR binding site of the ureA promoter demonstrates its role in acid-dependent activation of the promoter. In addition, acid-induced transcription of the ureA gene is unaltered in a nikR mutant, providing evidence that pH-responsive regulation and nickel-responsive regulation of the ureA promoter are mediated by independent mechanisms involving the ArsR response regulator and the NikR protein.

Effect of different organic acids (citric, malic and ascorbic) on intragastric urease activity

BACKGROUND

The mechanism of citric acid-enhanced Helicobacter pylori urease activity remains unclear.

AIM

To compare ascorbic, citric and malic acid given at the same concentration and pH on intragastric urease activity.

METHODS

Volunteers received 40 mg of famotidine the evening prior to breath testing. After an overnight fast volunteers were randomized to receive 100 mL of water or 100 mm citric, malic, or ascorbic acid, pH 2.3 containing 75 mg of 13C-urea. At 15 min a second 100 mL solution of one of the test solutions was taken without added urea.

RESULTS

Twelve volunteers were studied (eight men, four women, age 19-57, median 50.7) in a randomized-crossover study. The mean breath test result at 30 min with ascorbic (17.5 +/- 5), malic (25.8 +/- 5) and citric acid (29.5 +/- 5) were all significantly greater than with water (9.5 +/- 3). Citric and malic acid were similar (P = 0.699) and significantly greater than ascorbic acid (P < 0.02). When the ascorbic acid was followed by citric acid, the result was similar to that with citrate alone (25.8 +/- 4) and greater than with ascorbic then ascorbic (P = 0.026).

CONCLUSIONS

Enhancement of H. pylori urease activity is not strictly a function of the pH. We propose the effect is related to differential effects of the availability of nickel, which is required for urease activity. Citric acid and malic acid were essentially equivalent such that malic acid could substitute for citric acid in the UBT; ascorbic acid would be a poor choice.

The periplasmic alpha-carbonic anhydrase activity of Helicobacter pylori is essential for acid acclimation

The role of the periplasmic alpha-carbonic anhydrase (alpha-CA) (HP1186) in acid acclimation of Helicobacter pylori was investigated. Urease and urea influx through UreI have been shown to be essential for gastric colonization and for acid survival in vitro. Intrabacterial urease generation of NH3 has a major role in regulation of periplasmic pH and inner membrane potential under acidic conditions, allowing adequate bioenergetics for survival and growth. Since alpha-CA catalyzes the conversion of CO2 to HCO3-, the role of CO2 in periplasmic buffering was studied using an alpha-CA deletion mutant and the CA inhibitor acetazolamide. Western analysis confirmed that alpha-CA was bound to the inner membrane. Immunoblots and PCR confirmed the absence of the enzyme and the gene in the alpha-CA knockout. In the mutant or in the presence of acetazolamide, there was an approximately 3 log10 decrease in acid survival. In acid, absence of alpha-CA activity decreased membrane integrity, as observed using membrane-permeant and -impermeant fluorescent DNA dyes. The increase in membrane potential and cytoplasmic buffering following urea addition to wild-type organisms in acid was absent in the alpha-CA knockout mutant and in the presence of acetazolamide, although UreI and urease remained fully functional. At low pH, the elevation of cytoplasmic and periplasmic pH with urea was abolished in the absence of alpha-CA activity. Hence, buffering of the periplasm to a pH consistent with viability depends not only on NH3 efflux from the cytoplasm but also on the conversion of CO2, produced by urease, to HCO3- by the periplasmic alpha-CA.

Flow cytometry analysis of germinating Bacillus spores, using membrane potential dye

Germination of Bacillus anthracis spores is necessary for the transcription of plasmidic genes essential to the infection. Assessing germination potential is crucial to predict the risk associated with pathogenic Bacillus exposure. The aim of this study was to set up a viability assay based on membrane potential in order to predict the earliest germination event of spores. B. cereus and two strains of B. subtilis were used. The spores were isolated with a sodium bromide gradient. Approximately 10(7) spores were incubated at 37 degrees C in tryptic soy broth (TSB). Aliquots were harvested at predetermined times and stained with 3,3'-dihexyloxacarbocyanine iodide [DiOC(6)(3)] or with bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC(4)(3)]. Fluorescence characteristics were obtained using flow cytometry. The earliest detectable activation of membrane potential occurred after 15 min of incubation in TSB using DiOC(6)(3). Using DiBAC(4)(3), the earliest detectable signal was after 4 h of incubation. Control experiments using carbonyl cyanide m-chlorophenylhydrazone (CCCP)-treated spores did not show any change in the fluorescence intensity over time. Since no membrane potential and no germination were detected in CCCP-treated spores, the activation of membrane potential seems to be associated with germination. DiOC(6)(3) can be used as an early membrane potential indicator for spores. DiBAC(4)(3), by contrast, is not a early membrane potential marker.

A model for the study of Helicobacter pylori interaction with human gastric acid secretion

We present a comprehensive mathematical model describing Helicobacter pylori interaction with the human gastric acid secretion system. We use the model to explore host and bacterial conditions that allow persistent infection to develop and be maintained. Our results show that upon colonization, there is a transient period (day 1-20 post-infection) prior to the establishment of persistence. During this period, changes to host gastric physiology occur including elevations in positive effectors of acid secretion (such as gastrin and histamine). This is promoted by reduced somatostatin levels, an inhibitor of acid release. We suggest that these changes comprise compensatory mechanisms aimed at restoring acid to pre-infection levels. We also show that ammonia produced by bacteria sufficiently buffers acid promoting bacteria survival and growth.

Acid-responsive gene induction of ammonia-producing enzymes in Helicobacter pylori is mediated via a metal-responsive repressor cascade

Although the adaptive mechanisms allowing the gastric pathogen Helicobacter pylori to survive acid shocks have been well documented, the mechanisms allowing growth at mildly acidic conditions (pH approximately 5.5) are still poorly understood. Here we demonstrate that H. pylori strain 26695 increases the transcription and activity of its urease, amidase, and formamidase enzymes four- to ninefold in response to growth at pH 5.5. Supplementation of growth medium with NiCl2 resulted in a similar induction of urease activity (at low NiCl2 concentration) and amidase activity (at > or = 500 micro M NiCl2) but did not affect formamidase activity. Mutation of the fur gene, which encodes an iron-responsive repressor of both amidases, resulted in a constitutively high level of amidase and formamidase activity at either pH but did not affect urease activity at pH 7.0 or pH 5.5. In contrast, mutation of the nikR gene, encoding the nickel-responsive activator of urease expression, resulted in a significant reduction of acid-responsive induction of amidase and formamidase activity. Finally, acid-responsive repression of fur transcription was absent in the H. pylori nikR mutant, whereas transcription of the nikR gene itself was increased at pH 5.5 in wild-type H. pylori. We hypothesize that H. pylori uses a repressor cascade to respond to low pH, with NikR initiating the response directly via the urease operon and indirectly via the members of the Fur regulon.

False negative urea breath tests with H2-receptor antagonists: interactions between Helicobacter pylori density and pH

BACKGROUND

We studied the effects of famotidine, sodium bicarbonate, and citric acid on the 13C-urea breath test (UBT).

METHODS

Helicobacter pylori-infected volunteers received a UBT, 40 mg of famotidine at bedtime, and a second UBT (pudding test meal, 648 mg NaHCO3 tablet then 125 mg of urea in 200 ml of water containing 650 mg of NaHCO3). Experiment 2 consisted of four UBTs. Two were standard citric acid UBTs with 75 mg of urea and 2 g citric acid and two were sequential bicarbonate-citric acid UBTs. Sequential UBTs consisted of administration of a 648 mg bicarbonate tablet with 50 g of Polycose in 200 ml of water. Five minutes later, 125 mg of 13C-urea was given in 75 ml of water containing 650 mg of NaHCO3. Breath samples were collected after 15 minutes. Then, to acutely acidify the stomach, 4 g of citric acid was given in 200 ml of water. A second breath sample was collected 15 minutes after the citric acid. The standard UBTs were done before and after 6 days of famotidine (40 mg b.i.d.). Sequential UBTs were done after 1 and 6 days of famotidine therapy. Gastric biopsies for histology, culture, and mucosal cytokines were assessed before and after 6 days of famotidine.

RESULTS

Eighteen subjects participated, 10 in each experiment; seven had endoscopy with biopsy. Famotidine/ bicarbonate resulted an approximately 50% fall in UBT values (p = .021) with 10% becoming negative. The gastric pH increased from 5.1 +/- 0.5 to 6.7 +/- 0.2 (p = .03) although no pH value predicted the occurrence of false negative results. Under famotidine acid suppression, NaHCO3 reduced the delta over baseline (DOB) by 63% (p = .021). This was reversed with citric acid. Histology showed a H2-receptor antagonist-associated increase in the depth of gastric corpus inflammation.

CONCLUSIONS

H2-receptor antagonists differ from proton pump inhibitors as high intragastric pH may cause a reduction in urease activity, unrelated to a reduced bacterial load and reversed by citric acid.

Mechanism of proton gating of a urea channel

The size and complexity of many pH-gated channels have frustrated the development of specific structural models. The small acid-activated six-membrane segment urea channel of Helicobacter hepaticus (HhUreI), homologous to the essential UreI of the gastric pathogen Helicobacter pylori, enables identification of all the periplasmic sites of proton gating by site-directed mutagenesis. Exposure to external acidity enhances [(14)C]urea uptake by Xenopus oocytes expressing HhUreI, with half-maximal activity (pH(0.5)) at pH 6.8. A downward shift of pH(0.5) in single site mutants identified four of six protonatable periplasmic residues (His-50 at the boundary of the second transmembrane segment TM2, Glu-56 in the first periplasmic loop, Asp-59 at the boundary of TM3, and His-170 at the boundary of TM6) that affect proton gating. Asp-59 was the only site at which a protonatable residue appeared to be essential for pH gating. Mutation of Glu-110 or Glu-114 in PL2 did not affect the pH(0.5) of gating. A chimera, where the entire periplasmic domain of HhUreI was fused to the membrane domain of Streptococcus salivarius UreI (SsUreI), retained the pH-independent properties of SsUreI. Hence, proton gating of HhUreI likely depends upon the formation of hydrogen bonds by periplasmic residues that in turn produce conformational changes of the transmembrane domain. Further studies on HhUreI may facilitate understanding of other physiologically important pH-responsive channels.

Acid-adaptive genes of Helicobacter pylori

Helicobacter pylori is the only neutralophile that has been able to colonize the human stomach by using a variety of acid-adaptive mechanisms. One of the adaptive mechanisms is increased buffering due to expression of an acid-activated inner membrane urea channel, UreI, and a neutral pH-optimum intrabacterial urease. To delineate other possible adaptive mechanisms, changes in gene expression in response to acid exposure were examined using genomic microarrays of H. pylori exposed to different levels of external pH (7.4, 6.2, 5.5, and 4.5) for 30 min in the absence and presence of 5 mM urea. Gene expression was correlated with intrabacterial pH measured using 2',7'-bis-(2-carboxyethyl)-5-carboxyfluorescein and compared to that observed with exposure to 42 degrees C for 30 min. Microarrays containing the 1,534 open reading frames of H. pylori strain 26695 were hybridized with cDNAs from control (pH 7.4; labeled with Cy3) and acidic (labeled with Cy5) conditions. The intrabacterial pH was 8.1 at pH 7.4, fell to 5.3 at pH 4.5, and rose to 6.2 with urea. About 200 genes were up-regulated and approximately 100 genes were down-regulated at pH 4.5 in the absence of urea, and about half that number changed in the presence of urea. These genes included pH-homeostatic, transcriptional regulatory, motility, cell envelope, and pathogenicity genes. The up-regulation of some pH-homeostatic genes was confirmed by real-time PCR. There was little overlap with the genes induced by temperature stress. These results suggest that H. pylori has evolved multifaceted acid-adaptive mechanisms enabling it to colonize the stomach that may be novel targets for eliminating infection.

The gastric biology of Helicobacter pylori

Helicobacter pylori is a neutralophilic, gram-negative, ureolytic organism that is able to colonize the human stomach but does not survive in a defined medium with a pH <4.0 unless urea is present. In order to live in the gastric environment, it has developed a repertoire of acid resistance mechanisms that can be classified into time-independent, acute, and chronic responses. Time-independent acid resistance depends on the structure of the organism's inner and outer membrane proteins that have a high isoelectric point, thereby reducing their proton permeability. Acute acid resistance depends on the constitutive synthesis of a neutral pH optimum urease that is an oligomeric Ni(2+)-containing heterodimer of UreA and UreB subunits. Gastric juice urea is able to rapidly access intrabacterial urease when the periplasmic pH falls below approximately 6.2 owing to pH-gating of a urea channel, UreI. This results in the formation of NH3, which then neutralizes the bacterial periplasm to provide a pH of approximately 6.2 and an inner membrane potential of -101 mV, giving a proton motive force of approximately -200 mV. UreI is a six-transmembrane segment protein, with homology to the amiS genes of the amidase gene cluster and to UreI of Helicobacter hepaticus and Streptococcus salivarius. Expression of these UreI proteins in Xenopus oocytes has shown that UreI of H. pylori and H. hepaticus can transport urea only at acidic pH, whereas that of S. salivarius is open at both neutral and acidic pH. Site-directed mutagenesis and chimeric analysis have identified amino acids implicated in maintaining the closed state of the channel at neutral pH and other amino acids that play a structural role in channel function. Deletion of ureI abolishes the ability of the organism to survive in acid and also to colonize the mouse or gerbil stomach. However, if acid secretion is inhibited in gerbils, the deletion mutants do colonize but are eradicated when acid secretion is allowed to return, showing that UreI is essential for gastric survival and that the habitat of H. pylori at the gastric surface must fall to pH 3.5 or below. The chronic response is from increased Ni(2+) insertion into the apo-enzyme, which results in a threefold increase in urease, which is also dependent on expression of UreI. This allows the organism to live in either gastric fundus or gastric antrum depending on the level of acidity at the gastric surface. There are other effects of acid on transcript stability that may alter levels of protein synthesis in acid. Incubation of the organism at acidic pH also results in regulation of expression of a variety of genes, such as some outer membrane proteins, that constitutes an acid tolerance response. Understanding of these acid resistance and tolerance responses should provide novel eradication therapies for this carcinogenic gastric pathogen.

Acinetobacter lwoffii infection and gastritis

About 25% of humans with chronic gastritis are negative for Helicobacter pylori, suggesting that other bacteria are capable of causing inflammation. Bacterial overgrowth may occur in the stomach under conditions of reduced acid secretion. In this review, we will explore what is generally known about non-H. pylori organisms and their ability to induce gastritis, with particular focus on Acinetobacter lwoffi.

Medium pH-dependent redistribution of the urease of Helicobacter pylori

Helicobacter pylori is an aetiological agent of gastric disease. Although the role of urease in gastric colonization of H. pylori has been shown, it remains unclear as to where urease is located in this bacterial cell. The purpose of this study was to define the urease-associated apparatus in the H. pylori cytoplasm. H. pylori was incubated at both a neutral and an acidic pH in the presence or absence of urea and examined by double indirect immunoelectron microscopy. The density of gold particles for UreA was greatest in the inner portion of the wild-type H. pylori cytoplasm at neutral pH but was greatest in the outer portion at acidic pH. This difference was independent of the presence of urea and was not observed in the ureI-deletion mutant. Also, the eccentric shift of urease in acidic pH was not observed in UreI. After a 2 day incubation period at acidic pH, it was observed that the urease gold particles in H. pylori assembled and were associated with UreI gold particles. Urease immunoreactivity shifted from the inner to the outer portion of H. pylori as a result of an extracellular decrease in pH. This shift was urea-independent and UreI-dependent, suggesting an additional role of UreI in urease-dependent acid resistance. This is the first report of the intracellular transport of molecules in bacteria in response to changes in the extracellular environment.

Interactions among the seven Helicobacter pylori proteins encoded by the urease gene cluster

Survival of Helicobacter pylori in acid depends on intrabacterial urease. This urease is a Ni(2+)-containing oligomeric heterodimer. Regulation of its activity and assembly is important for gastric habitation by this neutralophile. The gene complex encodes catalytic subunits (ureA/B), an acid-gated urea channel (ureI), and accessory assembly proteins (ureE-H). With the use of yeast two-hybrid analysis for determining protein-protein interactions, UreF as bait identified four interacting sequences encoding UreH, whereas UreG as bait detected five UreE sequences. These results were confirmed by coimmunoprecipitation and beta-galactosidase assays. Native PAGE immunoblotting of H. pylori inner membranes showed interaction of UreA/B with UreI, whereas UreI deletion mutants lacked this protein interaction. Deletion of ureE-H did not affect this interaction with UreI. Hence, the accessory proteins UreE/G and UreF/H form dimeric complexes and UreA/B form a membrane complex with UreI, perhaps enabling assembly of the urease apoenzyme at the membrane surface and immediate urea access to intrabacterial urease to allow rapid periplasmic neutralization.

Current trends in the treatment of upper gastrointestinal disease

The past 25 years have seen an amazing improvement in the treatment and understanding of acid-related disorders. In particular, the introduction of selective histamine receptor antagonists and proton pump inhibitors has made the medical control of acid secretion an effective means of therapy. The demonstration that infection with Helicobacter pylori is responsible for most cases of peptic ulcer disease resulted in another major improvement in therapy in these areas as a result of the eradication of the organism. Research continues in an attempt to find improved means of acid control and better methods for the eradication of H. pylori based on unique proteins expressed by the organism to resist gastric acidity.

Mechanisms of acid resistance due to the urease system of Helicobacter pylori

BACKGROUND & AIMS

Helicobacter pylori, a neutralophile, uses acid neutralization by urease to combat gastric acidity, allowing gastric colonization. Both acute and chronic acid resistance mechanisms are present. Acute mechanisms of acid adaptation could be due to surface urease, increased inner-membrane urea permeability via UreI, or both. Slower mechanisms may involve increased nickel insertion into apoenzyme, posttranscriptional regulation, or increased enzyme synthesis. The aim of this study was to further define regulation of urease under acidic conditions.

METHODS

Surface-bound urease was analyzed by measurement of free and bound urease after centrifugation through a step gradient and by quantitative urease immunostaining of intact and fixed bacteria. Changes in urease synthesis or assembly were determined by incubation of the organisms at pH 5.5 or 7.0 in the absence and presence of chloramphenicol, urea, or nickel chelator and in ureI-positive and -negative organisms.

RESULTS

The amount of surface urease was below detection limits with either centrifugation washing or immunostaining. Total bacterial urease activity was increased 3-5-fold by incubation at pH 5.5 in the presence of chloramphenicol but not in nickel-free medium or in ureI knockout organisms. There was also a 3-fold increase in survival of acid shock in acid-adapted organisms.

CONCLUSIONS

Surface-bound urease is too low to contribute to acid resistance. Acidic medium pH induces UreI-dependent nickel incorporation into apoenzyme. This augmentation of urease activity increases survival in acid and is part of the gastric colonization strategy of the organism.

Energetics of Helicobacter pylori and its implications for the mechanism of urease-dependent acid tolerance at pH 1

In the presence of urea the neutrophilic human pathogen Helicobacter pylori survives for several hours at pH 1 with concomitant cytoplasmic pH homeostasis. To study this effect in detail, the transmembrane proton motive force and cytoplasmic urease activity of H. pylori were determined at various pH values. In the absence of urea, the organism maintained a close-to-neutral cytoplasm and an internally negative membrane potential at external pH values greater than 4 to 5. In the presence of urea, H. pylori accomplished cytoplasmic pH homeostasis down to an external pH of 1.2. At this external pH, the cytoplasmic pH was 4.9 and the membrane potential was slightly negative inside. The latter finding is in contrast to the situation in acidophiles, which develop inside-positive membrane potentials under similar conditions. Measurements of the time course of the membrane potential confirmed that addition of urea to the cells led to hyperpolarization. Most likely, this effect was due to electrogenic export of ammonium cations from the cytoplasm. The urease activity of intact cells increased nearly exponentially with decreasing external pH. This activation was not due to enhanced gene expression at low external pH values. In cell extracts the pH optimum of urease activity was dependent on the buffer system and was about pH 5 in sodium citrate buffer. Since this is the cytoplasmic pH of the cells at pH 1 to 2, we propose that cytoplasmic pH is a factor in the in vivo activation of the urease at low external pH values. The mechanism by which urease activity leads to cytoplasmic pH homeostasis in H. pylori is discussed.

Changes in gastric mucosa and luminal environment during acid-suppressive therapy: a review in depth

Acid-suppressive therapy and subsequent changes in gastric mucosa and luminal environment rank highly amongst the investigated issues in gastroenterology over the past two to three decades. Herewith, we present an overview of these intragastric changes, particularly during long-term administration of acid-suppresive medication and concurrent infection with Helicobacter pylori. Current evidence indicates that: i) Long-term acid suppression facilitates the development of fundic ECL cell hyperplasia, especially in the presence of Helicobacter pylori. No neoplastic changes directly attributable to acid suppression have so far been demonstrated in humans. ii) Acid-suppressive therapy increases the risk of enteric infections. iii) Acid-suppressive therapy does not alter fat and mineral bioavailability, but may decrease the absorption of protein-bound vitamin B12. iv) Acid suppression invariably results in intragastric overgrowth of non-Helicobacter pylori bacterial species. The concurrent infection with Helicobacter pylori may promote this bacterial overgrowth and the intragastric formation of N-nitrosamines. v) Acid-suppressive therapy alters the natural course of Helicobacter pylori gastritis, transforming the antral-predominant pattern into a body-predominant pattern, which in turn may progress to body gland atrophy. The pathophysiology of this phenomenon is currently under investigation. vi) In view of the potential adverse effects of acid suppression in the presence of Helicobacter pylori, the screen-and-treat strategy is advocated for Helicobacter pylori in subjects considered for long-term treatment.

Citric acid-enhanced Helicobacter pylori urease activity in vivo is unrelated to gastric emptying

BACKGROUND

In a previous study, the use of a citric acid test meal produced a rapid dose-dependent increase in urease activity that was significantly greater than that resulting from a pudding meal, ascorbic acid or sodium citrate. The mechanism was hypothesized to be related to the ability of citric acid to delay gastric emptying and possibly to enhance intragastric distribution of the urea.

OBJECTIVE

To compare the effects of sodium citrate, two doses of citric acid and a pudding meal on gastric motor function.

METHOD

Eleven normal healthy volunteers were investigated using non-invasive techniques to measure gastric emptying and gastric motility. We evaluated gastric emptying using the Meretek 13Ceebiscuit solid phase gastric emptying breath test, which employs a 340-calorie biscuit containing 200 mg of the edible 13C-blue-green alga Spirulina platensis, after the administration of test meals of pudding, 2 g and 4 g of citric acid and 2 g of sodium citrate. Electrogastrograms (Digitrapper EGG) were also recorded for 30 min before and 180 min after the test meal.

RESULTS

Gastric emptying, as assessed by the half-time (T1/2), was delayed similarly with the pudding (136.8 +/- 9 min) and with 4 g of citric acid (144.5 +/- 7 min) (P > 0.7). Sodium citrate (108.7 +/- 6 min) and 2 g of citric acid (110.1 +/- 6 min) had similar effects on gastric emptying (P=0.986), and were significantly less effective in delaying gastric emptying (P < 0.01) compared to pudding or 4 g of citric acid. The electrogastrograms remained normal and there were no differences among meals and no relation with the gastric emptying results.

CONCLUSIONS

The increased intragastric urea hydrolysis associated with citric acid test meals cannot be attributed to delayed gastric emptying. Changes in the intragastric distribution of urea or a direct effect of citric acid on the bacteria (e.g. via the cytoplasmic protein, UreI) are more likely to be responsible.

The Helicobacter pylori UreI protein: role in adaptation to acidity and identification of residues essential for its activity and for acid activation

Helicobacter pylori is a human gastric pathogen that survives the strong acidity of the stomach by virtue of its urease activity. This activity produces ammonia, which neutralizes the bacterial microenvironment. UreI, an inner membrane protein, is essential for resistance to low pH and for the gastric colonization of mice by H. pylori. In the heterologous Xenopus oocytes expression system, UreI behaves like an H+-gated urea channel, and His-123 was found to be important for low pH activation. We investigated the role of UreI directly in H. pylori and showed that, in the presence of urea, strains expressing wild-type UreI displayed very rapid stimulation of extracellular ammonia production upon exposure to pH </= 5. This response was not observed when acetamide was used as a source of ammonia; therefore, it is specific for urea hydrolysis. To identify residues critical for UreI activity or activation, we constructed H. pylori strains carrying individual chromosomal mutations of UreI (i) in the four conserved histidine residues (H71, H123, H131, H193) and (ii) in a conserved region of the third intracellular loop (L165, G166, K167, F168). The distal H193 (and not H123) was found to be crucial for stimulating the production of ammonia at low pH; a single mutation in this residue uncoupled the UreI activity from its acid activation. The third intracellular loop of UreI was shown to be important for UreI activity. Thus, in H. pylori, UreI is necessary for the adaptation of urease activity to the extracellular pH. UreI behaves like a novel type of urea transporter, and the identification of residues essential for its function in H. pylori provides new insight into the unusual molecular mechanism of low pH activation.

pathogenesis of Helicobacter pylori-induced gastric inflammation

Helicobacter pylori causes persistent inflammation in the human stomach, yet only a minority of persons harbouring this organism develop peptic ulcer disease or gastric malignancy. An important question is why such variation exists among colonized individuals. Recent evidence has demonstrated that H. pylori isolates possess substantial phenotypic and genotypic diversity, which may engender differential host inflammatory responses that influence clinical outcome. For example, H. pylori strains that possess the cag pathogenicity island induce more severe gastritis and augment the risk for developing peptic ulcer disease and distal gastric cancer. An alternative, but not exclusive, hypothesis is that enhanced inflammation and injury is a consequence of an inappropriate host immune response to the chronic presence of H. pylori within the gastric niche. Investigations that precisely delineate the mechanisms responsible for induction of gastritis will ultimately help to define which H. pylori-colonized persons bear the highest risk for subsequent development of clinical disease, and thus, enable physicians to focus eradication therapy.

Cloning, expression, and catalytic activity of Helicobacter hepaticus urease

Helicobacter hepaticus causes disease in the liver and lower intestinal tract of mice. It is strongly urease positive, although it does not live in an acidic environment. The H. hepaticus urease gene cluster was expressed in Escherichia coli with and without coexpression of the Helicobacter pylori nickel transporter NixA. As for H. pylori, it was difficult to obtain enzymatic activity from recombinant H. hepaticus urease; special conditions including NiCl2 supplementation were required. The H. hepaticus urease cluster contains a homolog of each gene in the H. pylori urease cluster, including the urea transporter gene ureI. Downstream genes were homologs of the nik nickel transport operon of E. coli. Nongastric H. hepaticus produces urease similar to that of H. pylori.

Nickel-responsive induction of urease expression in Helicobacter pylori is mediated at the transcriptional level

The nickel-containing enzyme urease is an essential colonization factor of the gastric pathogen Helicobacter pylori, as it allows the bacterium to survive the acidic conditions in the gastric mucosa. Although urease can represents up to 10% of the total protein content of H. pylori, expression of urease genes is thought to be constitutive. Here it is demonstrated that H. pylori regulates the expression and activity of its urease enzyme as a function of the availability of the cofactor nickel. Supplementation of brucella growth medium with 1 or 100 microM NiCl(2) resulted in up to 3.5-fold-increased expression of the urease subunit proteins UreA and UreB and up to 12-fold-increased urease enzyme activity. The induction was specific for nickel, since the addition of cadmium, cobalt, copper, iron, manganese, or zinc did not affect the expression of urease. Both Northern hybridization studies and a transcriptional ureA::lacZ fusion demonstrated that the observed nickel-responsive regulation of urease is mediated at the transcriptional level. Mutation of the HP1027 gene, encoding the ferric uptake regulator (Fur), did not affect the expression of urease in unsupplemented medium but reduced the nickel induction of urease expression to only twofold. This indicates that Fur is involved in the modulation of urease expression in response to nickel. These data demonstrate nickel-responsive regulation of H. pylori urease, a phenomenon likely to be of importance during the colonization and persistence of H. pylori in the gastric mucosa.

Cell lysis is responsible for the appearance of extracellular urease in Helicobacter pylori

BACKGROUND

Helicobacter pylori is a neutralophilic bacterium that colonizes the acidic human gastric surface using the neutralizing capacity of a constitutively produced urease. Urease is present both in the cytoplasm and bound to the outside surface of the bacteria. The origin of the surface urease continues to be controversial. This study provides additional evidence that the origin of surface urease is cell lysis, not secretion.

METHODS

H. Pylori was transformed with a plasmid encoding green fluorescent protein (GFP), a non-native cytoplasmic protein. Cultures supplemented with beta-cyclodextrin or horse serum were collected over various time periods and spun through a ficoll cushion to gently separate whole bacteria from released protein. The pellet and supernatant fractions were analyzed by fluorimetry, SDS-PAGE and Coomassie blue or Western analysis.

RESULTS

GFP fluorescence and antigenic reactivity in the supernatant increased at each time point. GFP, the non-native cytoplasmic protein, and UreB, a native cytoplasmic protein, increased over time in the supernatant and both proteins were always present in the pellet fraction. UreI, an inner membrane protein, was only present in the pellet fraction. beta-galactosidase, a protein not found in H. pylori, was used as a negative control.

CONCLUSIONS

Since it is unlikely that there is an intrinsic secretion system for GFP, a non-native protein, its increasing presence over time in the supernate fraction along with UreB, and retention of UreI in the pellet fraction implies that cell lysis accounts for the presence of urease on the surface of H. pylori.

Sites of pH regulation of the urea channel of Helicobacter pylori

Helicobacter pylori (Hp) and Streptococcus salivarius (Ss) require intrabacterial urease for acid resistance and express a urea channel, UreI. The presence of UreI was shown to increase urea permeability approximately 300-fold over that of a non-polar ureI deletion mutant. Expression of SsUreI in Xenopus oocytes increased urea uptake pH independently, whereas HpUreI shows an acidic pH dependence, half-maximal at pH 6.0. Mutagenesis of all histidines, aspartates, glutamates and the lysine in the periplasmic domain of HpUreI showed that His-123, His-131, Asp-129, Asp-140, Glu-138 and Lys-132 in the second periplasmic loop (PL2) and His-193 in the C-terminus (Ct) were important for activation of transport. With the exception of a lysine that was shown to substitute for His-193 in HpUreI, these charged amino acids are absent in SsUreI. A chimera in which PL1 of HpUreI was replaced by PL1 of SsUreI retained activity at acidic pH and gained partial activity at neutral pH. Exchange of PL2 inactivated transport, whereas exchange of Ct had no effect. Chimeras, in which either PL1 or PL2 of HpUreI replaced those of SsUreI, retained wild-type transport, but replacement of the Ct or both loops inactivated transport. PL1 appears to be important for restricting transport through HpUreI at neutral pH, whereas protonation of three histidines in PL2 and Ct and the presence of three dicarboxylic amino acids in PL2 appears to be necessary to activate HpUreI at acidic pH.

Simplified 13C-urea breath test for detection of Helicobacter pylori infection

OBJECTIVE

The U.S. standard 13C-urea breath test (13C-UBT) has proven to be extremely reliable but entails several complicated performance requirements and a test period of approximately 1 h. The aim of this study was to compare the standard 13C-UBT with a simplified version embodying modifications of test meal, duration of fasting, amount of 13C-urea, method of breath collection, and duration of test.

METHODS

This was a randomized, three-way, crossover study of the standard U.S. 13C-UBT, which contains 125 mg of 13C-urea and a pudding test meal. The final breath sample is taken 30 min after urea ingestion. This test was compared with a formulation containing 75 mg of 13C-urea, a 2.5-g citric acid test meal (UBT-Lite), and a final breath sample taken by direct exhalation into tubes 15 min after urea ingestion. We also compared the effect of prior meals versus fasting on the test outcome with the UBT-Lite.

RESULTS

A total of 259 subjects were enrolled in the trial, and 249 completed all three urea breath tests. There was excellent agreement between the three versions of the UBT with >98% of subjects having concordant results. Using predetermined criteria, there was substantial equivalence between the tests. Neither solid and/or liquid food up to 1 h before performing the UBT-Lite affected outcome.

CONCLUSION

The UBT-Lite formulation of the 13C-UBT proved to be an improved version of the U.S. standard 13C-UBT offering less expensive ingredients, shorter test duration, and a simplified breath test collection method, without sacrificing accuracy.

Prolonged survival and cytoplasmic pH homeostasis of Helicobacter pylori at pH 1

In the presence of urea, Helicobacter pylori survived for at least 3 h at pH 1. Under these conditions, the cells maintained their cytoplasmic pH at 5.8. De novo protein synthesis during acid shock was not essential for survival of H. pylori at pH 1.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori infects over half of the world population. Infection with the bacterium causes gastritis and peptic ulcer disease and is associated with the development of gastric cancers. However, only a small proportion of individuals develop these complications of infection. Therefore, identification of both host and bacterial factors that mediate disease is an intense area of current research interest. This review highlights recent advances in understanding of the mechanisms underlying disease pathophysiology following infection with H. pylori.

Helicobacter pylori adhesins: review and perspectives

It is highly unlikely that chronic infection with H. pylori could occur in the absence of adhesin-host cell interactions. Also, there is no evidence that any of the serious outcomes of H. pylori infection such as gastric and duodenal ulcers, gastric cancer or mucosa-associated lymphoid tissue (MALT) lymphoma could occur without prior colonization of the gastric epithelium mediated by H. pylori adhesins. H. pylori is highly adaptable, as evidenced by the fact that it can occupy a single host for decades. An important facet of this adaptability is its ability to physically interact with various types of host cells and also with host mucins and extracellular matrix proteins using a number of different adhesins displaying a variety of unique receptor specificities. Thus it is highly unlikely that any one particular H. pylori adhesin will ever be proven responsible for a particular outcome such as duodenal ulcer, MALT lymphoma, or adenocarcinoma. Also, while the search for additional H. pylori adhesins should and certainly will continue, we suggest that the scope of this effort should be expanded to include investigations into the patterns of expression and interaction between individual outer membrane proteins. Which of the numerous H. pylori outer membrane proteins (OMPs) actually function as adhesins (i.e., have receptor-binding sites) and which OMPs are simply necessary for optimal display of the adhesive OMPs? There are many other important questions about H. pylori adhesins waiting to be answered. For example, which adhesins are responsible for loose adherence to host cells and which adhesins are responsible for intimate, or membrane-to-membrane, adherence, and do these adhesins normally work in concert or in a sequential fashion? Also, is a specific type of adhesin necessary for type IV protein translocation into host cells and, if so, is adhesin expression coregulated with the effector protein export?

Review article: the control of gastric acid and Helicobacter pylori eradication

This review focuses on the gastric acid pump as a therapeutic target for the control of acid secretion in peptic ulcer and gastro-oesophageal reflux disease. The mechanism of the proton pump inhibitors is discussed as well as their clinical use. The biology of Helicobacter pylori as a gastric denizen is then discussed, with special regard to its mechanisms of acid resistance. Here the properties of the products of the urease gene clusters, ureA, B and ureI, E, F, G and H are explored in order to explain the unique location of this pathogen. The dominant requirement for acid resistance is the presence of a proton gated urea transporter, UreI, which increases access of gastric juice urea to the intrabacterial urease 300-fold. This enables rapid and continuous buffering of the bacterial periplasm to approximately pH 6.0, allowing acid resistance and growth at acidic pH in the presence of 1 mM urea. A hypothesis for the basis of combination therapy for eradication is also presented.

Local pH elevation mediated by the intrabacterial urease of Helicobacter pylori cocultured with gastric cells

Helicobacter pylori resists gastric acidity by modulating the proton-gated urea channel UreI, allowing for pH(out)-dependent regulation of urea access to intrabacterial urease. We employed pH- and Ca(2+)-sensitive fluorescent dyes and confocal microscopy to determine the location, rate, and magnitude of pH changes in an H. pylori-AGS cell coculture model, comparing wild-type bacteria with nonpolar ureI-deletion strains (ureI-ve). Addition of urea at pH 5.5 to the coculture resulted first in elevation of bacterial periplasmic pH, followed by an increase of medium pH and then pH in AGS cells. No change in periplasmic pH occurred in ureI-deletion mutants, which also induced a slower increase in the pH of the medium. Pretreatment of the mutant bacteria with the detergent C(12)E(8) before adding urea resulted in rapid elevation of bacterial cytoplasmic pH and medium pH. UreI-dependent NH(3) generation by intrabacterial urease buffers the bacterial periplasm, enabling acid resistance at the low urea concentrations found in gastric juice. Perfusion of AGS cells with urea-containing medium from coculture at pH 5.5 did not elevate pH(in) or [Ca(2+)](in), unless the conditioned medium was first neutralized to elevate the NH(3)/NH(4)(+) ratio. Therefore, cellular effects of intrabacterial ammonia generation under acidic conditions are indirect and not through a type IV secretory complex. The pH(in) and [Ca(2+)](in) elevation that causes the NH(3)/NH(4)(+) ratio to increase after neutralization of infected gastric juice may contribute to the gastritis seen with H. pylori infection.

Identification of the urease operon in Helicobacter pylori and its control by mRNA decay in response to pH

We investigated the transcription of the urease gene cluster ureABIEFGH in Helicobacter pylori to determine the regulation of gene expression of the highly produced enzyme urease. Northern blot hybridization analysis demonstrated that cells of the wild-type strain grown in an ordinary broth had transcripts of ureAB, ureABI, ureI, ureIE' and ure'FGH, but cells of a ureI-disrupted mutant had only the ureAB transcript. When the wild-type cells were exposed to pH 8 for 30 min, very little mRNA was detected. However, when exposed to pH 6, a large amount of the ureIE" transcript, which was longer than the ureIE' transcript, together with the additional transcripts ureABIEFGH and ure'EFGH were detected. Rifampicin addition experiments demonstrated that urease mRNAs, and the ureIE' transcripts in particular, are more stable at pH 5.5 than at pH 7. In accord with these results, urease activity in the crude cell extract of the pH 5.5 culture was twice as much as that of the pH 7 culture, although the amounts of UreA and UreB detected by immunoblot analysis were similar. The transcription start point of ureI was identified by primer extension using a ureA promoter-deleted mutant, and a consensus sequence of RpoD-RNA polymerase was found in the ureI promoter. The 3' end of the ureIE" mRNA, determined using S1 nuclease mapping, revealed that the transcript is able to cover the majority of the ureE open reading frame (ORF) that might be sufficient for UreE activity. Based on the above results, we conclude that the urease gene cluster of H. pylori consists of two operons, ureAB and ureIEFGH, and that primary transcripts of the latter as well as the read-through transcript, ureABIEFGH, are cleaved to produce several species of mRNA. It has been suggested that the ureIEFGH operon is regulated post-transcriptionally by mRNA decay in response to environmental pH. We are tempted to speculate that the ureE" transcript present in acidic pH may contribute to produce an active product that can proceed the nickel incorporation to the active centre, the final step of urease biosynthesis.