Expression of the Helicobacter pylori adhesin SabA is controlled via phase variation and the ArsRS signal transduction system

The role of variable DNA tandem repeats in bacterial adaptation

DNA tandem repeats (TRs), also designated as satellite DNA, are inter- or intragenic nucleotide sequences that are repeated two or more times in a head-to-tail manner. Because TR tracts are prone to strand-slippage replication and recombination events that cause the TR copy number to increase or decrease, loci containing TRs are hypermutable. An increasing number of examples illustrate that bacteria can exploit this instability of TRs to reversibly shut down or modulate the function of specific genes, allowing them to adapt to changing environments on short evolutionary time scales without an increased overall mutation rate. In this review, we discuss the prevalence and distribution of inter- and intragenic TRs in bacteria and the mechanisms of their instability. In addition, we review evidence demonstrating a role of TR variations in bacterial adaptation strategies, ranging from immune evasion and tissue tropism to the modulation of environmental stress tolerance. Nevertheless, while bioinformatic analysis reveals that most bacterial genomes contain a few up to several dozens of intra- and intergenic TRs, only a small fraction of these have been functionally studied to date.

Divergent mechanisms of interaction of Helicobacter pylori and Campylobacter jejuni with mucus and mucins

Helicobacter pylori and Campylobacter jejuni colonize the stomach and intestinal mucus, respectively. Using a combination of mucus-secreting cells, purified mucins, and a novel mucin microarray platform, we examined the interactions of these two organisms with mucus and mucins. H. pylori and C. jejuni bound to distinctly different mucins. C. jejuni displayed a striking tropism for chicken gastrointestinal mucins compared to mucins from other animals and preferentially bound mucins from specific avian intestinal sites (in order of descending preference: the large intestine, proximal small intestine, and cecum). H. pylori bound to a number of animal mucins, including porcine stomach mucin, but with less avidity than that of C. jejuni for chicken mucin. The strengths of interaction of various wild-type strains of H. pylori with different animal mucins were comparable, even though they did not all express the same adhesins. The production of mucus by HT29-MTX-E12 cells promoted higher levels of infection by C. jejuni and H. pylori than those for the non-mucus-producing parental cell lines. Both C. jejuni and H. pylori bound to HT29-MTX-E12 mucus, and while both organisms bound to glycosylated epitopes in the glycolipid fraction of the mucus, only C. jejuni bound to purified mucin. This study highlights the role of mucus in promoting bacterial infection and emphasizes the potential for even closely related bacteria to interact with mucus in different ways to establish successful infections.

Life in the human stomach: persistence strategies of the bacterial pathogen Helicobacter pylori

The bacterial pathogen Helicobacter pylori has co-evolved with humans and colonizes approximately 50% of the human population, but only causes overt gastric disease in a subset of infected hosts. In this Review, we discuss the pathogenesis of H. pylori and the mechanisms it uses to promote persistent colonization of the gastric mucosa, with a focus on recent insights into the role of the virulence factors vacuolating cytotoxin (VacA), cytotoxin-associated gene A (CagA) and CagL. We also describe the immunobiology of H. pylori infection and highlight how this bacterium manipulates the innate and adaptive immune systems of the host to promote its own persistence.

Structural, enzymatic and biochemical studies on Helicobacter pylori arginase

Arginase is an enzyme involved in the last step of the urea cycle, where it catalyses the hydrolysis of l-arginine to generate l-ornithine and urea. Compared to the well-characterised arginases from animals, yeast and other bacteria, Helicobacter pylori arginase, or RocF, is unique in at least three aspects. Firstly, it has been identified as an important factor in evasion of the host's immune system and thus contributes to persistent infection by the bacterium. Secondly, the optimal catalytic conditions of RocF are different from those of other arginases. Finally, sequence alignment indicates that RocF possesses considerable differences at its N- and C-terminal from other arginases and harbours an insertion of 13 residues in the middle of the sequence. To better understand these unique biochemical and enzymatic properties, we therefore have embarked on determining the structure of RocF. In this study, the crystal structure of RocF was solved with the molecular replacement method. Based on the structure and systematic mutagenesis studies, we confirmed that the inserted residues form a helix that was not observed in other arginases and was able to raise the arginase activity by 30% probably by change the conformation of the substrate binding pocket. Six residues were involved in Mn(2+) binding, all of which were essential for arginase activity. The C-terminal motif is not sufficient in establishing the oligomeric state of RocF, and no disulphide bonds were observed in RocF.

Effect of gastric environment on Helicobacter pylori adhesion to a mucoadhesive polymer

Helicobacter pylori infection has been associated with several gastric diseases. This bacterium colonizes the gastric mucosa of half of the world's population, and available treatments are unsuccessful in practically one in every five patients. Mucoadhesive polymers, such as chitosan, are being investigated as gastric drug delivery systems. However, since chitosan is also known for its antimicrobial properties, this work aims to evaluate H. pylori interactions with chitosan under simulated gastric environments, namely using various pHs (2.6, 4 and 6), pepsin and urea. To enable the visualization of adherent bacteria, ultrathin chitosan films were produced by spin-coating on gold/glass surfaces, cross-linked with genipin and characterized by Fourier transform infrared reflection absorption spectroscopy, ellipsometry and electrokinetic analysis. Films with homogeneous thickness of 11.7±0.6 nm were produced, and were stable and protonated at all the pHs used. Furthermore, they adsorbed pepsin in all these pHs, in contrast to urea, of which a small adsorption was only observed at pH 6. H. pylori binding to chitosan was higher at pH2.6 although most of adherent bacteria were dead. The presence of pepsin decreased bacterial adhesion, but increased its viability while in a more stressed morphology (coccoid form). The presence of urea did not affect the amount, morphology or viability of chitosan-adherent bacteria. In suspension, the decrease in pH changed H. pylori zeta potential from negative to positive. Moreover, bacteria were only culturable when incubated in pH 6 with and without urea (without pepsin). This work demonstrates that chitosan has the capacity to bind and kill H. pylori in a range of pHs independently of urea. This opens new perspectives for the application of chitosan-based materials to the elimination of H. pylori gastric colonization, though pepsin might appear to be an obstacle.

Molecular analysis of Helicobacter pylori virulent-associated genes in hepatobiliary patients

OBJECTIVES

The Helicobacter pylori virulence-associated genes in hepatobiliary patients, including vacA, iceA, babA2, cagA and cagE, have not been reported. The aim of this study was to investigate these genes and the association of those and the clinical outcomes in hepatobiliary diseases.

METHODS

Eighty H. pylori-PCR-positive cases were obtained from hepatobiliary patients, representing both cholangiocarcinoma (CCA) (n= 58) and cholelithiasis (n= 22). The diversity of virulence genes was examined by polymerase chain reaction and DNA sequencing. Phylogenetic analysis of cagA was determined using the maximum parsimony method.

RESULTS

The vacAs1a + c/m1, iceA1 and babA2 genes were the most predominant genotypes in both CCA and cholelithiasis patients. The cagA and cagE genes were found significantly more frequently in patients with CCA than those with cholelithiasis (P < 0.05). The cagA positive samples were the Western-type cagA and showed that almost all of the detected sequences in Thai hepatobiliary and Thai gastric cancer patients were classified in the same cluster but separated from the cluster of Japan and other countries.

CONCLUSIONS

The cagA and cagE genes may be associated in the pathogenesis of hepatobiliary diseases, especially of CCA. Besides the bacterial variation, other host factors may be involved in the pathogenesis of hepatobiliary cancer.

Length of thymidine homopolymeric repeats modulates promoter activity of sabA in Helicobacter pylori

BACKGROUND

 Helicobacter pylori uses SabA to interact with sialyl-Lewis x on the gastric mucosal surface to establish persistent colonization. The number of CT repeats in sabA is variable and thus influences SabA translation, but the expression of SabA determined by Western blotting does not fully match with a CT sequence-based prediction. Furthermore, a homopolymeric thymidine (polyT) tract located upstream of sabA has been observed, but its role in regulating sabA expression is still unknown.

METHODS

 The transcriptional start site (TSS) of sabA in strains J99 and Hp258 was determined by 5' RACE. One hundred and fifteen clinical isolates were sequenced to analyze the distribution of the polyT tract length and promoter sequence. Finally, RT-PCR and an E. coli-lux reporter system were used to determine the sabA promoter activity with different lengths of the polyT tract.

RESULTS

 The TSS of sabA was located at 66 or 64 bp upstream of the translational start codon in J99 and Hp258, respectively. The polyT tract close to the -35 element varied from T₁₀ to T₂₈ in 115 clinical isolates, and 70% of the isolates contained a stretch of 14-19 Ts. The sabA gene displayed slipped strand mispairing (SSM) of the polyT tract, generating varying genotypes in J99 (16-18 Ts) and Hp258 (14-15 Ts). Furthermore, J99 with lengths of T₁₆ and T₃₀, had higher sabA promoter activity than the common length of T₁₈.

CONCLUSION

 Our findings indicate that the sabA promoter region modulates its transcriptional activity through a variable polyT tract, and SSM generates mixed genotypes in the population.

Regulation of Helicobacter pylori adherence by gene conversion

Genetic diversification of Helicobacter pylori adhesin genes may allow adaptation of adherence properties to facilitate persistence despite host defences. The sabA gene encodes an adhesin that binds sialyl-Lewis antigens on inflamed gastric tissue. We found variability in the copy number and locus of the sabA gene and the closely related sabB and omp27 genes due to gene conversion among 51 North American paediatric H. pylori strains. We determined that sabB to sabA gene conversion is predominantly the result of intra-genomic recombination and RecA, RecG and AddA influence the rate at which it occurs. Although all clinical strains had at least one sabA gene copy, sabA and sabB were lost due to gene conversion at similar rates in vitro, suggesting host selection to maintain the sabA gene. sabA gene duplication resulted in increased SabA protein production and increased adherence to sialyl-Lewis antigens and mouse gastric tissue. In conclusion, gene conversion is a mechanism for H. pylori to regulate sabA expression level and adherence.

Helicobacter pylori adhesion to gastric epithelial cells is mediated by glycan receptors

Helicobacter pylori adhesion to gastric epithelial cells constitutes a key step in the establishment of a successful infection of the gastric mucosa. The high representation of outer membrane proteins in the bacterial genome suggests the relevance of those proteins in the establishment of profitable interactions with the host gastric cells. Gastric epithelial cells are protected by a mucous layer gel, mainly consisting of the MUC5AC and MUC6 mucins. In addition to this protective role, mucins harbor glycan-rich domains that constitute preferential binding sites of many pathogens. In this article we review the main players in the process of H. pylori adhesion to gastric epithelial cells, which contribute decisively to the high prevalence and chronicity of H. pylori infection. The BabA adhesin recognizes both H-type 1 and Lewis b blood-group antigens expressed on normal gastric mucosa of secretor individuals, contributing to the initial steps of infection. Upon colonization, persistent infection induces an inflammatory response with concomitant expression of sialylated antigens. The SabA adhesin mediates H. pylori binding to inflamed gastric mucosa by recognizing sialyl-Lewis a and sialyl-Lewis x antigens. The expression of the BabA and SabA adhesins is tightly regulated, permitting the bacteria to rapidly adapt to the changes of glycosylation of the host gastric mucosa that occur during infection, as well as to escape from the inflammatory response. The growing knowledge of the interactions between the bacterial adhesins and the host receptors will contribute to the design of alternative strategies for eradication of the infection.

Analysis of protein expression regulated by the Helicobacter pylori ArsRS two-component signal transduction system

Previous studies have shown that the Helicobacter pylori ArsRS two-component signal transduction system contributes to acid-responsive gene expression. To identify additional members of the ArsRS regulon and further investigate the regulatory role of the ArsRS system, we analyzed protein expression in wild-type and arsS null mutant strains. Numerous proteins were differentially expressed in an arsS mutant strain compared to a wild-type strain when the bacteria were cultured at pH 5.0 and also when they were cultured at pH 7.0. Genes encoding 14 of these proteins were directly regulated by the ArsRS system, based on observed binding of ArsR to the relevant promoter regions. The ArsRS-regulated proteins identified in this study contribute to acid resistance (urease and amidase), acetone metabolism (acetone carboxylase), resistance to oxidative stress (thioredoxin reductase), quorum sensing (Pfs), and several other functions. These results provide further definition of the ArsRS regulon and underscore the importance of the ArsRS system in regulating expression of H. pylori proteins during bacterial growth at both neutral pH and acidic pH.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori induces chronic inflammation of the gastric mucosa, but only a proportion of infected individuals develop peptic ulcer disease or gastric carcinoma. Reasons underlying these observations include differences in bacterial pathogenicity as well as in host susceptibility. Numerous studies published in the last year provided new insight into H. pylori virulence factors, their interaction with the host and consequences in pathogenesis. These include the role of bacterial genetic diversity in host colonization and persistence, outer membrane proteins and modulation of adhesin expression, new aspects of VacA functions, and CagA and its phosphorylation-dependent and -independent cellular effects. This article will also review the recent novel findings on the interactions of H. pylori with diverse host epithelial signaling pathways and events involved in the initiation of carcinogenesis, including genetic instability and dysregulation of DNA repair.

Outer membrane protein expression profile in Helicobacter pylori clinical isolates

The gram-negative gastric pathogen Helicobacter pylori is equipped with an extraordinarily large set of outer membrane proteins (OMPs), whose role in the infection process is not well understood. The Hop (Helicobacter outer membrane porins) and Hor (Hop-related proteins) groups constitute a large paralogous family consisting of 33 members. The OMPs AlpA, AlpB, BabA, SabA, and HopZ have been identified as adhesins or adherence-associated proteins. To better understand the relevance of these and other OMPs during infection, we analyzed the expression of eight different omp genes (alpA, alpB, babA, babB, babC, sabA, hopM, and oipA) in a set of 200 patient isolates, mostly from symptomatic children or young adults. Virtually all clinical isolates produced the AlpA and AlpB proteins, supporting their essential function. All other OMPs were produced at extremely variable rates, ranging from 35% to 73%, indicating a function in close adaptation to the individual host or gastric niche. In 11% of the isolates, BabA was produced, and SabA was produced in 5% of the isolates, but the strains failed to bind their cognate substrates. Interleukin-8 (IL-8) expression in gastric cells was strictly dependent on the presence of the cag pathogenicity island, whereas the presence of OipA clearly enhanced IL-8 production. The presence of the translocated effector protein CagA correlated well with BabA and OipA production. In conclusion, we found unexpectedly diverse omp expression profiles in individual H. pylori strains and hypothesize that this reflects the selective pressure for adhesion, which may differ across different hosts as well as within an individual over time.