Helicobacter pullorum: An Emerging Zoonotic Pathogen

Helicobacter pullorum (H.pullorum) commonly colonizes the gastrointestinal tract of poultry causing gastroenteritis. The bacterium may be transmitted to humans through contaminated meat where it has been associated with colitis and hepatitis. Despite the high prevalence of H. pullorum observed in poultry, little is known about the mechanisms by which this bacterium establishes infection in host and its virulence determinants. In this article we aim to provide an overview of this emerging zoonotic pathogen; its general characteristics, hosts, prevalence, and transmission as well as its pathogenic potential. We also discuss possible control strategies and risk of disease emergence.

Insight into the essential role of the Helicobacter pylori HP1043 orphan response regulator: genome-wide identification and characterization of the DNA-binding sites

Many bacterial regulatory genes appear to be dispensable, as they can be deleted from the genome without loss of bacterial functionalities. In Helicobacter pylori, the hp1043 gene, also known as hsrA, is one of the transcriptional regulator that is essential for cell viability. This gene could not be deleted, nor the amount of protein modulated, supporting the hypothesis that HP1043 could be involved in the regulation of crucial cellular processes. Even though detailed structural data are available for the HP1043 protein, its targets are still ill-defined. Using Chromatin Immunoprecipitation-sequencing (ChIP-seq), one of the most powerful approaches to characterize protein-DNA interactions in vivo, we were able to identify genome-wide several new HP1043 binding sites. Moreover, in vitro DNA binding assays enabled precise mapping of the HP1043 binding sites on the new targets, revealing the presence of a conserved nucleotide sequence motif. Intriguingly, a significant fraction of the newly identified binding sites overlaps promoter regions controlling the expression of genes involved in translation. Accordingly, when protein translation was blocked, a significant induction of almost all HP1043 target genes was detected. These observations prompted us to propose HP1043 as a key regulator in H. pylori, likely involved in sensing and in coordinating the response to environmental conditions that provoke an arrest of protein synthesis. The essential role of HP1043 in coordinating central cellular processes is discussed.

Divergent distribution of the sensor kinase CosS in non-thermotolerant campylobacter species and its functional incompatibility with the response regulator CosR of Campylobacter jejuni

Two-component signal transduction systems are commonly composed of a sensor histidine kinase and a cognate response regulator, modulating gene expression in response to environmental changes through a phosphorylation-dependent process. CosR is an OmpR-type response regulator essential for the viability of Campylobacter jejuni, a major foodborne pathogenic species causing human gastroenteritis. Although CosR is a response regulator, its cognate sensor kinase has not been identified in C. jejuni. In this study, DNA sequence analysis of the cosR flanking regions revealed that a gene encoding a putative sensor kinase, which we named cosS, is prevalent in non-thermotolerant Campylobacter spp., but not in thermotolerant campylobacters. Phosphorylation assays indicated that C. fetus CosS rapidly autophosphorylates and then phosphorylates C. fetus CosR, suggesting that the CosRS system constitutes a paired two-component signal transduction system in C. fetus. However, C. fetus CosS does not phosphorylate C. jejuni CosR, suggesting that CosR may have different regulatory cascades between thermotolerant and non-thermotolerant Campylobacter species. Comparison of CosR homolog amino acid sequences showed that the conserved phosphorylation residue (D51), which is present in all non-thermotolerant Campylobacter spp., is absent from the CosR homologs of thermotolerant Campylobacter species. However, C. jejuni CosR was not phosphorylated by C. fetus CosS even after site-directed mutagenesis of N51D, implying that C. jejuni CosR may possibly function phosphorylation-independently. In addition, the results of cosS mutational analysis indicated that CosS is not associated with the temperature dependence of the Campylobacter spp. despite its unique divergent distribution only in non-thermotolerant campylobacters. The findings in this study strongly suggest that thermotolerant and non-thermotolerant Campylobacter spp. have different signal sensing mechanisms associated with the CosR regulation.

Response to metronidazole and oxidative stress is mediated through homeostatic regulator HsrA (HP1043) in Helicobacter pylori

Metronidazole (MTZ) is often used in combination therapies to treat infections caused by the gastric pathogen Helicobacter pylori. Resistance to MTZ results from loss-of-function mutations in genes encoding RdxA and FrxA nitroreductases. MTZ-resistant strains, when cultured at sub-MICs of MTZ (5 to 20 μg/ml), show dose-dependent defects in bacterial growth; depressed activities of many Krebs cycle enzymes, including pyruvate:ferredoxin oxidoreductase (PFOR); and low transcript levels of porGDAB (primer extension), phenotypes consistent with an involvement of a transcriptional regulator. Using a combination of protein purification steps, electrophoretic mobility shift assays (EMSAs), and mass spectrometry analyses of proteins bound to porG promoter sequences, we identified HP1043, an essential homeostatic global regulator (HsrA [for homeostatic stress regulator]). Competition EMSAs and supershift analyses with HsrA-enriched protein fractions confirmed specific binding to porGDAB and hsrA promoter sequences. Exposure to MTZ resulted in >10-fold decreases in levels of HsrA and in levels of the HsrA-regulated gene products PFOR and TlpB. Exposure to paraquat (PQ), hydrogen peroxide, or organic peroxides showed near equivalence with MTZ, revealing a common oxidative stress response pathway. Finally, direct superoxide dismutase (SOD) assays showed an inverse relationship between HsrA levels and SOD activity, suggesting that HsrA may serve as a repressor of sodB. As a homeostatic sentinel, HsrA appears to be ideally positioned to enable rapid shutdown of genes associated with metabolism and growth while activating (directly or indirectly) oxidative defense genes in response to low levels of toxic metabolites (MTZ or oxygen) before they reach DNA-damaging levels.

Gastric and enterohepatic non-Helicobacter pylori Helicobacters

A substantial number of reports published in the last year have contributed to a better understanding of both human and animal infection with non-Helicobacter pylori Helicobacter species (NHPH). Gastric infection of humans with Helicobacter suis and Helicobacter felis as well as unidentified NHPH has been described to cause a chronic gastritis and a variety of clinical symptoms, whereas enterohepatic NHPH, including Helicobacter cinaedi, Helicobacter bilis, and Helicobacter canis, have been reported to be associated with human diseases such as bacteremia, cellulitis, cutaneous diseases, and fever of unknown origin in immunocompromised hosts. In various animal species, including dogs and laboratory mice, high rates of infection with NHPH were described. For gastric NHPH, mainly H. suis and H. felis infection was studied, revealing that differences in the immune response evoked in the host do exist when compared to Helicobacter pylori. Pathogenic mechanisms of infection with Helicobacter pullorum, H. bilis, and Helicobacter hepaticus were investigated, as well as immune responses involved in H. bilis-, Helicobacter typhlonius-, and H. hepaticus-induced intestinal inflammation. Complete genome sequences of Helicobacter heilmannii strain ASB1 and a H. cinaedi strain isolated in a case of human bacteremia were published, as well as comparative genomics of a human-derived Helicobacter bizzozeronii strain and proteome or secretome analyses for H. hepaticus and Helicobacter trogontum, respectively. Molecular analysis has revealed a function for type VI secretion systems of H. hepaticus and H. pullorum, the Helicobacter mustelae iron urease, and several other functional components of NHPH. In each section of this chapter, new findings on gastric NHPH will first be discussed, followed by those on enterohepatic Helicobacter species.