Genetic organization and conjugal plasmid DNA transfer of pHP69, a plasmid from a Korean isolate of Helicobacter pylori

Four Chromosomal Type IV Secretion Systems in Helicobacter pylori: Composition, Structure and Function

The pathogenic bacterium Helicobacter pylori is genetically highly diverse and a major risk factor for the development of peptic ulcer disease and gastric adenocarcinoma in humans. During evolution, H. pylori has acquired multiple type IV secretion systems (T4SSs), and then adapted for various purposes. These T4SSs represent remarkable molecular transporter machines, often associated with an extracellular pilus structure present in many bacteria, which are commonly composed of multiple structural proteins spanning the inner and outer membranes. By definition, these T4SSs exhibit central functions mediated through the contact-dependent conjugative transfer of mobile DNA elements, the contact-independent release and uptake of DNA into and from the extracellular environment as well as the secretion of effector proteins in mammalian host target cells. In recent years, numerous features on the molecular functionality of these T4SSs were disclosed. H. pylori encodes up to four T4SSs on its chromosome, namely the Cag T4SS present in the cag pathogenicity island (cagPAI), the ComB system, as well as the Tfs3 and Tfs4 T4SSs, some of which exhibit unique T4SS functions. The Cag T4SS facilitates the delivery of the CagA effector protein and pro-inflammatory signal transduction through translocated ADP-heptose and chromosomal DNA, while various structural pilus proteins can target host cell receptors such as integrins or TLR5. The ComB apparatus mediates the import of free DNA from the extracellular milieu, whereas Tfs3 may accomplish the secretion or translocation of effector protein CtkA. Both Tfs3 and Tfs4 are furthermore presumed to act as conjugative DNA transfer machineries due to the presence of tyrosine recombinases with cognate recognition sequences, conjugational relaxases, and potential origins of transfer (oriT) found within the tfs3 and tfs4 genome islands. In addition, some extrachromosomal plasmids, transposons and phages have been discovered in multiple H. pylori isolates. The genetic exchange mediated by DNA mobilization events of chromosomal genes and plasmids combined with recombination events could account for much of the genetic diversity found in H. pylori. In this review, we highlight our current knowledge on the four T4SSs and the involved mechanisms with consequences for H. pylori adaptation to the hostile environment in the human stomach.

Two novel transcriptional reporter systems for monitoring Helicobacter pylori stress responses

Helicobacter pylori, a human pathogen linked to many stomach diseases, is well adapted to colonize aggressive gastric environments, and its virulence factors contribute this adaptation. Here, we report the construction of two novel H. pylori vectors, pSv2 and pSv4, carrying a reporter gene fused to the promoters of virulence factor genes for monitoring the response of single H. pylori cells to various stresses. H. pylori cryptic plasmids were modified by the introduction of the Escherichia coli origin of replication, chloramphenicol resistance cassette, and promoterless gfp gene to produce E. coli/H. pylori shuttle vectors. The promoter regions of vacA and ureA genes encoding well-characterized H. pylori virulence factors were fused to the promoterless gfp gene. Recording the GFP fluorescence signal from the genetically modified H. pylori cells immobilized in specifically designed microfluidic devices revealed the response of transcriptional reporter systems to osmotic stress, acidic stress, elevated Ni²⁺ concentration or iron chelation. Our observations validate the utility of the pSv2 and pSv4 vectors to monitor the regulation of virulence factor genes in diverse strains and clinical isolates of H. pylori.

Recent Advances in Helicobacter pylori Replication: Possible Implications in Adaptation to a Pathogenic Lifestyle and Perspectives for Drug Design

DNA replication is an important step in the life cycle of every cell that ensures the continuous flow of genetic information from one generation to the next. In all organisms, chromosome replication must be coordinated with overall cell growth. Helicobacter pylori growth strongly depends on its interaction with the host, particularly with the gastric epithelium. Moreover, H. pylori actively searches for an optimal microniche within a stomach, and it has been shown that not every microniche equally supports growth of this bacterium. We postulate that besides nutrients, H. pylori senses different, unknown signals, which presumably also affect chromosome replication to maintain H. pylori propagation at optimal ratio allowing H. pylori to establish a chronic, lifelong infection. Thus, H. pylori chromosome replication and particularly the regulation of this process might be considered important for bacterial pathogenesis. Here, we summarize our current knowledge of chromosome and plasmid replication in H. pylori and discuss the mechanisms responsible for regulating this key cellular process. The results of extensive studies conducted thus far allow us to propose common and unique traits in H. pylori chromosome replication. Interestingly, the repertoire of proteins involved in replication in H. pylori is significantly different to that in E. coli, strongly suggesting that novel factors are engaged in H. pylori chromosome replication and could represent attractive drug targets.

DNA transfer in the gastric pathogen Helicobacter pylori

The gastric pathogen Helicobacter pylori is one of the most genetically diverse bacteria. Recombination and DNA transfer contribute to its genetic variability and enhance host adaptation. Among the strategies described to increase genetic diversity in bacteria, DNA transfer by conjugation is one of the best characterized. Using this mechanism, a fragment of DNA from a donor cell can be transferred to a recipient, always mediated by a conjugative nucleoprotein complex, which is evolutionarily related to type IV secretion systems (T4SSs). Interestingly, the H. pylori chromosomes can encode up to four T4SSs, including the cagPAI, comB, tfs3, and tfs4 genes, some of which are known to promote chronic H. pylori infection. The T4SS encoded by the cagPAI mediates the injection of the effector protein CagA and proinflammatory signaling, and the comB system is involved in DNA uptake from the environment. However, the role of tfs3 and tfs4 is not yet clear. The presence of a functional XerD tyrosine recombinase and 5'AAAGAATG-3' border sequences as well as two putative conjugative relaxases (Rlx1 and Rlx2), a coupling protein (TraG), and a chromosomal region carrying a putative origin of transfer (oriT) suggest the existence of a DNA transfer apparatus in tfs4. Moreover, a conjugation-like DNA transfer mechanism in H. pylori has already been described in vitro, but whether this occurs in vivo is still unknown. Some extrachromosomal plasmids and phages are also present in various H. pylori strains. Genetic exchange among plasmids and chromosomes, and involved DNA mobilization events, could explain part of H. pylori's genetic diversity. Here, we review our knowledge about the possible DNA transfer mechanisms in H. pylori and its implications in bacterial adaptation to the host environment.