Role of the Pst system in plaque formation by the intracellular pathogen Shigella flexneri

Regulation of OmpA Translation and Shigella dysenteriae Virulence by an RNA Thermometer

RNA thermometers are cis-acting riboregulators that mediate the posttranscriptional regulation of gene expression in response to environmental temperature. Such regulation is conferred by temperature-responsive structural changes within the RNA thermometer that directly result in differential ribosomal binding to the regulated transcript. The significance of RNA thermometers in controlling bacterial physiology and pathogenesis is becoming increasingly clear. This study combines in silico, molecular genetics, and biochemical analyses to characterize both the structure and function of a newly identified RNA thermometer within the ompA transcript of Shigella dysenteriae First identified by in silico structural predictions, genetic analyses have demonstrated that the ompA RNA thermometer is a functional riboregulator sufficient to confer posttranscriptional temperature-dependent regulation, with optimal expression observed at the host-associated temperature of 37°C. Structural studies and ribosomal binding analyses have revealed both increased exposure of the ribosomal binding site and increased ribosomal binding to the ompA transcript at permissive temperatures. The introduction of site-specific mutations predicted to alter the temperature responsiveness of the ompA RNA thermometer has predictable consequences for both the structure and function of the regulatory element. Finally, in vitro tissue culture-based analyses implicate the ompA RNA thermometer as a bona fide S. dysenteriae virulence factor in this bacterial pathogen. Given that ompA is highly conserved among Gram-negative pathogens, these studies not only provide insight into the significance of riboregulation in controlling Shigella virulence, but they also have the potential to facilitate further understanding of the physiology and/or pathogenesis of a wide range of bacterial species.

Molecular and Cellular Mechanisms of Shigella flexneri Dissemination

The intracellular pathogen Shigella flexneri is the causative agent of bacillary dysentery in humans. The disease is characterized by bacterial invasion of intestinal cells, dissemination within the colonic epithelium through direct spread from cell to cell, and massive inflammation of the intestinal mucosa. Here, we review the mechanisms supporting S. flexneri dissemination. The dissemination process primarily relies on actin assembly at the bacterial pole, which propels the pathogen throughout the cytosol of primary infected cells. Polar actin assembly is supported by polar expression of the bacterial autotransporter family member IcsA, which recruits the N-WASP/ARP2/3 actin assembly machinery. As motile bacteria encounter cell-cell contacts, they form plasma membrane protrusions that project into adjacent cells. In addition to the ARP2/3-dependent actin assembly machinery, protrusion formation relies on formins and myosins. The resolution of protrusions into vacuoles occurs through the collapse of the protrusion neck, leading to the formation of an intermediate membrane-bound compartment termed vacuole-like protrusions (VLPs). VLP formation requires tyrosine kinase and phosphoinositide signaling in protrusions, which relies on the integrity of the bacterial type 3 secretion system (T3SS). The T3SS is also required for escaping double membrane vacuoles through the activity of the T3SS translocases IpaB and IpaC, and the effector proteins VirA and IcsB. Numerous factors supporting envelope biogenesis contribute to IcsA exposure and maintenance at the bacterial pole, including LPS synthesis, membrane proteases, and periplasmic chaperones. Although less characterized, the assembly and function of the T3SS in the context of bacterial dissemination also relies on factors supporting envelope biogenesis. Finally, the dissemination process requires the adaptation of the pathogen to various cellular compartments through transcriptional and post-transcriptional mechanisms.

NtrBC and Nac contribute to efficient Shigella flexneri intracellular replication

Shigella flexneri two-component regulatory systems (TCRS) are responsible for sensing changes in environmental conditions and regulating gene expression accordingly. We examined 12 TCRS that were previously uncharacterized for potential roles in S. flexneri growth within the eukaryotic intracellular environment. We demonstrate that the TCRS EvgSA, NtrBC, and RstBA systems are required for wild-type plaque formation in cultured epithelial cells. The phenotype of the NtrBC mutant depended in part on the Nac transcriptional regulator. Microarray analysis was performed to identify S. flexneri genes differentially regulated by the NtrBC system or Nac in the intracellular environment. This study contributes to our understanding of the transcriptional regulation necessary for Shigella to effectively adapt to the mammalian host cell.

Comparative genomics of Campylobacter concisus isolates reveals genetic diversity and provides insights into disease association

Background

In spite of its association with gastroenteritis and inflammatory bowel diseases, the isolation of Campylobacter concisus from both diseased and healthy individuals has led to controversy regarding its role as an intestinal pathogen. One proposed reason for this is the presence of high genetic diversity among the genomes of C. concisus strains.

Results

In this study the genomes of six C. concisus strains were sequenced, assembled and annotated including two strains isolated from Crohn’s disease patients (UNSW2 and UNSW3), three from gastroenteritis patients (UNSW1, UNSWCS and ATCC 51562) and one from a healthy individual (ATCC 51561). The genomes of C. concisus BAA-1457 and UNSWCD, available from NCBI, were included in subsequent comparative genomic analyses. The Pan and Core genomes for the sequenced C. concisus strains consisted of 3254 and 1556 protein coding genes, respectively.

Conclusion

Genes were identified with specific conservation in C. concisus strains grouped by phenotypes such as invasiveness, adherence, motility and diseased states. Phylogenetic trees based on ribosomal RNA sequences and concatenated host-related pathways for the eight C. concisus strains were generated using the neighbor-joining method, of which the 16S rRNA gene and peptidoglycan biosynthesis grouped the C. concisus strains according to their pathogenic phenotypes. Furthermore, 25 non-synonymous amino acid changes with 14 affecting functional domains, were identified within proteins of conserved host-related pathways, which had possible associations with the pathogenic potential of C. concisus strains. Finally, the genomes of the eight C. concisus strains were compared to the nine available genomes of the well-established pathogen Campylobacter jejuni, which identified several important differences in the respiration pathways of these two species. Our findings indicate that C. concisus strains are genetically diverse, and suggest the genomes of this bacterium contain respiration pathways and modifications in the peptidoglycan layer that may play an important role in its virulence.

Increased Pho regulon activation correlates with decreased virulence of an avian pathogenic Escherichia coli O78 strain

Avian pathogenic Escherichia coli (APEC) strains are associated with respiratory infections, septicemia, cellulitis, peritonitis, and other conditions, since colibacillosis manifests in many ways. The Pho regulon is jointly controlled by the two-component regulatory system PhoBR and by the phosphate-specific transport (Pst) system. To determine the specific roles of the PhoBR regulon and the Pst system in the pathogenesis of the APEC O78 strain χ7122, different phoBR and pst mutant strains were tested in vivo in chickens and in vitro for virulence traits. Mutations resulting in constitutive activation of the Pho regulon rendered strains more sensitive than the wild type to hydrogen peroxide and to the bactericidal effects of rabbit serum. In addition, production of type 1 fimbriae was also impaired in these strains. Using a chicken competitive infection model, all PhoB constitutive mutants were outcompeted by the wild-type parent, including strains containing a functional Pst system. Cumulative inactivation of the Pst system and the PhoB regulator resulted in a restoration of virulence. In addition, loss of the PhoB regulator alone did not affect virulence in the chicken infection model. Interestingly, the level of attenuation of the mutant strains correlated directly with the level of activation of the Pho regulon. Overall, results indicate that activation of the Pho regulon rather than phosphate transport by the Pst system plays a major role in the attenuation of the APEC O78 strain χ7122.

The high-affinity phosphate transporter Pst in Proteus mirabilis HI4320 and its importance in biofilm formation

Proteus mirabilis causes urinary tract infections (UTIs) in individuals requiring long-term indwelling catheterization. The pathogenesis of this uropathogen is mediated by a number of virulence factors and the formation of crystalline biofilms. In addition, micro-organisms have evolved complex systems for the acquisition of nutrients, including the phosphate-specific transport system, which has been shown to be important in biofilm formation and pathogenesis. A functional Pst system is important during UTIs caused by P. mirabilis HI4320, since transposon mutants in the PstS periplasmic binding protein and the PstA permease protein were attenuated in the CBA mouse model of UTI. These mutants displayed a defect in biofilm formation when grown in human urine. This study focuses on a comparison of the proteomes during biofilm and planktonic growth in phosphate-rich medium and human urine, and microscopic investigations of biofilms formed by the pst mutants. Our data suggest that (i) the Deltapst mutants, and particularly the DeltapstS mutant, are defective in biofilm formation, and (ii) the proteomes of these mutants differ significantly from that of the wild-type. Therefore, since the Pst system of P. mirabilis HI4320 negatively regulates biofilm formation, this system is important for the pathogenesis of these organisms during complicated UTIs.

The pst operon of enteropathogenic Escherichia coli enhances bacterial adherence to epithelial cells

Enteropathogenic Escherichia coli (EPEC) adheres in vivo and in vitro to epithelial cells. Two main adhesins, the bundle-forming pilus and intimin, encoded by the bfp operon and eae, respectively, are responsible for the localized and the intimate adherence phenotypes. Deletion of the pst operon of EPEC abolishes the transport of inorganic phosphate through the phosphate-specific transport system and causes the constitutive expression of the PHO regulon genes. In the absence of pst there is a decrease in the expression of the main EPEC adhesins and a reduction in bacterial adherence to epithelial cells in vitro. This effect is not related to PHO constitutivity, because a Deltapst phoB double mutant that is defective in the transcription of the PHO genes also displayed low levels of adherence and expression of adhesins. Likewise, a PHO-constitutive phoR mutation did not affect bacterial adherence. The expression of the per operon, which encodes the bfp and ler regulators PerA and PerC, is also negatively affected by the pst deletion. Overall, the data presented here demonstrate that the pst operon of EPEC plays a positive role in the bacterial adherence mechanism by increasing the expression of perA and perC and consequently the transcription of bfp and eae.

The phosphate regulon and bacterial virulence: a regulatory network connecting phosphate homeostasis and pathogenesis

Bacterial pathogens regulate virulence factor gene expression coordinately in response to environmental stimuli, including nutrient starvation. The phosphate (Pho) regulon plays a key role in phosphate homeostasis. It is controlled by the PhoR/PhoB two-component regulatory system. PhoR is an integral membrane signaling histidine kinase that, through an interaction with the ABC-type phosphate-specific transport (Pst) system and a protein called PhoU, somehow senses environmental inorganic phosphate (P(i)) levels. Under conditions of P(i) limitation (or in the absence of a Pst component or PhoU), PhoR activates its partner response regulator PhoB by phosphorylation, which, in turn, up- or down-regulates target genes. Single-cell profiling of PhoB activation has shown recently that Pho regulon gene expression exhibits a stochastic, "all-or-none" behavior. Recent studies have also shown that the Pho regulon plays a role in the virulence of several bacteria. Here, we present a comprehensive overview of the role of the Pho regulon in bacterial virulence. The Pho regulon is clearly not a simple regulatory circuit for controlling phosphate homeostasis; it is part of a complex network important for both bacterial virulence and stress response.

The high-affinity phosphate transporter Pst is a virulence factor for Proteus mirabilis during complicated urinary tract infection

Proteus mirabilis is a ubiquitous bacterium associated with complicated urinary tract infection (UTI). Mutagenesis studies of the wild-type strain HI4320 in the CBA mouse model of ascending UTIs have identified attenuated mutants with transposon insertions in genes encoding the high-affinity phosphate transporter Pst (pstS, pstA). The transcription of the pst operon (pstSCAB-phoU) and other members of the phosphate regulon of Escherichia coli, including alkaline phosphatase (AP), are regulated by the two-component regulatory system PhoBR and are repressed until times of phosphate starvation. This normal suppression was relieved in pstS::Tn5 and pstA::Tn5 mutants, which constitutively produced AP regardless of growth conditions. No significant growth defects were observed in vitro for the pst mutants during the independent culture or coculture studies in rich broth, phosphate-limiting minimal salts medium, or human urine. Mutants complemented with the complete pst operon repressed AP synthesis in vitro and colonized the mouse bladder in numbers comparable to the wild-type strain HI4320. Therefore, the Pst transport system imparts a significant in vivo advantage to wild-type P. mirabilis that is not required for in vitro growth. Thus, the Pst transporter has satisfied molecular Koch's postulates as a virulence factor in the pathogenesis of urinary tract infection caused by P. mirabilis.