Deletion of a Vibrio cholerae ClC channel results in acid sensitivity and enhanced intestinal colonization

Abundant Monovalent Ions as Environmental Signposts for Pathogens during Host Colonization

Host colonization by a pathogen requires proper sensing and response to local environmental cues, to ensure adaptation and continued survival within the host. The ionic milieu represents a critical potential source of environmental cues, and indeed, there has been extensive study of the interplay between host and pathogen in the context of metals such as iron, zinc, and manganese, vital ions that are actively sequestered by the host. The inherent non-uniformity of the ionic milieu also extends, however, to "abundant" ions such as chloride and potassium, whose concentrations vary greatly between tissue and cellular locations, and with the immune response. Despite this, the concept of abundant ions as environmental cues and key players in host-pathogen interactions is only just emerging. Focusing on chloride and potassium, this review brings together studies across multiple bacterial and parasitic species that have begun to define both how these abundant ions are exploited as cues during host infection, and how they can be actively manipulated by pathogens during host colonization. The close links between ion homeostasis and sensing/response to different ionic signals, and the importance of studying pathogen response to cues in combination, are also discussed, while considering the fundamental insight still to be uncovered from further studies in this nascent area of inquiry.

In vivo repressed genes of Vibrio cholerae reveal inverse requirements of an H+/Cl- transporter along the gastrointestinal passage

The facultative human pathogen Vibrio cholerae changes its transcriptional profile upon oral ingestion by the host to facilitate survival and colonization fitness. Here, we used a modified version of recombination-based in vivo expression technology to investigate gene silencing during the in vivo passage, which has been understudied. Using a murine model of cholera, we screened a V. cholerae transposon library composed of 10,000 randomly generated reporter fusions and identified 101 in vivo repressed (ivr) genes. Our data indicate that constitutive expression of ivr genes reduces colonization fitness, highlighting the necessity to down-regulate these genes in vivo. For example, the ivr gene clcA, encoding an H⁺/Cl^- transporter, could be linked to the acid tolerance response against hydrochloric acid. In a chloride-dependent manner, ClcA facilitates survival under low pH (e.g., the stomach), but its presence becomes detrimental under alkaline conditions (e.g., lower gastrointestinal tract). This pH-dependent clcA expression is controlled by the LysR-type activator AphB, which acts in concert with AphA to initiate the virulence cascade in V. cholerae after oral ingestion. Thus, transcriptional networks dictating induction of virulence factors and the repression of ivr genes overlap to regulate in vivo colonization dynamics. Overall, the results presented herein highlight the impact of spatiotemporal gene silencing in vivo. The molecular characterization of the underlying mechanisms can provide important insights into in vivo physiology and virulence network regulation.

The LysR-type regulator LeuO regulates the acid tolerance response in Vibrio cholerae

Vibrio cholerae is a neutrophilic enteric pathogen that is extremely sensitive to acid. As V. cholerae passages through the host gastrointestinal tract it is exposed to a variety of environmental stresses including low pH and volatile fatty acids. Exposure to acidic environments induces expression of the V. cholerae acid tolerance response. A key component of the acid tolerance response is the cad system, which is encoded by cadC and the cadBA operon. CadB is a lysine/cadaverine antiporter and CadA is a lysine decarboxylase and these function together to counter low intracellular and extracellular pH. CadC is a membrane-associated transcription factor that activates cadBA expression in response to acidic conditions. Herein we investigated the role of the LysR-type transcriptional regulator LeuO in the V. cholerae acid tolerance response. Transcriptional reporter assays revealed that leuO expression repressed cadC transcription, indicating that LeuO was a cadC repressor. Consistent with this, leuO expression was inversely linked to lysine decarboxylase production and leuO overexpression resulted in increased sensitivity to organic acids. Overexpression of leuO in a cadA mutant potentiated killing by organic acids, suggesting that the function of leuO in the acid tolerance response extended beyond its regulation of the cad system. Collectively, these studies have identified a new physiological role for LeuO in V. cholerae acid tolerance.

Efficient responses to host and bacterial signals during Vibrio cholerae colonization

Vibrio cholerae, the microorganism responsible for the diarrheal disease cholera, is able to sense and respond to a variety of changing stimuli in both its aquatic and human gastrointestinal environments. Here we present a review of research efforts aimed toward understanding the signals this organism senses in the human host. V. cholerae's ability to sense and respond to temperature and pH, bile, osmolarity, oxygen and catabolite levels, nitric oxide, and mucus, as well as the quorum sensing signals produced in response to these factors will be discussed. We also review the known quorum sensing regulatory pathways and discuss their importance with regard to the regulation of virulence and colonization during infection.

The crystal structure of AphB, a virulence gene activator from Vibrio cholerae, reveals residues that influence its response to oxygen and pH

Expression of the two critical virulence factors of Vibrio cholerae, toxin-coregulated pilus and cholera toxin, is initiated at the tcpPH promoter by the regulators AphA and AphB. AphA is a winged helix DNA-binding protein that enhances the ability of AphB, a LysR-type transcriptional regulator, to activate tcpPH expression. We present here the 2.2 Å X-ray crystal structure of full-length AphB. As reported for other LysR-type proteins, AphB is a tetramer with two distinct subunit conformations. Unlike other family members, AphB must undergo a significant conformational change in order to bind to DNA. We have found five independent mutations in the putative ligand-binding pocket region that allow AphB to constitutively activate tcpPH expression at the non-permissive pH of 8.5 and in the presence of oxygen. These findings indicate that AphB is responsive to intracellular pH as well as to anaerobiosis and that residues in the ligand-binding pocket of the protein influence its ability to respond to both of these signals. We have solved the structure of one of the constitutive mutants, and observe conformational changes that would allow DNA binding. Taken together, these results describe a pathway of conformational changes allowing communication between the ligand and DNA binding regions of AphB.

The LysR-type virulence activator AphB regulates the expression of genes in Vibrio cholerae in response to low pH and anaerobiosis

AphB is a LysR-type activator that initiates the expression of the virulence cascade in Vibrio cholerae by cooperating with the quorum-sensing-regulated activator AphA at the tcpPH promoter on the Vibrio pathogenicity island (VPI). To identify the ancestral chromosomal genes in V. cholerae regulated by AphB, we carried out a microarray analysis and show here that AphB influences the expression of a number of genes that are not associated with the VPI. One gene strongly activated by AphB is cadC, which encodes the ToxR-like transcriptional activator responsible for activating the expression of lysine decarboxylase, which plays an important role in survival at low pH. Other genes activated by AphB encode a Na(+)/H(+) antiporter, a carbonic anhydrase, a member of the ClC family of chloride channels, and a member of the Gpr1/Fun34/YaaH family. AphB influences each of these genes directly by recognizing a conserved binding site within their promoters, as determined by gel mobility shift assays. Transcriptional lacZ fusions indicate that AphB activates the expression of these genes under aerobic conditions in response to low pH and also under anaerobic conditions at neutral pH. Further experiments show that the regulation of cadC by AphB in response to low pH and anaerobiosis is mirrored in the heterologous organism Escherichia coli, is independent of the global regulators Fnr and ArcAB, and depends upon the region of the promoter that contains the AphB binding site. These results raise the possibility that the activity of AphB is influenced by the pH and oxygen tension of the environment.

A ClC chloride channel homolog and ornithine-containing membrane lipids of Rhizobium tropici CIAT899 are involved in symbiotic efficiency and acid tolerance

Rhizobium tropici CIAT899 is highly tolerant to several environmental stresses and is a good competitor for nodule occupancy of common bean plants in acid soils. Random transposon mutagenesis was performed to identify novel genes of this strain involved in symbiosis and stress tolerance. Here, we present a genetic analysis of the locus disrupted by the Tn5 insertion in mutant 899-PV9, which lead to the discovery of sycA, a homolog of the ClC family of chloride channels and Cl-/H+ exchange transporters. A nonpolar deletion in this gene caused serious deficiencies in nodule development, nodulation competitiveness, and N2 fixation on Phaseolus vulgaris plants, probably due to its reduced ability to invade plant cells and to form stable symbiosomes, as judged by electron transmission microscopy. A second gene (olsC), found downstream of sycA, is homologous to aspartyl/asparaginyl beta-hydroxylases and modifies two species of ornithine-containing lipids in vivo, presumably by hydroxylation at a still-unknown position. A mutant carrying a nonpolar deletion in olsC is symbiotically defective, whereas overexpressed OlsC in the complemented strain provokes an acid-sensitive phenotype. This is the first report of a ClC homolog being essential for the establishment of a fully developed N2-fixing root nodule symbiosis and of a putative beta-hydroxylase that modifies ornithine-containing membrane lipids of R. tropici CIAT899, which, in turn, are contributing to symbiotic performance and acid tolerance.

Hfq is essential for Vibrio cholerae virulence and downregulates σE expression

Hfq is an RNA-binding protein that interacts with both small untranslated RNAs (sRNAs) and mRNAs to modulate gene expression post-transcriptionally. In Escherichia coli and Salmonella typhimurium, Hfq is required for efficient expression of the stationary phase sigma factor sigma(S), and consequently is critical for Salmonella virulence. We have found that Hfq is also essential for the virulence of Vibrio cholerae, as strains lacking hfq fail to colonize the suckling mouse intestine. Deletion of the V. cholerae hfq does not prevent production of sigma(S), nor does it prevent expression of TCP, V. cholerae's primary colonization factor. The expression and activity of the alternative sigma factor sigma(E) are dramatically increased in a V. cholerae hfq mutant. Comparison of the transcriptome of an hfq mutant with that of an rseA mutant, which also overexpresses sigma(E), revealed that sigma(E) controls approximately half the genes found to be upregulated in the hfq mutant. However, increased sigma(E) does not appear to account for this strain's reduced virulence. It is likely that sRNAs, in conjunction with Hfq, are critical regulators of V. cholerae pathogenicity.