ToxR activates the Vibrio cholerae virulence genes by tethering DNA to the membrane through versatile binding to multiple sites.

ToxR, a Vibrio cholerae transmembrane one-component signal transduction factor, lies within a regulatory cascade that results in the expression of ToxT, toxin coregulated pilus, and cholera toxin. While ToxR has been extensively studied for its ability to activate or repress various genes in V. cholerae, here we present the crystal structures of the ToxR cytoplasmic domain bound to DNA at the toxT and ompU promoters. The structures confirm some predicted interactions, yet reveal other unexpected promoter interactions with implications for other potential regulatory roles for ToxR. We show that ToxR is a versatile virulence regulator that recognizes diverse and extensive, eukaryotic-like regulatory DNA sequences, that relies more on DNA structural elements than specific sequences for binding. Using this topological DNA recognition mechanism, ToxR can bind both in tandem and in a twofold inverted-repeat-driven manner. Its regulatory action is based on coordinated multiple binding to promoter regions near the transcription start site, which can remove the repressing H-NS proteins and prepares the DNA for optimal interaction with the RNA polymerase.

In vitro Streptococcus mutans adhesion and biofilm formation on different esthetic orthodontic archwires.

OBJECTIVES: To evaluate the ability of different esthetic archwires to retain oral biofilms in vitro. MATERIALS AND METHODS: Seven different brands of coated orthodontic archwires were tested: two epoxy coated, two polytetrafluoroethylene coated, two rhodium coated, and one silver plus polymer coated. Conventional uncoated metallic archwires were used as controls. Streptococus mutans adherence to archwires was quantified by colony count following 24 hours of biolfilm growth, and total wire-associated biofilm was measured using a crystal violet staining assay. For both tests, two conditions were used: 0% sucrose and 3% sucrose. For statistical analysis, P < .05 was considered as statistically significant. RESULTS: For S. mutans colony forming units per biofilm, there were no statistically significant differences among the various archwires (P = .795 for 0% sucrose; P = .905 for 3% sucrose). Regarding total biofilm formed on archwires in the 3% sucrose condition, there were statistically significant differences in crystal violet staining only for the comparison between Niti Micro Dental White and Copper Ni-Ti wires (P < .05). CONCLUSIONS: The clinical use of esthetic-coated orthodontic wires may be considered to have similar risks as uncoated archwires for biofilm retention.

Characterization of Vibrio cholerae isolates from freshwater sources in northwest Ohio.

Vibrio cholerae is a natural inhabitant of aquatic ecosystems worldwide, typically residing in coastal or brackish water. While more than 200 serogroups have been identified, only serogroups O1 and O139 have been associated with epidemic cholera. However, infections other than cholera can be caused by nonepidemic, non-O1/non-O139 V. cholerae strains, including gastroenteritis and extraintestinal infections. While V. cholerae can also survive in freshwater, that is typically only observed in regions of the world where cholera is endemic. We recently isolated V. cholerae from several locations in lakes and rivers in northwest Ohio. These isolates were all found to be non-O1/non-O139 V. cholerae strains, that would not cause cholera. However, these isolates contained a variety of virulence genes, including ctxA, rtxA, rtxC, hlyA, and ompU. Therefore, it is possible that some of these isolates have the potential to cause gastroenteritis or other infections in humans. We also investigated the relative motility of the isolates and their ability to form biofilms as this is important for V. cholerae survival in the environment. We identified one isolate that forms very robust biofilms, up to 4x that of our laboratory strains. Finally, we investigated the susceptibility of these isolates to a panel of antibiotics. We found that many of the isolates showed decreased susceptibility to some of the antibiotics tested, which could be of concern. While we do not know if these isolates are pathogenic to humans, increased surveillance to better understand the public health risk to the local community should be considered.

The wing of the ToxR winged helix-turn-helix domain is required for DNA binding and activation of toxT and ompU.

ToxR and TcpP, two winged helix-turn-helix (w-HTH) family transcription factors, co-activate expression of the toxT promoter in Vibrio cholerae. ToxT then directly regulates a number of genes required for virulence. In addition to co-activation of toxT, ToxR can directly activate the ompU promoter and repress the ompT promoter. Based on a previous study suggesting that certain wing residues of ToxR are preferentially involved in toxT co-activation compared to direct ompU activation, we employed alanine-scanning mutagenesis to determine which residues in the wing of ToxR are required for activation of each promoter. All of the ToxR wing residues tested that were critical for transcriptional activation of toxT and/or ompU were also critical for DNA binding. While some ToxR wing mutants had reduced interaction with TcpP, that reduced interaction did not correlate with a specific defect in toxT activation. Rather, such mutants also affected ompU activation and DNA binding. Based on these findings we conclude that the primary role of the wing of ToxR is to bind DNA, along with the DNA recognition helix of ToxR, and this function is required both for direct activation of ompU and co-activation of toxT.

Ail provides multiple mechanisms of serum resistance to Yersinia pestis.

Ail, a multifunctional outer membrane protein of Yersinia pestis, confers cell binding, Yop delivery and serum resistance activities. Resistance to complement proteins in serum is critical for the survival of Y. pestis during the septicemic stage of plague infections. Bacteria employ a variety of tactics to evade the complement system, including recruitment of complement regulatory factors, such as factor H, C4b-binding protein (C4BP) and vitronectin (Vn). Y. pestis Ail interacts with the regulatory factors Vn and C4BP, and Ail homologs from Y. enterocolitica and Y. pseudotuberculosis recruit factor H. Using co-sedimentation assays, we demonstrate that two surface-exposed amino acids, F80 and F130, are required for the interaction of Y. pestis Ail with Vn, factor H and C4BP. However, although Ail-F80A/F130A fails to interact with these complement regulatory proteins, it still confers 10,000-fold more serum resistance than a Δail strain and prevents C9 polymerization, potentially by directly interfering with MAC assembly. Using site-directed mutagenesis, we further defined this additional mechanism of complement evasion conferred by Ail. Finally, we find that at Y. pestis concentrations reflective of early-stage septicemic plague, Ail weakly recruits Vn and fails to recruit factor H, suggesting that this alternative mechanism of serum resistance may be essential during plague infection.

Conjugated Linoleic Acid Reduces Cholera Toxin Production In Vitro and In Vivo by Inhibiting Vibrio cholerae ToxT Activity.

The severe diarrheal disease cholera is endemic in over 50 countries. Current therapies for cholera patients involve oral and/or intravenous rehydration, often combined with the use of antibiotics to shorten the duration and intensity of the disease. However, as antibiotic resistance increases, treatment options will become limited. Linoleic acid has been shown to be a potent negative effector of V. cholerae virulence that acts on the major virulence transcription regulator protein, ToxT, to inhibit virulence gene expression. ToxT activates transcription of the two major virulence factors required for disease, cholera toxin (CT) and toxin-coregulated pilus (TCP). A conjugated form of linoleic acid (CLA) is currently sold over the counter as a dietary supplement and is generally recognized as safe by the U.S. Food and Drug Administration. This study examined whether CLA could be used as a new therapy to reduce CT production, which, in turn, would decrease disease duration and intensity in cholera patients. CLA could be used in place of traditional antibiotics and would be very unlikely to generate resistance, as it affects only virulence factor production and not bacterial growth or survival.

Mechanism for inhibition of Vibrio cholerae ToxT activity by the unsaturated fatty acid components of bile.

UNLABELLED: The Gram-negative curved bacillus Vibrio cholerae causes the severe diarrheal illness cholera. During host infection, a complex regulatory cascade results in production of ToxT, a DNA-binding protein that activates the transcription of major virulence genes that encode cholera toxin (CT) and toxin-coregulated pilus (TCP). Previous studies have shown that bile and its unsaturated fatty acid (UFA) components reduce virulence gene expression and therefore are likely important signals upon entering the host. However, the mechanism for the bile-mediated reduction of TCP and CT expression has not been clearly defined. There are two likely hypotheses to explain this reduction: (i) UFAs decrease DNA binding by ToxT, or (ii) UFAs decrease dimerization of ToxT. The work presented here elucidates that bile or UFAs directly affect DNA binding by ToxT. UFAs, specifically linoleic acid, can enter V. cholerae when added exogenously and are present in the cytoplasm, where they can then interact with ToxT. Electrophoretic mobility shift assays (EMSAs) with ToxT and various virulence promoters in the presence or absence of UFAs showed a direct reduction in ToxT binding to DNA, even in promoters with only one ToxT binding site. Virstatin, a synthetic ToxT inhibitor, was previously shown to reduce ToxT dimerization. Here we show that virstatin affects DNA binding only at ToxT promoters with two binding sites, unlike linoleic acid, which affects ToxT binding promoters having either one or two ToxT binding sites. This suggests a mechanism in which UFAs, unlike virstatin, do not affect dimerization but affect monomeric ToxT binding to DNA. IMPORTANCE: Vibrio cholerae must produce the major virulence factors cholera toxin (CT) and toxin-coregulated pilus (TCP) to cause cholera. CT and TCP production depends on ToxT, the major virulence transcription activator. ToxT activity is negatively regulated by unsaturated fatty acids (UFAs) present in the lumen of the upper small intestine. This study investigated the mechanism for inhibition of ToxT activity by UFAs and found that UFAs directly reduce specific ToxT binding to DNA at virulence promoters and subsequently reduce virulence gene expression. UFAs inhibit ToxT monomers from binding DNA. This differs from the inhibitory mechanism of a synthetic ToxT inhibitor, virstatin, which inhibits ToxT dimerization. Understanding the mechanisms for inhibition of virulence could lead to better cholera therapeutics.

[14C] Linoleic Acid Uptake and Fractionation Assay in Vibrio cholerae.

The gram-negative curved bacillus Vibrio cholerae (V. cholerae) causes the severe diarrheal illness cholera. The work presented here is to assess whether unsaturated fatty acids (UFAs), such as linoleic acid, have the potential to directly affect proteins involved in DNA binding because they are able to enter the cell. In this protocol, we show how to measure linoleic acid entering V. cholerae when added exogenously and determine whether it is able to enter the cytoplasm. This protocol will quantify how much linoleic acid is able to enter the cell and then identify the amount of linoleic acid that stays in the membrane or ultimately enters the cytoplasm.

A small unstructured region in Vibrio cholerae ToxT mediates the response to positive and negative effectors and ToxT proteolysis.

Vibrio cholerae is the causative agent of the severe diarrheal disease cholera. The production of the virulence factors that are required for human disease is controlled by a complex network of transcriptional and posttranscriptional regulators. ToxT is the transcription regulator that directly controls the production of the two major virulence factors, toxin-coregulated pilus (TCP) and cholera toxin (CT). The solved crystal structure of ToxT revealed an unstructured region in the N-terminal domain between residues 100 and 110. This region and the surrounding amino acids have been previously implicated in ToxT proteolysis, resistance to inhibition by negative effectors, and ToxT dimerization. To better characterize this region, site-directed mutagenesis was performed to assess the effects on ToxT proteolysis and bile sensitivity. This analysis identified specific mutations within this unstructured region that prevent ToxT proteolysis and other mutations that reduce inhibition by bile and unsaturated fatty acids. In addition, we found that mutations that affect the sensitivity of ToxT to bile also affect the sensitivity of ToxT to its positive effector, bicarbonate. These results suggest that a small unstructured region in the ToxT N-terminal domain is involved in multiple aspects of virulence gene regulation and response to human host signals.