Small-molecule inhibitor of Vibrio cholerae virulence and intestinal colonization

Selenium reduces enterohemorrhagic Escherichia coli O157:H7 verotoxin production and globotriaosylceramide receptor expression on host cells

Aim: This study investigated the efficacy of selenium (Se) in reducing Escherichia coli O157:H7 verotoxin production and toxin gene expression. Additionally, the effect of Se on globotriaosylceramide (Gb3) receptor in human lymphoma cells was determined. Materials & methods: The effect of Se on verotoxin synthesis was determined by standard ELISA, whereas its effect on Gb3 receptor was determined by flow cytometry and real-time quantitative PCR. Results & conclusions: Se reduced extracellular and intracellular verotoxin concentration by 40–60% and 80–90%, respectively (p < 0.05), and downregulated verotoxin genes (p < 0.05). Se reduced Gb3 receptor synthesis in lymphoma cells, and real-time quantitative PCR data revealed a significant downregulation of LacCer synthase gene (GalT2) involved in Gb3 synthesis. Further studies are warranted to validate these results in an appropriate animal model.

The effects of antibiotics on the microbiome throughout development and alternative approaches for therapeutic modulation

The widespread use of antibiotics in the past 80 years has saved millions of human lives, facilitated technological progress and killed incalculable numbers of microbes, both pathogenic and commensal. Human-associated microbes perform an array of important functions, and we are now just beginning to understand the ways in which antibiotics have reshaped their ecology and the functional consequences of these changes. Mounting evidence shows that antibiotics influence the function of the immune system, our ability to resist infection, and our capacity for processing food. Therefore, it is now more important than ever to revisit how we use antibiotics. This review summarizes current research on the short-term and long-term consequences of antibiotic use on the human microbiome, from early life to adulthood, and its effect on diseases such as malnutrition, obesity, diabetes, and Clostridium difficile infection. Motivated by the consequences of inappropriate antibiotic use, we explore recent progress in the development of antivirulence approaches for resisting infection while minimizing resistance to therapy. We close the article by discussing probiotics and fecal microbiota transplants, which promise to restore the microbiota after damage of the microbiome. Together, the results of studies in this field emphasize the importance of developing a mechanistic understanding of gut ecology to enable the development of new therapeutic strategies and to rationally limit the use of antibiotic compounds.

Beyond Antibiotics: New Therapeutic Approaches for Bacterial Infections

The utility of conventional antibiotics for the treatment of bacterial infections has become increasingly strained due to increased rates of resistance coupled with reduced rates of development of new agents. As a result, multidrug-resistant, extensively drug-resistant, and pandrug-resistant bacterial strains are now frequently encountered. This has led to fears of a "postantibiotic era" in which many bacterial infections will be untreatable. Alternative nonantibiotic treatment strategies need to be explored to ensure that a robust pipeline of effective therapies is available to clinicians. In this review, we highlight some of the recent developments in this area, such as the targeting of bacterial virulence factors, utilization of bacteriophages to kill bacteria, and manipulation of the microbiome to combat infections.

Activity of Norspermidine on Bacterial Biofilms of Multidrug-Resistant Clinical Isolates Associated with Persistent Extremity Wound Infections

Biofilm formation is a major virulence factor for numerous pathogenic bacteria and is cited as a central event in the pathogenesis of chronic human infections, which is in large part due to excessive extracellular matrix secretion and metabolic changes that occur within the biofilm rendering them highly tolerant to antimicrobial treatments. Polyamines, including norspermidine, play central roles in bacterial biofilm development, but have also recently been shown to inhibit biofilm formation in select strains of various pathogenic bacteria. The aim of this study was to evaluate in vitro the biofilm dispersive and inhibitory activities of norspermidine against multidrug-resistant clinical isolates of Acinetobacter baumannii(n = 4), Klebsiella pneumoniae (n = 3), Pseudomonas aeruginosa (n = 5) and Staphylococcus aureus (n = 4) associated with chronic extremity wound infections using the semi-quantitative 96-well plate method and confocal laser microscopy. In addition to the antibiofilm activity, biocompatibility of norspermidine was also evaluated by measuring toxicity in vitro to human cell lines and whole porcine tissue explants using MTT viability assay and histological analysis. Norspermidine (5-20 mM) had variable dispersive and inhibitory activity on biofilms which was dependent on both the strain and species. Of the clinical bacterial species evaluated herein, A. baumannii isolates were the most sensitive to the effect of norspermidine, which was in part due to the inhibitory effects of norspermidine on bacterial motility and expression of genes involved in the production of homoserine lactones and quorum sensing molecules both essential for biofilm formation. Importantly, exposure of cell lines and whole tissues to norspermidine for prolonged periods of time (≥24 h) was observed to reduce viability and alter tissue histology in a time and concentration dependent manner, with 20 mM exposure having the greatest negative effects on both tissues and individual cell lines. Collectively our findings demonstrate that, similar to other polyamines, norspermidine displays both inhibitory and dispersive activities on biofilms of clinical multidrug-resistant bacterial isolates, in particular for strains of A. baumannii. Additionally our findings suggest that direct application may be considered on tissues, albeit for limited exposure times.

Suppression of Virulence of Toxigenic Vibrio cholerae by Anethole through the Cyclic AMP (cAMP)-cAMP Receptor Protein Signaling System

Use of natural compounds as antivirulence drugs could be an alternative therapeutic approach to modify the outcome of bacterial infections, particularly in view of growing resistance to available antimicrobials. Here, we show that sub-bactericidal concentration of anethole, a component of sweet fennel seed, could suppress virulence potential in O1 El Tor biotype strains of toxigenic Vibrio cholerae, the causative agent of the ongoing 7^th cholera pandemic. The expression of cholera toxin (CT) and toxin coregulated pilus (TCP), the major virulence factors of V. cholerae, is controlled through a regulatory cascade involving activation of ToxT with synergistic coupling interaction of ToxR/ToxS with TcpP/TcpH. We present evidence that anethole inhibits in vitro expression of CT and TCP in a toxT-dependent but toxR/toxS-independent manner and through repression of tcpP/tcpH, by using bead-ELISA, western blotting and quantitative real-time RT-PCR assays. The cyclic AMP (cAMP)-cAMP receptor protein (CRP) is a well-studied global signaling system in bacterial pathogens, and this complex is known to suppress expression of tcpP/tcpH in V. cholerae. We find that anethole influences the virulence regulatory cascade by over-expressing cyaA and crp genes. Moreover, suppression of toxigenic V. cholerae-mediated fluid accumulation in ligated ileum of rabbit by anethole demonstrates its potentiality as an antivirulence drug candidate against the diseases caused by toxigenic V. cholerae. Taken altogether, these results revealing a mechanism of virulence inhibition in V. cholerae by the natural compound anethole, may have relevance in designing antivirulence compounds, particularly against multiple antibiotic resistant bacterial pathogens.

Targeting virulence not viability in the search for future antibacterials

New antibacterials need new approaches to overcome the problem of rapid antibiotic resistance. Here we review the development of potential new antibacterial drugs that do not kill bacteria or inhibit their growth, but combat disease instead by targeting bacterial virulence.

Mechanism of Action and Initial, In Vitro SAR of an Inhibitor of the Shigella flexneri Virulence Regulator VirF

Shigella spp. are among the main causative agents of acute diarrheal illness and claim more than 1 million lives per year worldwide. There are multiple bacterial genes that control the pathogenesis of Shigella, but the virF gene may be the most important. This gene, located on the primary pathogenicity island of Shigella, encodes VirF, an AraC-family transcriptional activator that is responsible for initiating the pathogenesis cycle in Shigella. We have previously shown that it is possible to attenuate the virulence of Shigella flexneri via small molecule inhibition of VirF. In this study, we probed the mechanism of action of our small molecule inhibitors of VirF. To enable these studies, we have developed a homologous and efficient expression and purification system for VirF and have optimized two different in vitro VirF-DNA binding assays. We have determined that one of our HTS hit compounds inhibits VirF binding to DNA with a calculated Ki similar to the effective doses seen in our transcriptional activation and virulence screens. This is consistent with inhibition of DNA binding as the mechanism of action of this hit compound. We have also screened 15 commercially sourced analogs of this compound and deduced an initial SAR from the approximately 100-fold range in activities. Our four other HTS hit compounds do not inhibit DNA binding and yet they do block VirF activity. This suggests that multiple agents with different molecular mechanisms of inhibition of VirF could be developed. Pursuing hits with different mechanisms of action could be a powerful approach to enhance activity and to circumvent resistance that could develop to any one of these agents.

Amoebae-Based Screening Reveals a Novel Family of Compounds Restricting Intracellular Legionella pneumophila

The causative agent of Legionnaires' disease, Legionella pneumophila, grows in environmental amoebae and mammalian macrophages within a distinct compartment, the 'Legionella-containing vacuole' (LCV). Intracellular bacteria are protected from many antibiotics, and thus are notoriously difficult to eradicate. To identify novel compounds that restrict intracellular bacterial replication, we previously developed an assay based on a coculture of amoebae and GFP-producing L. pneumophila. This assay was used to screen a pathway-based, highly diverse chemical library, referred to as the Sinergia library. In this work, we chose to focus on a group of 11 hit compounds, the majority of which originated from the query molecule CN585, a compound that targets the protein phosphatase calcineurin. Further studies on 78 related compound variants revealed crucial structural attributes, namely a triple-ring scaffold with a central triazine moiety, substituted in positions 3 and 5 by two piperidine or pyrrolidine rings, and in position 1 by an amine group bearing a single aliphatic chain moiety. The most effective compound, ZINC00615682, inhibited intracellular replication of L. pneumophila with an IC50 of approximately 20 nM in Acanthamoeba castellanii and slightly less efficiently in Dictyostelium discoideum or macrophages. Pharmacological and genetic attempts to implicate calcineurin in the intracellular replication of L. pneumophila failed. Taken together, these results show that the amoebae-based screen and structure-activity relationship analysis is suitable for the identification of novel inhibitors of the intracellular replication of L. pneumophila. The most potent compound identified in this study targets (an) as yet unidentified host factor(s).

Precision-guided antimicrobial peptide as a targeted modulator of human microbial ecology

One major challenge to studying human microbiome and its associated diseases is the lack of effective tools to achieve targeted modulation of individual species and study its ecological function within multispecies communities. Here, we show that C16G2, a specifically targeted antimicrobial peptide, was able to selectively kill cariogenic pathogen Streptococcus mutans with high efficacy within a human saliva-derived in vitro oral multispecies community. Importantly, a significant shift in the overall microbial structure of the C16G2-treated community was revealed after a 24-h recovery period: several bacterial species with metabolic dependency or physical interactions with S. mutans suffered drastic reduction in their abundance, whereas S. mutans' natural competitors, including health-associated Streptococci, became dominant. This study demonstrates the use of targeted antimicrobials to modulate the microbiome structure allowing insights into the key community role of specific bacterial species and also indicates the therapeutic potential of C16G2 to achieve a healthy oral microbiome.

The Carbonic Anhydrase Inhibitor Ethoxzolamide Inhibits the Mycobacterium tuberculosis PhoPR Regulon and Esx-1 Secretion and Attenuates Virulence

Mycobacterium tuberculosis must sense and adapt to host environmental cues to establish and maintain an infection. The two-component regulatory system PhoPR plays a central role in sensing and responding to acidic pH within the macrophage and is required for M. tuberculosis intracellular replication and growth in vivo. Therefore, the isolation of compounds that inhibit PhoPR-dependent adaptation may identify new antivirulence therapies to treat tuberculosis. Here, we report that the carbonic anhydrase inhibitor ethoxzolamide inhibits the PhoPR regulon and reduces pathogen virulence. We show that treatment of M. tuberculosis with ethoxzolamide recapitulates phoPR mutant phenotypes, including downregulation of the core PhoPR regulon, altered accumulation of virulence-associated lipids, and inhibition of Esx-1 protein secretion. Quantitative single-cell imaging of a PhoPR-dependent fluorescent reporter strain demonstrates that ethoxzolamide inhibits PhoPR-regulated genes in infected macrophages and mouse lungs. Moreover, ethoxzolamide reduces M. tuberculosis growth in both macrophages and infected mice. Ethoxzolamide inhibits M. tuberculosis carbonic anhydrase activity, supporting a previously unrecognized link between carbonic anhydrase activity and PhoPR signaling. We propose that ethoxzolamide may be pursued as a new class of antivirulence therapy that functions by modulating expression of the PhoPR regulon and Esx-1-dependent virulence.

Narrow-spectrum inhibitors of Campylobacter jejuni flagellar expression and growth

Campylobacter jejuni is a major cause of food-borne illness due to its ability to reside within the gastrointestinal tracts of chickens. Multiple studies have identified the flagella of C. jejuni as a major determinant of chicken colonization. An inhibitor screen of approximately 147,000 small molecules was performed to identify compounds that are able to inhibit flagellar expression in a reporter strain of C. jejuni. Several compounds that modestly inhibited motility of wild-type C. jejuni in standard assays were identified, as were a number of small molecules that robustly inhibited C. jejuni growth, in vitro. Examination of similar bacterial screens found that many of these small molecules inhibited only the growth of C. jejuni. Follow-up assays demonstrated inhibition of other strains of C. jejuni and Campylobacter coli but no inhibition of the closely related Helicobacter pylori. The compounds were determined to be bacteriostatic and nontoxic to eukaryotic cells. Preliminary results from a day-of-hatch chick model of colonization suggest that at least one of the compounds demonstrates promise for reducing Campylobacter colonization loads in vivo, although further medicinal chemistry may be required to enhance bioavailability.

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

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.

Bacterial fitness shapes the population dynamics of antibiotic-resistant and -susceptible bacteria in a model of combined antibiotic and anti-virulence treatment

Bacterial resistance to antibiotic treatment is a huge concern: introduction of any new antibiotic is shortly followed by the emergence of resistant bacterial isolates in the clinic. This issue is compounded by a severe lack of new antibiotics reaching the market. The significant rise in clinical resistance to antibiotics is especially problematic in nosocomial infections, where already vulnerable patients may fail to respond to treatment, causing even greater health concern. A recent focus has been on the development of anti-virulence drugs as a second line of defence in the treatment of antibiotic-resistant infections. This treatment, which weakens bacteria by reducing their virulence rather than killing them, should allow infections to be cleared through the body׳s natural defence mechanisms. In this way there should be little to no selective pressure exerted on the organism and, as such, a predominantly resistant population should be less likely to emerge. However, before the likelihood of resistance to these novel drugs emerging can be predicted, we must first establish whether such drugs can actually be effective. Many believe that anti-virulence drugs would not be powerful enough to clear existing infections, restricting their potential application to prophylaxis. We have developed a mathematical model that provides a theoretical framework to reveal the circumstances under which anti-virulence drugs may or may not be successful. We demonstrate that by harnessing and combining the advantages of antibiotics with those provided by anti-virulence drugs, given infection-specific parameters, it is possible to identify treatment strategies that would efficiently clear bacterial infections, while preventing the emergence of antibiotic-resistant subpopulations. Our findings strongly support the continuation of research into anti-virulence drugs and demonstrate that their applicability may reach beyond infection prevention.

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.

Chemical biology applied to the study of bacterial pathogens

In recent years, chemical biology and chemical genomics have been increasingly applied to the field of microbiology to uncover new potential therapeutics as well as to probe virulence mechanisms in pathogens. The approach offers some clear advantages, as identified compounds (i) can serve as a proof of principle for the applicability of drugs to specific targets; (ii) can serve as conditional effectors to explore the function of their targets in vitro and in vivo; (iii) can be used to modulate gene expression in otherwise genetically intractable organisms; and (iv) can be tailored to a narrow or broad range of bacteria. This review highlights recent examples from the literature to illustrate how the use of small molecules has advanced discovery of novel potential treatments and has been applied to explore biological mechanisms underlying pathogenicity. We also use these examples to discuss practical considerations that are key to establishing a screening or discovery program. Finally, we discuss the advantages and challenges of different approaches and the methods that are emerging to address these challenges.

Elucidation of the RamA regulon in Klebsiella pneumoniae reveals a role in LPS regulation

Klebsiella pneumoniae is a significant human pathogen, in part due to high rates of multidrug resistance. RamA is an intrinsic regulator in K. pneumoniae established to be important for the bacterial response to antimicrobial challenge; however, little is known about its possible wider regulatory role in this organism during infection. In this work, we demonstrate that RamA is a global transcriptional regulator that significantly perturbs the transcriptional landscape of K. pneumoniae, resulting in altered microbe-drug or microbe-host response. This is largely due to the direct regulation of 68 genes associated with a myriad of cellular functions. Importantly, RamA directly binds and activates the lpxC, lpxL-2 and lpxO genes associated with lipid A biosynthesis, thus resulting in modifications within the lipid A moiety of the lipopolysaccharide. RamA-mediated alterations decrease susceptibility to colistin E, polymyxin B and human cationic antimicrobial peptide LL-37. Increased RamA levels reduce K. pneumoniae adhesion and uptake into macrophages, which is supported by in vivo infection studies, that demonstrate increased systemic dissemination of ramA overexpressing K. pneumoniae. These data establish that RamA-mediated regulation directly perturbs microbial surface properties, including lipid A biosynthesis, which facilitate evasion from the innate host response. This highlights RamA as a global regulator that confers pathoadaptive phenotypes with implications for our understanding of the pathogenesis of Enterobacter, Salmonella and Citrobacter spp. that express orthologous RamA proteins.

Bacteria can rapidly evolve under antibiotic pressure to develop resistance, which occurs when target genes mutate, or when resistance-encoding genes are transferred. Alternatively, microbes can simply alter the levels of intrinsic proteins that allow the organism to “buy” time to resist antibiotic pressure. Klebsiella pneumoniae is a pathogen that causes significant blood stream or respiratory infections, but more importantly is a bacterium that is increasingly being reported as multidrug resistant. Our data demonstrate that RamA can trigger changes on the bacterial surface that allow Klebsiella to survive both antibiotic challenge, degradation by host immune peptides and resist phagocytosis. We demonstrate that the molecular basis of increased survival of ramA overexpressing K. pneumoniae, against host-derived factors is associated with RamA-driven alterations of the lipid A moiety of Klebsiella LPS. This modification is likely to be linked to Klebsiella’s ability to resist the host response so that it remains undetected by the immune system. The relevance of our work extends beyond RamA in Klebsiella as other pathogens such as Enterobacter spp and Salmonella spp. also produce this protein. Thus our overarching conclusion is that the intrinsic regulator, RamA perturbs host-microbe and microbe-drug interactions.

Small molecule inhibitors of Clostridium difficile toxin B-induced cellular damage

Clostridium difficile causes life-threatening diarrhea through the actions of its homologous toxins TcdA and TcdB on human colonocytes. Therapeutic agents that block toxin-induced damage are urgently needed to prevent the harmful consequences of toxin action that are not addressed with current antibiotic-based treatments. Here, we developed an imaging-based phenotypic screen to identify small molecules that protected human cells from TcdB-induced cell rounding. A series of structurally diverse compounds with antitoxin activity were identified and found to act through one of a small subset of mechanisms, including direct binding and sequestration of TcdB, inhibition of endosomal maturation, and noncompetitive inhibition of the toxin glucosyltransferase activity. Distinct classes of inhibitors were used further to dissect the determinants of the toxin-mediated necrosis phenotype occurring at higher doses of toxin. These findings validate and inform novel targeting strategies for discovering small molecule agents to treat C. difficile infection.

Anti-virulence Strategies to Target Bacterial Infections

Resistance of important bacterial pathogens to common antimicrobial therapies and the emergence of multidrug-resistant bacteria are increasing at an alarming rate and constitute one of our greatest challenges in the combat of bacterial infection and accompanied diseases. The current shortage of effective drugs, lack of successful prevention measures and only a few new antibiotics in the clinical pipeline demand the development of novel treatment options and alternative antimicrobial therapies. Our increasing understanding of bacterial virulence strategies and the induced molecular pathways of the infectious disease provides novel opportunities to target and interfere with crucial pathogenicity factors or virulence-associated traits of the bacteria while bypassing the evolutionary pressure on the bacterium to develop resistance. In the past decade, numerous new bacterial targets for anti-virulence therapies have been identified, and structure-based tailoring of intervention strategies and screening assays for small-molecule inhibitors of such pathways were successfully established. In this chapter, we will take a closer look at the bacterial virulence-related factors and processes that present promising targets for anti-virulence therapies, recently discovered inhibitory substances and their promises and discuss the challenges, and problems that have to be faced.

Colonization factors of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) is a major cause of life-threatening diarrheal disease around the world. The major aspects of ETEC virulence are colonization of the small intestine and the secretion of enterotoxins which elicit diarrhea. Intestinal colonization is mediated, in part, by adhesins displayed on the bacterial cell surface. As colonization of the intestine is the critical first step in the establishment of an infection, it represents a potential point of intervention for the prevention of infections. Therefore, colonization factors (CFs) have been important subjects of research in the field of ETEC virulence. Research in this field has revealed that ETEC possesses a large array of serologically distinct CFs that differ in composition, structure, and function. Most ETEC CFs are pili (fimbriae) or related fibrous structures, while other adhesins are simple outer membrane proteins lacking any macromolecular structure. This chapter reviews the genetics, structure, function, and regulation of ETEC CFs and how such studies have contributed to our understanding of ETEC virulence and opened up potential opportunities for the development of preventive and therapeutic interventions.

Glucose- but not rice-based oral rehydration therapy enhances the production of virulence determinants in the human pathogen Vibrio cholerae

Despite major attempts to prevent cholera transmission, millions of people worldwide still must address this devastating disease. Cholera research has so far mainly focused on the causative agent, the bacterium Vibrio cholerae, or on disease treatment, but rarely were results from both fields interconnected. Indeed, the treatment of this severe diarrheal disease is mostly accomplished by oral rehydration therapy (ORT), whereby water and electrolytes are replenished. Commonly distributed oral rehydration salts also contain glucose. Here, we analyzed the effects of glucose and alternative carbon sources on the production of virulence determinants in the causative agent of cholera, the bacterium Vibrio cholerae during in vitro experimentation. We demonstrate that virulence gene expression and the production of cholera toxin are enhanced in the presence of glucose or similarly transported sugars in a ToxR-, TcpP- and ToxT-dependent manner. The virulence genes were significantly less expressed if alternative non-PTS carbon sources, including rice-based starch, were utilized. Notably, even though glucose-based ORT is commonly used, field studies indicated that rice-based ORT performs better. We therefore used a spatially explicit epidemiological model to demonstrate that the better performing rice-based ORT could have a significant impact on epidemic progression based on the recent outbreak of cholera in Haiti. Our results strongly support a change of carbon source for the treatment of cholera, especially in epidemic settings.

Cholera research has so far mainly focused on the causative agent, the bacterium Vibrio cholerae, or on disease treatment, but rarely were results from both fields interconnected. Indeed, the treatment of this severe diarrheal disease is mostly accomplished by oral rehydration therapy (ORT). ORT aims at rehydrating patients through the provision of water and oral rehydration salts; the latter being composed of electrolytes as well as glucose as a carbon source. Although glucose-based ORS is commonly used to treat diarrheal diseases and is recommended by the WHO, field studies on cholera indicated that rice-based ORT performs better than glucose-based ORT. Here, we investigated the impact that glucose, starch, or other carbon sources exert on V. cholerae. We demonstrated that glucose leads to an increased expression of the major virulence genes in the pathogen and, accordingly, to an enhanced production of cholera toxin during in vitro experimentation. Because the cholera toxin is primarily responsible for the severe symptoms that are associated with the disease, our study highlights the negative effects of glucose-based ORT. Next, we used a spatially explicit epidemiological model to demonstrate that the better performing rice-based ORS could have a significant impact on epidemic progression based on the recent outbreak of cholera in Haiti.

Bicarbonate increases binding affinity of Vibrio cholerae ToxT to virulence gene promoters

The major Vibrio cholerae virulence gene transcription activator, ToxT, is responsible for the production of the diarrhea-inducing cholera toxin (CT) and the major colonization factor, toxin coregulated pilus (TCP). In addition to the two primary virulence factors mentioned, ToxT is responsible for the activation of accessory virulence genes, such as aldA, tagA, acfA, acfD, tcpI, and tarAB. ToxT activity is negatively modulated by bile and unsaturated fatty acids found in the upper small intestine. Conversely, previous work identified another intestinal signal, bicarbonate, which enhances the ability of ToxT to activate production of CT and TCP. The work presented here further elucidates the mechanism for the enhancement of ToxT activity by bicarbonate. Bicarbonate was found to increase the activation of ToxT-dependent accessory virulence promoters in addition to those that produce CT and TCP. Bicarbonate is taken up into the V. cholerae cell, where it positively affects ToxT activity by increasing DNA binding affinity for the virulence gene promoters that ToxT activates regardless of toxbox configuration. The increase in ToxT binding affinity in the presence of bicarbonate explains the elevated level of virulence gene transcription.

Alkaloids: an overview of their antibacterial, antibiotic-enhancing and antivirulence activities

With reports of pandrug-resistant bacteria causing untreatable infections, the need for new antibacterial therapies is more pressing than ever. Alkaloids are a large and structurally diverse group of compounds that have served as scaffolds for important antibacterial drugs such as metronidazole and the quinolones. In this review, we highlight other alkaloids with development potential. Natural, semisynthetic and synthetic alkaloids of all classes are considered, looking first at those with direct antibacterial activity and those with antibiotic-enhancing activity. Potent examples include CJ-13,136, a novel actinomycete-derived quinolone alkaloid with a minimum inhibitory concentration of 0.1 ng/mL against Helicobacter pylori, and squalamine, a polyamine alkaloid from the dogfish shark that renders Gram-negative pathogens 16- to >32-fold more susceptible to ciprofloxacin. Where available, information on toxicity, structure-activity relationships, mechanisms of action and in vivo activity is presented. The effects of alkaloids on virulence gene regulatory systems such as quorum sensing and virulence factors such as sortases, adhesins and secretion systems are also described. The synthetic isoquinoline alkaloid virstatin, for example, inhibits the transcriptional regulator ToxT in Vibrio cholerae, preventing expression of cholera toxin and fimbriae and conferring in vivo protection against intestinal colonisation. The review concludes with implications and limitations of the described research and directions for future research.

Computer based screening for novel inhibitors against Vibrio cholerae using NCI diversity set-II: an alternative approach by targeting transcriptional activator ToxT

Cholera is a severe diarrheal disease caused by Vibrio cholerae and remains as a major health risk in developing countries. The emergence and spread of multi-drug resistant V. cholerae strains during the past two decades is now a major problem in the treatment of cholera and have created the urgent need for the development of novel therapeutic agents. Targeting transcriptional factor is now a novel approach to tackle the development of multi-drug resistant strain. In the recent year virtual high throughput screening has emerged as a widely accepted powerful technology in the identification of novel and diverse lead. This study provides new insight to the search for new potent and selective inhibitors that still remains necessary to avoid the risk of possible resistance and reduce toxicity and side effects of currently available cholera drugs. The publications of high resolution X-ray structure of V. cholerae ToxT has open the way to the structure based virtual screening to identify new small molecular inhibitors which still remain necessary to avoid the risk of possible resistance and reduce toxicity and side effects of currently available cholera drugs. In this study we have performed structure based virtual screening approach using NCI diversity set-II to look for novel inhibitor of ToxT and proposed eight candidate compounds with high scoring function. Thus from complex scoring and binding ability it is elucidated that these compounds could be the promising inhibitors or could be developed as novel lead compounds for drug design against cholera.

Challenges and future prospects of antibiotic therapy: from peptides to phages utilization

Bacterial infections are raising serious concern across the globe. The effectiveness of conventional antibiotics is decreasing due to global emergence of multi-drug-resistant (MDR) bacterial pathogens. This process seems to be primarily caused by an indiscriminate and inappropriate use of antibiotics in non-infected patients and in the food industry. New classes of antibiotics with different actions against MDR pathogens need to be developed urgently. In this context, this review focuses on several ways and future directions to search for the next generation of safe and effective antibiotics compounds including antimicrobial peptides, phage therapy, phytochemicals, metalloantibiotics, lipopolysaccharide, and efflux pump inhibitors to control the infections caused by MDR pathogens.

Antibiotic alternatives: the substitution of antibiotics in animal husbandry?

It is a common practice for decades to use of sub-therapeutic dose of antibiotics in food-animal feeds to prevent animals from diseases and to improve production performance in modern animal husbandry. In the meantime, concerns over the increasing emergence of antibiotic-resistant bacteria due to the unreasonable use of antibiotics and an appearance of less novelty antibiotics have prompted efforts to develop so-called alternatives to antibiotics. Whether or not the alternatives could really replace antibiotics remains a controversial issue. This review summarizes recent development and perspectives of alternatives to antibiotics. The mechanism of actions, applications, and prospectives of the alternatives such as immunity modulating agents, bacteriophages and their lysins, antimicrobial peptides, pro-, pre-, and synbiotics, plant extracts, inhibitors targeting pathogenicity (bacterial quorum sensing, biofilm, and virulence), and feeding enzymes are thoroughly discussed. Lastly, the feasibility of alternatives to antibiotics is deeply analyzed. It is hard to conclude that the alternatives might substitute antibiotics in veterinary medicine in the foreseeable future. At the present time, prudent use of antibiotics and the establishment of scientific monitoring systems are the best and fastest way to limit the adverse effects of the abuse of antibiotics and to ensure the safety of animal-derived food and environment.

Bio-inspired detoxification using 3D-printed hydrogel nanocomposites

Rationally designed nanoparticles that can bind toxins show great promise for detoxification. However, the conventional intravenous administration of nanoparticles for detoxification often leads to nanoparticle accumulation in the liver, posing a risk of secondary poisoning especially in liver-failure patients. Here we present a liver-inspired three-dimensional (3D) detoxification device. This device is created by 3D printing of designer hydrogels with functional polydiacetylene nanoparticles installed in the hydrogel matrix. The nanoparticles can attract, capture and sense toxins, while the 3D matrix with a modified liver lobule microstructure allows toxins to be trapped efficiently. Our results show that the toxin solution completely loses its virulence after treatment using this biomimetic detoxification device. This work provides a proof-of-concept of detoxification by a 3D-printed biomimetic nanocomposite construct in hydrogel, and could lead to the development of alternative detoxification platforms.

Targeting virulence: can we make evolution-proof drugs?

Antivirulence drugs are a new type of therapeutic drug that target virulence factors, potentially revitalising the drug-development pipeline with new targets. As antivirulence drugs disarm the pathogen, rather than kill or halt pathogen growth, it has been hypothesized that they will generate much weaker selection for resistance than traditional antibiotics. However, recent studies have shown that mechanisms of resistance to antivirulence drugs exist, seemingly damaging the 'evolution-proof' claim. In this Opinion article, we highlight a crucial distinction between whether resistance can emerge and whether it will spread to a high frequency under drug selection. We argue that selection for resistance can be reduced, or even reversed, using appropriate combinations of target and treatment environment, opening a path towards the development of evolutionarily robust novel therapeutics.

Proteomic analysis of Vibrio cholerae outer membrane vesicles

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria provide an interesting research material for defining cell-envelope proteins without experimental cell disruption. OMVs are also promising immunogenic platforms and may play important roles in bacterial survival and pathogenesis. We used in-solution trypsin digestion coupled to mass spectrometry to identify 90 proteins present in OMVs of Vibrio cholerae when grown under conditions that activate the TCP pilus virulence regulatory protein (ToxT) virulence regulon. The ToxT expression profile and potential contribution to virulence of these proteins were assessed using ToxT and in vivo RNA-seq, Tn-seq, and cholera stool proteomic and other genome-wide data sets. Thirteen OMV-associated proteins appear to be essential for cell growth, and therefore may represent antibacterial drug targets. Another 12 nonessential OMV proteins, including DegP protease, were required for intestinal colonization in rabbits. Comparative proteomics of a degP mutant revealed the importance of DegP in the incorporation of nine proteins into OMVs, including ones involved in biofilm matrix formation and various substrates of the type II secretion system. Taken together, these results suggest that DegP plays an important role in determining the content of OMVs and also affects phenotypes such as intestinal colonization, proper function of the type II secretion system, and formation of biofilm matrix.

Virstatin inhibits biofilm formation and motility of Acinetobacter baumannii

BACKGROUND

Acinetobacter baumannii has emerged as an opportunistic nosocomial pathogen causing infections worldwide. One reason for this emergence is due to its natural ability to survive in the hospital environment, which may be explained by its capacity to form biofilms. Cell surface appendages are important determinants of the A. baumannii biofilm formation and as such constitute interesting targets to prevent the development of biofilm-related infections. A chemical agent called virstatin was recently described to impair the virulence of Vibrio cholerae by preventing the expression of its virulence factor, the toxin coregulated pilus (type IV pilus). The objective of this work was to investigate the potential effect of virstatin on A. baumannii biofilms.

RESULTS

After a dose-response experiment, we determined that 100 μM virstatin led to an important decrease (38%) of biofilms formed by A. baumannii ATCC17978 grown under static mode. We demonstrated that the production of biofilms grown under dynamic mode was also delayed and reduced. The biofilm susceptibility to virstatin was then tested for 40 clinical and reference A. baumannii strains. 70% of the strains were susceptible to virstatin (with a decrease of 10 to 65%) when biofilms grew in static mode, whereas 60% of strains respond to the treatment when their biofilms grew in dynamic mode. As expected, motility and atomic force microscopy experiments showed that virstatin acts on the A. baumannii pili biogenesis.

CONCLUSIONS

By its action on pili biogenesis, virstatin demonstrated a very promising antibiofilm activity affecting more than 70% of the A. baumannii clinical isolates.

Carvacrol and trans-cinnamaldehyde reduce Clostridium difficile toxin production and cytotoxicity in vitro

Clostridium difficile is a nosocomial pathogen that causes a serious toxin-mediated enteric disease in humans. Reducing C. difficile toxin production could significantly minimize its pathogenicity and improve disease outcomes in humans. This study investigated the efficacy of two, food-grade, plant-derived compounds, namely trans-cinnamaldehyde (TC) and carvacrol (CR) in reducing C. difficile toxin production and cytotoxicity in vitro. Three hypervirulent C. difficile isolates were grown with or without the sub-inhibitory concentrations of TC or CR, and the culture supernatant and the bacterial pellet were collected for total toxin quantitation, Vero cell cytotoxicity assay and RT-qPCR analysis of toxin-encoding genes. The effect of CR and TC on a codY mutant and wild type C. difficile was also investigated. Carvacrol and TC substantially reduced C. difficile toxin production and cytotoxicity on Vero cells. The plant compounds also significantly down-regulated toxin production genes. Carvacrol and TC did not inhibit toxin production in the codY mutant of C. difficile, suggesting a potential codY-mediated anti-toxigenic mechanism of the plant compounds. The antitoxigenic concentrations of CR and TC did not inhibit the growth of beneficial gut bacteria. Our results suggest that CR and TC could potentially be used to control C. difficile, and warrant future studies in vivo.

Something old, something new: revisiting natural products in antibiotic drug discovery

Antibiotic discovery is in crisis. Despite a growing need for new drugs resulting from the increasing number of multi-antibiotic-resistant pathogens, there have been only a handful of new antibiotics approved for clinical use in the past 2 decades. Faced with scientific, economic, and regulatory challenges, the pharmaceutical sector seems unable to respond to what has been called an “apocalyptic” threat. Natural products produced by bacteria and fungi are genetically encoded products of natural selection that have been the mainstay sources of the antibiotics in current clinical use. The pharmaceutical industry has largely abandoned these compounds in favor of large libraries of synthetic molecules because of difficulties in identifying new natural product antibiotics scaffolds. Advances in next-generation genome sequencing, bioinformatics, and analytical chemistry are combining to overcome barriers to natural products. Coupled with new strategies in antibiotic discovery, including inhibition of resistance, novel drug combinations, and new targets, natural products are poised for a renaissance to address what is a pressing health care crisis.

Potential novel antibiotics from HTS targeting the virulence-regulating transcription factor, VirF, from Shigella flexneri

VirF is an AraC-type transcriptional regulator responsible for activating the transcription of virulence genes required for the intracellular invasion and cell-to-cell spread of Shigella flexneri. Gene disruption studies have validated VirF as a potential target for an anti-virulence therapy to treat shigellosis by determining that VirF is necessary for virulence, but not required for bacterial viability. Using a bacteria-based, β-galactosidase reporter assay we completed a high-throughput screening (HTS) campaign monitoring VirF activity in the presence of over 140,000 small molecules. From our screening campaign we identified five lead compounds to pursue in tissue-culture-based invasion and cell-to-cell spread assays and toxicity screens. Our observations of activity in these models for infection have validated our approach of targeting virulence regulation and have allowed us to identify a promising chemical scaffold from our HTS for hit-to-lead development. Interestingly, differential effects on invasion versus cell-to-cell spread suggest that the compounds’ efficacies may depend, in part, on the specific promoter that VirF is recognizing.

Enriched protein screening of human bone marrow mesenchymal stromal cell secretions reveals MFAP5 and PENK as novel IL-10 modulators

The secreted proteins from a cell constitute a natural biologic library that can offer significant insight into human health and disease. Discovering new secreted proteins from cells is bounded by the limitations of traditional separation and detection tools to physically fractionate and analyze samples. Here, we present a new method to systematically identify bioactive cell-secreted proteins that circumvent traditional proteomic methods by first enriching for protein candidates by differential gene expression profiling. The bone marrow stromal cell secretome was analyzed using enriched gene expression datasets in combination with potency assay testing. Four proteins expressed by stromal cells with previously unknown anti-inflammatory properties were identified, two of which provided a significant survival benefit to mice challenged with lethal endotoxic shock. Greater than 85% of secreted factors were recaptured that were otherwise undetected by proteomic methods, and remarkable hit rates of 18% in vitro and 9% in vivo were achieved.

Active efflux influences the potency of quorum sensing inhibitors in Pseudomonas aeruginosa

Many bacteria regulate gene expression through a cell-cell signaling process called quorum sensing (QS). In proteobacteria, QS is largely mediated by signaling molecules known as N-acylated L-homoserine lactones (AHLs) and their associated intracellular LuxR-type receptors. The design of non-native small molecules capable of inhibiting LuxR-type receptors (and thereby QS) in proteobacteria is an active area of research, and numerous lead compounds are AHL derivatives that mimic native AHL molecules. Much of this previous work has focused on the pathogen Pseudomonas aeruginosa, which controls an arsenal of virulence factors and biofilm formation through QS. The MexAB-OprM efflux pump has been shown to play a role in the secretion of the major AHL signal in P. aeruginosa, N-(3-oxododecanoyl) L-homoserine lactone. In the current study, we show that a variety of non-native AHLs and related derivatives capable of inhibiting LuxR-type receptors in P. aeruginosa display significantly higher potency in a P. aeruginosa Δ(mexAB-oprM) mutant, suggesting that MexAB-OprM also recognizes these compounds as substrates. We also demonstrate that the potency of 5,6-dimethyl-2-aminobenzimidazole, recently shown to be a QS and biofilm inhibitor in P. aeruginosa, is not affected by the presence/absence of the MexAB-OprM pump. These results have implications for the use of non-native AHLs and related derivatives as QS modulators in P. aeruginosa and other bacteria, and provide a potential design strategy for the development of new QS modulators that are resistant to active efflux.

Liposome-like Nanostructures for Drug Delivery

Liposomes are a class of well-established drug carriers that have found numerous therapeutic applications. The success of liposomes, together with recent advancements in nanotechnology, has motivated the development of various novel liposome-like nanostructures with improved drug delivery performance. These nanostructures can be categorized into five major varieties, namely: (1) polymer-stabilized liposomes, (2) nanoparticle-stabilized liposomes, (3) core-shell lipid-polymer hybrid nanoparticles, (4) natural membrane-derived vesicles, and (5) natural membrane coated nanoparticles. They have received significant attention and have become popular drug delivery platforms. Herein, we discuss the unique strengths of these liposome-like platforms in drug delivery, with a particular emphasis on how liposome-inspired novel designs have led to improved therapeutic efficacy, and review recent progress made by each platform in advancing healthcare.

Antibiotic resistance-the need for global solutions

The causes of antibiotic resistance are complex and include human behaviour at many levels of society; the consequences affect everybody in the world. Similarities with climate change are evident. Many efforts have been made to describe the many different facets of antibiotic resistance and the interventions needed to meet the challenge. However, coordinated action is largely absent, especially at the political level, both nationally and internationally. Antibiotics paved the way for unprecedented medical and societal developments, and are today indispensible in all health systems. Achievements in modern medicine, such as major surgery, organ transplantation, treatment of preterm babies, and cancer chemotherapy, which we today take for granted, would not be possible without access to effective treatment for bacterial infections. Within just a few years, we might be faced with dire setbacks, medically, socially, and economically, unless real and unprecedented global coordinated actions are immediately taken. Here, we describe the global situation of antibiotic resistance, its major causes and consequences, and identify key areas in which action is urgently needed.

Cholera: pathophysiology and emerging therapeutic targets

Cholera is a diarrheal disease that remains an important global health problem with several hundreds of thousands of reported cases each year. This disease is caused by intestinal infection with Vibrio cholerae, which is a highly motile gram-negative bacterium with a single-sheathed flagellum. In the course of cholera pathogenesis, V. cholerae expresses a transcriptional activator ToxT, which subsequently transactivates expressions of two crucial virulence factors: toxin-coregulated pilus and cholera toxin (CT). These factors are responsible for intestinal colonization of V. cholerae and induction of fluid secretion, respectively. In intestinal epithelial cells, CT binds to GM1 ganglioside receptors on the apical membrane and undergoes retrograde vesicular trafficking to endoplasmic reticulum, where it exploits endoplasmic reticulum-associated protein degradation systems to release a catalytic A1 subunit of CT (CT A1) into cytoplasm. CT A1, in turn, catalyzes ADP ribosylation of α subunits of stimulatory G proteins, leading to a persistent activation of adenylate cyclase and an elevation of intracellular cAMP. Increased intracellular cAMP in human intestinal epithelial cells accounts for pathogenesis of profuse diarrhea and severe fluid loss in cholera. This review provides an overview of the pathophysiology of cholera diarrhea and discusses emerging drug targets for cholera, which include V. cholerae virulence factors, V. cholerae motility, CT binding to GM1 receptor, CT internalization and intoxication, as well as cAMP metabolism and transport proteins involved in cAMP-activated Cl(-) secretion. Future directions and perspectives of research on drug discovery and development for cholera are discussed.

Caenorhabditis elegans-based in vivo screening of bioactives from marine sponge-associated bacteria against Vibrio alginolyticus

AIM

To establish Caenorhabditis elegans based in vivo method for screening bioactives from marine sponge associated bacteria (SAB) against Vibrio species.

METHODS AND RESULTS

About 256 SAB isolates were screened for their ability to rescue C. elegans infected with Vibrio species. The chloroform extract of the positive isolate was subjected to column fractionation and purity of the active fraction was analysed using HPLC. Further, the components were elucidated using GC/MS. The active fraction was tested for its in vivo rescue activity, antibacterial and anti-QS activity. In vivo colonization reduction and biofilm inhibition efficiency were assessed using GFP-tagged V. alginolyticus using confocal laser scanning microscopy (CLSM). The ability of the active fraction in modulating expression of V. alginolyticus quorum sensing (QS) regulators luxT and lafK was measured using real-time PCR. The results indicated that the chloroform extract of SAB4.2 displayed significant rescue activity against V. alginolyticus by inhibiting the QS pathway. HPLC analysis of the active fraction revealed a single major peak and GC/MS analysis suggested Pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl) as the major constituent. The potent bacterial isolate was identified as Alcaligenes faecalis.

CONCLUSIONS

In vivo screening using C. elegans identified a marine isolate that inhibits the virulence of V. alginolyticus by interrupting the QS pathway.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study provides a C. elegans based in vivo screening method for identifying bioactives from natural resources by overcoming the disadvantages of traditional in vitro plate assays.

Angiotensin II receptor type 1--a novel target for preventing neonatal meningitis in mice by Escherichia coli K1

The increasing incidence of Escherichia coli K1 meningitis due to escalating antibiotic resistance warrants alternate treatment options to prevent this deadly disease. We screened a library of small molecules from the National Institutes of Health clinical collection and identified telmisartan, an angiotensin II receptor type 1 (AT1R) blocker, as a potent inhibitor of E. coli invasion into human brain microvascular endothelial cells (HBMECs). Immunoprecipitation studies revealed that AT1R associates with endothelial cell gp96, the receptor in HBMECs for E. coli outer membrane protein A. HBMECs pretreated with telmisartan or transfected with AT1R small interfering RNA were resistant to E. coli invasion because of downregulation of protein kinase C-α phosphorylation. Administration of a soluble derivative of telmisartan to newborn mice before infection with E. coli prevented the onset of meningitis and suppressed neutrophil infiltration and glial cell migration in the brain. Therefore, telmisartan has potential as an alternate treatment option for preventing E. coli meningitis.

Disarming bacterial virulence through chemical inhibition of the DNA binding domain of an AraC-like transcriptional activator protein

The misuse of antibiotics during past decades has led to pervasive antibiotic resistance in bacteria. Hence, there is an urgent need for the development of new and alternative approaches to combat bacterial infections. In most bacterial pathogens the expression of virulence is tightly regulated at the transcriptional level. Therefore, targeting pathogens with drugs that interfere with virulence gene expression offers an effective alternative to conventional antimicrobial chemotherapy. Many Gram-negative intestinal pathogens produce AraC-like proteins that control the expression of genes required for infection. In this study we investigated the prototypical AraC-like virulence regulator, RegA, from the mouse attaching and effacing pathogen, Citrobacter rodentium, as a potential drug target. By screening a small molecule chemical library and chemical optimization, we identified two compounds that specifically inhibited the ability of RegA to activate its target promoters and thus reduced expression of a number of proteins required for virulence. Biophysical, biochemical, genetic, and computational analyses indicated that the more potent of these two compounds, which we named regacin, disrupts the DNA binding capacity of RegA by interacting with amino acid residues within a conserved region of the DNA binding domain. Oral administration of regacin to mice, commencing 15 min before or 12 h after oral inoculation with C. rodentium, caused highly significant attenuation of intestinal colonization by the mouse pathogen comparable to that of an isogenic regA-deletion mutant. These findings demonstrate that chemical inhibition of the DNA binding domains of transcriptional regulators is a viable strategy for the development of antimicrobial agents that target bacterial pathogens.

Small-molecule inhibitors of toxT expression in Vibrio cholerae

Vibrio cholerae, a Gram-negative bacterium, infects humans and causes cholera, a severe disease characterized by vomiting and diarrhea. These symptoms are primarily caused by cholera toxin (CT), whose production by V. cholerae is tightly regulated by the virulence cascade. In this study, we designed and carried out a high-throughput chemical genetic screen to identify inhibitors of the virulence cascade. We identified three compounds, which we named toxtazin A and toxtazin B and B', representing two novel classes of toxT transcription inhibitors. All three compounds reduce production of both CT and the toxin-coregulated pilus (TCP), an important colonization factor. We present evidence that toxtazin A works at the level of the toxT promoter and that toxtazins B and B' work at the level of the tcpP promoter. Treatment with toxtazin B results in a 100-fold reduction in colonization in an infant mouse model of infection, though toxtazin A did not reduce colonization at the concentrations tested. These results add to the growing body of literature indicating that small-molecule inhibitors of virulence genes could be developed to treat infections, as alternatives to antibiotics become increasingly needed.

IMPORTANCE

V. cholerae caused more than 580,000 infections worldwide in 2011 alone (WHO, Wkly. Epidemiol. Rec. 87:289-304, 2012). Cholera is treated with an oral rehydration therapy consisting of water, glucose, and electrolytes. However, as V. cholerae is transmitted via contaminated water, treatment can be difficult for communities whose water source is contaminated. In this study, we address the need for new therapeutic approaches by targeting the production of the main virulence factor, cholera toxin (CT). The high-throughput screen presented here led to the identification of two novel classes of inhibitors of the virulence cascade in V. cholerae, toxtazin A and toxtazins B and B'. We demonstrate that (i) small-molecule inhibitors of virulence gene production can be identified in a high-throughput screen, (ii) targeting virulence gene production is an effective therapeutic strategy, and (iii) small-molecule inhibitors can uncover unknown layers of gene regulation, even in well-studied regulatory cascades.

The human microbiome: from symbiosis to pathogenesis

The human microbiota is a complex assemblage of the microbes inhabiting many sites in the human body. Recent advances in technology have enabled deep sequencing and analysis of the members and structures of these communities. Two sites, the vagina and gastrointestinal tract, are highlighted to exemplify how technological advances have enhanced our knowledge of the host-microbiota system. These examples represent low- and high-complexity communities, respectively. In each example, certain community structures are identified that can be extrapolated to larger collections representing multiple individuals and potential disease or health states. One common feature is the unexpected diversity of the microbiota at any of these locations, which poses a challenge for relating the microbiota to health and disease. However, we anticipate microbiota compositional measurements could become standard clinical practice in the future and may become diagnostic for certain diseases or increased susceptibility to certain disorders. The microbiota of a number of disease states are currently being examined to identify potential correlations. In line with these predictions, it is possible that existing conditions may be resolved by altering the microbiota in a positive way.

Platforms for antibiotic discovery

The spread of resistant bacteria, leading to untreatable infections, is a major public health threat but the pace of antibiotic discovery to combat these pathogens has slowed down. Most antibiotics were originally isolated by screening soil-derived actinomycetes during the golden era of antibiotic discovery in the 1940s to 1960s. However, diminishing returns from this discovery platform led to its collapse, and efforts to create a new platform based on target-focused screening of large libraries of synthetic compounds failed, in part owing to the lack of penetration of such compounds through the bacterial envelope. This article considers strategies to re-establish viable platforms for antibiotic discovery. These include investigating untapped natural product sources such as uncultured bacteria, establishing rules of compound penetration to enable the development of synthetic antibiotics, developing species-specific antibiotics and identifying prodrugs that have the potential to eradicate dormant persisters, which are often responsible for hard-to-treat infections.