Identification of novel host-targeted compounds that protect from anthrax lethal toxin-induced cell death

Tauroursodeoxycholic Acid Inhibits Clostridioides difficile Toxin-Induced Apoptosis

C. difficile infection (CDI) is a highly inflammatory disease mediated by the production of two large toxins that weaken the intestinal epithelium and cause extensive colonic tissue damage. Antibiotic alternative therapies for CDI are urgently needed as current antibiotic regimens prolong the perturbation of the microbiota and lead to high disease recurrence rates. Inflammation is more closely correlated with CDI severity than bacterial burden, thus therapies that target the host response represent a promising yet unexplored strategy for treating CDI. Intestinal bile acids are key regulators of gut physiology that exert cytoprotective roles in cellular stress, inflammation, and barrier integrity, yet the dynamics between bile acids and host cellular processes during CDI have not been investigated. Here we show that several bile acids are protective against apoptosis caused by C. difficile toxins in Caco-2 cells and that protection is dependent on conjugation of bile acids. Out of 20 tested bile acids, taurine conjugated ursodeoxycholic acid (TUDCA) was the most potent inhibitor, yet unconjugated UDCA did not alter toxin-induced apoptosis. TUDCA treatment decreased expression of genes in lysosome associated and cytokine signaling pathways. TUDCA did not affect C. difficile growth or toxin activity in vitro whereas UDCA significantly reduced toxin activity in a Vero cell cytotoxicity assay and decreased tcdA gene expression. These results demonstrate that bile acid conjugation can have profound effects on C. difficile as well as the host and that conjugated and unconjugated bile acids may exert different therapeutic mechanisms against CDI.

Clostridioides difficile toxins: mechanisms of action and antitoxin therapeutics

Clostridioides difficile is a Gram-positive anaerobe that can cause a spectrum of disorders that range in severity from mild diarrhoea to fulminant colitis and/or death. The bacterium produces up to three toxins, which are considered the major virulence factors in C. difficile infection. These toxins promote inflammation, tissue damage and diarrhoea. In this Review, we highlight recent biochemical and structural advances in our understanding of the mechanisms that govern host-toxin interactions. Understanding how C. difficile toxins affect the host forms a foundation for developing novel strategies for treatment and prevention of C. difficile infection.

Antibiotic resistance modifying ability of phytoextracts in anthrax biological agent Bacillus anthracis and emerging superbugs: a review of synergistic mechanisms

Background and objectives

The chemotherapeutic management of infections has become challenging due to the global emergence of antibiotic resistant pathogenic bacteria. The recent expansion of studies on plant-derived natural products has lead to the discovery of a plethora of phytochemicals with the potential to combat bacterial drug resistance via various mechanisms of action. This review paper summarizes the primary antibiotic resistance mechanisms of bacteria and also discusses the antibiotic-potentiating ability of phytoextracts and various classes of isolated phytochemicals in reversing antibiotic resistance in anthrax agent Bacillus anthracis and emerging superbug bacteria.

Methods

Growth inhibitory indices and fractional inhibitory concentration index were applied to evaluate the in vitro synergistic activity of phytoextract-antibiotic combinations in general.

Findings

A number of studies have indicated that plant-derived natural compounds are capable of significantly reducing the minimum inhibitory concentration of standard antibiotics by altering drug-resistance mechanisms of B. anthracis and other superbug infection causing bacteria. Phytochemical compounds allicin, oleanolic acid, epigallocatechin gallate and curcumin and Jatropha curcas extracts were exceptional synergistic potentiators of various standard antibiotics.

Conclusion

Considering these facts, phytochemicals represents a valuable and novel source of bioactive compounds with potent antibiotic synergism to modulate bacterial drug-resistance.

Anti-Virulence Strategy against the Honey Bee Pathogenic Bacterium Paenibacillus larvae via Small Molecule Inhibitors of the Bacterial Toxin Plx2A

American Foulbrood, caused by Paenibacillus larvae, is the most devastating bacterial honey bee brood disease. Finding a treatment against American Foulbrood would be a huge breakthrough in the battle against the disease. Recently, small molecule inhibitors against virulence factors have been suggested as candidates for the development of anti-virulence strategies against bacterial infections. We therefore screened an in-house library of synthetic small molecules and a library of flavonoid natural products, identifying the synthetic compound M3 and two natural, plant-derived small molecules, Acacetin and Baicalein, as putative inhibitors of the recently identified P. larvae toxin Plx2A. All three inhibitors were potent in in vitro enzyme activity assays and two compounds were shown to protect insect cells against Plx2A intoxication. However, when tested in exposure bioassays with honey bee larvae, no effect on mortality could be observed for the synthetic or the plant-derived inhibitors, thus suggesting that the pathogenesis strategies of P. larvae are likely to be too complex to be disarmed in an anti-virulence strategy aimed at a single virulence factor. Our study also underscores the importance of not only testing substances in in vitro or cell culture assays, but also testing the compounds in P. larvae-infected honey bee larvae.

Medicinal plant extracts protect epithelial cells from infection and DNA damage caused by colibactin-producing Escherichia coli, and inhibit the growth of bacteria

AIMS

To investigate the biological activity of Thai medicinal plant extracts and their active substances on the inhibition of growth and the transcription of colibactin genes of colibactin-producing Escherichia coli, and effect on the pathogenesis from colibactin toxin including transient infections and colibactin-induced DNA damage.

METHODS AND RESULTS

Among 16 medicinal plants examined, aqueous extracts of Terminalia catappa, Psidium guajava and Sandoricum koetjape demonstrated the growth inhibition against E. coli ATCC 25922, which is known to produce colibactin toxin. These plant extracts contain the active phytochemical compounds, tannin and quercetin, which are able to inhibit the growth of E. coli ATCC 25922. Interestingly, the extracts of T. catappa, P. guajava and S. koetjape, and their compounds tannin and quercetin, protected the eukaryotic epithelial cells of Vero cells and Caco-2 cells from infection and DNA damage by E. coli ATCC 25922. Moreover, these plant extracts and compounds exhibited efficacy to downregulate the expression of five genes (clbA, clbB, clbM, clbN and clbP) that are required for colibactin biosynthesis.

CONCLUSIONS

The extracts of T. catappa, P. guajava and S. koetjape, and their compounds of tannin and quercetin had ability to inhibit the growth and transcription of colibactin genes of colibactin-producing Escherichia coli. Hence, these plant extracts and compounds could protect the transient infection and DNA damage of the eukaryotic epithelial cells.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first of its kind to report on the enhancement of the biological properties of T. catappa, P. guajava and S. koetjape, and to support the exogenous compound usage of tannin and quercetin, which may be able to protect against the transient infection and DNA damage of eukaryotic cells from E. coli carrying colibactin toxin.

Gold-catalyzed [4+1]-annulation reactions between anthranils and 4-methoxy-1,2-dienyl-5-ynes involving a 1,2-allene shift

Gold catalyzed [4+1]-annulations of 4-methoxy-1,2-dien-5-ynes with anthranils are described; the reaction mechanism involves initial formation of α-imino gold carbenes, followed by a subsequent 1,2-allene shift.

Host-targeted niclosamide inhibits C. difficile virulence and prevents disease in mice without disrupting the gut microbiota

Clostridium difficile is the leading cause of nosocomial diarrhea and colitis in the industrialized world. Disruption of the protective gut microbiota by antibiotics enables colonization by multidrug-resistant C. difficile, which secrete up to three different protein toxins that are responsible for the gastrointestinal sequelae. Oral agents that inhibit the damage induced by toxins, without altering the gut microbiota, are urgently needed to prevent primary disease and break the cycle of antibiotic-induced disease recurrence. Here, we show that the anthelmintic drug, niclosamide, inhibits the pathogenesis of all three toxins by targeting a host process required for entry into colonocytes by each toxin. In mice infected with an epidemic strain of C. difficile, expressing all three toxins, niclosamide reduced both primary disease and recurrence, without disrupting the diversity or composition of the gut microbiota. Given its excellent safety profile, niclosamide may address an important unmet need in preventing C. difficile primary and recurrent diseases.

Clostridium difficile causes diarrhea and colitis by producing up to three different protein toxins. Here, Tam et al. show that an anthelmintic drug, niclosamide, inhibits the pathogenesis of all three toxins by targeting a host process required for toxin entry into host cells, without disrupting the gut microbiota.

A Copper-Catalyzed Tandem Cyclization Reaction of Aminoalkynes with Alkynes for the Construction of Tetrahydropyrrolo[1,2-a]quinolines Scaffold

A synthetic method for diversely substituted tetrahydropyrrolo[1,2-a]quinolines was developed via CuCl-catalyzed cascade transformation of internal aminoalkynes with alkynes under microwave- irradiation.

Clostridium difficile infection

Infection of the colon with the Gram-positive bacterium Clostridium difficile is potentially life threatening, especially in elderly people and in patients who have dysbiosis of the gut microbiota following antimicrobial drug exposure. C. difficile is the leading cause of health-care-associated infective diarrhoea. The life cycle of C. difficile is influenced by antimicrobial agents, the host immune system, and the host microbiota and its associated metabolites. The primary mediators of inflammation in C. difficile infection (CDI) are large clostridial toxins, toxin A (TcdA) and toxin B (TcdB), and, in some bacterial strains, the binary toxin CDT. The toxins trigger a complex cascade of host cellular responses to cause diarrhoea, inflammation and tissue necrosis - the major symptoms of CDI. The factors responsible for the epidemic of some C. difficile strains are poorly understood. Recurrent infections are common and can be debilitating. Toxin detection for diagnosis is important for accurate epidemiological study, and for optimal management and prevention strategies. Infections are commonly treated with specific antimicrobial agents, but faecal microbiota transplants have shown promise for recurrent infections. Future biotherapies for C. difficile infections are likely to involve defined combinations of key gut microbiota.

Small Molecules Take A Big Step Against Clostridium difficile

Effective treatment of Clostridium difficile infections demands a shift away from antibiotics towards toxin-neutralizing agents. Work by Bender et al., using a drug that attenuates toxin action in vivo without affecting bacterial survival, demonstrates the exciting potential of small molecules as a new modality in the fight against C. difficile.

Inhibiting Microbial Toxins Using Plant-Derived Compounds and Plant Extracts

Many pathogenic bacteria and fungi produce potentially lethal toxins that cause cytotoxicity or impaired cellular function either at the site of colonization or other locations in the body through receptor-mediated interactions. Various factors, including biotic and abiotic environments, competing microbes, and chemical cues affect toxin expression in these pathogens. Recent work suggests that several natural compounds can modulate toxin production in pathogenic microbes. However, studies explaining the mechanistic basis for their effect are scanty. This review discusses the potential of various plant-derived compounds for reducing toxin production in foodborne and other microbes. In addition, studies highlighting their anti-toxigenic mechanism(s) are discussed.

Unconventional screening approaches for antibiotic discovery

The dramatic rise in microbial drug resistance in recent years has led to ongoing searches for novel drugs to add to the armory against infectious disease. Nevertheless, a paucity of new antibacterial drugs in discovery and development pipelines using traditional approaches has prompted a variety of unconventional and disruptive strategies for antibacterial drug discovery. Herein, we review recent nontraditional approaches that have been piloted for early drug discovery efforts. These unique methodologies open new avenues for finding the next generation of antimicrobials.

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.

Chemical genetics reveals a kinase-independent role for protein kinase R in pyroptosis

Formation of the inflammasome, a scaffolding complex that activates caspase-1, is important in numerous diseases. Pyroptotic cell death induced by anthrax lethal toxin (LT) is a model for inflammasome-mediated caspase-1 activation. We discovered 7-desacetoxy-6,7-dehydrogedunin (7DG) in a phenotypic screen as a small molecule that protects macrophages from LT-induced death. Using chemical proteomics, we identified protein kinase R (PKR) as the target of 7DG and show that RNAi knockdown of PKR phenocopies treatment with 7DG. Further, we show that PKR's role in ASC assembly and caspase-1 activation induced by several different inflammasome stimuli is independent of PKR's kinase activity, demonstrating that PKR has a previously uncharacterized role in caspase-1 activation and pyroptosis that is distinct from its reported kinase-dependent roles in apoptosis and inflammasome formation in lipopolysaccharide-primed cells. Remarkably, PKR has different roles in two distinct cell death pathways and has a broad role in inflammasome function relevant in other diseases.