The selenophosphate synthetase, selD, is important for Clostridioides difficile physiology

Inhibition of selenoprotein synthesis is not the mechanism by which auranofin inhibits growth of Clostridioides difficile

Clostridioides difficile infections (CDIs) are responsible for a significant number of antibiotic-associated diarrheal cases. The standard-of-care antibiotics for C. difficile are limited to fidaxomicin and vancomycin, with the recently obsolete metronidazole recommended if both are unavailable. No new antimicrobials have been approved for CDI since fidaxomicin in 2011, despite varying rates of treatment failure among all standard-of-care drugs. Drug repurposing is a rational strategy to generate new antimicrobials out of existing therapeutics approved for other indications. Auranofin is a gold-containing anti-rheumatic drug with antimicrobial activity against C. difficile and other microbes. In a previous report, our group hypothesized that inhibition of selenoprotein biosynthesis was auranofin's primary mechanism of action against C. difficile. However, in this study, we discovered that C. difficile mutants lacking selenoproteins are still just as sensitive to auranofin as their respective wild-type strains. Moreover, we found that selenite supplementation dampens the activity of auranofin against C. difficile regardless of the presence of selenoproteins, suggesting that selenite's neutralization of auranofin is not because of compensation for a chemically induced selenium deficiency. Our results clarify the findings of our original study and may aid drug repurposing efforts in discovering the compound's true mechanism of action against C. difficile.

Food for thought-The link between Clostridioides difficile metabolism and pathogenesis

Clostridioides difficile (C. difficile) is an opportunistic pathogen that leads to antibiotic-associated diarrhoea and is a leading cause of morbidity and mortality worldwide. Antibiotic usage is the main risk factor leading to C. difficile infection (CDI), as a dysbiotic gut environment allows colonisation and eventual pathology manifested by toxin production. Although colonisation resistance is mediated by the action of secondary bile acids inhibiting vegetative outgrowth, nutrient competition also plays a role in preventing CDI as the gut microbiota compete for nutrient niches inhibiting C. difficile growth. C. difficile is able to metabolise carbon dioxide, the amino acids proline, hydroxyproline, and ornithine, the cell membrane constituent ethanolamine, and the carbohydrates trehalose, cellobiose, sorbitol, and mucin degradation products as carbon and energy sources through multiple pathways. Zinc sequestration by the host response mediates metabolic adaptation of C. difficile by perhaps signalling an inflamed gut allowing it to acquire abundant nutrients. Persistence within the gut environment is also mediated by the by-products of metabolism through the production of p-cresol, which inhibit gut commensal species growth promoting dysbiosis. This review aims to explore and describe the various metabolic pathways of C. difficile, which facilitate its survival and pathogenesis within the colonised host gut.

d-Proline Reductase Underlies Proline-Dependent Growth of Clostridioides difficile

Clostridioides difficile is a nosocomial pathogen that colonizes the gut and causes diarrhea, colitis, and severe inflammation. Recently, C. difficile has been shown to use toxin-mediated inflammation to promote host collagen degradation, which releases several amino acids into the environment. Amino acids act as electron donors and acceptors in Stickland metabolism, an anaerobic process involving redox reactions between pairs of amino acids. Proline, glycine, and hydroxyproline are the three main constituents of collagen and are assumed to act as electron acceptors, but their exact effects on the growth and physiology of C. difficile are still unclear. Using three standard culture media (supplemented brain heart infusion [BHIS], tryptone-yeast [TY], and C. difficile minimal medium [CDMM]) supplemented with proline, glycine, or hydroxyproline, we grew C. difficile strains R20291, JIR8094, and a panel of mutants unable to express the Stickland selenoenzymes d-proline reductase and glycine reductase. In the wild-type strains, growth yields in rich media (BHIS and TY) were higher with proline and hydroxyproline but not glycine; moreover, proline-stimulated growth yields required the activity of d-proline reductase, whereas hydroxyproline-stimulated growth yields were independent of its activity. While assumed to be a proline auxotroph, C. difficile could surprisingly grow in a defined medium (CDMM) without proline but only if d-proline reductase was absent. We believe the mere presence of this enzyme ultimately determines the organism's strict dependence on proline and likely defines the bioenergetic priorities for thriving in the host. Finally, we demonstrated that addition of proline and hydroxyproline to the culture medium could reduce toxin production but not in cells lacking selenoproteins. IMPORTANCE Stickland metabolism is a core facet of C. difficile physiology that likely plays a major role in host colonization. Here, we carefully delineate the effects of each amino acid on the growth of C. difficile with respect to the selenoenzymes d-proline reductase and glycine reductase. Moreover, we report that d-proline reductase forces C. difficile to strictly depend on proline for growth. Finally, we provide evidence that proline and hydroxyproline suppress toxin production and that selenoproteins are involved in this mechanism. Our findings highlight the significance of selenium-dependent Stickland reactions and may provide insight on what occurs during host infection, especially as it relates to the decision to colonize based on proline as a nutrient.

Gut associated metabolites and their roles in Clostridioides difficile pathogenesis

The nosocomial pathogen Clostridioides difficile is a burden to the healthcare system. Gut microbiome disruption, most commonly by broad-spectrum antibiotic treatment, is well established to generate a state that is susceptible to CDI. A variety of metabolites produced by the host and/or gut microbiota have been shown to interact with C. difficile. Certain bile acids promote/inhibit germination while other cholesterol-derived compounds and amino acids used in the Stickland metabolic pathway affect growth and CDI colonization. Short chain fatty acids maintain intestinal barrier integrity and a myriad of other metabolic compounds are used as nutritional sources or used by C. difficile to inhibit or outcompete other bacteria in the gut. As the move toward non-antibiotic CDI treatment takes place, a deeper understanding of interactions between C. difficile and the host's gut microbiome and metabolites becomes more relevant.

Host and Clostridioides difficile-Response Modulated by Micronutrients and Glutamine: An Overview

Changes in intestinal microbiota are integral to development of Clostridioides difficile (C. difficile)—associated nosocomial diarrhea. Certain diets, especially Western diets, increase susceptibility to C. difficile infection (CDI). Here, we discuss recent findings regarding how nutrients modulate response of the host and C. difficile during infection. Calcium has a role in the sporulation and germination process. Selenium is effective in reducing the total amount of C. difficile toxin A (TcdA) and toxin B (TcdB) and in decreasing its cytotoxicity. In addition, selenium phosphate synthetase deficiency reduces C. difficile growth and spore production. On the other hand, iron has a dual role in C. difficile growth. For instance, high intracellular levels can generate reactive hydroxyl radicals, whereas low levels can reduce its growth. In humans, zinc deficiency appears to be related to the recurrence of CDI, in contrast, in the CDI model in mice a diet rich in zinc increased the toxin's activity. Low vitamin D levels contribute to C. difficile colonization, toxin production, and inflammation. Furthermore, glutamine appears to protect intestinal epithelial cells from the deleterious effects of TcdA and TcdB. In conclusion, nutrients play an important role in modulating host and pathogen response. However, further studies are needed to better understand the mechanisms and address some controversies.