Nutrient-Dependent Efficacy of the Antifungal Protein YvgO Correlates to Cellular Proliferation Rate in Candida albicans 3153A and Byssochlamys fulva H25

Baicalein Acts against Candida albicans by Targeting Eno1 and Inhibiting Glycolysis

Baicalein (BE) is a promising antifungal small-molecule compound with an extended antifungal spectrum, good synergy with fluconazole, and low toxicity, but its target protein and antifungal mechanism remain elusive. In this study, we found that BE can function against Candida albicans by disrupting glycolysis through targeting Eno1 and inhibiting its function. Eno1 acts as a key therapeutic target of the drug, as BE had no antifungal activity against the eno1 null mutant in a Galleria mellonella model of C. albicans infection. To investigate the mechanism of action, we solved the crystal structure of C. albicans Eno1(CaEno1) and then compared the difference between this structure and that of Eno1 from humans. The predicted primary binding site of BE on CaEno1 is between amino acids D261 and W274, with D263, S269, and K273 playing critical roles in the interaction with BE. Both positions S269 and K273 have different residues in the human Eno1 (hEno1). This finding suggests that BE may bind selectively to CaEno1, which would limit the potential for side effects in humans. Our findings demonstrate that Eno1 is a target protein of BE and thus may serve as a novel target for the development of antifungal therapeutics acting through the inhibition of glycolysis.

IMPORTANCE Baicalein (BE) is a promising antifungal agent which has been well characterized, but its target protein is still undiscovered. The protein Eno1 plays a crucial role in the survival of Candida albicans. However, there are few antifungal agents which inhibit the functions of Eno1. Here, we found that BE can function against Candida albicans by disrupting glycolysis through targeting Eno1 and inhibiting its function. We further solved the crystal structure of C. albicans Eno1(CaEno1) and predicted that the primary binding site of BE on CaEno1 is between amino acids D261 and W274, with D263, S269, and K273 playing critical roles in the interaction with BE. Our findings will be helpful to get specific small-molecule inhibitors of CaEno1 and open the way for the development of new antifungal therapeutics targeted at inhibiting glycolysis.

Sugar Sensing and Signaling in Candida albicans and Candida glabrata

Candida species, such as Candida albicans and Candida glabrata, cause infections at different host sites because they adapt their metabolism depending on the available nutrients. They are able to proliferate under both nutrient-rich and nutrient-poor conditions. This adaptation is what makes these fungi successful pathogens. For both species, sugars are very important nutrients and as the sugar level differs depending on the host niche, different sugar sensing systems must be present. Saccharomyces cerevisiae has been used as a model for the identification of these sugar sensing systems. One of the main carbon sources for yeast is glucose, for which three different pathways have been described. First, two transporter-like proteins, ScSnf3 and ScRgt2, sense glucose levels resulting in the induction of different hexose transporter genes. This situation is comparable in C. albicans and C. glabrata, where sensing of glucose by CaHgt4 and CgSnf3, respectively, also results in hexose transporter gene induction. The second glucose sensing mechanism in S. cerevisiae is via the G-protein coupled receptor ScGpr1, which causes the activation of the cAMP/PKA pathway, resulting in rapid adaptation to the presence of glucose. The main components of this glucose sensing system are also conserved in C. albicans and C. glabrata. However, it seems that the ligand(s) for CaGpr1 are not sugars but lactate and methionine. In C. glabrata, this pathway has not yet been investigated. Finally, the glucose repression pathway ensures repression of respiration and repression of the use of alternative carbon sources. This pathway is not well characterized in Candida species. It is important to note that, apart from glucose, other sugars and sugar-analogs, such as N-acetylglucosamine in the case of C. albicans, are also important carbon sources. In these fungal pathogens, sensing sugars is important for a number of virulence attributes, including adhesion, oxidative stress resistance, biofilm formation, morphogenesis, invasion, and antifungal drug tolerance. In this review, the sugar sensing and signaling mechanisms in these Candida species are compared to S. cerevisiae.

Variable Efficacy of the Proteinaceous Antifungal YvgO in Select Fruit Juices and Teas as a Complement with UV Methods of Food Protection

Heat-resistant fungal spores present a processing challenge for beverages and fruit juices, as thermal and UV strategies are often inadequate in reducing heat-resistant fungal burdens to acceptable levels. While effective against pathogenic or invasive bacteria, germicidal UV light treatments also fail to achieve an appreciable reduction of heat-resistant fungal spores. As an alternative, the efficacy of the antifungal protein YvgO was examined across a selection of fruit juices and teas, as well as solid model matrices. Compared with its efficacy in analogous liquid matrices, the apparent efficacy of YvgO was diminished on acidified solid matrices due to a reduction in YvgO diffusion. Using an XTT [2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide] tetrazolium dye cytotoxicity assay, the effective concentrations to reduce growth by 50% were elucidated in samples challenged with Byssochlamys fulva H25. The MICs were determined and ranged from 2 ppm in apple juice and acidified teas to approximately 3 to 12 ppm for lemonade and orange, white cranberry, blueberry, prune, cherry, and grape juices. Apple cider and nonacidified teas showed reduced efficacy, with MICs exceeding 100 ppm. Tannin-rich products readily removed YvgO from the product, impairing its efficacy. Adding bovine serum albumin as a competitive inhibitor effectively reversed the YvgO-tannin association and restored efficacy in black but not green tea matrices. When challenged with a 5-log CFU inoculum of B. fulva, the shelf lives of the products were extended for various times up to 28 days in a concentrationdependent manner. However, initial efficacy was not predictive of shelf life extension, as some products exhibited improved protection at just two- and fourfold concentrations above the MIC, while others only exhibited long-term stability when concentrations exceeded 20 times the MIC. As such, YvgO may be an attractive alternative to currently available protection strategies and will provide needed diversity for natural food protectants.