Small-molecule inhibitors of the budded-to-hyphal-form transition in the pathogenic yeast Candida albicans

In vitro co-culture model of Trichomonas vaginalis, Candida albicans, and Lactobacillus crispatus: a system for assessing antimicrobial activity and microorganism interactions in vaginitis

Trichomonas vaginalis is a flagellated protozoan causing trichomoniasis, the most common non-viral sexually transmitted infection. It is associated with various complications, particularly in asymptomatic carriers. Another major cause of vaginitis is Candida albicans, a normal member of the vaginal microbiota, which causes vulvovaginal candidiasis when immune imbalances occur, leading to recurrent infections. Treatment-resistant strains of these pathogens pose a significant challenge. Lactobacillus crispatus, a dominant species in the vaginal microbiota, produces antimicrobial compounds that help protect the vaginal mucosa. This study establishes an in vitro co-culture of T. vaginalis, C. albicans, and L. crispatus to simulate the vaginal microenvironment at the site of infection. MRS medium was chosen for the co-culture, with initial cell densities determined as follows: T. vaginalis at 1.0 × 106 trophozoites/mL (counted using a hemocytometer), 3.33 × 104 CFU/mL for C. albicans, and either 5.53 × 106 CFU/mL (for co-culture with the ATCC isolate) or 5.53 × 107 CFU/mL (for co-culture with a fresh clinical isolate) for L. crispatus. The cell densities of C. albicans and L. crispatus were quantified as colony-forming units (CFU) on selective agar. The incubation period for co-culture, ensuring optimal growth of all microorganisms, was 24 hours. In co-culture, L. crispatus at both tested densities acidified the medium. The co-culture system demonstrated lower MIC values for metronidazole (50 µM in the ATCC isolate co-culture and 25 µM with the fresh clinical isolate) and lower MFC values for fluconazole (6.25 µM), compared to monocultures of T. vaginalis (100 µM) and C. albicans (12.50 µM). Furthermore, the triple co-culture increased the cytotoxicity to vaginal cell and erythrocytes for the ATCC isolate while significantly inhibited both biofilm formation and metabolic activity of C. albicans (by up to 92% and 90%, respectively), as well as its yeast-to-hyphae transition (by up to 70%). SEM analyses highlighted the morphological differences among T. vaginalis, C. albicans, and L. crispatus, including isolate-specific size variations in the protozoan. These findings suggest that this in vitro co-culture system is a valuable tool for evaluating the antimicrobial efficacy of novel compounds against vaginitis pathogens and for studying interactions within the vaginal microenvironment.

Structure-Based Optimization of Novel Sterol 24-C-Methyltransferase Inhibitors for the Treatment of Candida albicans Infections

Interfering with sterol biosynthesis is an important strategy for developing safe and effective antifungal drugs. We previously identified compound H55 as an allosteric inhibitor of the fungal-specific C-24 sterol methyltransferase Erg6 for treating Candida albicans infections. Herein, 62 derivatives of H55 were designed and synthesized based on target-ligand interactions to identify more active candidates. Among them, d28 displayed the most potent antivirulence ability (MHIC50 = 0.25 μg/mL) by targeting Erg6, exhibiting an 8-fold increase in potency compared with H55. Moreover, d28 significantly outperformed H55 in inhibiting cell adhesion and biofilm formation, and exhibited minimal cytotoxicity and negligible potential to induce drug resistance. Of note, the coadministration of d28 and other sterol biosynthesis inhibitors, such as tridemorph or terbinafine, demonstrated a strong synergistic antifungal action in vitro and in vivo in a murine skin infection model. These results support the potential application of d28 in the treatment of C. albicans infections.

Inhibition of Candida albicans Biofilm Formation and Attenuation of Its Virulence by Liriope muscari

(1) Background: Although Candida albicans accounts for the majority of fungal infections, therapeutic options are limited and require alternative antifungal agents with new targets; (2) Methods: A biofilm formation assay with RPMI1640 medium was performed with Liriope muscari extract. A combination antifungal assay, dimorphic transition assay, and adhesion assay were performed under the biofilm formation condition to determine the anti-biofilm formation effect. qRT-PCR analysis was accomplished to confirm changes in gene expression; (3) Results: L. muscari extract significantly reduces biofilm formation by 51.65% at 1.56 μg/mL use and therefore increases susceptibility to miconazole. L. muscari extract also inhibited the dimorphic transition of Candida; nearly 50% of the transition was inhibited when 1.56 μg/mL of the extract was treated. The extract of L. muscari inhibited the expression of genes related to hyphal development and extracellular matrix of 34.4% and 36.0%, respectively, as well as genes within the Ras1-cAMP-PKA, Cph2-Tec1, and MAP kinase signaling pathways of 25.58%, 7.1% and 15.8%, respectively, at 1.56 μg/mL of L. muscari extract treatment; (4) Conclusions: L. muscari extract significantly reduced Candida biofilm formation, which lead to induced antifungal susceptibility to miconazole. It suggests that L. muscari extract is a promising anti-biofilm candidate of Candida albicans since the biofilm formation of Candida albicans is an excellent target for candidiasis regulation.

Quinalizarin as a potential antifungal drug for the treatment of Candida albicans fungal infection in cancer patients

This study aims to analyze the antifungal properties of quinalizarin, a plant-derived compound with proven anticancer effects. Quinalizarin exhibited antifungal activity against opportunistic pathogenic Candida species and Geotrichum capitatum. The treatment with this anthraquinone reduced hyphal growth, inhibited biofilm formation, and damaged mature Candida albicans biofilms. Real-time RT-PCR revealed that quinalizarin downregulated the expression of hyphae-related and biofilm-specific genes. The flow cytometry method used in the study showed that both apoptosis and necrosis were the physiological mechanisms of quinalizarin-induced C. albicans cell death, depending on the dose of the antifungal agent. A further study revealed an increase in the levels of intracellular reactive oxygen species and alterations in mitochondrial membrane potential after treatment with quinalizarin. Finally, quinalizarin was found to have low toxicity in a hemolytic test using human erythrocytes. In conclusion, we have identified quinalizarin as a potential antifungal compound.IMPORTANCEThis article is a study to determine the antifungal activity of quinalizarin (1,2,5,8-tetrahydroxyanthraquinone). Quinalizarin has potential antitumor properties and is effective in different types of tumor cells. The aim of the present study was to prove that quinalizarin can be used simultaneously in the treatment of cancer and in the treatment of intercurrent fungal infections. Quinalizarin was identified as a novel antifungal compound with low toxicity. These results may contribute to the development of a new drug with dual activity in the treatment of cancer-associated candidiasis.

Metal-free thiolation of sulfonyl hydrazone with thiophenol: synthesis of 4-thio-chroman and diarylmethyl thioethers

A simple, efficient, and transition metal-free approach was developed for accessing 4-thio-substituted chroman and diarylmethyl thioethers from sulfonyl hydrazones. This protocol provides straightforward access to a class of diarylmethane derivatives with good to excellent yields. This operationally simple protocol exhibited good tolerance for labile functional groups, providing biologically relevant chemical libraries. This safe and feasible route is applicable to the large-scale synthesis of 4-thio-substituted chromans, which are of great synthetic interest because of their further reaction potential.

Synthesis of diarylmethyl thioethers via a DABCO-catalyzed 1,6-conjugate addition reaction of p-quinone methides with organosulfur reagents

A rapid and simple method was developed for the synthesis of diarylmethyl thioethers via a DABCO-catalyzed 1,6-conjugate addition reaction of para-quinone methides (p-QMs) with organosulfur reagents. A series of diarylmethyl thioethers were synthesized at 13-85% yields by this method. After that, the antibacterial activities of synthesized diarylmethyl thioethers and their derivatives were evaluated. The MIC range (μg mL^-1) against Staphylococcus aureus ATCC 25923 and clinically isolated methicillin-resistant S. aureus was 8-128 and 64-128, respectively.

Targeting Virulence Factors of Candida albicans with Natural Products

Natural products derived from natural resources, including nutritional functional food, play an important role in human health. In recent years, the study of anti-fungal and other properties of agri-foods and derived functional compounds has been a hot research topic. Candida albicans is a parasitic fungus that thrives on human mucosal surfaces, which are colonized through opportunistic infection. It is the most prevalent cause of invasive fungal infection in immunocompromised individuals, resulting in a wide variety of clinical symptoms. Moreover, the efficacy of classical therapeutic medications such as fluconazole is often limited by the development of resistance. There is an ongoing need for the development of novel and effective antifungal therapy and medications. Infection of C. albicans is influenced by a great quantity of virulence factors, like adhesion, invasion-promoting enzymes, mycelial growth, and phenotypic change, and among others. Furthermore, various natural products especially from food sources that target C. albicans virulence factors have been researched, providing promising prospects for C. albicans prevention and treatment. In this review, we discuss the virulence factors of C. albicans and how functional foods and derived functional compounds affect them. Our hope is that this review will stimulate additional thoughts and suggestions regarding nutritional functional food and therapeutic development for patients afflicted with C. albicans.

Synergistic Antibiofilm Effects of Pseudolaric Acid A Combined with Fluconazole against Candida albicans via Inhibition of Adhesion and Yeast-To-Hypha Transition

Candida albicans biofilms are resistant to several clinical antifungal agents. Thus, it is necessary to develop new antibiofilm intervention measures. Pseudolaric acid A (PAA), a diterpenoid mainly derived from the pine bark of Pseudolarix kaempferi, has been reported to have an inhibitory effect on C. albicans. The primary aim of the current study was to investigate the antibiofilm effect of PAA when combined with fluconazole (FLC) and explore the underlying mechanisms. Biofilm activity was assessed by tetrazolium {XTT [2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide salt]} reduction assays. PAA (4 μg/mL) combined with FLC (0.5 μg/mL) significantly inhibited early, developmental, and mature biofilm formation compared with the effect of PAA or FLC alone (P < 0.05). Furthermore, PAA (4 μg/mL) combined with FLC (0.5 μg/mL) produced a 56% reduction in C. albicans biofilm adhesion. The combination of PAA (4 μg/mL) and FLC (0.5 μg/mL) also performed well in inhibiting yeast-to-hypha transition. Transcriptome analysis using RNA sequencing and quantitative reverse transcription PCR indicated that the PAA-FLC combination treatment produced a strong synergistic inhibitory effect on the expression of genes involved in adhesion (ALS1, ALS4, and ALS2) and yeast-to-hypha transition (ECE1, PRA1, and TEC1). Notably, PAA, rather than FLC, may have a primary role in suppressing the expression of ALS1. In conclusion, these findings demonstrate, for the first time, that the combination of PAA and FLC has an improved antibiofilm effect against the formation of C. albicans biofilms by inhibiting adhesion and yeast-to-hypha transition; this may provide a novel therapeutic strategy for treating C. albicans biofilm-associated infection.

IMPORTANCE Biofilms are the primary cause of antibiotic-resistant candida infections associated with medical implants and devices worldwide. Treating biofilm-associated infections is a challenge for clinicians because these infections are intractable and persistent. Candida albicans readily forms extensive biofilms on the surface of medical implants and mucosa. In this study, we demonstrated, for the first time, an inhibitory effect of pseudolaric acid A alone and in combination with fluconazole on C. albicans biofilms. Moreover, pseudolaric acid A in combination with fluconazole exerted an antibiofilm effect through multiple pathways, including inhibition of yeast-to-hypha transition and adhesion. This research not only provides new insights into the synergistic mechanisms of antifungal drug combinations but also brings new possibilities for addressing C. albicans drug resistance.

Repurposing the FDA-approved anticancer agent ponatinib as a fluconazole potentiator by suppression of multidrug efflux and Pma1 expression in a broad spectrum of yeast species

Fungal infections have emerged as a major global threat to human health because of the increasing incidence and mortality rates every year. The emergence of drug resistance and limited arsenal of antifungal agents further aggravates the current situation resulting in a growing challenge in medical mycology. Here, we identified that ponatinib, an FDA-approved antitumour drug, significantly enhanced the activity of the azole fluconazole, the most widely used antifungal drug. Further detailed investigation of ponatinib revealed that its combination with fluconazole displayed broad-spectrum synergistic interactions against a variety of human fungal pathogens such as Candida albicans, Saccharomyces cerevisiae and Cryptococcus neoformans. Mechanistic insights into the mode of action unravelled that ponatinib reduced the efflux of fluconazole via Pdr5 and suppressed the expression of the proton pump, Pma1. Taken together, our study identifies ponatinib as a novel antifungal that enhances drug activity of fluconazole against diverse fungal pathogens.

Could Candida Overgrowth Be Involved in the Pathophysiology of Autism?

The purpose of this review is to summarize the current acquiredknowledge of Candida overgrowth in the intestine as a possible etiology of autism spectrum disorder (ASD). The influence of Candida sp. on the immune system, brain, and behavior of children with ASD isdescribed. The benefits of interventions such as a carbohydrates-exclusion diet, probiotic supplementation, antifungal agents, fecal microbiota transplantation (FMT), and microbiota transfer therapy (MTT) will be also discussed. Our literature query showed that the results of most studies do not fully support the hypothesis that Candida overgrowth is correlated with gastrointestinal (GI) problems and contributes to autism behavioral symptoms occurrence. On the one hand, it was reported that the modulation of microbiota composition in the gut may decrease Candida overgrowth, help reduce GI problems and autism symptoms. On the other hand, studies on humans suggesting the beneficial effects of a sugar-free diet, probiotic supplementation, FMT and MTT treatment in ASD are limited and inconclusive. Due to the increasing prevalence of ASD, studies on the etiology of this disorder are extremely needed and valuable. However, to elucidate the possible involvement of Candida in the pathophysiology of ASD, more reliable and well-designed research is certainly required.

Hedera rhombea inhibits the biofilm formation of Candida, thereby increases the susceptibility to antifungal agent, and reduces infection

Candida is an opportunistic pathogen and a common cause of fungal infections worldwide. Anti-fungal use against Candida infections has resulted in the appearance of resistant strains. The limited choice of anti-fungal therapy means alternative strategies are needed to control fungal infectious diseases. The aim of this study was to evaluate the inhibition of Candida biofilm formation by Hedera rhombea (Korean name: songak) extract. Biofilm formation was assessed using the crystal violet assay which showed a dose dependent reduction in the presence of extract with the biofilm formation inhibitory concentration of C. albicans (IC50 = 12.5μg/ml), C. tropicalis var. tropicalis (IC50 = 25μg/ml), C. parapsilosis var. parapsilosis (IC50 = 6.25μg/ml), C. glabrata (IC50 = 6.25μg/ml), C. tropicalis (IC50 = 12.5μg/ml), and C. parapsilosis (IC50 = 12.5μg/ml) without directly reducing Candida growth. Treatment with 6.25μg/mL of extract increased the antifungal susceptibility to miconazole from 32% decreasing of fungal growth to 98.8% of that based on the fungal growth assay. Treatment of extract dose-dependently reduced the dimorphic transition of Candida based on the dimorphic transition assay and treatment of 3.125μg/mL of extract completely blocked the adherence of Candida to the HaCaT cells. To know the molecular mechanisms of biofilm formation inhibition by extract, qRT-PCR analysis was done, and the extract was found to dose dependently reduce the expression of hyphal-associated genes (ALS3, ECE1, HWP1, PGA50, and PBR1), extracellular matrix genes (GSC1, ZAP1, ADH5, and CSH1), Ras1-cAMP-PKA pathway genes (CYR1, EFG1, and RAS1), Cph2-Tec1 pathway gene (TEC1) and MAP kinases pathway gene (HST7). In this study, Hedera rhombea extract showed inhibition of fungal biofilm formation, activation of antifungal susceptibility, and reduction of infection. These results suggest that fungal biofilm formation is good screen for developing the antifungal adjuvant and Hedera rhombea extract should be a good candidate against biofilm-related fungal infection.

Mechanistic Understanding of Candida albicans Biofilm Formation and Approaches for Its Inhibition

In recent years, the demand for novel antifungal therapies has increased several- folds due to its potential to treat severe biofilm-associated infections. Biofilms are made by the sessile microorganisms attached to the abiotic or biotic surfaces, enclosed in a matrix of exopolymeric substances. This results in new phenotypic characteristics and intrinsic resistance from both host immune response and antimicrobial drugs. Candida albicans biofilm is a complex association of hyphal cells that are associated with both abiotic and animal tissues. It is an invasive fungal infection and acts as an important virulent factor. The challenges linked with biofilm-associated diseases have urged scientists to uncover the factors responsible for the formation and maturation of biofilm. Several strategies have been developed that could be adopted to eradicate biofilm-associated infections. This article presents an overview of the role of C. albicans biofilm in its pathogenicity, challenges it poses and threats associated with its formation. Further, it discusses strategies that are currently available or under development targeting prostaglandins, quorum-sensing, changing surface properties of biomedical devices, natural scaffolds, and small molecule-based chemical approaches to combat the threat of C. albicans biofilm. This review also highlights the recent developments in finding ways to increase the penetration of drugs into the extracellular matrix of biofilm using different nanomaterials against C. albicans.

Synthesis and antifungal activity of polycyclic pyridone derivatives with anti-hyphal and biofilm formation activity against Candida albicans

Thirty-five pyridone derivatives were synthesized, with derivatization conducted on polycyclic pyridone scaffolds, including cis- or trans-oxydecalin and other cyclic structures, by domino-Knoevenagel-electrocyclic reactions. The anti-fungal activities of the synthesized compounds were tested against Candida albicans. Ten compounds inhibited hyphal formation without inhibiting growth. Pyridones with anti-hyphal formation activity (4c, 6d, 12a and 12c) were tested for their ability to inhibit biofilm formation. Compound 6d showed both anti-hyphal and biofilm inhibition activity.

Comprehensive genetic analysis of adhesin proteins and their role in virulence of Candida albicans

Candida albicans is a microbial fungus that exists as a commensal member of the human microbiome and an opportunistic pathogen. Cell surface-associated adhesin proteins play a crucial role in C. albicans' ability to undergo cellular morphogenesis, develop robust biofilms, colonize, and cause infection in a host. However, a comprehensive analysis of the role and relationships between these adhesins has not been explored. We previously established a CRISPR-based platform for efficient generation of single- and double-gene deletions in C. albicans, which was used to construct a library of 144 mutants, comprising 12 unique adhesin genes deleted singly, and every possible combination of double deletions. Here, we exploit this adhesin mutant library to explore the role of adhesin proteins in C. albicans virulence. We perform a comprehensive, high-throughput screen of this library, using Caenorhabditis elegans as a simplified model host system, which identified mutants critical for virulence and significant genetic interactions. We perform follow-up analysis to assess the ability of high- and low-virulence strains to undergo cellular morphogenesis and form biofilms in vitro, as well as to colonize the C. elegans host. We further perform genetic interaction analysis to identify novel significant negative genetic interactions between adhesin mutants, whereby combinatorial perturbation of these genes significantly impairs virulence, more than expected based on virulence of the single mutant constituent strains. Together, this study yields important new insight into the role of adhesins, singly and in combinations, in mediating diverse facets of virulence of this critical fungal pathogen.

Inhibition of the filamentation of Candida albicans by Borojoa patinoi silver nanoparticles

Candida albicans is fungus capable of changing from yeast to filamentous form when it’s transformed from a normal commensal to an opportunistic pathogen. The development of alternatives that interfere with this transition could be an effective way to reduce candidiasis. In this regard, evaluate the inhibitory effect of two Borojoa patinoi silver nanoparticles (AgNPs) produced by green synthesis at 5 °C and 25 °C on the process of filamentation of Candida albicans. The percentage of inhibition of filamentous forms of C. albicans ATCC10231 and C. albicans SC5314 with AgNPs was determined. Results showed that temperature of synthesis affected both the shape and size of silver nanoparticles synthesized using Borojoa patinoi extracts. The inhibition percentage of filamentous forms of Candida albicans ATCC10231 when treated with silver nanoparticles synthesized at 5 °C was 85.9% and at 25 °C it was 40%. C. albicans SC5314 when treated with AgNP synthesized at 5 °C was 97.2% and at 25 °C it was 64%. Cell toxicity assay showed that at 100ng/ml, AgNPs synthesized at 25 °C were safe in MES-OV CRL-3272 cell line. Our results showed that the silver nanoparticles obtained from Borojoa patinoi are inhibitors of the filamentous process of C. albicans.

Metal-Free, Acid/Phosphine-Induced Regioselective Thiolation of p-Quinone Methides with Sodium Aryl/Alkyl Sulfinates

A simple and efficient method for the regioselective thiolation of p-quinone methides with sodium aryl/alkyl sulfinates has been established using an acid/phosphine-induced radical route under transition-metal-free conditions. A broad range of sodium aryl/alkyl sulfinates and p-quinone methides (p-QMs) are compatible for the reaction, giving the expected products with good to excellent yields. Control experiments were also performed to gain insights into the generation mechanism of thiyl radicals and hydrogen-atom transfer process. This protocol provides a safe and feasible way for the formation of carbon-sulfur bonds.

Inhibition of Distinct Proline- or N-Acetylglucosamine-Induced Hyphal Formation Pathways by Proline Analogs in Candida albicans

Candida albicans undergoes a yeast-to-hyphal transition that has been recognized as a virulence property as well as a turning point leading to biofilm formation associated with candidiasis. It is known that yeast-to-hyphal transition is induced under complex environmental conditions including temperature (above 35°C), pH (greater than 6.5), CO₂, N-acetylglucosamine (GlcNAc), amino acids, RPMI-1640 synthetic culture medium, and blood serum. To identify the hyphal induction factor in the RPMI-1640 medium, we examined each component of RPMI-1640 and established a simple hyphal induction condition, that is, incubation in L-proline solution at 37°C. Incubation in GlcNAc solution alone, which is not contained in RPMI-1640, without any other materials was also identified as another simple hyphal induction condition. To inhibit hyphal formation, proline and GlcNAc analogs were examined. Among the proline analogs used, L-azetidine-2-carboxylic acid (AZC) inhibited hyphal induction under both induction conditions, but L-4-thiazolidinecarboxylic acid (T4C) specifically inhibited proline-induced hyphal formation only, while α-N-methyl-L-proline (mPro) selectively inhibited GlcNAc-induced hyphal formation. Hyphal formation in fetal bovine serum was also inhibited by AZC or T4C together with mPro without affecting the proliferation of yeast form. These results indicate that these proline analogs are ideal inhibitors of yeast-to-hyphal transition in C. albicans.

Antibiotic-induced gut metabolome and microbiome alterations increase the susceptibility to Candida albicans colonization in the gastrointestinal tract

Antibiotic-induced alterations in the gut ecosystem increases the susceptibility to Candida albicans, yet the mechanisms involved remains poorly understood. Here we show that mice treated with the broad-spectrum antibiotic cefoperazone promoted the growth, morphogenesis and gastrointestinal (GI) colonization of C. albicans. Using metabolomics, we revealed that the cecal metabolic environment of the mice treated with cefoperazone showed a significant alteration in intestinal metabolites. Levels of carbohydrates, sugar alcohols and primary bile acids increased, whereas carboxylic acids and secondary bile acids decreased in antibiotic treated mice susceptible to C. albicans. Furthermore, using in-vitro assays, we confirmed that carbohydrates, sugar alcohols and primary bile acids promote, whereas carboxylic acids and secondary bile acids inhibit the growth and morphogenesis of C. albicans. In addition, in this study we report changes in the levels of gut metabolites correlated with shifts in the gut microbiota. Taken together, our in-vivo and in-vitro results indicate that cefoperazone-induced metabolome and microbiome alterations favor the growth and morphogenesis of C. albicans, and potentially play an important role in the GI colonization of C. albicans.

G-protein-coupled Receptors in Fungi

G-protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in fungi. These receptors have an important role in the transduction of extracellular signals into intracellular sites in response to diverse stimuli. They enable fungi to coordinate cell function and metabolism, thereby promoting their survival and propagation, and sense certain fundamentally conserved elements, such as nutrients, pheromones, and stress, for adaptation to their niches, environmental stresses, and host environment, causing disease and pathogen virulence. This chapter highlights the role of GPCRs in fungi in coordinating cell function and metabolism. Fungal cells sense the molecular interactions between extracellular signals. Their respective sensory systems are described here in detail.

Two New 1,3,4-Oxadiazoles With Effective Antifungal Activity Against Candida albicans

Candida infections have become a serious public health problem with high mortality rates, especially in immunocompromised patients, since Candida albicans is the major opportunistic pathogen responsible for systemic or invasive candidiasis. Commercially available antifungal agents are restricted and fungal resistance to such drugs has increased; therefore, the development of a more specific antifungal agent is necessary. Using assays for antifungal activity, here we report that two new compounds of 1,3,4-oxadiazoles class (LMM5 and LMM11), which were discovered by in silico methodologies as possible thioredoxin reductase inhibitors, were effective against C. albicans. Both compounds had in vitro antifungal activity with MIC 32 μg/ml. Cytotoxicity in vitro demonstrated that LMM5 and LMM11 were non-toxic in the cell lines evaluated. The kinetic of the time-kill curve suggested a fungistatic profile and showed an inhibitory effect of LMM5 and LMM11 in 12 h that remained for 24 and 36 h, which is better than fluconazole. In the murine systemic candidiasis model by C. albicans, the two compounds significantly reduced the renal and spleen fungal burden. According to the SEM and TEM images, we hypothesize that the mechanism of action of LMM5 and LMM11 is directly related to the inhibition of the enzyme thioredoxin reductase and internally affect the fungal cell. In view of all in vitro and in vivo results, LMM5 and LMM11 are effective therapeutic candidates for the development of new antifungal drugs addressing the treatment of human infections caused by C. albicans.

Linking Cellular Morphogenesis with Antifungal Treatment and Susceptibility in Candida Pathogens

Fungal infections are a growing public health concern, and an increasingly important cause of human mortality, with Candida species being amongst the most frequently encountered of these opportunistic fungal pathogens. Several Candida species are polymorphic, and able to transition between distinct morphological states, including yeast, hyphal, and pseudohyphal forms. While not all Candida pathogens are polymorphic, the ability to undergo morphogenesis is linked with the virulence of many of these pathogens. There are also many connections between Candida morphogenesis and antifungal drug treatment and susceptibility. Here, we review how Candida morphogenesis-a key virulence trait-is linked with antifungal drugs and antifungal drug resistance. We highlight how antifungal therapeutics are able to modulate morphogenesis in both sensitive and drug-resistant Candida strains, the shared signaling pathways that mediate both morphogenesis and the cellular response to antifungal drugs and drug resistance, and the connection between Candida morphology, drug resistance, and biofilm growth. We further review the development of anti-virulence drugs, and targeting Candida morphogenesis as a novel therapeutic strategy to target fungal pathogens. Together, this review highlights important connections between fungal morphogenesis, virulence, and susceptibility to antifungals.

A phenotypic small-molecule screen identifies halogenated salicylanilides as inhibitors of fungal morphogenesis, biofilm formation and host cell invasion

A poorly exploited paradigm in the antimicrobial therapy field is to target virulence traits for drug development. In contrast to target-focused approaches, antivirulence phenotypic screens enable identification of bioactive molecules that induce a desirable biological readout without making a priori assumption about the cellular target. Here, we screened a chemical library of 678 small molecules against the invasive hyphal growth of the human opportunistic yeast Candida albicans. We found that a halogenated salicylanilide (N1-(3,5-dichlorophenyl)-5-chloro-2-hydroxybenzamide) and one of its analogs, Niclosamide, an FDA-approved anthelmintic in humans, exhibited both antifilamentation and antibiofilm activities against C. albicans and the multi-resistant yeast C. auris. The antivirulence activity of halogenated salicylanilides were also expanded to C. albicans resistant strains with different resistance mechanisms. We also found that Niclosamide protected the intestinal epithelial cells against invasion by C. albicans. Transcriptional profiling of C. albicans challenged with Niclosamide exhibited a signature that is characteristic of the mitochondria-to-nucleus retrograde response. Our chemogenomic analysis showed that halogenated salicylanilides compromise the potential-dependant mitochondrial protein translocon machinery. Given the fact that the safety of Niclosamide is well established in humans, this molecule could represent the first clinically approved antivirulence agent against a pathogenic fungus.

Inhibition of yeast-to-filamentous growth transitions in Candida albicans by a small molecule inducer of mammalian apoptosis

The opportunistic fungal pathogen of humans Candida albicans is able to grow in different morphological forms such as round or oval yeasts and filamentous hyphae and pseudohyphae. Morphogenesis, the ability to switch between the yeast and filamentous growth forms, is important for adapting to new microenvironments in the human host and for pathogenesis. The molecular pathways governing morphogenesis are complex and incompletely understood. Previously, we identified several small organic molecules that specifically inhibit the initiation of hyphal growth in C. albicans without affecting cell viability or budded growth. One molecule from that screen is known to induce apoptosis in mammalian cells. In this study, we have screened additional inducers of mammalian apoptosis and identified BH3I-1, as well as several structural derivatives of BH3I-1, that act as specific inhibitors of morphogenesis under a variety of environmental conditions. Chemical epistasis experiments suggest that BH3I-1 acts downstream of the hypha-specific gene regulators Rfg1, Nrg1 and Ume6.

Chemical inhibitors of Candida albicans hyphal morphogenesis target endocytosis

Candida albicans is an opportunistic pathogen, typically found as a benign commensal yeast living on skin and mucosa, but poised to invade injured tissue to cause local infections. In debilitated and immunocompromised individuals, C. albicans may spread to cause life-threatening systemic infections. Upon contact with serum and at body temperature, C. albicans performs a regulated switch to filamentous morphology, characterized by emergence of a germ tube from the yeast cell followed by mold-like growth of branching hyphae. The ability to switch between growth morphologies is an important virulence factor of C. albicans. To identify compounds able to inhibit hyphal morphogenesis, we screened libraries of existing drugs for inhibition of the hyphal switch under stringent conditions. Several compounds that specifically inhibited hyphal morphogenesis were identified. Chemogenomic analysis suggested an interaction with the endocytic pathway, which was confirmed by direct measurement of fluid-phase endocytosis in the presence of these compounds. These results suggest that the activity of the endocytic pathway, which is known to be particularly important for hyphal growth, represents an effective target for hyphae-inhibiting drugs.

17β-Estradiol inhibits estrogen binding protein-mediated hypha formation in Candida albicans

Candida albicans is one of the most prevalent and clinically important fungal pathogens. The ability to change form depending on environmental stress is an important microbial virulence factor. A survey of compounds that inhibit this morphological change identified various steroids, including 17β-estradiol. Interestingly, C. albicans has proteins capable of binding to steroids, including estrogen binding protein (Ebp1). Estrogens regulate cell differentiation and proliferation in humans through estrogen receptor proteins. To determine whether EBP1 regulates a virulence factor, we investigated the effect of 17β-estradiol on the morphological transition of C. albicans using an ebp1 deletion mutant. Treatment with 10 μg/mL of 17β-estradiol inhibited hypha formation, whereas its effect on the ebp1 deletion mutant was decreased compared to that on the wild-type and revertant strains. These data suggest a new pathway for the yeast-to-hypha transition via EBP1 in C. albicans.

Minocycline Inhibits Candida albicans Budded-to-Hyphal-Form Transition and Biofilm Formation

Candida albicans frequently causes bloodstream infections; its budded-to-hyphalform transition (BHT) and biofilm formation are major contributors to virulence. During an analysis of antibacterial compounds that inhibit C. albicans BHT, we found that the tetracycline derivative minocycline inhibited BHT and subsequent biofilm formation. Minocycline decreased expression of hypha-specific genes HWP1 and ECE1, and adhesion factor gene ALS3 of C. albicans. In addition, minocycline decreased cell surface hydrophobicity and the extracellular β-glucan level in biofilms. Minocycline has been widely used for catheter antibiotic lock therapy to prevent bacterial infection; this compound may also be prophylactically effective against Candida infection.

Tackling Fungal Resistance by Biofilm Inhibitors

The high incidence and mortality of invasive fungal infections and serious drug resistance have become a global public health issue. The ability of fungal cells to form biofilms is an important reason for the emergence of severe resistance to most clinically available antifungal agents. Targeting fungal biofilm formation by small molecules represents a promising new strategy for the development of novel antifungal agents. This perspective will provide a comprehensive review of fungal biofilm inhibitors. In particular, discovery strategies, chemical structures, antibiofilm/antifungal activities, and structure-activity relationship studies will be discussed. Development of inhibitors to treat biofilm-related resistant fungal infections is a new yet clinically unexploited paradigm, and there is still a long way to go to clinical application. Better understanding of fungal biofilms in combination with systematic drug discovery efforts will pave the way for potential clinical applications.

Propolis: a potential natural product to fight Candida species infections

AIM

To evaluate the effect of propolis against Candida species planktonic cells and its counterpart's biofilms.

MATERIALS & METHODS

The MIC values, time-kill curves and filamentation form inhibition were determined in Candida planktonic cells. The effect of propolis on Candida biofilms was assessed through quantification of CFUs.

RESULTS

MIC values, ranging from 220 to 880 µg/ml, demonstrated higher efficiency on C. albicans and C. parapsilosis than on C. tropicalis cells. In addition, propolis was able to prevent Candida species biofilm's formation and eradicate their mature biofilms, coupled with a significant reduction on C. tropicalis and C. albicans filamentation.

CONCLUSION

Propolis is an inhibitor of Candida virulence factors and represents an innovative alternative to fight candidiasis.

Targeting Candida albicans filamentation for antifungal drug development

Candida albicans remains the main etiological agent of candidiasis, as this otherwise normal commensal of humans is capable of causing active infection in immune- and medically-compromised patients. The high morbidity and mortality rates associated with candidiasis, coupled with the emergence of drug resistance demand the development of novel therapeutic strategies. However, there is a paucity of selective targets that can be exploited in the development of new antifungals. Contrary to conventional antibiotics that kill or curtail growth, specifically targeting virulence mechanisms represents an attractive option for antifungal drug development. In C. albicans, a growing body of research over the last few decades has provided important insights into its virulence factors and their contribution to the pathogenesis of candidiasis. Of these, filamentation is the one that has received the most attention and perhaps shows the most promise as a target for new anti-virulence strategies to combat C. albicans infections.

Effects of magnolol and honokiol on adhesion, yeast-hyphal transition, and formation of biofilm by Candida albicans

Background

The first step in infection by Candida albicans is adhesion to host cells or implanted medical devices and this followed by hyphal growth and biofilm formation. Yeast-to-hyphal transition has long been identified as a key factor in fungal virulence. Following biofilm formation, C. albicans is usually less sensitive or insensitive to antifungals. Therefore, development of new antifungals with inhibitory action on adhesion, yeast-hyphal transition and biofilm formation by C. albicans is very necessary.

Methods

The effects of magnolol and honokiol on hypha growth were investigated using different induction media. Their inhibitory effects were determined using the 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5- carboxanilide assay, and biofilm thickness and viability were observed by a confocal scanning laser microscope. Mammalian cells were used in adhesion assays. Genes related to hyphae development and cell adhesions were analyzed by real-time reverse transcription-polymerase chain reaction. The exogenous cyclic adenosine monophosphate was used to determine the mechanisms of action of magnolol and honokiol. Caenorhabditis elegans was used as an in vivo model to estimate the antifungal activities of magnolol and honokiol.

Results and conclusions

Magnolol and honokiol inhibited adhesion, the transition from yeast to hypha, and biofilm formation by C. albicans through the Ras1-cAMP-Efg1 pathway. Moreover, magnolol and honokiol prolonged the survival of nematodes infected by C. albicans. Magnolol and honokiol have potential inhibitory effects against biofilm formation by C. albicans.

General Significance

This study provides useful information towards the development of new strategies to reduce the incidence of C. albicans biofilm-associated infection.

Novel high-throughput screening method for identification of fungal dimorphism blockers

Invasive mycoses have been increasing worldwide, with Candida spp. being the most prevalent fungal pathogen causing high morbidity and mortality in immunocompromised individuals. Only few antimycotics exist, often with severe side effects. Therefore, new antifungal drugs are urgently needed. Because the identification of antifungal compounds depends on fast and reliable assays, we present a new approach based on high-throughput image analysis to define cell morphology. Candida albicans and other fungi of the Candida clade switch between different growth morphologies, from budding yeast to filamentous hyphae. Yeasts are considered proliferative, whereas hyphae are required for invasion and dissemination. Thus, morphotype switching in many Candida spp. is connected to virulence and pathogenesis. It is, consequently, reasonable to presume that morphotype blockers interfere with the virulence, thereby preventing hazardous colonization. Our method efficiently differentiates yeast from hyphal cells using a combination of automated microscopy and image analysis. We selected the parameters length/width ratio and mean object shape to quantitatively discriminate yeasts and hyphae. Notably, Z' factor calculations for these parameters confirmed the suitability of our method for high-throughput screening. As a second stage, we determined cell viability to discriminate morphotype-switching inhibitors from those that are fungicidal. Thus, our method serves as a basis for the identification of candidates for next-generation antimycotics.

In pursuit of the ideal antifungal agent for Candida infections: high-throughput screening of small molecules

Candida infections have created a great burden on the public healthcare sector. The situation is worsened by recent epidemiological changes. Furthermore, the current arsenal of antifungal agents is limited and associated with undesirable drawbacks. Therefore, new antifungal agents that surpass the existing ones are urgently needed. High-throughput screening of small molecule libraries enables rapid hit identification and, possibly, increases hit rate. Moreover, the identified hits could be associated with unrecognized or multiple drug targets, which would provide novel insights into the biological processes of the pathogen. Hence, it is proposed that high-throughput screening of small molecules is particularly important in the pursuit of the ideal antifungal agents for Candida infections.

In vitro and in vivo activity of a novel antifungal small molecule against Candida infections

Candida is the most common fungal pathogen of humans worldwide and has become a major clinical problem because of the growing number of immunocompromised patients, who are susceptible to infection. Moreover, the number of available antifungals is limited, and antifungal-resistant Candida strains are emerging. New and effective antifungals are therefore urgently needed. Here, we discovered a small molecule with activity against Candida spp. both in vitro and in vivo. We screened a library of 50,240 small molecules for inhibitors of yeast-to-hypha transition, a major virulence attribute of Candida albicans. This screening identified 20 active compounds. Further examination of the in vitro antifungal and anti-biofilm properties of these compounds, using a range of Candida spp., led to the discovery of SM21, a highly potent antifungal molecule (minimum inhibitory concentration (MIC) 0.2 – 1.6 µg/ml). In vitro, SM21 was toxic to fungi but not to various human cell lines or bacterial species and was active against Candida isolates that are resistant to existing antifungal agents. Moreover, SM21 was relatively more effective against biofilms of Candida spp. than the current antifungal agents. In vivo, SM21 prevented the death of mice in a systemic candidiasis model and was also more effective than the common antifungal nystatin at reducing the extent of tongue lesions in a mouse model of oral candidiasis. Propidium iodide uptake assay showed that SM21 affected the integrity of the cell membrane. Taken together, our results indicate that SM21 has the potential to be developed as a novel antifungal agent for clinical use.

Cancer drugs inhibit morphogenesis in the human fungal pathogen, Candida albicans

Candida infections are very common in cancer patients and it is a common practice to prescribe antifungal antibiotics along with anticancer drugs. Yeast to hyphal form switching is considered to be important in invasive candidiasis. Targeting morphogenetic switching may be useful against invasive candidiasis. In this study, we report the antimorphogenetic properties of thirty cancer drugs.

Candidiasis drug discovery and development: new approaches targeting virulence for discovering and identifying new drugs

INTRODUCTION

Targeting pathogenetic mechanisms, rather than essential processes, represents a very attractive alternative for the development of new antibiotics. This may be particularly important in the case of antimycotics, due to the urgent need for novel antifungal drugs and the paucity of selective fungal targets. The opportunistic pathogenic fungus Candida albicans is the main etiological agent of candidiasis, the most common human fungal infection. These infections carry unacceptably high mortality rates, a clear reflection of the many shortcomings of current antifungal therapy, including the limited armamentarium of antifungal agents, their toxicity and the emergence of resistance. Moreover, the antifungal pipeline is mostly dry.

AREAS COVERED

This review covers some of the most recent progress toward understanding C. albicans pathogenetic processes and how to harness this information for the development of anti-virulence agents. The two principal areas covered are filamentation and biofilm formation, as C. albicans pathogenicity is intimately linked to its ability to undergo morphogenetic conversions between yeast and filamentous morphologies and to its ability to form biofilms.

EXPERT OPINION

Filamentation and biofilm formation represent high value targets, yet are clinically unexploited, for the development of novel anti-virulence approaches against candidiasis. Although this has proved a difficult task despite increasing understanding at the molecular level of C. albicans virulence, there are some opportunities and prospects for antifungal drug development targeting these two important biological processes.

Purpurin suppresses Candida albicans biofilm formation and hyphal development

A striking and clinically relevant virulence trait of the human fungal pathogen Candida albicans is its ability to grow and switch reversibly among different morphological forms. Inhibition of yeast-to-hypha transition in C. albicans represents a new paradigm for antifungal intervention. We have previously demonstrated the novel antifungal activity of purpurin against Candida fungi. In this study, we extended our investigation by examining the in vitro effect of purpurin on C. albicans morphogenesis and biofilms. The susceptibility of C. albicans biofilms to purpurin was examined quantitatively by 2,3-bis(2-methoxy-4-nitro-5-sulfo-phenyl)-2H-tetrazolium-5-carboxanilide reduction assay. Hyphal formation and biofilm ultrastructure were examined qualitatively by scanning electron microscopy (SEM). Quantitative reverse transcription-PCR (qRT-PCR) was used to evaluate the expression of hypha-specific genes and hyphal regulator in purpurin-treated fungal cells. The results showed that, at sub-lethal concentration (3 µg/ml), purpurin blocked the yeast-to-hypha transition under hypha-inducing conditions. Purpurin also inhibited C. albicans biofilm formation and reduced the metabolic activity of mature biofilms in a concentration-dependent manner. SEM images showed that purpurin-treated C. albicans biofilms were scanty and exclusively consisted of aggregates of blastospores. qRT-PCR analyses indicated that purpurin downregulated the expression of hypha-specific genes (ALS3, ECE1, HWP1, HYR1) and the hyphal regulator RAS1. The data strongly suggested that purpurin suppressed C. albicans morphogenesis and caused distorted biofilm formation. By virtue of the ability to block these two virulence traits in C. albicans, purpurin may represent a potential candidate that deserves further investigations in the development of antifungal strategies against this notorious human fungal pathogen in vivo.

The plant defensin RsAFP2 induces cell wall stress, septin mislocalization and accumulation of ceramides in Candida albicans

The antifungal plant defensin RsAFP2 isolated from radish interacts with fungal glucosylceramides and induces apoptosis in Candida albicans. To further unravel the mechanism of RsAFP2 antifungal action and tolerance mechanisms, we screened a library of 2868 heterozygous C. albicans deletion mutants and identified 30 RsAFP2-hypersensitive mutants. The most prominent group of RsAFP2 tolerance genes was involved in cell wall integrity and hyphal growth/septin ring formation. Consistent with these genetic data, we demonstrated that RsAFP2 interacts with the cell wall of C. albicans, which also contains glucosylceramides, and activates the cell wall integrity pathway. Moreover, we found that RsAFP2 induces mislocalization of septins and blocks the yeast-to-hypha transition in C. albicans. Increased ceramide levels have previously been shown to result in apoptosis and septin mislocalization. Therefore, ceramide levels in C. albicans membranes were analysed following RsAFP2 treatment and, as expected, increased accumulation of phytoC24-ceramides in membranes of RsAFP2-treated C. albicans cells was detected. This is the first report on the interaction of a plant defensin with glucosylceramides in the fungal cell wall, causing cell wall stress, and on the effects of a defensin on septin localization and ceramide accumulation.

E1210, a new broad-spectrum antifungal, suppresses Candida albicans hyphal growth through inhibition of glycosylphosphatidylinositol biosynthesis

Continued research toward the development of new antifungals that act via inhibition of glycosylphosphatidylinositol (GPI) biosynthesis led to the design of E1210. In this study, we assessed the selectivity of the inhibitory activity of E1210 against Candida albicans GWT1 (Orf19.6884) protein, Aspergillus fumigatus GWT1 (AFUA_1G14870) protein, and human PIG-W protein, which can catalyze the inositol acylation of GPI early in the GPI biosynthesis pathway, and then we assessed the effects of E1210 on key C. albicans virulence factors. E1210 inhibited the inositol acylation activity of C. albicans Gwt1p and A. fumigatus Gwt1p with 50% inhibitory concentrations (IC(50)s) of 0.3 to 0.6 μM but had no inhibitory activity against human Pig-Wp even at concentrations as high as 100 μM. To confirm the inhibition of fungal GPI biosynthesis, expression of ALS1 protein, a GPI-anchored protein, on the surfaces of C. albicans cells treated with E1210 was studied and shown to be significantly lower than that on untreated cells. However, the ALS1 protein levels in the crude extract and the RHO1 protein levels on the cell surface were found to be almost the same. Furthermore, E1210 inhibited germ tube formation, adherence to polystyrene surfaces, and biofilm formation of C. albicans at concentrations above its MIC. These results suggested that E1210 selectively inhibited inositol acylation of fungus-specific GPI which would be catalyzed by Gwt1p, leading to the inhibition of GPI-anchored protein maturation, and also that E1210 suppressed the expression of some important virulence factors of C. albicans, through its GPI biosynthesis inhibition.

Small molecule inhibitors of the Candida albicans budded-to-hyphal transition act through multiple signaling pathways

The ability of the pathogenic yeast Candida albicans to interconvert between budded and hyphal growth states, herein termed the budded-to-hyphal transition (BHT), is important for C. albicans development and virulence. The BHT is under the control of multiple cell signaling pathways that respond to external stimuli, including nutrient availability, high temperature, and pH. Previous studies identified 21 small molecules that could inhibit the C. albicans BHT in response to carbon limitation in Spider media. However, the studies herein show that the BHT inhibitors had varying efficacies in other hyphal-inducing media, reflecting their varying abilities to block signaling pathways associated with the different media. Chemical epistasis analyses suggest that most, but not all, of the BHT inhibitors were acting through either the Efg1 or Cph1 signaling pathways. Notably, the BHT inhibitor clozapine, a FDA-approved drug used to treat atypical schizophrenia by inhibiting G-protein-coupled dopamine receptors in the brain, and several of its functional analogs were shown to act at the level of the Gpr1 G-protein-coupled receptor. These studies are the first step in determining the target and mechanism of action of these BHT inhibitors, which may have therapeutic anti-fungal utility in the future.

Small-molecule inhibitors of biofilm formation in laboratory and clinical isolates of Candida albicans

Candida albicans cells have the ability to form biofilms on biotic and abiotic surfaces, such as indwelling medical devices. C. albicans cells can interconvert between budded and hyphal growth forms, herein termed the budded-to-hyphal transition (BHT), which is important for the formation of mature biofilms. Previous work identified 23 small organic molecules that could inhibit the BHT but did not affect C. albicans cell viability or budded cell growth. These BHT inhibitors were proposed to inhibit multiple signalling pathways regulating the BHT, many of which also regulate biofilm formation. However, only three of the BHT inhibitors, buhytrinA, ETYA and CGP-37157, were capable of inhibiting in vitro biofilm formation of wild-type laboratory C. albicans strains. When clinical C. albicans isolates were examined for their ability to form biofilms, only 11 of the 28 clinical isolates tested (39%) were capable of forming biofilms. Although buhytrinA, ETYA and CGP-37157 could inhibit the BHT of all 28 clinical isolates, they were only able to inhibit biofilm formation of a subset of these clinical isolates, with ETYA having 100% efficacy. These data indicate that the biofilm-forming capability of laboratory and clinical isolates of C. albicans, as well as the efficacy of BHT inhibitors against these different isolates, can differ dramatically. These differences between laboratory and clinical isolates should be an important aspect to consider when examining potentially new antifungal therapeutics.

Modulation of morphogenesis in Candida albicans by various small molecules

The pathogenic yeast Candida albicans, a member of the mucosal microbiota, is responsible for a large spectrum of infections, ranging from benign thrush and vulvovaginitis in both healthy and immunocompromised individuals to severe, life-threatening infections in immunocompromised patients. A striking feature of C. albicans is its ability to grow as budding yeast and as filamentous forms, including hyphae and pseudohyphae. The yeast-to-hypha transition contributes to the overall virulence of C. albicans and may even constitute a target for the development of antifungal drugs. Indeed, impairing morphogenesis in C. albicans has been shown to be a means to treat candidiasis. Additionally, a large number of small molecules such as farnesol, fatty acids, rapamycin, geldanamycin, histone deacetylase inhibitors, and cell cycle inhibitors have been reported to modulate the yeast-to-hypha transition in C. albicans. In this minireview, we take a look at molecules that modulate morphogenesis in this pathogenic yeast. When possible, we address experimental findings regarding their mechanisms of action and their therapeutic potential. We discuss whether or not modulating morphogenesis constitutes a strategy to treat Candida infections.

Conjugated linoleic acid inhibits hyphal growth in Candida albicans by modulating Ras1p cellular levels and downregulating TEC1 expression

The polymorphic yeast Candida albicans exists in yeast and filamentous forms. Given that the morphogenetic switch coincides with the expression of many virulence factors, the yeast-to-hypha transition constitutes an attractive target for the development of new antifungal agents. Since an untapped therapeutic potential resides in small molecules that hinder C. albicans filamentation, we characterized the inhibitory effect of conjugated linoleic acid (CLA) on hyphal growth and addressed its mechanism of action. CLA inhibited hyphal growth in a dose-dependent fashion in both liquid and solid hypha-inducing media. The fatty acid blocked germ tube formation without affecting cellular growth rates. Global transcriptional profiling revealed that CLA downregulated the expression of hypha-specific genes and abrogated the induction of several regulators of hyphal growth, including TEC1, UME6, RFG1, and RAS1. However, neither UME6 nor RFG1 was necessary for CLA-mediated hyphal growth inhibition. Expression analysis showed that the downregulation of TEC1 expression levels by CLA depended on RAS1. In addition, while RAS1 transcript levels remained constant in CLA-treated cells, its protein levels declined with time. With the use of a strain expressing GFP-Ras1p, CLA treatment was also shown to affect Ras1p localization to the plasma membrane. These findings suggest that CLA inhibits hyphal growth by affecting the cellular localization of Ras1p and blocking the increase in RAS1 mRNA and protein levels. Combined, these effects should prevent the induction of the Ras1p signaling pathway. This study provides the biological and molecular explanations that underlie CLA's ability to inhibit hyphal growth in C. albicans.

Antifungal activity of tamoxifen: in vitro and in vivo activities and mechanistic characterization

Tamoxifen (TAM), an estrogen receptor antagonist used primarily to treat breast cancer, has well-recognized antifungal properties, but the activity of TAM has not been fully characterized using standardized (i.e., CLSI) in vitro susceptibility testing, nor has it been demonstrated in an in vivo model of fungal infection. In addition, its mechanism of action remains to be clearly defined at the molecular level. Here, we report that TAM displays in vitro activity (MIC, 8 to 64 microg/ml) against pathogenic yeasts (Candida albicans, other Candida spp., and Cryptococcus neoformans). In vivo, 200 mg/kg of body weight per day TAM reduced kidney fungal burden (-1.5 log(10) CFU per g tissue; P = 0.008) in a murine model of disseminated candidiasis. TAM is a known inhibitor of mammalian calmodulin, and TAM-treated yeast show phenotypes consistent with decreased calmodulin function, including lysis, decreased new bud formation, disrupted actin polarization, and decreased germ tube formation. The overexpression of calmodulin suppresses TAM toxicity, hypofunctional calmodulin mutants are hypersensitive to TAM, and TAM interferes with the interaction between Myo2p and calmodulin, suggesting that TAM targets calmodulin as part of its mechanism of action. Taken together, these experiments indicate that the further study of compounds related to TAM as antifungal agents is warranted.