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

Ethyl caffeate combined with fluconazole exhibits efficacy against azole-resistant oropharyngeal candidiasis via the EFGR/JNK/c-JUN signaling pathway

Ethyl caffeate (EC) is a phenylpropanoid compound derived from Elephantopus scaber. In our previous work, EC was investigated to have a strong synergistic antifungal effect against azole-resistant strains of Candida albicans when combined with fluconazole (FLU). However, the protective effect and mechanism of EC + FLU on oropharyngeal candidiasis (OPC) caused by drug-resistant strains of C. albicans have not been investigated. This study aimed to investigate the protective effect and mechanism of EC combined with FLU against C. albicans-resistant strains that lead to OPC. An OPC mouse model revealed that EC + FLU treatment reduced fungal load and massive hyphal invasion of tongue tissues, and ameliorated the integrity of the tongue mucosa. Periodic acid-Schiff staining results showed more structural integrity of the tongue tissues and reduced inflammatory cell infiltration after EC + FLU treatment. Phosphorylation of EGFR (epidermal growth factor receptor) and other proteins in the EFGR/JNK (c-Jun N-terminal kinase)/c-JUN (transcription factor Jun) signaling pathway was significantly downregulated by EC + FLU. EGFR and S100A9 mRNA expression were also reduced. The above results were verified in FaDu cells. ELISA results showed that the concentration of inflammatory factors in the cell supernatant was significantly reduced after EC combined with FLU treatment. Molecular docking revealed that EC exhibited high binding energy to EGFR. In conclusion, EC enhances the susceptibility of azole-resistant C. albicans to FLU, and the underlying mechanism is related to the inhibition of the EGFR/JNK/c-JUN signaling pathway. This result suggests that EC has potential to be developed as an antifungal sensitizer to treat OPC caused by azole-resistant C. albicans.

Controlling antifungal activity with light: Optical regulation of fungal ergosterol biosynthetic pathway with photo-responsive CYP51 inhibitors

Invasive fungal infections (IFIs) have been associated with high mortality, highlighting the urgent need for developing novel antifungal strategies. Herein the first light-responsive antifungal agents were designed by optical control of fungal ergosterol biosynthesis pathway with photocaged triazole lanosterol 14α-demethylase (CYP51) inhibitors. The photocaged triazoles completely shielded the CYP51 inhibition. The content of ergosterol in fungi before photoactivation and after photoactivation was 4.4% and 83.7%, respectively. Importantly, the shielded antifungal activity (MIC₈₀ ≥ 64 μg/mL) could be efficiently recovered (MIC₈₀ = 0.5-8 μg/mL) by light irradiation. The new chemical tools enable optical control of fungal growth arrest, morphological conversion and biofilm formation. The ability for high-precision antifungal treatment was validated by in vivo models. The light-activated compound A1 was comparable to fluconazole in prolonging survival in Galleria mellonella larvae with a median survival of 14 days and reducing fungal burden in the mouse skin infection model. Overall, this study paves the way for precise regulation of antifungal therapy with improved efficacy and safety.

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.

Novel Carboline Fungal Histone Deacetylase (HDAC) Inhibitors for Combinational Treatment of Azole-Resistant Candidiasis

Due to the evolution and development of antifungal drug resistance, limited efficacy of existing drugs has led to high mortality in patients with serious fungal infections. To develop novel antifungal therapeutic strategies, herein a series of carboline fungal histone deacetylase (HDAC) inhibitors were designed and synthesized, which had potent synergistic effects with fluconazole against resistant Candida albicans infection. In particular, compound D12 showed excellent in vitro and in vivo synergistic antifungal efficacy with fluconazole to treat azole-resistant candidiasis. It cooperated with fluconazole in reducing the virulence of C. albicans by blocking morphological mutual transformation and inhibiting biofilm formation. Mechanism studies revealed that the reversion of drug resistance was due to downregulation of the expression of the azole target gene ERG11 and efflux gene CDR1. Taken together, fungal HDAC inhibitor D12 offered a promising lead compound for combinational treatment of azole-resistant candidiasis.

Role of arthroconidia in biofilm formation by Trichosporon asahii

BACKGROUND

Trichosporon asahii is the major causative agent of disseminated and deep-seated trichosporonosis. It is capable of forming biofilms on surfaces, leading to medical device-related infection.Trichosporon asahii may be present as yeast form, hyphae and/or arthroconidia; however, the relationship between its biofilm-forming ability and its morphological transition is unclear.

OBJECTIVES

We investigated whether the T. asahii morphological transition contributes to its biofilm formation. We also determined the conditions required to induce each of the morphologies.

METHODS

Three high- and three low-biofilm-producing strains (HBS and LBS, respectively) were selected using a biofilm formation assay, and the cell surface hydrophobicity of these six strains was measured. For each strain, the morphology was observed and the number of each morphological form (yeast form, hypha and arthroconidium) was counted to calculate the ratio. Finally, the ability of cells each morphological type to adhere to the polystyrene substrate was evaluated.

RESULTS

The HBS exhibited abundant arthroconidia and hyphae; in contrast, the LBS produced mainly hyphae with few or no arthroconidia. The production of hyphae was increased by nitrogen-containing medium, and the production of arthroconidia was increased by nitrogen-deficient medium. Cells incubated under nitrogen-deficient conditions showed higher adherence to a polystyrene surface than those incubated in the presence of nitrogen.

CONCLUSION

Arthroconidia of T. asahii play a key role in biofilm formation by promoting cellular adhesion.

Association of the hypha-related protein Pra1 and zinc transporter Zrt1 with biofilm formation by the pathogenic yeast Candida albicans

Bloodstream infection by the pathogenic fungus Candida albicans is a major health problem. Candidemia is often associated with medical devices, which can act as substrates for biofilm development. Biofilm-related infections are relatively difficult to treat because of their resistance to antimicrobial agents. It is therefore important to explore the mechanisms of biofilm formation. Dimorphism is a major contributor to biofilm formation in C. albicans. To determine whether the hypha-related proteins Pra1 (pH-regulated antigen) and Zrt1 (zinc transporter) are responsible for biofilm formation, the ability of pra1 and zrt1 deletion mutants to form biofilms was investigated. Biofilm formation by both deletion mutants was less than that of the wild-type strain. Because Pra1 and Zrt1 are also related to the zinc homeostasis system, the effects of adding zinc on biofilm formation were also examined. Biofilm formation was increased in the presence of zinc. These data suggest that Pra1 and Zrt1 regulate biofilm formation through zinc homeostasis.

Alizarin and Chrysazin Inhibit Biofilm and Hyphal Formation by Candida albicans

Candida albicans is one of the most common pathogen causes fungal infections. This opportunistic pathogen can form biofilms comprised of yeast, hyphae and pseudo hyphal elements, and the hyphal form C. albicans considered as probable virulence factor. We investigated the antibiofilm activities of 13 quinones and anthraquinones related compounds against C. albicans biofilms by using crystal violet and 2,3-bis (2-Methoxy-4-Nitro-5-Sulfo-phenyl)-2H-Tetrazolium-5-Carboxanilide (XTT) reduction assays to assess inhibitions of biofilm growth. Morphological changes in biofilms and biofilm thicknesses were determined by scanning electron microscopy and confocal laser scanning microscopy, respectively. It was found alizarin (1,2-dihydroxyanthraquinone) and chrysazin (1,8-dihydroxyanthraquinone) suppressed C. albicans biofilm formation. Interestingly, alizarin and chrysazin at only 2 μg/ml effectively inhibited hyphal formation and prolonged the survival of C. albicans infected Caenorhabditis elegans, thus showing a distinct antivirulent potential. A structural activity relationship study of alizarin and 6 other anthraquinones showed the presence of a hydroxyl group at C-1 position which is important for antibiofilm and antifilamentation activities. Transcriptomic analyses revealed that alizarin downregulated the expression of several hypha-specific and biofilm related genes (ALS3, ECE1, ECE2, and RBT1). Furthermore, unlike the commercial antifungal drug fluconazole, no acute toxic effect was observed when uninfected nematodes were exposed to alizarin at concentrations up to 1 mg/ml. The results of this study indicate alizarin suppresses the virulence of C. albicans in vivo which suggests alizarin may be considered as a potential candidate for further investigations to develop antifungal agent against fungal pathogen in vivo.

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.

From Biology to Drug Development: New Approaches to Combat the Threat of Fungal Biofilms

Fungal infections constitute a major threat to an escalating number of critically ill patients. Fungi are eukaryotic organisms and, as such, there is a limited armamentarium of antifungal drugs, which leads to high mortality rates. Moreover, fungal infections are often associated with the formation of biofilms, which contribute to virulence and further complicate treatment due to the high level of antifungal drug resistance displayed by sessile cells within these microbial communities. Thus, the treatment of fungal infections associated with a biofilm etiology represents a formidable and unmet clinical challenge. The increasing importance and awareness of fungal biofilms is reflected by the fact that this is now an area of very active research. Studies in the last decade have provided important insights into fungal biofilm biology, physiology, and pathology, as well as into the molecular basis of biofilm resistance. Here we discuss how this accumulated knowledge may inform the development of new antibiofilm strategies and therapeutics that are urgently needed.

In vitro analysis of finasteride activity against Candida albicans urinary biofilm formation and filamentation

Candida albicans is the 3rd most common cause of catheter-associated urinary tract infections, with a strong propensity to form drug-resistant catheter-related biofilms. Due to the limited efficacy of available antifungals against biofilms, drug repurposing has been investigated in order to identify novel agents with activities against fungal biofilms. Finasteride is a 5-α-reductase inhibitor commonly used for the treatment of benign prostatic hyperplasia, with activity against human type II and III isoenzymes. We analyzed the Candida Genome Database and identified a C. albicans homolog of type III 5-α-reductase, Dfg10p, which shares 27% sequence identity and 41% similarity to the human type III 5-α-reductase. Thus, we investigated finasteride for activity against C. albicans urinary biofilms, alone and in combination with amphotericin B or fluconazole. Finasteride alone was highly effective in the prevention of C. albicans biofilm formation at doses of ≥16 mg/liter and the treatment of preformed biofilms at doses of ≥128 mg/liter. In biofilm checkerboard analyses, finasteride exhibited synergistic activity in the prevention of biofilm formation in a combination of 4 mg/liter finasteride with 2 mg/liter fluconazole. Finasteride inhibited filamentation, thus suggesting a potential mechanism of action. These results indicate that finasteride alone is highly active in the prevention of C. albicans urinary biofilms in vitro and has synergistic activity in combination with fluconazole. Further investigation of the clinical utility of finasteride in the prevention of urinary candidiasis is warranted.

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.

Small-molecule suppressors of Candida albicans biofilm formation synergistically enhance the antifungal activity of amphotericin B against clinical Candida isolates

A new class of fungal biofilm inhibitors represented by shearinines D (3) and E (4) were obtained from a Penicillium sp. isolate. The inhibitory activities of 3 and 4 were characterized using a new imaging flow-cytometer technique, which enabled the rapid phenotypic analysis of Candida albicans cell types (budding yeast cells, germ tube cells, pseudohyphae, and hyphae) in biofilm populations. The results were confirmed by experimental data obtained from three-dimensional confocal laser scanning microscopy and 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) assays. These data indicate that 3 and 4 inhibited C. albicans biofilm formation by blocking the outgrowth of hyphae at a relatively late stage of biofilm development (IC50 = 8.5 and 7.6 μM, respectively). However, 3 and 4 demonstrated comparatively weak activity at disrupting existing biofilms. Compounds 3 and 4 also exhibited synergistic activities with amphotericin B against C. albicans and other clinical Candida isolates by enhancing the potency of amphotericin B up to 8-fold against cells in both developing and established biofilms. These data suggest that the Candida biofilm disruption and amphotericin B potentiating effects of 3 and 4 could be mediated through multiple biological targets. The shearinines are good tools for testing the potential advantages of using adjunctive therapies in combination with antifungals.