Synergistic effect of calcineurin inhibitors and fluconazole against Candida albicans biofilms

Anti-dermatophytic activity of marine sponge, Sigmadocia carnosa (Dendy) on clinically isolated fungi

OBJECTIVE

To screen the anti-fungal effects and find out the active metabolites from sponge, Sigmadocia carnosa (S. carnosa) against four dermatophytic fungi.

METHODS

The methanol, ethyl acetate and acetone extract of marine sponge, S. carnosa was examined against Trichophyton mentagrophytes (T. mentagrophytes), Trichophyton rubrum (T. rubrum), Epidermophyton floccosum (E. floccosum) and Microsporum gypseum (M. gypseum) and qualitative analysed to find out the active molecules.

RESULTS

The methanol extract of sponge was expressed significant activity than ethyl acetate and acetone. The minimum inhibitory concentration (MIC) of methanol extract of sponge that resulted in complete growth inhibition of T. mentagrophytes, T. rubrum, E. floccosum and M. gypseum were found to 125, 250, 250 and 250 µg/mL respectively. But, 100 % inhibition of fungal spore germination was observed in T. mentagrophytes at 500 µg/mL concentration followed by T. rubrum, E. floccosum and M. gypseum at 1 000 µg/mL concentration. Other two extracts showed weak anti spore germination activity against the tested dermatophytic fungi. Methanol extracts showed presence of terpenoids, steroids, alkaloids, saponins and glycosides.

CONCLUSION

Based on the literature, this is the first study which has conducted to inhibit the growth and spore germination of dermatophytic fungi with S. carnosa. Further research also needs to purify and characterize the secondary metabolites from the sponge, S. carnosa for the valuable source of novel substances for future drug discovery.

Overcoming antifungal resistance

Fungal infections have become one of the major causes of morbidity and mortality in immunocompromised patients. Despite increased awareness and improved treatment strategies, the frequent development of resistance to the antifungal drugs used in clinical settings contributes to the increasing toll of mycoses. Although a natural phenomenon, antifungal drug resistance can compromise advances in the development of effective diagnostic techniques and novel antifungals. In this review, we will discuss the advent of cellular-micro- arrays, microfluidics, genomics, proteomics and other state-of-the art technologies in conquering antifungal drug resistance.

Novel strategies to fight Candida species infection

In recent years, there has been a significant increase in the incidence of human fungal infections. The increase in cases of infection caused by Candida species, and the consequent excessive use of antimicrobials, has favored the emergence of resistance to conventional antifungal agents over the past decades. Consequently, Candida infections morbidity and mortality are also increasing. Therefore, new approaches are needed to improve the outcome of patients suffering from Candida infections, because it seems unlikely that the established standard treatments will drastically lower the morbidity of mucocutaneous Candida infections and the high mortality associated with invasive candidiasis. This review aims to present the last advances in the traditional antifungal therapy, and present an overview of novel strategies that are being explored for the treatment of Candida infections, with a special focus on combined antifungal agents, antifungal therapies with alternative compounds (plant extracts and essential oils), adjuvant immunotherapy, photodynamic therapy and laser therapy.

Resistance reversal induced by a combination of fluconazole and tacrolimus (FK506) in Candida glabrata

There is an increasing concern about Candida glabrata due to its high isolation frequency in candidiasis recently and notorious drug resistance to fluconazole. Drug combination is one effective approach to counteract drug resistance. This study aimed to test whether a combination of fluconazole and tacrolimus (FK506) had a synergistic effect on C. glabrata, and to seek the potential mechanisms underlying the synergistic effects. In vitro effects of fluconazole and FK506 against C. glabrata with different susceptibilities were investigated by a chequerboard method and a time-kill curve method. The mechanistic studies against the resistant C. glabrata were performed from two aspects: quantification of expression levels of fluconazole resistance genes (ERG11, CDR1, PDH1 and SNQ2) by real-time quantitative PCR and functional assays of drug efflux pumps. The addition of FK506 resulted in a decrease in the MIC of fluconazole from 32 to 8 µg ml(-1) against the dose-dependent susceptible C. glabrata, and from 256 to 16 µg ml(-1) against the resistant C. glabrata, respectively. The synergy was further confirmed by the time-kill assay. The expression levels of the ERG11 and SNQ2 genes were significantly downregulated after exposure to the drug combination, whereas that of the CDR1 gene was significantly upregulated, and no significant change in expression of PDH1 gene was observed. Flow cytometric assays showed that FK506 reduced the efflux of fluconazole. Tacrolimus enhanced the susceptibility of fluconazole against resistant C. glabrata by reducing the expression levels of the ERG11 and SNQ2 genes and inhibiting fluconazole efflux.

In vitro interactions of calcineurin inhibitors with conventional antifungal agents against the yeast form of Penicillium marneffei

Penicillium marneffei can cause a life-threatening disseminated mycosis in immunocompromised hosts. However, therapeutic strategies for the treatment of this infectious disease are limited. Reports of other fungi suggest that calcineurin inhibitors interact with antifungal agents to improve the treatment outcomes. Here, we evaluated the in vitro interaction of the calcineurin inhibitors cyclosporine A and tacrolimus (FK506) combined with conventional antifungal agents against the pathogenic yeast form of P. marneffei. We demonstrate that the combination of cyclosporine A with amphotericin B, itraconazole, or fluconazole was synergistic for 85, 65, and 30 % of P. marneffei strains, respectively. In contrast, no synergism was observed in all the combinations containing tacrolimus. Furthermore, antagonism was not observed for any combination. In conclusion, the therapeutic potential of a combinatory approach using the calcineurin inhibitor cyclosporine A with conventional antifungal drugs may lead to improved treatment regimens for P. marneffei infections. We propose that mechanism of action studies with cyclosporine A and antifungal agents is needed.

The calcineurin inhibitor cyclosporin A exhibits synergism with antifungals against Candida parapsilosis species complex

Candida parapsilosis complex comprises three closely related species, C. parapsilosis sensu stricto, Candida metapsilosis and Candida orthopsilosis. In the last decade, antifungal resistance to azoles and caspofungin among C. parapsilosis sensu lato strains has been considered a matter of concern worldwide. In the present study, we evaluated the synergistic potential of antifungals and the calcineurin inhibitor cyclosporin A (Cys) against planktonic and biofilms of C. parapsilosis complex from clinical sources. Susceptibility assays with amphotericin, fluconazole, voriconazole, caspofungin and Cys were performed by microdilution in accordance with Clinical and Laboratory Standards Institute guidelines. Synergy testing against planktonic cells of C. parapsilosis sensu lato strains was assessed by the chequerboard method. Combinations formed by antifungals with Cys were evaluated against mature biofilms in microtitre plates. No differences in the antifungal susceptibility pattern among species were observed, but C. parapsilosis sensu stricto strains were more susceptible to Cys than C. orthopsilosis and C. metapsilosis. Synergism between antifungals and Cys was observed in C. parapsilosis sensu lato strains. Combinations formed by antifungals and Cys were able to prevent biofilm formation and showed an inhibitory effect against mature biofilms of C. parapsilosis sensu stricto, C. metapsilosis and C. orthopsilosis. These results strengthen the potential of calcineurin inhibition as a promising approach to enhance the efficiency of antifungal drugs.

Mechanisms of Candida biofilm drug resistance

Candida commonly adheres to implanted medical devices, growing as a resilient biofilm capable of withstanding extraordinarily high antifungal concentrations. As currently available antifungals have minimal activity against biofilms, new drugs to treat these recalcitrant infections are urgently needed. Recent investigations have begun to shed light on the mechanisms behind the profound resistance associated with the biofilm mode of growth. This resistance appears to be multifactorial, involving both mechanisms similar to conventional, planktonic antifungal resistance, such as increased efflux pump activity, as well as mechanisms specific to the biofilm lifestyle. A unique biofilm property is the production of an extracellular matrix. Two components of this material, β-glucan and extracellular DNA, promote biofilm resistance to multiple antifungals. Biofilm formation also engages several stress response pathways that impair the activity of azole drugs. Resistance within a biofilm is often heterogeneous, with the development of a subpopulation of resistant persister cells. In this article we review the molecular mechanisms underlying Candida biofilm antifungal resistance and their relative contributions during various growth phases.

FK520 interacts with the discrete intrahelical amino acids of multidrug transporter Cdr1 protein and acts as antagonist to selectively chemosensitize azole-resistant clinical isolates of Candida albicans

FK520, a homolog of antifungal FK506, displays fungicidal synergism with azoles in Candida albicans and inhibits drug efflux mediated by ABC multidrug transporter. This study establishes the molecular basis of interaction of FK520 with Cdr1 protein, which is one of the major ABC multidrug transporters of C. albicans. For this, we have exploited an in-house library of Cdr1 protein consisting of 252 mutant variants where the entire primary structure of the two transmembrane domains comprising of 12 transmembrane helices was subjected to alanine scanning. With these mutant variants of Cdr1 protein, we could identify the critical amino acids of the transporter protein, which if replaced with alanine, not only abrogated FK520-dependent competitive inhibition of drug efflux but simultaneously decreased susceptibility to azoles. Notably, the replacement of most of the residues with alanine was inconsequential; however, there were close to 13% mutant variants, which showed abrogation of drug efflux and reversal of fungicidal synergy with azoles. Of note, all the intrahelical residues of Cdr1 protein, which abrogated inhibitor's ability to block the efflux and reversed fungicidal synergy, were common. Taken together, our results provide evidence of cross-talk of FK520 with Cdr1 by interacting with the select intrahelical residues of the protein to chemosensitize isolates of Candida.

Future directions for anti-biofilm therapeutics targeting Candida

While proliferating in its most common mode of growth, a biofilm, Candida spp. exhibit increased resistance to available antifungal agents. These adherent communities are difficult to eradicate and often responsible for treatment failures. New therapies are urgently needed to treat a variety of Candida biofilm infections in the medical setting. This review discusses the medical relevance of Candida biofilms, the drug resistance associated with this mode of growth, and approaches to combat these resilient infections.

In vitro screening of antifungal compounds able to counteract biofilm development

Fungi are able to grow as a single-species or a more complex biofilm attached to inert surfaces (catheters…) or tissues (lung…). This last form is a microbial niche which must be considered as a major risk factor of developing a human fungal infection. Nowadays, only a few therapeutic agents have been shown to be active against fungal biofilms in vitro and/or in vivo. So there is a real need to find new anti-biofilm molecules. Here we describe in detail some rapid, 96-well microtiter plate-based methods, for the screening of compounds with anti-biofilm activity against Candida spp. yeasts. Two approaches will be considered: prophylactic or curative effects of the tested compounds by producing biofilms on two supports - polystyrene well surfaces and catheter sections.

The effect of antifungal combination on transcripts of a subset of drug-resistance genes in clinical isolates of Candida species induced biofilms

Biofilm formation is often associated with increased Candida resistance toward antifungal agents. Therefore, the current study aimed to assess the incidence of biofilm formation among Candida isolates and to investigate the effect of high doses of fluconazole {FLC}, voriconazole {VOC} and amphotericin B {AMB}, singly and in combination on mature biofilms. Moreover, it aimed to assess the expression of selected genes (CDR1, KRE1 and SKN1) responsible for Candida biofilm resistance. The study included 49 patients; samples were collected from the King Khalid Hospital, Riyadh, Saudi Arabia. Isolates were prepared for biofilm formation and quantification using 0.4% (w/v) crystal violet. Minimum Inhibitory concentration (MIC) and fractional inhibitory concentration (FIC) were conducted by the broth microdilution method. Biofilm eradication was evaluated using counting, XTT stain intensity and observed under the inverted microscope. Selected genes were evaluated in Candida biofilms under the effect of antifungal exposure using QPCR. The major isolates were Candida albicans (65.3%) followed by Candida tropicalis and Candida glabrata. 77.6% of the strains were biofilm formers. AMB showed susceptibility in 87.8% of isolates, followed by VOC (77.6%) and FLC (67.3%). MIC50 and MIC90 were (0.03, 0.125), (0.5, 8), (2, >128) μg/ml for AMB, VOC and FLC, respectively. 34.7% and 18.4% of the isolates were antagonistic to AMB/FLC and AMB/VOC, respectively. Mature biofilms of ten selected isolates were found resistant to FLC (1000 μg/ml). VOR and AMB concentration required to inhibit biofilm formation was 16-250 fold higher than the MIC for planktonic cells. Isolates showed significant reduction with antifungal combination when compared with the untreated controls (p value ⩽ 0.01), or using fluconazole alone (p value ⩽ 0.05). High doses of the antifungals were employed to assess the effect on the persisters' selected gene expression. Marked over expression of SKN1 and to a lesser extent KRE1 was noticed among the mature biofilms treated with AMB alone or in combination after 1 h of exposure, and SKN1 expression was even more sharply induced after 24 h. No statistically significant over expression of CDR1 was observed in biofilms after exposure to high doses of FLC, VOC or any of the combinations used.

In vitro study of sequential fluconazole and caspofungin treatment against Candida albicans biofilms

Candida albicans biofilms are generally considered to be resistant to azole antifungal agents but susceptible to echinocandins. We demonstrate that in a sequential therapy regimen, treatment with fluconazole first followed by caspofungin leads to a significant decrease of the efficacy of this echinocandin. Cellular stress responses induced by high fluconazole concentrations and mediated by Hsp90 and calcineurin play an important role in this phenomenon.

Structural optimization of berberine as a synergist to restore antifungal activity of fluconazole against drug-resistant Candida albicans

We have conducted systematic structural modification, deconstruction, and reconstruction of the berberine core with the aim of lowering its cytotoxicity, investigating its pharmacophore, and ultimately, seeking novel synergistic agents to restore the effectiveness of fluconazole against fluconazole-resistant Candida albicans. A structure-activity relationship study of 95 analogues led us to identify the novel scaffold of N-(2-(benzo[d][1,3]dioxol-5-yl)ethyl)-2-(substituted phenyl)acetamides 7 a-l, which exhibited remarkable levels of in vitro synergistic antifungal activity. Compound 7 d (N-(2-(benzo[d][1,3]dioxol-5-yl)ethyl)-2-(2-fluorophenyl)acetamide) significantly decreased the MIC₈₀ values of fluconazole from 128.0 μg mL⁻¹ to 0.5 μg mL⁻¹ against fluconazole-resistant C. albicans and exhibited much lower levels of cytotoxicity than berberine toward human umbilical vein endothelial cells.

In vitro analysis of flufenamic acid activity against Candida albicans biofilms

In a recent high-throughput screen against specific Candida albicans drug targets, several compounds that exhibited non-specific antifungal activity were identified, including the non-steroidal anti-inflammatory drug flufenamic acid (FFA). This study sought to determine the effect of different doses of FFA, alone or in combination with fixed concentrations of the standard antifungal agents amphotericin B (AmB), caspofungin (CAS) or fluconazole (FLU), for the prevention and treatment of C. albicans biofilms. Biofilms were formed in a 96-well microplate followed by evaluation of antifungal activity using the XTT assay. FFA concentrations of ≥512mg/L demonstrated >80% prevention of biofilm formation. FFA concentrations of 1024mg/L demonstrated >85% reduction of mature biofilms. When FFA (≥8mg/L) was used in combination with FLU (32mg/L), antifungal activity increased to 99% for the prevention of biofilm formation. Similarly, when a FFA concentration of ≥8mg/L was used in combination with either AmB (0.25mg/L) or CAS (0.125mg/L), antifungal activity also increased up to 99% for the prevention of biofilm formation. The inhibitory effect of FFA on C. albicans biofilms has not been reported previously, therefore these findings suggest that FFA in combination with traditional antifungals might be useful for the treatment and prevention of C. albicans biofilms.

Antifungal therapy with an emphasis on biofilms

Fungal infections are on the rise as advances in modern medicine prolong the lives of severely ill patients. Fungi are eukaryotic organisms and there are a limited number of targets for antifungal drug development; as a result the antifungal arsenal is exceedingly limited. Azoles, polyenes and echinocandins constitute the mainstay of antifungal therapy for patients with life-threatening mycoses. One of the main factors complicating antifungal therapy is the formation of fungal biofilms, microbial communities displaying resistance to most antifungal agents. A better understanding of fungal biofilms provides for new opportunities for the development of urgently needed novel antifungal agents and strategies.

Recent insights into Candida albicans biofilm resistance mechanisms

Like other microorganisms, free-living Candida albicans is mainly present in a three-dimensional multicellular structure, which is called a biofilm, rather than in a planktonic form. Candida albicans biofilms can be isolated from both abiotic and biotic surfaces at various locations within the host. As the number of abiotic implants, mainly bloodstream and urinary catheters, has been increasing, the number of biofilm-associated bloodstream or urogenital tract infections is also strongly increasing resulting in a raise in mortality. Cells within a biofilm structure show a reduced susceptibility to specific commonly used antifungals and, in addition, it has recently been shown that such cells are less sensitive to killing by components of our immune system. In this review, we summarize the most important insights in the mechanisms underlying biofilm-associated antifungal drug resistance and immune evasion strategies, focusing on the most recent advances in this area of research.

High-throughput nano-biofilm microarray for antifungal drug discovery

Micro- and nanoscale technologies have radically transformed biological research from genomics to tissue engineering, with the relative exception of microbial cell culture, which is still largely performed in microtiter plates and petri dishes. Here, we present nanoscale culture of the opportunistic fungal pathogen Candida albicans on a microarray platform. The microarray consists of 1,200 individual cultures of 30 nl of C. albicans biofilms (“nano-biofilms”) encapsulated in an inert alginate matrix. We demonstrate that these nano-biofilms are similar to conventional macroscopic biofilms in their morphological, architectural, growth, and phenotypic characteristics. We also demonstrate that the nano-biofilm microarray is a robust and efficient tool for accelerating the drug discovery process: (i) combinatorial screening against a collection of 28 antifungal compounds in the presence of immunosuppressant FK506 (tacrolimus) identified six drugs that showed synergistic antifungal activity, and (ii) screening against the NCI challenge set small-molecule library identified three heretofore-unknown hits. This cell-based microarray platform allows for miniaturization of microbial cell culture and is fully compatible with other high-throughput screening technologies.

Microorganisms are typically still grown in petri dishes, test tubes, and Erlenmeyer flasks in spite of the latest advances in miniaturization that have benefitted other allied research fields, including genomics and proteomics. Culturing microorganisms in small scale can be particularly valuable in cutting down time, cost, and reagent usage. This paper describes the development, characterization, and application of nanoscale culture of an opportunistic fungal pathogen, Candida albicans. Despite a more than 2,000-fold reduction in volume, the growth characteristics and drug response profiles obtained from the nanoscale cultures were comparable to the industry standards. The platform also enabled rapid identification of new drug candidates that were effective against C. albicans biofilms, which are a major cause of mortality in hospital-acquired infections.

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.

Chloroquine sensitizes biofilms of Candida albicans to antifungal azoles

Biofilms formed by Candida albicans, a human pathogen, are known to be resistant to different antifungal agents. Novel strategies to combat the biofilm associated Candida infections like multiple drug therapy are being explored. In this study, potential of chloroquine to be a partner drug in combination with four antifungal agents, namely fluconazole, voriconazole, amphotericin B, and caspofungin, was explored against biofilms of C. albicans. Activity of various concentrations of chloroquine in combination with a particular antifungal drug was analyzed in a checkerboard format. Growth of biofilm in presence of drugs was analyzed by XTT-assay, in terms of relative metabolic activity compared to that of drug free control. Results obtained by XTT-metabolic assay were confirmed by scanning electron microscopy. The interactions between chloroquine and four antifungal drugs were determined by calculating fractional inhibitory concentration indices. Azole resistance in biofilms was reverted significantly (p < 0.05) in presence of 250 μg/mL of chloroquine, which resulted in inhibition of biofilms at very low concentrations of antifungal drugs. No significant alteration in the sensitivity of biofilms to caspofungin and amphotericin B was evident in combination with chloroquine. This study for the first time indicates that chloroquine potentiates anti-biofilm activity of fluconazole and voriconazole.

Genetic and genomic architecture of the evolution of resistance to antifungal drug combinations

The evolution of drug resistance in fungal pathogens compromises the efficacy of the limited number of antifungal drugs. Drug combinations have emerged as a powerful strategy to enhance antifungal efficacy and abrogate drug resistance, but the impact on the evolution of drug resistance remains largely unexplored. Targeting the molecular chaperone Hsp90 or its downstream effector, the protein phosphatase calcineurin, abrogates resistance to the most widely deployed antifungals, the azoles, which inhibit ergosterol biosynthesis. Here, we evolved experimental populations of the model yeast Saccharomyces cerevisiae and the leading human fungal pathogen Candida albicans with azole and an inhibitor of Hsp90, geldanamycin, or calcineurin, FK506. To recapitulate a clinical context where Hsp90 or calcineurin inhibitors could be utilized in combination with azoles to render resistant pathogens responsive to treatment, the evolution experiment was initiated with strains that are resistant to azoles in a manner that depends on Hsp90 and calcineurin. Of the 290 lineages initiated, most went extinct, yet 14 evolved resistance to the drug combination. Drug target mutations that conferred resistance to geldanamycin or FK506 were identified and validated in five evolved lineages. Whole-genome sequencing identified mutations in a gene encoding a transcriptional activator of drug efflux pumps, PDR1, and a gene encoding a transcriptional repressor of ergosterol biosynthesis genes, MOT3, that transformed azole resistance of two lineages from dependent on calcineurin to independent of this regulator. Resistance also arose by mutation that truncated the catalytic subunit of calcineurin, and by mutation in LCB1, encoding a sphingolipid biosynthetic enzyme. Genome analysis revealed extensive aneuploidy in four of the C. albicans lineages. Thus, we identify molecular determinants of the transition of azole resistance from calcineurin dependence to independence and establish multiple mechanisms by which resistance to drug combinations evolves, providing a foundation for predicting and preventing the evolution of drug resistance.

Fungal infections are a leading cause of mortality worldwide and are difficult to treat due to the limited number of antifungal drugs, whose effectiveness is compromised by the emergence of drug resistance. A powerful strategy to combat drug resistance is combination therapy. Inhibiting the molecular chaperone Hsp90 or its downstream effector calcineurin cripples fungal stress responses and abrogates drug resistance. Here we provide the first analysis of the genetic and genomic changes that underpin the evolution of resistance to antifungal drug combinations in the leading human fungal pathogen, Candida albicans, and model yeast, Saccharomyces cerevisiae. We evolved experimental populations with combinations of inhibitors of Hsp90 or calcineurin and the most widely used antifungal in the clinic, the azoles, which inhibit ergosterol biosynthesis. We harnessed whole-genome sequencing to identify diverse resistance mutations among the 14 lineages that evolved resistance to the drug combination. These included mutations in genes encoding the drug targets, a transcriptional regulator of multidrug transporters, a transcriptional repressor of ergosterol biosynthesis enzymes, and a regulator of sphingolipid biosynthesis. We also identified extensive aneuploidies in several C. albicans lineages. Our study reveals multiple mechanisms by which resistance to drug combination can evolve, suggesting new strategies to combat drug resistance.

Hsp21 potentiates antifungal drug tolerance in Candida albicans

Systemic infections of humans with the fungal pathogen Candida albicans are associated with a high mortality rate. Currently, efficient treatment of these infections is hampered by the relatively low number of available antifungal drugs. We recently identified the small heat shock protein Hsp21 in C. albicans and demonstrated its fundamental role for environmental stress adaptation and fungal virulence. Hsp21 was found in several pathogenic Candida species but not in humans. This prompted us to investigate the effects of a broad range of different antifungal drugs on an Hsp21-null C. albicans mutant strain. Our results indicate that combinatorial therapy targeting Hsp21, together with specific antifungal drug targets, has strong synergistic potential. In addition, we demonstrate that Hsp21 is required for tolerance to ethanol-induced stress and induction of filamentation in response to pharmacological inhibition of Hsp90. These findings might pave the way for the development of new treatment strategies against Candida infections.

Antimicrobial photodynamic inactivation inhibits Candida albicans virulence factors and reduces in vivo pathogenicity

The objective of this study was to evaluate whether Candida albicans exhibits altered pathogenicity characteristics following sublethal antimicrobial photodynamic inactivation (APDI) and if such alterations are maintained in the daughter cells. C. albicans was exposed to sublethal APDI by using methylene blue (MB) as a photosensitizer (0.05 mM) combined with a GaAlAs diode laser (λ 660 nm, 75 mW/cm(2), 9 to 27 J/cm(2)). In vitro, we evaluated APDI effects on C. albicans growth, germ tube formation, sensitivity to oxidative and osmotic stress, cell wall integrity, and fluconazole susceptibility. In vivo, we evaluated C. albicans pathogenicity with a mouse model of systemic infection. Animal survival was evaluated daily. Sublethal MB-mediated APDI reduced the growth rate and the ability of C. albicans to form germ tubes compared to untreated cells (P < 0.05). Survival of mice systemically infected with C. albicans pretreated with APDI was significantly increased compared to mice infected with untreated yeast (P < 0.05). APDI increased C. albicans sensitivity to sodium dodecyl sulfate, caffeine, and hydrogen peroxide. The MIC for fluconazole for C. albicans was also reduced following sublethal MB-mediated APDI. However, none of those pathogenic parameters was altered in daughter cells of C. albicans submitted to APDI. These data suggest that APDI may inhibit virulence factors and reduce in vivo pathogenicity of C. albicans. The absence of alterations in daughter cells indicates that APDI effects are transitory. The MIC reduction for fluconazole following APDI suggests that this antifungal could be combined with APDI to treat C. albicans infections.

In vitro activity of verapamil alone and in combination with fluconazole or tunicamycin against Candida albicans biofilms

Calcium channels and pumps play important roles in morphogenesis, stress response and virulence in Candida albicans. We hypothesised that verapamil, a potent calcium channel blocker, may display an inhibitory effect on C. albicans biofilms. To test this hypothesis, the in vitro activity of verapamil was evaluated alone and in combination with fluconazole or tunicamycin against C. albicans biofilms using a 96-well microtitre plate model. As expected, verapamil exerted inhibitory activity against C. albicans biofilms. The combinations of verapamil/fluconazole and verapamil/tunicamycin yielded synergistic effects on biofilm formation and on pre-formed biofilms. Furthermore, verapamil alone or in combination with fluconazole or tunicamycin led to a significant decrease in the transcription level of ALS3, essential for biofilm development. Therefore, verapamil may be a potential agent to enhance the effect of antifungal drugs against C. albicans biofilms.

Functional characterization of calcineurin homologs PsCNA1/PsCNB1 in Puccinia striiformis f. sp. tritici using a host-induced RNAi system

Calcineurin plays a key role in morphogenesis, pathogenesis and drug resistance in most fungi. However, the function of calcineurin genes in Puccinia striiformis f. sp. tritici (Pst) is unclear. We identified and characterized the calcineurin genes PsCNA1 and PsCNB1 in Pst. Phylogenetic analyses indicate that PsCNA1 and PsCNB1 form a calcium/calmodulin regulated protein phosphatase belonging to the calcineurin heterodimers composed of subunits A and B. Quantitative RT-PCR analyses revealed that both PsCNA1 and PsCNB1 expression reached their maximum in the stage of haustorium formation, which is one day after inoculation. Using barely stripe mosaic virus (BSMV) as a transient expression vector in wheat, the expression of PsCNA1 and PsCNB1 in Pst was suppressed, leading to slower extension of fungal hyphae and reduced production of urediospores. The immune-suppressive drugs cyclosporin A and FK506 markedly reduced the germination rates of urediospores, and when germination did occur, more than two germtubes were produced. These results suggest that the calcineurin signaling pathway participates in stripe rust morphogenetic differentiation, especially the formation of haustoria during the early stage of infection and during the production of urediospores. Therefore PsCNA1 and PsCNB1 can be considered important pathogenicity genes involved in the wheat-Pst interaction.

Sensitization of Candida albicans biofilms to various antifungal drugs by cyclosporine A

Background

Biofilms formed by Candida albicans are resistant towards most of the available antifungal drugs. Therefore, infections associated with Candida biofilms are considered as a threat to immunocompromised patients. Combinatorial drug therapy may be a good strategy to combat C. albicans biofilms.

Methods

Combinations of five antifungal drugs- fluconazole (FLC), voriconazole (VOR), caspofungin (CSP), amphotericin B (AmB) and nystatin (NYT) with cyclosporine A (CSA) were tested in vitro against planktonic and biofilm growth of C. albicans. Standard broth micro dilution method was used to study planktonic growth, while biofilms were studied in an in vitro biofilm model. A chequerboard format was used to determine fractional inhibitory concentration indices (FICI) of combination effects. Biofilm growth was analyzed using XTT-metabolic assay.

Results

MICs of various antifungal drugs for planktonic growth of C. albicans were lowered in combination with CSA by 2 to 16 fold. Activity against biofilm development with FIC indices of 0.26, 0.28, 0.31 and 0.25 indicated synergistic interactions between FLC-CSA, VOR-CSA, CSP-CSA and AmB-CSA, respectively. Increase in efficacy of the drugs FLC, VOR and CSP against mature biofilms after addition of 62.5 μg/ml of CSA was evident with FIC indices 0.06, 0.14 and 0.37, respectively.

Conclusions

The combinations with CSA resulted in increased susceptibility of biofilms to antifungal drugs. Combination of antifungal drugs with CSA would be an effective prophylactic and therapeutic strategy against biofilm associated C. albicans infections.

A Candida biofilm-induced pathway for matrix glucan delivery: implications for drug resistance

Extracellular polysaccharides are key constituents of the biofilm matrix of many microorganisms. One critical carbohydrate component of Candida albicans biofilms, β-1,3 glucan, has been linked to biofilm protection from antifungal agents. In this study, we identify three glucan modification enzymes that function to deliver glucan from the cell to the extracellular matrix. These enzymes include two predicted glucan transferases and an exo-glucanase, encoded by BGL2, PHR1, and XOG1, respectively. We show that the enzymes are crucial for both delivery of β-1,3 glucan to the biofilm matrix and for accumulation of mature matrix biomass. The enzymes do not appear to impact cell wall glucan content of biofilm cells, nor are they necessary for filamentation or biofilm formation. We demonstrate that mutants lacking these genes exhibit enhanced susceptibility to the commonly used antifungal, fluconazole, during biofilm growth only. Transcriptional analysis and biofilm phenotypes of strains with multiple mutations suggest that these enzymes act in a complementary fashion to distribute matrix downstream of the primary β-1,3 glucan synthase encoded by FKS1. Furthermore, our observations suggest that this matrix delivery pathway works independently from the C. albicans ZAP1 matrix formation regulatory pathway. These glucan modification enzymes appear to play a biofilm-specific role in mediating the delivery and organization of mature biofilm matrix. We propose that the discovery of inhibitors for these enzymes would provide promising anti-biofilm therapeutics.

Viral protease inhibitors affect the production of virulence factors in Cryptococcus neoformans

The effects of the protease inhibitors saquinavir, darunavir, ritonavir, and indinavir on growth inhibition, protease and phospholipase activities, as well as capsule thickness of Cryptococcus neoformans were investigated. Viral protease inhibitors did not reduce fungal growth when tested in concentrations ranging from 0.001 to 1.000 mg/L. A tendency toward increasing phospholipase activity was observed with the highest tested drug concentration in a strain-specific pattern. However, these drugs reduced protease activity as well as capsule production. Our results confirm a previous finding that antiretroviral drugs affect the production of important virulence factors of C. neoformans.

Amino acid-derived 1,2-benzisothiazolinone derivatives as novel small-molecule antifungal inhibitors: identification of potential genetic targets

We have identified four synthetic compounds (DFD-VI-15, BD-I-186, DFD-V-49, and DFD-V-66) from an amino acid-derived 1,2-benzisothiazolinone (BZT) scaffold that have reasonable MIC(50) values against a panel of fungal pathogens. These compounds have no structural similarity to existing antifungal drugs. Three of the four compounds have fungicidal activity against Candida spp., Cryptococcus neoformans, and several dermatophytes, while one is fungicidal to Aspergillus fumigatus. The kill rates of our compounds are equal to those in clinical usage. The BZT compounds remain active against azole-, polyene-, and micafungin-resistant strains of Candida spp. A genetics-based approach, along with phenotype analysis, was used to begin mode of action (MOA) studies of one of these compounds, DFD-VI-15. The genetics-based screen utilized a homozygous deletion collection of approximately 4,700 Saccharomyces cerevisiae mutants. We identified mutants that are both hypersensitive and resistant. Using FunSpec, the hypersensitive mutants and a resistant ace2 mutant clustered within a category of genes related directly or indirectly to mitochondrial functions. In Candida albicans, the functions of the Ace2p transcription factor include the regulation of glycolysis. Our model is that DFD-VI-15 targets a respiratory pathway that limits energy production. Supporting this hypothesis are phenotypic data indicating that DFD-VI-15 causes increased cell-reactive oxidants (ROS) and a decrease in mitochondrial membrane potential. Also, the same compound has activity when cells are grown in a medium containing glycerol (mitochondrial substrate) but is much less active when cells are grown anaerobically.

Global analysis of the evolution and mechanism of echinocandin resistance in Candida glabrata

The evolution of drug resistance has a profound impact on human health. Candida glabrata is a leading human fungal pathogen that can rapidly evolve resistance to echinocandins, which target cell wall biosynthesis and are front-line therapeutics for Candida infections. Here, we provide the first global analysis of mutations accompanying the evolution of fungal drug resistance in a human host utilizing a series of C. glabrata isolates that evolved echinocandin resistance in a patient treated with the echinocandin caspofungin for recurring bloodstream candidemia. Whole genome sequencing identified a mutation in the drug target, FKS2, accompanying a major resistance increase, and 8 additional non-synonymous mutations. The FKS2-T1987C mutation was sufficient for echinocandin resistance, and associated with a fitness cost that was mitigated with further evolution, observed in vitro and in a murine model of systemic candidemia. A CDC6-A511G(K171E) mutation acquired before FKS2-T1987C(S663P), conferred a small resistance increase. Elevated dosage of CDC55, which acquired a C463T(P155S) mutation after FKS2-T1987C(S663P), ameliorated fitness. To discover strategies to abrogate echinocandin resistance, we focused on the molecular chaperone Hsp90 and downstream effector calcineurin. Genetic or pharmacological compromise of Hsp90 or calcineurin function reduced basal tolerance and resistance. Hsp90 and calcineurin were required for caspofungin-dependent FKS2 induction, providing a mechanism governing echinocandin resistance. A mitochondrial respiration-defective petite mutant in the series revealed that the petite phenotype does not confer echinocandin resistance, but renders strains refractory to synergy between echinocandins and Hsp90 or calcineurin inhibitors. The kidneys of mice infected with the petite mutant were sterile, while those infected with the HSP90-repressible strain had reduced fungal burden. We provide the first global view of mutations accompanying the evolution of fungal drug resistance in a human host, implicate the premier compensatory mutation mitigating the cost of echinocandin resistance, and suggest a new mechanism of echinocandin resistance with broad therapeutic potential.

The evolution of drug resistance poses a severe threat to human health. Candida glabrata is a leading cause of mortality due to fungal infections worldwide. It can rapidly evolve resistance to drugs such as echinocandins, which target the fungal cell wall and are front-line therapeutics for Candida infections. We harness whole genome sequencing to provide a global view of mutations that accumulate in C. glabrata during the evolution of echinocandin resistance in a human host. Nine non-synonymous mutations were identified, including one in the echinocandin target. A mutation in an additional gene conferred a small resistance increase and another was in a gene whose dosage mitigated the fitness cost of resistance. We further discovered that compromising function of the molecular chaperone Hsp90 abrogates drug resistance and reduces kidney fungal burden in a mouse model of infection. Hsp90 and its downstream effector calcineurin are required for induction of the drug target in response to drug. Thus, we reveal the first global portrait of antifungal resistance mutations that evolve in a human host, identify the first compensatory mutation that mitigates the cost of echinocandin resistance, and suggest a new mechanism of echinocandin resistance that can be exploited to treat life-threatening fungal infections.

Identification and characterization of antifungal compounds using a Saccharomyces cerevisiae reporter bioassay

New antifungal drugs are urgently needed due to the currently limited selection, the emergence of drug resistance, and the toxicity of several commonly used drugs. To identify drug leads, we screened small molecules using a Saccharomyces cerevisiae reporter bioassay in which S. cerevisiae heterologously expresses Hik1, a group III hybrid histidine kinase (HHK) from Magnaporthe grisea. Group III HHKs are integral in fungal cell physiology, and highly conserved throughout this kingdom; they are absent in mammals, making them an attractive drug target. Our screen identified compounds 13 and 33, which showed robust activity against numerous fungal genera including Candida spp., Cryptococcus spp. and molds such as Aspergillus fumigatus and Rhizopus oryzae. Drug-resistant Candida albicans from patients were also highly susceptible to compounds 13 and 33. While the compounds do not act directly on HHKs, microarray analysis showed that compound 13 induced transcripts associated with oxidative stress, and compound 33, transcripts linked with heavy metal stress. Both compounds were highly active against C. albicans biofilm, in vitro and in vivo, and exerted synergy with fluconazole, which was inactive alone. Thus, we identified potent, broad-spectrum antifungal drug leads from a small molecule screen using a high-throughput, S. cerevisiae reporter bioassay.

Fungal biofilm resistance

Fungal biofilm infections have become increasingly recognised as a significant clinical problem. One of the major reasons behind this is the impact that these have upon treatment, as antifungal therapy often fails and surgical intervention is required. This places a large financial burden on health care providers. This paper aims to illustrate the importance of fungal biofilms, particularly Candida albicans, and discusses some of the key fungal biofilm resistance mechanisms that include, extracellular matrix (ECM), efflux pump activity, persisters, cell density, overexpression of drug targets, stress responses, and the general physiology of the cell. The paper demonstrates the multifaceted nature of fungal biofilm resistance, which encompasses some of the newest data and ideas in the field.

A D-octapeptide drug efflux pump inhibitor acts synergistically with azoles in a murine oral candidiasis infection model

Clinical management of patients undergoing treatment of oropharyngeal candidiasis with azole antifungals can be impaired by azole resistance. High-level azole resistance is often caused by the overexpression of Candida albicans efflux pump Cdr1p. Inhibition of this pump therefore represents a target for combination therapies that reverse azole resistance. We assessed the therapeutic potential of the D-octapeptide derivative RC21v3, a Cdr1p inhibitor, in the treatment of murine oral candidiasis caused by either the azole-resistant C. albicans clinical isolate MML611 or its azole-susceptible parental strain MML610. RC21v3, fluconazole (FLC), or a combination of both drugs were administered orally to immunosuppressed ICR mice at 3, 24, and 27 h after oral inoculation with C. albicans. FLC protected the mice inoculated with MML610 from oral candidiasis, but was only partially effective in MML611-infected mice. The co-application of RC21v3 (0.02 μmol per dose) potentiated the therapeutic performance of FLC for mice infected with either strain. It caused a statistically significant decrease in C. albicans cfu isolated from the oral cavity of the infected mice and reduced oral lesions. RC21v3 also enhanced the therapeutic activity of itraconazole against MML611 infection. These results indicate that RC21v3 in combination with azoles has potential as a therapy against azole-resistant oral candidiasis.

Modeling of fungal biofilms using a rat central vein catheter

Candida frequently grows as a biofilm, or an adherent community of cells protected from both the host immune system and antimicrobial therapies. Biofilms represent the predominant mode of growth for many clinical infections, including those associated with placement of a medical device. Here, we describe a model for Candida biofilm infection of one important clinical niche, a venous catheter. This animal model system incorporates the anatomical site, immune components, and fluid dynamics of a patient venous catheter infection and can be used for study of biofilm formation, drug resistance, and gene expression.

Interface of Candida albicans biofilm matrix-associated drug resistance and cell wall integrity regulation

Candida albicans frequently infects medical devices by growing as a biofilm, i.e., a community of adherent organisms entrenched in an extracellular matrix. During biofilm growth, Candida spp. acquire the ability to resist high concentrations of antifungal drugs. One recently recognized biofilm resistance mechanism involves drug sequestration by matrix β-1,3 glucan. Using a candidate gene approach, we investigated potential C. albicans β-1,3-glucan regulators, based on their homology to Saccharomyces cerevisiae, including SMI1 and protein kinase C (PKC) pathway components. We identified a role for the SMI1 in biofilm matrix glucan production and development of the associated drug resistance phenotype. This pathway appears to act through transcription factor Rlmp and glucan synthase Fks1p. The phenotypes of these mutant biofilms mimicked those of the smi1Δ/smi1Δ biofilm, and overexpression of FKS1 in the smi1Δ/smi1Δ mutant restored the biofilm resistant phenotype. However, control of this pathway is distinct from that of the upstream PKC pathway because the pkc1Δ/pkc1Δ, bck1Δ/bck1Δ, mkk2Δ/mkk2Δ, and mkc1Δ/mkc1Δ biofilms retained the resistant phenotype of the parent strain. In addition, resistance to cell-perturbing agents and gene expression data do not support a significant role for the cell wall integrity pathway during the biofilm formation. Here we show that Smi1p functions in conjunction with Rlm1p and Fks1p to produce drug-sequestering biofilm β-glucan. Our work provides new insight into how the C. albicans biofilm matrix production and drug resistance pathways intersect with the planktonic cell wall integrity pathway. This novel connection helps explain how pathogens in a multicellular biofilm community are protected from anti-infective therapy.

[Chronic fungal infection, up-to-date]

Deep-seated mycosis usually occurred in severe immunocompromised patients and is sometimes fatal. Hence, chronic fungal infection occurred in the patients with mild to moderate immunocompromised status and persists for longer period. Biofilm formation is one of the factors related to persisting infection of Candida spp. Biofilm formation resists to the antifungals in catheter-related Candida infection and selection of appropriate antifungals will be an important key to achieve good outcome. Although azoles possessed excellent antifungal activity against planktonic Candida spp., they show lower activity against biofilm-formed Candida spp. Overexpression of efflux pump of Candida spp. is reported to be involved in lowered activity of azoles. Amphotericin B, liposomal amphotericin B, and micafungin however, are expected to have high antifungal activity against biofilm-formed Candida spp. Recently, Aspergillus is also reported to possess potential of forming biofilm. Biofilm formation of Aspergillus is considered to be related to pathogenesis of chronic pulmonary aspergillosis. General antifungals are not highly active to biofilm-formed Aspergillus as same as Candida, and only amphotericin B and its liposomal formulation are expected to be effective in vitro.

Hsp90 governs dispersion and drug resistance of fungal biofilms

Fungal biofilms are a major cause of human mortality and are recalcitrant to most treatments due to intrinsic drug resistance. These complex communities of multiple cell types form on indwelling medical devices and their eradication often requires surgical removal of infected devices. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. We previously established that in the leading human fungal pathogen, Candida albicans, Hsp90 enables the emergence and maintenance of drug resistance in planktonic conditions by stabilizing the protein phosphatase calcineurin and MAPK Mkc1. Hsp90 also regulates temperature-dependent C. albicans morphogenesis through repression of cAMP-PKA signalling. Here we demonstrate that genetic depletion of Hsp90 reduced C. albicans biofilm growth and maturation in vitro and impaired dispersal of biofilm cells. Further, compromising Hsp90 function in vitro abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal drugs, the azoles. Depletion of Hsp90 led to reduction of calcineurin and Mkc1 in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 levels led to a marked decrease in matrix glucan levels, providing a compelling mechanism through which Hsp90 might regulate biofilm azole resistance. Impairment of Hsp90 function genetically or pharmacologically transformed fluconazole from ineffectual to highly effective in eradicating biofilms in a rat venous catheter infection model. Finally, inhibition of Hsp90 reduced resistance of biofilms of the most lethal mould, Aspergillus fumigatus, to the newest class of antifungals to reach the clinic, the echinocandins. Thus, we establish a novel mechanism regulating biofilm drug resistance and dispersion and that targeting Hsp90 provides a much-needed strategy for improving clinical outcome in the treatment of biofilm infections.

Candida albicans and Aspergillus fumigatus are the most common causative agents of fungal infections worldwide. Both species can form biofilms on host tissues and indwelling medical devices that are highly resistant to antifungal treatment. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. Compromising Hsp90 function reduced biofilm formation of C. albicans in vitro and impaired dispersal of biofilm cells, potentially blocking their capacity to serve as reservoirs for infection. Further, compromise of Hsp90 function abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal, the azoles, both in vitro and in a mammalian model of catheter-associated candidiasis. Key drug resistance regulators were depleted upon reduction of Hsp90 levels in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 markedly reduced levels of matrix glucan, a carbohydrate important for C. albicans biofilm drug resistance. Inhibition of Hsp90 also reduced resistance of A. fumigatus biofilms to the newest class of antifungal, the echinocandins. Thus, targeting Hsp90 provides a promising strategy for the treatment of biofilm infections caused by diverse fungal species.

Direct effects of non-antifungal agents used in cancer chemotherapy and organ transplantation on the development and virulence of Candida and Aspergillus species

Conventional antineoplastic, novel immunosuppressive agents and antibiotics used in cancer treatment can directly affect the growth, development and virulence of Candida and Aspergillus species. Cytotoxic and cisplatin compounds have anti-Candida activity and may be synergistic with antifungal drugs; they also inhibit Candida and Aspergillus filamentation/conidation and effect increased virulence in vitro. Glucocorticoids enhance Candida adherence to epithelial cells, germination in serum and in vitro secretion of phospholipases and proteases, as well as growth of A. fumigatus. Calcineurin and target of rapamycin inhibitors perturb Candida and Aspergillus morphogenesis, stress responses and survival in serum, reduce azole tolerance in Candida, but yield conflicting in vivo data. Inhibition of candidal heat shock protein 90 and candidal-specific histone deacetylase represent feasible therapeutic approaches for candidiasis. Tyrosine kinase inhibitors inhibit fungal cell entry into epithelial cells and phagocytosis. Quinolone and other antibiotics may augment activity of azole and polyene agents. The correlation of in vitro effects with clinically meaningful in vivo systems is warranted. 

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Calcineurin controls drug tolerance, hyphal growth, and virulence in Candida dubliniensis

Candida dubliniensis is an emerging pathogenic yeast species closely related to Candida albicans and frequently found colonizing or infecting the oral cavities of HIV/AIDS patients. Drug resistance during C. dubliniensis infection is common and constitutes a significant therapeutic challenge. The calcineurin inhibitor FK506 exhibits synergistic fungicidal activity with azoles or echinocandins in the fungal pathogens C. albicans, Cryptococcus neoformans, and Aspergillus fumigatus. In this study, we show that calcineurin is required for cell wall integrity and wild-type tolerance of C. dubliniensis to azoles and echinocandins; hence, these drugs are candidates for combination therapy with calcineurin inhibitors. In contrast to C. albicans, in which the roles of calcineurin and Crz1 in hyphal growth are unclear, here we show that calcineurin and Crz1 play a clearly demonstrable role in hyphal growth in response to nutrient limitation in C. dubliniensis. We further demonstrate that thigmotropism is controlled by Crz1, but not calcineurin, in C. dubliniensis. Similar to C. albicans, C. dubliniensis calcineurin enhances survival in serum. C. dubliniensis calcineurin and crz1/crz1 mutants exhibit attenuated virulence in a murine systemic infection model, likely attributable to defects in cell wall integrity, hyphal growth, and serum survival. Furthermore, we show that C. dubliniensis calcineurin mutants are unable to establish murine ocular infection or form biofilms in a rat denture model. That calcineurin is required for drug tolerance and virulence makes fungus-specific calcineurin inhibitors attractive candidates for combination therapy with azoles or echinocandins against emerging C. dubliniensis infections.

Optimizing a Candida biofilm microtiter plate model for measurement of antifungal susceptibility by tetrazolium salt assay

Candida spp. infect medical devices, such as venous and urinary catheters, by adhering to the surface and forming a community of drug-resistant cells surrounded by a matrix. The ability to measure drug activity during this biofilm mode of growth is of interest for the investigation of resistance mechanisms and novel antifungal therapies. The tetrazolium salt (XTT) reduction assay is the test most commonly used to estimate viable biofilm growth and to examine the impact of biofilm therapies. The primary goal of the current experiments was to identify assay variables that affect the XTT assay result in order to improve assay reproducibility, sensitivity, and throughput for the study of antifungal activity. The species used in the current studies included Candida albicans, C. parapsilosis, and C. glabrata. The assay variables that were studied included the impact of culture conditions, the duration of biofilm growth, the timing and frequency of drug administration, the XTT source and concentration, and the duration of XTT incubation. The conditions that impacted the assay readout and altered assay sensitivity included the duration of biofilm growth, the frequency of drug dosing, and the duration of XTT incubation. Several factors were found to reduce time and assay expense, including the elimination of washing steps, the shortening of incubation times, and the use of lower XTT concentrations. A description of assay pitfalls and troubleshooting is included. Recognition of these technical variables should allow investigators to better design reproducible biofilm therapeutic studies.

In vitro synergy of eugenol and methyleugenol with fluconazole against clinical Candida isolates

The species Candida is a group of opportunistic pathogenic commensals in immune-compromised patients. Treatment of Candida infections is becoming increasingly difficult due to antifungal drug resistance, especially with fluconazole (FLC), which is a commonly used azole. In the present study the in vitro antifungal activity of eugenol (EUG) and methyleugenol (MEUG) alone and in combination against 64 FLC-sensitive and 34 FLC-resistant clinical Candida isolates is highlighted. All the strains were susceptible to both the naturally occurring phenyl propanoids. The nature of the interaction was studied from fractional inhibitory concentration indices (FICIs) for both EUG plus FLC, and MEUG plus FLC combinations calculated from chequerboard microdilution assays. FICI values depicted a high synergism of FLC with both compounds, which was greatest with MEUG. FLC-resistant Candida isolates showed high sensitivity to both compounds. No antagonistic activity was seen in the strains tested in the present study. From these results we suggest that EUG and MEUG have great potential as antifungals, and that FLC can be supplemented with EUG and MEUG to treat FLC-resistant Candida infections.

Anticandidal effect of Ocimum sanctum essential oil and its synergy with fluconazole and ketoconazole

Holy basil, Ocimum sanctum (L.) is time-honored for its medicinal properties; however its antimicrobial characteristics are used only in 'Ayurvedic medicines'. Attention has been drawn to antifungal activity and a possible synergistic antifungal effect of Ocimum sanctum essential oil (OSEO) and established azole antimycotics-fluconazole and ketoconazole. To put forward this approach, antifungal activity has been assessed in seventy four fluconazole-sensitive and sixteen fluconazole-resistant Candida isolates. Hemolytic activity on human erythrocytes was also studied to rule out the possibility of allied additional cytotoxicity. The observed selectively fungicidal characteristics signify a promising candidature of O. sanctum essential oil as an antifungal agent in combinational treatments for candidosis.

Development and validation of an in vivo Candida albicans biofilm denture model

The most common form of oral candidiasis, denture-associated stomatitis, involves biofilm growth on an oral prosthetic surface. Cells in this unique environment are equipped to withstand host defenses and survive antifungal therapy. Studies of the biofilm process on dentures have primarily been limited to in vitro models. We developed a rodent acrylic denture model and characterized the Candida albicans and mixed oral bacterial flora biofilm formation, architecture, and drug resistance in vivo, using time course quantitative culture experiments, confocal microscopy, scanning electron microscopy, and antifungal susceptibility assays. We also examined the utility of the model for measurement of C. albicans gene expression and tested the impact of a specific gene product (Bcr1p) on biofilm formation. Finally, we assessed the mucosal host response to the denture biofilm and found the mucosal histopathology to be consistent with that of acute human denture stomatitis, demonstrating fungal invasion and neutrophil infiltration. This current oral denture model mimics human denture stomatitis and should be useful for testing the impact of gene disruption on biofilm formation, studying the impact of anti-infectives, examining the biology of mixed Candida-oral bacterial flora biofilm infections, and characterizing the host immunologic response to this disease process.