Molecular Mechanisms of Resistance to 5-Fluorocytosine in Laboratory Mutants of Candida glabrata

Genomics insights of candidiasis: mechanisms of pathogenicity and drug resistance

Candidiasis, a prevalent class of human infections caused by fungi belonging to the Candida genus, is garnering increasing attention due to its pathogenicity and the emergence of drug resistance. The advancement of genomics technologies has offered powerful tools for investigating the pathogenic mechanisms and drug resistance characteristics of Candida. This comprehensive review provides an overview of the applications of genomics in candidiasis research, encompassing genome sequencing, comparative genomics, and functional genomics, along with the pathogenic features and core virulence factors of Candida. Moreover, this review highlights the role of genomic variations in the emergence of drug resistance, further elucidating the evolutionary and adaptive mechanisms of Candida. In conclusion, the review underscores the current state of research and prospective avenues for exploration of candidiasis, providing a theoretical basis for clinical treatments and public health strategies.

Genetic mutations in Cryptococcus neoformans pyrimidine salvage pathway enzymes contribute to reduced susceptibility against 5-fluorocytosine

Cryptococcal meningitis is a high-mortality infection. Adding 5-fluorocytosine (5-FC) to its treatment improves outcomes, but resistance to 5-FC presents a significant challenge. We conducted whole-genome sequencing on seven C. neoformans isolates with varying 5-FC susceptibility, along with proteomic and in silico analyses. Our findings indicate that mutations in genes of the pyrimidine salvage pathway are responsible for 5-FC resistance. Specifically, we identified an E64G missense mutation in the FUR1 gene, a large deletion in the FCY1 gene, and a point mutation in FCY1 leading to a truncated protein. The proteomic data indicated that these mutations resulted in the absence or reduction of crucial enzymes in resistant isolates. Genetic transformations confirmed the association between these mutations and 5-FC resistance. Resistance to 5-FC can develop during treatment and is closely tied to mutations in key metabolic enzymes. Understanding in vivo resistance development is crucial for combating resistance and enhancing patient outcomes.

Etiopathology, Epidemiology, Diagnosis, and Treatment of Fungal Keratitis

Fungal keratitis (FK) is a severe ocular condition resulting from corneal infection that is prevalent in tropical countries, particularly in developing regions of Asia and Africa. Factors like corneal lens misuse, inappropriate steroid use, and diagnostic challenges have provoked the epidemic. FK causes significant vision impairment, scarring, and ocular deformities. Accurate pathological diagnosis is crucial for effective therapeutic intervention. Topical antifungal therapy with surface healing medications proves effective in preventing fungal-borne ulcers. Managing FK requires a comprehensive understanding of fungal pathogenesis, guiding formulation strategies and preventive measures to curb global ocular blindness. This review provides in-depth insights into FK, covering etiology, epidemiology, pathogenesis, therapeutic interventions, antifungal resistance, limitations, prevention, and future perspectives on ocular surface disease management.

Antifungal drug resistance in Candida: a special emphasis on amphotericin B

Invasive fungal infections in humans caused by several Candida species, increased considerably in immunocompromised or critically ill patients, resulting in substantial morbidity and mortality. Candida albicans is the most prevalent species, although the frequency of these organisms varies greatly according to geographic region. Infections with C. albicans and non-albicans Candida species have become more common, especially in the past 20 years, as a result of aging, immunosuppressive medication use, endocrine disorders, malnourishment, extended use of medical equipment, and an increase in immunogenic diseases. Despite C. albicans being the species most frequently associated with human infections, C. glabrata, C. parapsilosis, C. tropicalis, and C. krusei also have been identified. Several antifungal drugs with different modes of action are approved for use in clinical settings to treat fungal infections. However, due to the common eukaryotic structure of humans and fungi, only a limited number of antifungal drugs are available for therapeutic use. Furthermore, drug resistance in Candida species has emerged as a result of the growing use of currently available antifungal drugs against fungal infections. Amphotericin B (AmB), a polyene class of antifungal drugs, is mainly used for the treatment of serious systemic fungal infections. AmB interacts with fungal plasma membrane ergosterol, triggering cellular ion leakage via pore formation, or extracting the ergosterol from the plasma membrane inducing cellular death. AmB resistance is primarily caused by changes in the content or structure of ergosterol. This review summarizes the antifungal drug resistance exhibited by Candida species, with a special focus on AmB.

Fungal Infections, Treatment and Antifungal Resistance: The Sub-Saharan African Context

Fungal pathogens cause a wide range of infections in humans, from superficial to disfiguring, allergic syndromes, and life-threatening invasive infections, affecting over a billion individuals globally. With an estimated 1.5 million deaths annually attributable to them, fungal pathogens are a major cause of mortality in humans, especially people with underlying immunosuppression. The continuous increase in the population of individuals at risk of fungal infections in sub-Saharan Africa, such as HIV patients, tuberculosis patients, intensive care patients, patients with haematological malignancies, transplant (haematopoietic stem cell and organ) recipients and the growing global threat of multidrug-resistant fungal strains, raise the need for an appreciation of the region's perspective on antifungal usage and resistance. In addition, the unavailability of recently introduced novel antifungal drugs in sub-Saharan Africa further calls for regular evaluation of resistance to antifungal agents in these settings. This is critical for ensuring appropriate and optimal use of the limited available arsenal to minimise antifungal resistance. This review, therefore, elaborates on the multifaceted nature of fungal resistance to the available antifungal drugs on the market and further provides insights into the prevalence of fungal infections and the use of antifungal agents in sub-Saharan Africa.

Molecular Mechanisms Associated with Antifungal Resistance in Pathogenic Candida Species

Candidiasis is a highly pervasive infection posing major health risks, especially for immunocompromised populations. Pathogenic Candida species have evolved intrinsic and acquired resistance to a variety of antifungal medications. The primary goal of this literature review is to summarize the molecular mechanisms associated with antifungal resistance in Candida species. Resistance can be conferred via gain-of-function mutations in target pathway genes or their transcriptional regulators. Therefore, an overview of the known gene mutations is presented for the following antifungals: azoles (fluconazole, voriconazole, posaconazole and itraconazole), echinocandins (caspofungin, anidulafungin and micafungin), polyenes (amphotericin B and nystatin) and 5-fluorocytosine (5-FC). The following mutation hot spots were identified: (1) ergosterol biosynthesis pathway mutations (ERG11 and UPC2), resulting in azole resistance; (2) overexpression of the efflux pumps, promoting azole resistance (transcription factor genes: tac1 and mrr1; transporter genes: CDR1, CDR2, MDR1, PDR16 and SNQ2); (3) cell wall biosynthesis mutations (FKS1, FKS2 and PDR1), conferring resistance to echinocandins; (4) mutations of nucleic acid synthesis/repair genes (FCY1, FCY2 and FUR1), resulting in 5-FC resistance; and (5) biofilm production, promoting general antifungal resistance. This review also provides a summary of standardized inhibitory breakpoints obtained from international guidelines for prominent Candida species. Notably, N. glabrata, P. kudriavzevii and C. auris demonstrate fluconazole resistance.

What 'Omics Can Tell Us About Antifungal Adaptation

Invasive candidiasis, the most frequent healthcare-associated invasive fungal infection, is commonly caused by Candida albicans. However, in recent years other antifungal-resistant Candida species—namely Candida glabrata and Candidaauris—have emerged as a serious matter of concern. Much of our understanding of the mechanisms regulating antifungal resistance and tolerance relies on studies utilizing C. albicans, C. glabrataand the model yeast Saccharomyces cerevisiae. ‘Omics studies have been used to describe alterations in metabolic, genomic and transcriptomic expression profiles upon antifungal treatment of fungal cells. The physiological changes identified by these approaches could significantly affect fungal fitness in the host and survival during antifungal challenge, as well as provide further understanding of clinical resistance. Thus, this review aims to comparatively address ‘omics data for C. albicans, C. glabrata andS. cerevisiae published from 2000 to 2021 to identify what these technologies can tell us regarding cellular responses to antifungal therapy. We will also highlight possible effects on pathogen survival and identify future avenues for antifungal research.

Molecular Mechanisms of 5-Fluorocytosine Resistance in Yeasts and Filamentous Fungi

Effective management and treatment of fungal diseases is hampered by poor diagnosis, limited options for antifungal therapy, and the emergence of antifungal drug resistance. An understanding of molecular mechanisms contributing to resistance is essential to optimize the efficacy of currently available antifungals. In this perspective, one of the oldest antifungals, 5-fluorocytosine (5-FC), has been the focus of recent studies applying advanced genomic and transcriptomic techniques to decipher the order of events at the molecular level that lead to resistance. These studies have highlighted the complexity of resistance and provided new insights that are reviewed in the present paper.

Antimycotic Drugs and their Mechanisms of Resistance to Candida Species

Fungal infections have shown an upsurge in recent decades, which is mainly because of the increasing number of immunocompromised patients and the occurrence of invasive candidiasis has been found to be 7-15 fold greater than that of invasive aspergillosis. The genus Candida comprises more than 150 distinct species, however, only a few of them are found to be pathogenic to humans. Mortality rates of Candida species are found to be around 45% and the reasons for this intensified mortality are inefficient diagnostic techniques and unfitting initial treatment strategies. There are only a few antifungal drug classes that are employed for the remedy of invasive fungal infections. which include azoles, polyenes, echinocandins, and pyrimidine analogs. During the last 2-3 decades, the usage of antifungal drugs has increased several folds due to which the reports of escalating antifungal drug resistance have also been recorded. The resistance is mostly to the triazole- based compounds. Due to the occurrence of antifungal drug resistance, the success rates of treatment have been reduced as well as major changes have been observed in the frequency of fungal infections. In this review, we have summarized the major molecular mechanisms for the development of antifungal drug resistance.

A Review on Molecular Mechanisms of Antifungal Resistance in Candida glabrata: Update and Recent Advances

Candida glabrata is the second frequent etiologic agent of mucosal and invasive candidiasis. Based on the recent developments in molecular methods, C. glabrata has been introduced as a complex composed of C. glabrata, Candida nivariensis, and Candida bracarensis. The four main classes of antifungal drugs effective against C. glabrata are pyrimidine analogs (flucytosine), azoles, echinocandins, and polyenes. Although the use of antifungal drugs is related to the predictable development of drug resistance, it is not clear why C. glabrata is able to rapidly resist against multiple antifungals in clinics. The enhanced incidence and antifungal resistance of C. glabrata and the high mortality and morbidity need more investigation regarding the resistance mechanisms and virulence associated with C. glabrata; additional progress concerning the drug resistance of C. glabrata has to be further prevented. The present review highlights the mechanism of resistance to antifungal drugs in C. glabrata.

Target shortage and less explored multiple targeting: hurdles in the development of novel antifungals but overcome/addressed effectively through structural bioinformatics

Billions of people are affected by fungal infection worldwide, which is a major cause of morbidity and mortality in humans. Regardless of development in the field of antifungal therapeutics over the last three decades, multidrug resistance and limited efficacy of available antifungal drugs are very prominent and still a great hurdle in the patient treatment. The current antifungal pipeline is dry, which is needed to be strengthened. Although several strategies have been implemented over time to discover novel promising antifungal leads, but very little emphasis has been given to address the gap of fungal target identification. Undeniably, the need for identifying novel cellular fungal targets is as vital as discovering novel antifungal leads and a structural bioinformatics approach could be an effective strategy in this regard. To address the issue, we have performed in silico screening to identify a few potent multiple targeting ligands and their respective antifungal targets. Thus, we offer a perspective on the phenomena of 'target shortage' and least explored 'multiple targeting' being the most underrated challenges in antifungal drug discovery. 'Structural bioinformatics' could be an effective approach in the recognition of new/innovative antifungal target and identification/development of novel antifungal lead molecule aiming multiple molecular targets of the fungal pathogen.

Carrier-Mediated Drug Uptake in Fungal Pathogens

Candida, Aspergillus, and Cryptococcus species are the most frequent cause of severe human fungal infections. Clinically relevant antifungal drugs are scarce, and their effectiveness are hampered by the ability of fungal cells to develop drug resistance mechanisms. Drug effectiveness and drug resistance in human pathogens is very often affected by their “transportome”. Many studies have covered a panoply of drug resistance mechanisms that depend on drug efflux pumps belonging to the ATP-Binding Cassette and Major Facilitator Superfamily. However, the study of drug uptake mechanisms has been, to some extent, overlooked in pathogenic fungi. This review focuses on discussing current knowledge on drug uptake systems in fungal pathogens, highlighting the need for further studies on this topic of great importance. The following subjects are covered: (i) drugs imported by known transporter(s) in pathogenic fungi; and (ii) drugs imported by known transporter(s) in the model yeast Saccharomyces cerevisiae or in human parasites, aimed at the identification of their homologs in pathogenic fungi. Besides its contribution to increase the understanding of drug-pathogen interactions, the practical implications of identifying drug importers in human pathogens are discussed, particularly focusing on drug development strategies.

Anticandidal agent for multiple targets: the next paradigm in the discovery of proficient therapeutics/overcoming drug resistance

Candida albicans is a prominent human fungal pathogen. Current treatments are suffering a massive gap due to emerging resistance against available antifungals. Therefore, there is an ardent need for novel antifungal candidates that essentially have more than one target, as most antifungal repertoires are single-target drugs. Exploration of multiple-drug targeting in antifungal therapeutics is still pending. An extensive literature survey was performed to categorize and comprehend relevant studies and the current therapeutic scenario that led researchers to preferentially consider multitarget drug-based Candida infection therapy. With this article, we identified and compiled a few potent antifungal compounds that are directed toward multiple virulent targets in C. albicans. Such compound(s) provide an optimistic platform of multiple targeting and could leave a substantial impact on the development of effective antifungals.

Microevolution of the pathogenic yeasts Candida albicans and Candida glabrata during antifungal therapy and host infection

Infections by the pathogenic yeasts Candida albicans and Candida glabrata are among the most common fungal diseases. The success of these species as human pathogens is contingent on their ability to resist antifungal therapy and thrive within the human host. C. glabrata is especially resilient to azole antifungal treatment, while C. albicans is best known for its wide array of virulence features. The core mechanisms that underlie antifungal resistance and virulence in these pathogens has been continuously addressed, but the investigation on how such mechanisms evolve according to each environment is scarcer. This review aims to explore current knowledge on micro-evolution experiments to several treatment and host-associated conditions in C. albicans and C. glabrata. The analysis of adaptation strategies that evolve over time will allow to better understand the mechanisms by which Candida species are able to achieve stable phenotypes in real-life scenarios, which are the ones that should constitute the most interesting drug targets.

Genome-Wide Response to Drugs and Stress in the Pathogenic Yeast Candida glabrata

Candida glabrata is the second most common cause of candidemia worldwide and its prevalence has continuously increased over the last decades. C. glabrata infections are especially worrisome in immunocompromised patients, resulting in serious systemic infections, associated to high mortality rates. Intrinsic resistance to azole antifungals, widely used drugs in the clinical setting, and the ability to efficiently colonize the human host and medical devices, withstanding stress imposed by the immune system, are thought to underlie the emergence of C. glabrata. There is a clear clinical need to understand drug and stress resistance in C. glabrata. The increasing prevalence of multidrug resistant isolates needs to be addressed in order to overcome the decrease of viable therapeutic strategies and find new therapeutic targets. Likewise, the understanding of the mechanisms underlying its impressive ability thrive under oxidative, nitrosative, acidic and metabolic stresses, is crucial to design drugs that target these pathogenesis features. The study of the underlying mechanisms that translate C. glabrata plasticity and its competence to evade the immune system, as well as survive host stresses to establish infection, will benefit from extensive scrutiny. This chapter provides a review on the contribution of genome-wide studies to uncover clinically relevant drug resistance and stress response mechanisms in the human pathogenic yeast C. glabrata.

The susceptibility of Candida albicans strains to selected anticancer drugs and flucytosine, relevance of the presence of self-splicing intron in 25S rDNA

BACKGROUND

The presence of intron 25S allows to divide the Candida albicans species into three subclasses (A, B, C). Intronless and intron harboring strains were reported to have different susceptibility to some drugs, for example to flucytosine and bleomycin.

OBJECTIVES

In this paper we tested the activity of selected antineoplastic agents, bleomycin, mitomycin C, dactinomycin and fluorouracil as well as antifungal drug flucytosine against 49 C. albicans isolates. Twenty-four strains used in this work contained intron, whereas twenty-five were intronless.

METHODS

The minimal inhibitory concentrations were determined by the standard microdilution method according to EUCAST.

RESULTS

All of the tested agents showed antifungal activity. Bleomycin was the strongest with an average minimal inhibitory concentration [MIC] of 15.5mg/L (range: 2-32), while the highest MIC was found for dactinomycin: 172.14mg/L (range: 128-256). Intron harboring strains seem to be more susceptible to bleomycin and flucytosine; however, differences were not statistically significant. The only two strains with elevated MICs for flucytosine were intronless. In contrast, the MIC of 5-fluorouracil was more than two times lower in intron harbouring strains comparing to intronless strains (P-value=0.0124). We found that the addition of folinate significantly increased the susceptibility of intronless strains to fluorouracil. MIC of fluorouracil decreased in this group from 58.24 (range: 16-256) to 16,78mg/L (2-64) after the supplementation of folinate.

CONCLUSION

The antifungal potential of tested substances, especially the simultaneous action of fluorouracil and folinate (combination used in oncology), is encouraging further research.

Dysfunction of Prohibitin 2 Results in Reduced Susceptibility to Multiple Antifungal Drugs via Activation of the Oxidative Stress-Responsive Transcription Factor Pap1 in Fission Yeast

The fight against resistance to antifungal drugs requires a better understanding of the underlying cellular mechanisms. In order to gain insight into the mechanisms leading to antifungal drug resistance, we performed a genetic screen on a model organism, Schizosaccharomyces pombe, to identify genes whose overexpression caused resistance to antifungal drugs, including clotrimazole and terbinafine. We identified the phb2 ⁺ gene, encoding a highly conserved mitochondrial protein, prohibitin (Phb2), as a novel determinant of reduced susceptibility to multiple antifungal drugs. Unexpectedly, deletion of the phb2 ⁺ gene also exhibited antifungal drug resistance. Overexpression of the phb2 ⁺ gene failed to cause drug resistance when the pap1 ⁺ gene, encoding an oxidative stress-responsive transcription factor, was deleted. Furthermore, pap1⁺ mRNA expression was significantly increased when the phb2 ⁺ gene was overexpressed or deleted. Importantly, either overexpression or deletion of the phb2 ⁺ gene stimulated the synthesis of NO and reactive oxygen species (ROS), as measured by the cell-permeant fluorescent NO probe DAF-FM DA (4-amino-5-methylamino-2',7'-difluorofluorescein diacetate) and the ROS probe DCFH-DA (2',7'-dichlorodihydrofluorescein diacetate), respectively. Taken together, these results suggest that Phb2 dysfunction results in reduced susceptibility to multiple antifungal drugs by increasing NO and ROS synthesis due to dysfunctional mitochondria, thereby activating the transcription factor Pap1 in fission yeast.

Commercial AHAS-inhibiting herbicides are promising drug leads for the treatment of human fungal pathogenic infections

The increased prevalence of drug-resistant human pathogenic fungal diseases poses a major threat to global human health. Thus, new drugs are urgently required to combat these infections. Here, we demonstrate that acetohydroxyacid synthase (AHAS), the first enzyme in the branched-chain amino acid biosynthesis pathway, is a promising new target for antifungal drug discovery. First, we show that several AHAS inhibitors developed as commercial herbicides are powerful accumulative inhibitors of Candida albicans AHAS (K _i values as low as 800 pM) and have determined high-resolution crystal structures of this enzyme in complex with several of these herbicides. In addition, we have demonstrated that chlorimuron ethyl (CE), a member of the sulfonylurea herbicide family, has potent antifungal activity against five different Candida species and Cryptococcus neoformans (with minimum inhibitory concentration, 50% values as low as 7 nM). Furthermore, in these assays, we have shown CE and itraconazole (a P450 inhibitor) can act synergistically to further improve potency. Finally, we show in Candida albicans-infected mice that CE is highly effective in clearing pathogenic fungal burden in the lungs, liver, and spleen, thus reducing overall mortality rates. Therefore, in view of their low toxicity to human cells, AHAS inhibitors represent a new class of antifungal drug candidates.

Evolutionary Emergence of Drug Resistance in Candida Opportunistic Pathogens

Fungal infections, such as candidiasis caused by Candida, pose a problem of growing medical concern. In developed countries, the incidence of Candida infections is increasing due to the higher survival of susceptible populations, such as immunocompromised patients or the elderly. Existing treatment options are limited to few antifungal drug families with efficacies that vary depending on the infecting species. In this context, the emergence and spread of resistant Candida isolates are being increasingly reported. Understanding how resistance can evolve within naturally susceptible species is key to developing novel, more effective treatment strategies. However, in contrast to the situation of antibiotic resistance in bacteria, few studies have focused on the evolutionary mechanisms leading to drug resistance in fungal species. In this review, we will survey and discuss current knowledge on the genetic bases of resistance to antifungal drugs in Candida opportunistic pathogens. We will do so from an evolutionary genomics perspective, focusing on the possible evolutionary paths that may lead to the emergence and selection of the resistant phenotype. Finally, we will discuss the potential of future studies enabled by current developments in sequencing technologies, in vitro evolution approaches, and the analysis of serial clinical isolates.

Candidiasis and the impact of flow cytometry on antifungal drug discovery

INTRODUCTION

Invasive candidiasis continues to be associated with significant morbidity and mortality as well as substantial health care costs nationally and globally. One of the contributing factors is the development of resistance to antifungal agents that are already in clinical use. Moreover, there are known treatment limitations with all of the available antifungal agents. Since traditional techniques in novel drug discovery are time consuming, high-throughput screening using flow cytometry presents as a potential tool to identify new antifungal agents that would be useful in the management of these patients. Areas covered: In this review, the authors discuss the use of automated high-throughput screening assays based upon flow cytometry to identify potential antifungals from a library comprised of a large number of bioactive compounds. They also review studies that employed the use of this research methodology that has identified compounds with antifungal activity. Expert opinion: High-throughput screening using flow cytometry has substantially decreased the processing time necessary for screening thousands of compounds, and has helped enhance our understanding of fungal pathogenesis. Indeed, the authors see this technology as a powerful tool to help scientists identify new antifungal agents that can be added to the clinician's arsenal in their fight against invasive candidiasis.

Vetiver Essential Oil in Cosmetics: What Is New?

Background: Vetiver is a key ingredient for the perfume industry nowadays. However, with the constant and rapid changes of personal tastes, this appeal could vanish and this sector could decline quite quickly. New dissemination paths need to be found to tap this valuable resource. Methods: In this way, its potential use in cosmetics either as an active ingredient per se (with cosmeceutical significance or presenting antimicrobial activity) has hence been explored in vitro. Results: In this contribution, we demonstrated that vetiver essential oil displays no particularly significant and innovative cosmetic potential value in formulations apart from its scent already largely exploited. However, evaluated against twenty bacterial strains and two Candida species using the in vitro microbroth dilution method, vetiver oil demonstrated notably some outstanding activities against Gram-positive strains and against one Candida glabrata strain. Conclusions: Based on these findings, vetiver essential oil appears to be an appropriate aspirant for the development of an antimicrobial agent for medicinal purposes and for the development of a cosmetic ingredient used for its scent and displaying antimicrobial activity as an added value.

Antifungal Therapy: New Advances in the Understanding and Treatment of Mycosis

The high rates of morbidity and mortality caused by fungal infections are associated with the current limited antifungal arsenal and the high toxicity of the compounds. Additionally, identifying novel drug targets is challenging because there are many similarities between fungal and human cells. The most common antifungal targets include fungal RNA synthesis and cell wall and membrane components, though new antifungal targets are being investigated. Nonetheless, fungi have developed resistance mechanisms, such as overexpression of efflux pump proteins and biofilm formation, emphasizing the importance of understanding these mechanisms. To address these problems, different approaches to preventing and treating fungal diseases are described in this review, with a focus on the resistance mechanisms of fungi, with the goal of developing efficient strategies to overcoming and preventing resistance as well as new advances in antifungal therapy. Due to the limited antifungal arsenal, researchers have sought to improve treatment via different approaches, and the synergistic effect obtained by the combination of antifungals contributes to reducing toxicity and could be an alternative for treatment. Another important issue is the development of new formulations for antifungal agents, and interest in nanoparticles as new types of carriers of antifungal drugs has increased. In addition, modifications to the chemical structures of traditional antifungals have improved their activity and pharmacokinetic parameters. Moreover, a different approach to preventing and treating fungal diseases is immunotherapy, which involves different mechanisms, such as vaccines, activation of the immune response and inducing the production of host antimicrobial molecules. Finally, the use of a mini-host has been encouraging for in vivo testing because these animal models demonstrate a good correlation with the mammalian model; they also increase the speediness of as well as facilitate the preliminary testing of new antifungal agents. In general, many years are required from discovery of a new antifungal to clinical use. However, the development of new antifungal strategies will reduce the therapeutic time and/or increase the quality of life of patients.

Future therapies targeted towards eliminating Candida biofilms and associated infections

INTRODUCTION

Candida species are common human commensals and cause either superficial or invasive opportunistic infections. The biofilm form of candida as opposed to its suspended, planktonic form, is predominantly associated with these infections. Alternative or adjunctive therapies are urgently needed to manage Candida infections as the currently available short arsenal of antifungal drugs has been compromised due to their systemic toxicity, cross-reactivity with other drugs, and above all, by the emergence of drug-resistant Candida species due to irrational drug use. Areas covered: Combination anti-Candida therapies, antifungal lock therapy, denture cleansers, and mouth rinses have all been proposed as alternatives for disrupting candidal biofilms on different substrates. Other suggested approaches for the management of candidiasis include the use of natural compounds, such as probiotics, plants extracts and oils, antifungal quorum sensing molecules, anti-Candida antibodies and vaccines, cytokine therapy, transfer of primed immune cells, photodynamic therapy, and nanoparticles. Expert commentary: The sparsity of currently available antifungals and the plethora of proposed anti-candidal therapies is a distinct indication of the urgent necessity to develop efficacious therapies for candidal infections. Alternative drug delivery approaches, such as probiotics, reviewed here is likely to be a reality in clinical settings in the not too distant future.

Genetic evidence for involvement of membrane trafficking in the action of 5-fluorouracil

To identify novel genes that mediate cellular sensitivity and resistance to 5-fluorouracil (5-FU), we performed a genome-wide genetic screening to identify altered susceptibility to 5-FU by Schizosaccharomyces pombe haploid nonessential gene deletion library containing 3004 deletion mutants. We identified 50 hypersensitive and 12 resistant mutants to this drug. Mutants sensitive or resistant to 5-FU were classified into various categories based on their putative functions. The largest group of the genes whose disruption renders cells altered susceptibility to 5-FU is involved in nucleic acid metabolism, but to our surprise, the second largest group is involved in membrane trafficking. In addition, several other membrane traffic mutants examined including gdi1-i11, ypt3-i5, Δryh1, Δric1, and Δaps1 exhibited hypersensitivity to 5-FU. Furthermore, we found that 5-FU in low concentration that generally do not affect cell growth altered the localization of Syb1, a secretory vesicle SNARE synaptobrevin which is cycled between the plasma membrane and the endocytic pathway. Notably, 5-FU at such low concentration also significantly inhibited the secretion of acid phosphatase. Altogether, our findings revealed the first evidence that 5-FU influences membrane trafficking as the potential underlying mechanism of the drug action.

Emerging Threats in Antifungal-Resistant Fungal Pathogens

The use of antifungal drugs in the therapy of fungal diseases can lead to the development of antifungal resistance. Resistance has been described for virtually all antifungal agents in diverse pathogens, including Candida and Aspergillus species. The majority of resistance mechanisms have also been elucidated at the molecular level in these pathogens. Drug resistance genes and genome mutations have been identified. Therapeutic choices are limited for the control of fungal diseases, and it is tempting to combine several drugs to achieve better therapeutic efficacy. In the recent years, several novel resistance patterns have been observed, including antifungal resistance originating from environmental sources in Aspergillus fumigatus and the emergence of simultaneous resistance to different antifungal classes (multidrug resistance) in different Candida species. This review will summarize these current trends.

Recent progress in the study of the interactions of amphotericin B with cholesterol and ergosterol in lipid environments

In the past decade substantial progress has been made in understanding the organization and biological activity of amphotericin B (AmB) in the presence of sterols in lipid environments. This review concentrates mainly on interactions of AmB with lipids and sterols, AmB channel formation in membranes, AmB aggregation, AmB modifications important for understanding its biological activity, and AmB models explaining its mechanism of action. Most of the reviewed studies concern monolayers at the water–gas interface, monolayers deposited on a solid substrate by use of the Langmuir–Blodgett technique, micelles, vesicles, and multi-bilayers. Liposomal AmB formulations and drug delivery are intentionally omitted, because several reviews dedicated to this subject are already available.

Candida infections, causes, targets, and resistance mechanisms: traditional and alternative antifungal agents

The genus Candida includes about 200 different species, but only a few species are human opportunistic pathogens and cause infections when the host becomes debilitated or immunocompromised. Candida infections can be superficial or invasive. Superficial infections often affect the skin or mucous membranes and can be treated successfully with topical antifungal drugs. However, invasive fungal infections are often life-threatening, probably due to inefficient diagnostic methods and inappropriate initial antifungal therapies. Here, we briefly review our current knowledge of pathogenic species of the genus Candida and yeast infection causes and then focus on current antifungal drugs and resistance mechanisms. An overview of new therapeutic alternatives for the treatment of Candida infections is also provided.

Progress in antifungal susceptibility testing of Candida spp. by use of Clinical and Laboratory Standards Institute broth microdilution methods, 2010 to 2012

Antifungal susceptibility testing of Candida has been standardized and refined and now may play a useful role in managing Candida infections. Important new developments include validation of 24-h reading times for all antifungal agents and the establishment of species-specific epidemiological cutoff values (ECVs) for the systemically active antifungal agents and both common and uncommon species of Candida. The clinical breakpoints (CBPs) for fluconazole, voriconazole, and the echinocandins have been revised to provide species-specific interpretive criteria for the six most common species. The revised CBPs not only are predictive of clinical outcome but also provide a more sensitive means of identifying those strains with acquired or mutational resistance mechanisms. This brief review serves as an update on the new developments in the antifungal susceptibility testing of Candida spp. using Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) methods.

Candida glabrata: Multidrug Resistance and Increased Virulence in a Major Opportunistic Fungal Pathogen

C. glabrata is widely acknowledged to be an important and potentially antifungal resistant cause of hematogenously disseminated candidiasis (HDC). In the United States (US) both the frequency of C. glabrata as a cause of HDC and in vitro resistance to fluconazole has increased steadily since 1992. Although this species is generally considered to be less virulent than C. albicans, recent findings suggest that gain of function (GOF) mutations in the transcriptional regulator CgPdr1p results not only in broad resistance to azole antifungals but also an increase in both fitness and virulence in animal models. Furthermore, case reports and case series suggest the emergence of multidrug resistance (MDR) in this species. Recent data from multicenter surveys conducted in the US have demonstrated the emergence of co-resistance to both azoles and echinocandins in clinical isolates of C. glabrata. These findings are highlighted in an effort to bring attention to this important development.

Wild-type MIC distributions and epidemiological cutoff values for amphotericin B, flucytosine, and itraconazole and Candida spp. as determined by CLSI broth microdilution

Clinical breakpoints (CBPs) and epidemiological cutoff values (ECVs) have been established for several Candida spp. and the newer triazoles and echinocandins but are not yet available for older antifungal agents, such as amphotericin B, flucytosine, or itraconazole. We determined species-specific ECVs for amphotericin B (AMB), flucytosine (FC) and itraconazole (ITR) for eight Candida spp. (30,221 strains) using isolates from 16 different laboratories in Brazil, Canada, Europe, and the United States, all tested by the CLSI reference microdilution method. The calculated 24- and 48-h ECVs expressed in μg/ml (and the percentages of isolates that had MICs less than or equal to the ECV) for AMB, FC, and ITR, respectively, were 2 (99.8)/2 (99.2), 0.5 (94.2)/1 (91.4), and 0.12 (95.0)/0.12 (92.9) for C. albicans; 2 (99.6)/2 (98.7), 0.5 (98.0)/0.5 (97.5), and 2 (95.2)/4 (93.5) for C. glabrata; 2 (99.7)/2 (97.3), 0.5 (98.7)/0.5 (97.8), and 05. (99.7)/0.5 (98.5) for C. parapsilosis; 2 (99.8)/2 (99.2), 0.5 (93.0)/1 (90.5), and 0.5 (97.8)/0.5 (93.9) for C. tropicalis; 2 (99.3)/4 (100.0), 32 (99.4)/32 (99.3), and 1 (99.0)/2 (100.0) for C. krusei; 2 (100.0)/4 (100.0), 0.5 (95.3)/1 (92.9), and 0.5 (95.8)/0.5 (98.1) for C. lusitaniae; -/2 (100.0), 0.5 (98.8)/0.5 (97.7), and 0.25 (97.6)/0.25 (96.9) for C. dubliniensis; and 2 (100.0)/2 (100.0), 1 (92.7)/-, and 1 (100.0)/2 (100.0) for C. guilliermondii. In the absence of species-specific CBP values, these wild-type (WT) MIC distributions and ECVs will be useful for monitoring the emergence of reduced susceptibility to these well-established antifungal agents.

Antifungal resistance and new strategies to control fungal infections

Despite improvement of antifungal therapies over the last 30 years, the phenomenon of antifungal resistance is still of major concern in clinical practice. In the last 10 years the molecular mechanisms underlying this phenomenon were extensively unraveled. In this paper, after a brief overview of currently available antifungals, molecular mechanisms of antifungal resistance will be detailed. It appears that major mechanisms of resistance are essential due to the deregulation of antifungal resistance effector genes. This deregulation is a consequence of point mutations occurring in transcriptional regulators of these effector genes. Resistance can also follow the emergence of point mutations directly in the genes coding antifungal targets. In addition we further describe new strategies currently undertaken to discover alternative therapy targets and antifungals. Identification of new antifungals is essentially achieved by the screening of natural or synthetic chemical compound collections. Discovery of new putative antifungal targets is performed through genome-wide approaches for a better understanding of the human pathogenic fungi biology.

Mutational analysis of flucytosine resistance in Candida glabrata

The antifungal flucytosine (5-fluorocytosine [5FC]) is a prodrug metabolized to its toxic form, 5-fluorouracil (5FU), only by organisms expressing cytosine deaminase. One such organism is Candida glabrata, which has emerged as the second most common agent of bloodstream and mucosal candidiasis. This emergence has been attributed to the high rate at which C. glabrata develops resistance to azole antifungals. As an oral agent, 5FC represents an attractive alternative or complement to azoles; however, the frequency of 5FC resistance mutations and the mechanisms by which these mutations confer resistance have been explored only minimally. On RPMI 1640 medium containing 1 μg/ml 5FC (32-fold above the MIC, but less than 1/10 of typical serum levels), resistant mutants occurred at a relatively low frequency (2 × 10⁻⁷). Three of six mutants characterized were 5FU cross-resistant, suggesting a mutation downstream of the Fcy1 gene (cytosine deaminase), which was confirmed by sequence analysis of the Fur1 gene (uracil phosphoribosyl transferase). The remaining three mutants had Fcy1 mutations. To ascertain the effects of 5FC resistance mutations on enzyme function, mutants were isolated in ura3 strains. Three of seven mutants harbored Fcy1 mutations and failed to grow in uridine-free, cytosine-supplemented medium, consistent with inactive Fcy1. The remainder grew in this medium and had wild-type Fcy1; further analysis revealed these to be mutated in the Fcy2L homolog of S. cerevisiae Fcy2 (purine-cytosine transporter). Based on this analysis, we characterized three 5FC-resistant clinical isolates, and mutations were identified in Fur1 and Fcy1. These data provide a framework for understanding 5FC resistance in C. glabrata and potentially in other fungal pathogens.