A nonsense mutation in the ERG6 gene leads to reduced susceptibility to polyenes in a clinical isolate of Candida glabrata

Alternative ergosterol biosynthetic pathways confer antifungal drug resistance in the human pathogens within the Mucor species complex

Mucormycoses are emerging fungal infections caused by a variety of heterogeneous species within the Mucorales order. Among the Mucor species complex, Mucor circinelloides is the most frequently isolated pathogen in mucormycosis patients and despite its clinical significance, there is an absence of established genome manipulation techniques to conduct molecular pathogenesis studies. In this study, we generated a spontaneous uracil auxotrophic strain and developed a genetic transformation procedure to analyze molecular mechanisms conferring antifungal drug resistance. With this new model, phenotypic analyses of gene deletion mutants were conducted to define Erg3 and Erg6a as key biosynthetic enzymes in the M. circinelloides ergosterol pathway. Erg3 is a C-5 sterol desaturase involved in growth, sporulation, virulence, and azole susceptibility. In other fungal pathogens, erg3 mutations confer azole resistance because Erg3 catalyzes the production of a toxic diol upon azole exposure. Surprisingly, M. circinelloides produces only trace amounts of this toxic diol and yet, it is still susceptible to posaconazole and isavuconazole due to alterations in membrane sterol composition. These alterations are severely aggravated by erg3Δ mutations, resulting in ergosterol depletion and consequently, hypersusceptibility to azoles. We also identified Erg6a as the main C-24 sterol methyltransferase, whose activity may be partially rescued by the paralogs Erg6b and Erg6c. Loss of Erg6a function diverts ergosterol synthesis to the production of cholesta-type sterols, resulting in resistance to amphotericin B. Our findings suggest that mutations or epimutations causing loss of Erg6 function may arise during human infections, resulting in antifungal drug resistance to first-line treatments against mucormycosis.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Triazoles, the most widely used class of antifungal drugs, inhibit the biosynthesis of ergosterol, a crucial component of the fungal plasma membrane. Inhibition of a separate ergosterol biosynthetic step, catalyzed by the sterol C-24 methyltransferase Erg6, reduces the virulence of pathogenic yeasts, but its effects on filamentous fungal pathogens like Aspergillus fumigatus remain unexplored. Here, we show that the lipid droplet-associated enzyme Erg6 is essential for the viability of A. fumigatus and other Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Downregulation of erg6 causes loss of sterol-rich membrane domains required for apical extension of hyphae, as well as altered sterol profiles consistent with the Erg6 enzyme functioning upstream of the triazole drug target, Cyp51A/Cyp51B. Unexpectedly, erg6-repressed strains display wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, we show that erg6 repression results in significant reduction in mortality in a murine model of invasive aspergillosis. Taken together with recent studies, our work supports Erg6 as a potentially pan-fungal drug target.

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.

Erg251 has complex and pleiotropic effects on azole susceptibility, filamentation, and stress response phenotypes

Ergosterol is essential for fungal cell membrane integrity and growth, and numerous antifungal drugs target ergosterol. Inactivation or modification of ergosterol biosynthetic genes can lead to changes in antifungal drug susceptibility, filamentation and stress response. Here, we found that the ergosterol biosynthesis gene ERG251 is a hotspot for point mutations during adaptation to antifungal drug stress within two distinct genetic backgrounds of Candida albicans. Heterozygous point mutations led to single allele dysfunction of ERG251 and resulted in azole tolerance in both genetic backgrounds. This is the first known example of point mutations causing azole tolerance in C. albicans. Importantly, single allele dysfunction of ERG251 in combination with recurrent chromosome aneuploidies resulted in bona fide azole resistance. Homozygous deletions of ERG251 caused increased fitness in low concentrations of fluconazole and decreased fitness in rich medium, especially at low initial cell density. Dysfunction of ERG251 resulted in transcriptional upregulation of the alternate sterol biosynthesis pathway and ZRT2, a Zinc transporter. Notably, we determined that overexpression of ZRT2 is sufficient to increase azole tolerance in C. albicans. Our combined transcriptional and phenotypic analyses revealed the pleiotropic effects of ERG251 on stress responses including cell wall, osmotic and oxidative stress. Interestingly, while loss of either allele of ERG251 resulted in similar antifungal drug responses, we observed functional divergence in filamentation regulation between the two alleles of ERG251 (ERG251-A and ERG251-B) with ERG251-A exhibiting a dominant role in the SC5314 genetic background. Finally, in a murine model of systemic infection, homozygous deletion of ERG251 resulted in decreased virulence while the heterozygous deletion mutants maintain their pathogenicity. Overall, this study provides extensive genetic, transcriptional and phenotypic analysis for the effects of ERG251 on drug susceptibility, fitness, filamentation and stress responses.

Population genomic analyses reveal high diversity, recombination and nosocomial transmission among Candida glabrata (Nakaseomyces glabrata) isolates causing invasive infections

Candida glabrata is a commensal yeast of the gastrointestinal tract and skin of humans. However, it causes opportunistic infections in immunocompromised patients, and is the second most common Candida pathogen causing bloodstream infections. Although there are many studies on the epidemiology of C. glabrata infections, the fine- and large-scale geographical nature of C. glabrata remain incompletely understood. Here we investigated both the fine- and large-scale population structure of C. glabrata through genome sequencing of 80 clinical isolates obtained from six tertiary hospitals in Qatar and by comparing with global collections. Our fine-scale analyses revealed high genetic diversity within the Qatari population of C. glabrata and identified signatures of recombination, inbreeding and clonal expansion within and between hospitals, including evidence for nosocomial transmission among coronavirus disease 2019 (COVID-19) patients. In addition to signatures of recombination at the population level, both MATa and MATα alleles were detected in most hospitals, indicating the potential for sexual reproduction in clinical environments. Comparisons with global samples showed that the Qatari C. glabrata population was very similar to those from other parts of the world, consistent with the significant role of recent anthropogenic activities in shaping its population structure. Genome-wide association studies identified both known and novel genomic variants associated with reduced susceptibilities to fluconazole, 5-flucytosine and echinocandins. Together, our genomic analyses revealed the diversity, transmission patterns and antifungal drug resistance mechanisms of C. glabrata in Qatar as well as the relationships between Qatari isolates and those from other parts of the world.

Molecular Characterization and Sterol Profiles Identify Nonsynonymous Mutations in ERG2 as a Major Mechanism Conferring Reduced Susceptibility to Amphotericin B in Candida kefyr

The molecular basis of reduced susceptibility to amphotericin B (rs-AMB) among any yeasts is poorly defined. Genetic alterations in genes involved in ergosterol biosynthesis and total cell sterols were investigated among clinical Candida kefyr isolates. C. kefyr isolates (n = 81) obtained from 74 patients in Kuwait and identified by phenotypic and molecular methods were analyzed. An Etest was initially used to identify isolates with rs-AMB. Specific mutations in ERG2 and ERG6 involved in ergosterol biosynthesis were detected by PCR sequencing. Twelve selected isolates were also tested by the SensiTitre Yeast One (SYO), and total cell sterols were evaluated by gas chromatography-mass spectrometry and ERG3 and ERG11 sequencing. Eight isolates from 8 patients showed rs-AMB by Etest, including 2 isolates with additional resistance to fluconazole or to all three antifungals. SYO correctly identified 8 of 8 rs-AMB isolates. A nonsynonymous mutation in ERG2 was detected in 6 of 8 rs-AMB isolates but also in 3 of 73 isolates with a wild-type AMB pattern. One rs-AMB isolate contained a deletion (frameshift) mutation in ERG2. One or more nonsynonymous mutations was detected in ERG6 in 11 of 81 isolates with the rs-AMB or wild-type AMB pattern. Among 12 selected isolates, 2 and 2 isolates contained a nonsynonymous mutation(s) in ERG3 and ERG11, respectively. Ergosterol was undetectable in 7 of 8 rs-AMB isolates, and the total cell sterol profiles were consistent with loss of ERG2 function in 6 rs-AMB isolates and loss of ERG3 activity in another rs-AMB isolate. Our data showed that ERG2 is a major target conferring rs-AMB in clinical C. kefyr isolates. IMPORTANCE Some yeast species exhibit intrinsic resistance or rapidly acquire resistance to azole antifungals. Despite >50 years of clinical use, resistance to amphotericin B (AMB) among yeast species has been extremely rarely reported until recently. Reduced susceptibility to AMB (rs-AMB) among yeast species is, therefore, a matter of serious concern due to the availability of only four classes of antifungal drugs. Recent studies in Candida glabrata, Candida lusitaniae, and Candida auris have identified ERG genes involved in ergosterol biosynthesis as the major targets conferring rs-AMB. The results of this study also show that nonsynonymous mutations in ERG2 impair its function, abolish ergosterol from C. kefyr, and confer rs-AMB. Thus, rapid detection of rs-AMB among clinical isolates will help in proper management of invasive C. kefyr infections.

The sterol C-24 methyltransferase encoding gene, erg6, is essential for viability of Aspergillus species

Ergosterol is a critical component of fungal plasma membranes. Although many currently available antifungal compounds target the ergosterol biosynthesis pathway for antifungal effect, current knowledge regarding ergosterol synthesis remains incomplete for filamentous fungal pathogens like Aspergillus fumigatus. Here, we show for the first time that the lipid droplet-associated sterol C-24 methyltransferase, Erg6, is essential for A. fumigatus viability. We further show that this essentiality extends to additional Aspergillus species, including A. lentulus, A. terreus, and A. nidulans. Neither the overexpression of a putative erg6 paralog, smt1, nor the exogenous addition of ergosterol could rescue erg6 deficiency. Importantly, Erg6 downregulation results in a dramatic decrease in ergosterol and accumulation in lanosterol and is further characterized by diminished sterol-rich plasma membrane domains (SRDs) at hyphal tips. Unexpectedly, erg6 repressed strains demonstrate wild-type susceptibility against the ergosterol-active triazole and polyene antifungals. Finally, repressing erg6 expression reduced fungal burden accumulation in a murine model of invasive aspergillosis. Taken together, our studies suggest that Erg6, which shows little homology to mammalian proteins, is potentially an attractive antifungal drug target for therapy of Aspergillus infections.

Molecular mechanisms governing antifungal drug resistance

Fungal pathogens are a severe public health problem. The leading causative agents of systemic fungal infections include species from the Candida, Cryptococcus, and Aspergillus genera. As opportunistic pathogens, these fungi are generally harmless in healthy hosts; however, they can cause significant morbidity and mortality in immunocompromised patients. Despite the profound impact of pathogenic fungi on global human health, the current antifungal armamentarium is limited to only three major classes of drugs, all of which face complications, including host toxicity, unfavourable pharmacokinetics, or limited spectrum of activity. Further exacerbating this issue is the growing prevalence of antifungal-resistant infections and the emergence of multidrug-resistant pathogens. In this review, we discuss the diverse strategies employed by leading fungal pathogens to evolve antifungal resistance, including drug target alterations, enhanced drug efflux, and induction of cellular stress response pathways. Such mechanisms of resistance occur through diverse genetic alterations, including point mutations, aneuploidy formation, and epigenetic changes given the significant plasticity observed in many fungal genomes. Additionally, we highlight recent literature surrounding the mechanisms governing resistance in emerging multidrug-resistant pathogens including Candida auris and Candida glabrata. Advancing our knowledge of the molecular mechanisms by which fungi adapt to the challenge of antifungal exposure is imperative for designing therapeutic strategies to tackle the emerging threat of antifungal resistance.

Whole-Genome Sequence Analysis of Candida glabrata Isolates from a Patient with Persistent Fungemia and Determination of the Molecular Mechanisms of Multidrug Resistance

Whole-genome sequencing (WGS) was used to determine the molecular mechanisms of multidrug resistance for 10 serial Candida glabrata bloodstream isolates obtained from a neutropenic patient during 82 days of amphotericin B (AMB) or echinocandin therapy. For WGS, a library was prepared and sequenced using a Nextera DNA Flex Kit (Illumina) and the MiseqDx (Illumina) instrument. All isolates harbored the same Msh2p substitution, V239L, associated with multilocus sequence type 7 and a Pdr1p substitution, L825P, that caused azole resistance. Of six isolates with increased AMB MICs (≥2 mg/L), three harboring the Erg6p A158fs mutation had AMB MICs ≥ 8 mg/L, and three harboring the Erg6p R314K, Erg3p G236D, or Erg3p F226fs mutation had AMB MICs of 2-3 mg/L. Four isolates harboring the Erg6p A158fs or R314K mutation had fluconazole MICs of 4-8 mg/L while the remaining six had fluconazole MICs ≥ 256 mg/L. Two isolates with micafungin MICs > 8 mg/L harbored Fks2p (I661_L662insF) and Fks1p (C499fs) mutations, while six isolates with micafungin MICs of 0.25-2 mg/L harbored an Fks2p K1357E substitution. Using WGS, we detected novel mechanisms of AMB and echinocandin resistance; we explored mechanisms that may explain the complex relationship between AMB and azole resistance.

Antifungal azoles trigger a xenobiotic detoxification pathway and chitin synthesis in Neurospora crassa

The presence of antifungal drugs is prompting the fungal microorganisms to react by mechanisms broader than the resistance. The fungi evolved mechanisms, by which they respond to various stress conditions, including the presence of antifungal compounds. In this work, we studied the response of model filamentous fungus Neurospora crassa to azole antifungals in the broader context of the adaptation mechanisms. We demonstrated the increase in expression of filamentous fungi-specific genes encoding cytochrome enzymes of CYP65 clan and plasma membrane-localized ABCC transporters. Azoles appear not to conjugate with glutathione. Surprisingly, the azoles caused changes in the hyphae organization and the amount of chitin in cell wall by the same manner that was thought to be echinocandin-specific. The response to individual azoles appeared to be influenced by the structure of azole compound (prochloraz - main outlier). Taken together, these findings demonstrate the importance of study of stress response mechanisms, specifically in filamentous fungi. Many aspects of the reaction within azoles seem to be similar, though specificities are occurring.

Semisynthetic Amides of Amphotericin B and Nystatin A1: A Comparative Study of In Vitro Activity/Toxicity Ratio in Relation to Selectivity to Ergosterol Membranes

Polyene antifungal amphotericin B (AmB) has been used for over 60 years, and remains a valuable clinical treatment for systemic mycoses, due to its broad antifungal activity and low rate of emerging resistance. There is no consensus on how exactly it kills fungal cells but it is certain that AmB and the closely-related nystatin (Nys) can form pores in membranes and have a higher affinity towards ergosterol than cholesterol. Notably, the high nephro- and hemolytic toxicity of polyenes and their low solubility in water have led to efforts to improve their properties. We present the synthesis of new amphotericin and nystatin amides and a comparative study of the effects of identical modifications of AmB and Nys on the relationship between their structure and properties. Generally, increases in the activity/toxicity ratio were in good agreement with increasing ratios of selective permeabilization of ergosterol- vs. cholesterol-containing membranes. We also show that the introduced modifications had an effect on the sensitivity of mutant yeast strains with alterations in ergosterol biosynthesis to the studied polyenes, suggesting a varying affinity towards intermediate ergosterol precursors. Three new water-soluble nystatin derivatives showed a prominent improvement in safety and were selected as promising candidates for drug development.

Erg6p is essential for antifungal drug resistance, plasma membrane properties and cell wall integrity in Candida glabrata

ERG6 gene encodes C-24 methyltransferase, one of the specific enzymes that differ in mammalian and yeast sterol biosynthesis. To explore the function of CgErg6p in the yeast pathogen Candida glabrata, we have constructed the Cgerg6Δ deletion mutant. We found that C. glabrata cells lacking CgErg6p exhibit reduced susceptibility to both antifungal azoles and polyenes. The reduced content of ergosterol in the Cgerg6 deletion mutant was accompanied by increased expression of genes encoding the last steps of the ergosterol biosynthetic pathway. The absence of CgErg6p leads to plasma membrane hyperpolarization and decrease in its fluidity compared to the parental C. glabrata strain. The absence of sterols containing C-24 alkyls influenced the susceptibility of Cgerg6Δ mutant cells to alkali metal cations and several other metabolic inhibitors. Our results thus show that sterols lacking C-24 alkyls are not sufficient substitutes for maintaining yeast plasma membrane function. The absence of CgErg6p influences also the cell wall integrity and calcineurin signaling in C. glabrata.

Media optimization of antimicrobial activity production and beta-glucan content of endophytic fungi Xylaria sp. BCC 1067

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Xylaria is an untapped resource for natural product discovery.

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Xylaria mycelial extract contains antimicrobials and immunomodulator beta-glucan.

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Achieved high mycelial biomass and antifungal activity using media-type selection.

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Media replacement approach lowers cultivation time and enhances bioactivity.

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Additive effect of mycelial extract and salicylic or citric acid against P. acne.

Fungi is a notable asset for drug discovery and production of pharmaceuticals; however, slow growth and poor product yields have hindered industrial utilization. Here, the mycelial biomass of Xylaria sp. BCC 1067 was examined in parallel with the assessment of antimicrobial properties by using media-type selection. To enhance both mycelial content and antifungal activity, the media replacement approach was successfully applied to stimulate fungal growth and successively switched to poorer malt-peptone extract media for metabolite production. This simple optimization reduced fungal cultivation time by 7 days and yielded 4-fold increased mycelial mass (32.59 g/L), with approximately 3-fold increased antifungal activity against the model yeast Saccharomyces cerevisiae strain. A high level of β-glucan (115.84 mg/g of cell dry weight) and additive antibacterial effect against Propionibacterium acnes were also reported. This simple strategy of culture media optimization allows for investigation of novel and rich source of health-promoting substances for effective microbial utilization.

Investigation of the Defective Growth Pattern and Multidrug Resistance in a Clinical Isolate of Candida glabrata Using Whole-Genome Sequencing and Computational Biology Applications

Candida glabrata is increasingly isolated from blood cultures, and multidrug-resistant isolates have important implications for therapy. This study describes a cholesterol-dependent clinical C. glabrata isolate (ML72254) that did not grow without blood (containing cholesterol) on routine mycological media and that showed azole and amphotericin B (AmB) resistance. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) and whole-genome sequencing (WGS) were used for species identification. A modified Etest method (Mueller-Hinton agar supplemented with 5% sheep blood) was used for antifungal susceptibility testing. WGS data were processed via the Galaxy platform, and the genomic variations of ML72254 were retrieved. A computational biology workflow utilizing web-based applications (PROVEAN, AlphaFold Colab, and Missense3D) was constructed to predict possible deleterious effects of these missense variations on protein functions. The predictive ability of this workflow was tested with previously reported missense variations in ergosterol synthesis genes of C. glabrata. ML72254 was identified as C. glabrata sensu stricto with MALDI-TOF, and WGS confirmed this identification. The MICs of fluconazole, voriconazole, and amphotericin B were >256, >32, and >32 μg/mL, respectively. A novel frameshift mutation in the ERG1 gene (Pro314fs) and many missense variations were detected in the ergosterol synthesis genes. None of the missense variations in the ML72254 ergosterol synthesis genes were deleterious, and the Pro314fs mutation was identified as the causative molecular change for a cholesterol-dependent and multidrug-resistant phenotype. This study verified that web-based computational biology solutions can be powerful tools for examining the possible impacts of missense mutations in C. glabrata. IMPORTANCE In this study, a cholesterol-dependent C. glabrata clinical isolate that confers azole and AmB resistance was investigated using artificial intelligence (AI) technologies and cloud computing applications. This is the first of the known cholesterol-dependent C. glabrata isolate to be found in Turkey. Cholesterol-dependent C. glabrata isolates are rarely isolated in clinical samples; they can easily be overlooked during routine laboratory procedures. Microbiologists therefore need to be alert when discrepancies occur between microscopic examination and growth on routine media. In addition, because these isolates confer antifungal resistance, patient management requires extra care.

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.

Candida glabrata Antifungal Resistance and Virulence Factors, a Perfect Pathogenic Combination

In recent years, a progressive increase in the incidence of invasive fungal infections (IFIs) caused by Candida glabrata has been observed. The objective of this literature review was to study the epidemiology, drug resistance, and virulence factors associated with the C. glabrata complex. For this purpose, a systematic review (January 2001-February 2021) was conducted on the PubMed, Scielo, and Cochrane search engines with the following terms: "C. glabrata complex (C. glabrata sensu stricto, C. nivariensis, C. bracarensis)" associated with "pathogenicity" or "epidemiology" or "antibiotics resistance" or "virulence factors" with language restrictions of English and Spanish. One hundred and ninety-nine articles were found during the search. Various mechanisms of drug resistance to azoles, polyenes, and echinocandins were found for the C. glabrata complex, depending on the geographical region. Among the mechanisms found are the overexpression of drug transporters, gene mutations that alter thermotolerance, the generation of hypervirulence due to increased adhesion factors, and modifications in vital enzymes that produce cell wall proteins that prevent the activity of drugs designed for its inhibition. In addition, it was observed that the C. glabrata complex has virulence factors such as the production of proteases, phospholipases, and hemolysins, and the formation of biofilms that allows the complex to evade the host immune response and generate fungal resistance. Because of this, the C. glabrata complex possesses a perfect pathogenetic combination for the invasion of the immunocompromised host.

Update on Candida krusei, a potential multidrug-resistant pathogen

Although Candida albicans remains the main cause of candidiasis, in recent years a significant number of infections has been attributed to non-albicans Candida (NAC) species, including Candida krusei. This epidemiological change can be partly explained by the increased resistance of NAC species to antifungal drugs. C. krusei is a diploid, dimorphic ascomycetous yeast that inhabits the mucosal membrane of healthy individuals. However, this yeast can cause life-threatening infections in immunocompromised patients, with hematologic malignancy patients and those using prolonged azole prophylaxis being at higher risk. Fungal infections are usually treated with five major classes of antifungal agents which include azoles, echinocandins, polyenes, allylamines, and nucleoside analogues. Fluconazole, an azole, is the most commonly used antifungal drug due to its low host toxicity, high water solubility, and high bioavailability. However, C. krusei possesses intrinsic resistance to this drug while also rapidly developing acquired resistance to other antifungal drugs. The mechanisms of antifungal resistance of this yeast involve the alteration and overexpression of drug target, reduction in intracellular drug concentration and development of a bypass pathway. Antifungal resistance menace coupled with the paucity of the antifungal arsenal as well as challenges involved in antifungal drug development, partly due to the eukaryotic nature of both fungi and humans, have left researchers to exploit alternative therapies. Here we briefly review our current knowledge of the biology, pathophysiology and epidemiology of a potential multidrug-resistant fungal pathogen, C. krusei, while also discussing the mechanisms of drug resistance of Candida species and alternative therapeutic approaches.

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.

Antifungal Drug Resistance: Molecular Mechanisms in Candida albicans and Beyond

Fungal infections are a major contributor to infectious disease-related deaths across the globe. Candida species are among the most common causes of invasive mycotic disease, with Candida albicans reigning as the leading cause of invasive candidiasis. Given that fungi are eukaryotes like their human host, the number of unique molecular targets that can be exploited for antifungal development remains limited. Currently, there are only three major classes of drugs approved for the treatment of invasive mycoses, and the efficacy of these agents is compromised by the development of drug resistance in pathogen populations. Notably, the emergence of additional drug-resistant species, such as Candida auris and Candida glabrata, further threatens the limited armamentarium of antifungals available to treat these serious infections. Here, we describe our current arsenal of antifungals and elaborate on the resistance mechanisms Candida species possess that render them recalcitrant to therapeutic intervention. Finally, we highlight some of the most promising therapeutic strategies that may help combat antifungal resistance, including combination therapy, targeting fungal-virulence traits, and modulating host immunity. Overall, a thorough understanding of the mechanistic principles governing antifungal drug resistance is fundamental for the development of novel therapeutics to combat current and emerging fungal threats.

Host-pathogen interaction in Candida glabrata infection: current knowledge and implications for antifungal therapy

INTRODUCTION

The opportunistic fungal pathogen Candida glabrata poses a clinical challenge in the successful treatment of invasive Candida infections, owing to its low inherent susceptibility toward azole antifungals and the recent acquisition of coresistance toward azole and echinocandin drugs. Compared to other prevalent Candida bloodstream pathogens, C. glabrata neither exhibits secreted proteolytic activity nor invokes a strong immune response in a variety of host cells and is less virulent. It also does not form true hyphae, and the success of C. glabrata, therefore, as a prevalent human fungal pathogen, appears to be built upon a distinct set of virulence attributes.

AREAS COVERED

The focus of this review is to outline, in brief, the interaction of C. glabrata with the host, deduced from the knowledge gained from different in vitro, ex vivo, and in vivo model systems. In addition, we briefly discuss the current antifungals, antifungal resistance mechanisms, and the development of new antifungal therapies, along with the available information on the host response.

EXPERT OPINION

A detailed understanding of stresses, selection pressures and differential immune responses in the presence and absence of antifungals that C. glabrata encounters in varied niches of the host, is required to design effective antifungal therapy.

Antifungal drug susceptibility, molecular basis of resistance to echinocandins and molecular epidemiology of fluconazole resistance among clinical Candida glabrata isolates in Kuwait

Candida glabrata readily develops resistance to echinocandins. Identification, antifungal susceptibility testing (AST) and resistance mechanism to echinocandins among C. glabrata was determined in Kuwait. C. glabrata isolates (n = 75) were tested by Vitek2, multiplex PCR and/or PCR-sequencing of rDNA. AST to fluconazole, caspofungin, micafungin and amphotericin B was determined by Etest and to micafungin by broth microdilution (BMD). Mutations in hotspot-1/hotspot-2 of FKS1/FKS2 and ERG11 were detected by PCR-sequencing. All isolates were identified as C. glabrata sensu stricto. Seventy isolates were susceptible and five were resistant to micafungin by Etest and BMD (essential agreement, 93%; categorical agreement, 100%). Three micafungin-resistant isolates were resistant and two were susceptible dose-dependent to caspofungin. Four and one micafungin-resistant isolate contained S663P and ∆659 F mutation, respectively, in hotspot-1 of FKS2. Micafungin-resistant isolates were genotypically distinct strains. Only one of 36 fluconazole-resistant isolate contained nonsynonymous ERG11 mutations. Thirty-four of 36 fluconazole-resistant isolates were genotypically distinct strains. Our data show that micafungin susceptibility reliably identifies echinocandin-resistant isolates and may serve as a surrogate marker for predicting susceptibility/resistance of C. glabrata to caspofungin. All micafungin-resistant isolates also harbored a nonsynonymous/deletion mutation in hotspot-1 of FKS2. Fingerprinting data showed that echinocandin/fluconazole resistance development in C. glabrata is not clonal.

Molecular Epidemiology of Candida auris in Colombia Reveals a Highly Related, Countrywide Colonization With Regional Patterns in Amphotericin B Resistance

Background

Candida auris is a multidrug-resistant yeast associated with hospital outbreaks worldwide. During 2015-2016, multiple outbreaks were reported in Colombia. We aimed to understand the extent of contamination in healthcare settings and to characterize the molecular epidemiology of C. auris in Colombia.

Methods

We sampled patients, patient contacts, healthcare workers, and the environment in 4 hospitals with recent C. auris outbreaks. Using standardized protocols, people were swabbed at different body sites. Patient and procedure rooms were sectioned into 4 zones and surfaces were swabbed. We performed whole-genome sequencing (WGS) and antifungal susceptibility testing (AFST) on all isolates.

Results

Seven of the 17 (41%) people swabbed were found to be colonized. Candida auris was isolated from 37 of 322 (11%) environmental samples. These were collected from a variety of items in all 4 zones. WGS and AFST revealed that although isolates were similar throughout the country, isolates from the northern region were genetically distinct and more resistant to amphotericin B (AmB) than the isolates from central Colombia. Four novel nonsynonymous mutations were found to be significantly associated with AmB resistance.

Conclusions

Our results show that extensive C. auris contamination can occur and highlight the importance of adherence to appropriate infection control practices and disinfection strategies. Observed genetic diversity supports healthcare transmission and a recent expansion of C. auris within Colombia with divergent AmB susceptibility.

The antiproliferative peptide Ctn[15-34] is active against multidrug-resistant yeasts Candida albicans and Cryptococcus neoformans

AIMS

Crotalicidin (Ctn), a cathelicidin-related antimicrobial peptide from the South American rattlesnake venom gland, and its C-terminal Ctn[15-34] fragment, have exhibited important activities against micro-organisms, trypanosomatid protozoa and certain lines of tumour cells. Herein, the activity against clinical strains of fluconazole-resistant Candida albicans and of amphotericin B and fluconazole-resistant Cryptococcus neoformans was investigated.

METHODS AND RESULTS

Microdilution and luminescent cell viability tests were used to evaluate and compare the susceptibility of pathogenic yeasts to these peptides. The time-kill curves of the most active Ctn[15-34] alone or in combination with fluconazole against drug-resistant yeasts were determined. Concomitantly, the fungicidal and/or fungistatic effects of Ctn[15-34] were visualized by the spotting test. The peptides were active against all strains, including those resistant to antifungal agents. The association of fluconazole with both Ctn and Ctn[15-34], although not synergic, was additive. In contrast, such pattern was not observed for C. neoformans.

CONCLUSIONS

Overall, Ctn and Ctn[15-34] are potential antifungal leads displaying anti-yeast activities against clinical isolates endowed with drug resistance mechanisms.

SIGNIFICANCE AND IMPACT OF THE STUDY

The effective peptide activity against resistant strains of pathogenic yeasts demonstrates that crotalicidin-derived peptides are promising templates to develop new antifungal pharmaceuticals.

Lack of detection of Candida nivariensis and Candida bracarensis among 440 clinical Candida glabrata sensu lato isolates in Kuwait

Occurrence of Candida nivariensis and Candida bracarensis, two species phenotypically similar to Candida glabrata sensu stricto, in human clinical samples from different geographical settings remains unknown. This study developed a low-cost multiplex PCR (mPCR) and three species-specific singleplex PCR assays. Reference strains of common Candida species were used during development and the performance of mPCR and singleplex PCR assays was evaluated with 440 clinical C. glabrata sensu lato isolates. The internal transcribed spacer (ITS) region of rDNA was also sequenced from 85 selected isolates and rDNA sequence variations were used for determining genetic relatedness among the isolates by using MEGA X software. Species-specific amplicons for C. glabrata (~360 bp), C. nivariensis (~250 bp) and C. bracarensis (~180 bp) were obtained in mPCR while no amplicon was obtained from other Candida species. The three singleplex PCR assays also yielded expected results with reference strains of Candida species. The mPCR amplified ~360 bp amplicon from all 440 C. glabrata sensu lato isolates thus identifying all clinical isolates in Kuwait as C. glabrata sensu stricto. The results of mPCR were confirmed for all 440 isolates as they yielded an amplicon only in C. glabrata sensu stricto-specific singleplex PCR assay. The rDNA sequence data identified 28 ITS haplotypes among 85 isolates with 18 isolates belonging to unique haplotypes and 67 isolates belonging to 10 cluster haplotypes. In conclusion, we have developed a simple, low-cost mPCR assay for rapid differentiation of C. glabrata sensu stricto from C. nivariensis and C. bracarensis. Our data obtained from a large collection of clinical C. glabrata sensu lato isolates show that C. nivariensis and C. bracarensis are rare pathogens in Kuwait. Considerable genetic diversity among C. glabrata sensu stricto isolates was also indicated by rDNA sequence analyses.

Genomewide Elucidation of Drug Resistance Mechanisms for Systemically Used Antifungal Drugs Amphotericin B, Caspofungin, and Voriconazole in the Budding Yeast

There are only a few antifungal drugs used systemically in treatment, and invasive fungal infections that are resistant to these drugs are an emerging problem in health care. In this study, we performed a high-copy-number genomic DNA (gDNA) library screening to find and characterize genes that reduce susceptibility to amphotericin B, caspofungin, and voriconazole in Saccharomyces cerevisiae We identified the PDR16 and PMP3 genes for amphotericin B, the RMD9 and SWH1 genes for caspofungin, and the MRS3 and TRI1 genes for voriconazole. The deletion mutants for PDR16 and PMP3 were drug susceptible, but the other mutants had no apparent susceptibility. Quantitative-PCR analyses suggested that the corresponding drugs upregulated expression of the PDR16, PMP3, SWH1, and MRS3 genes. To further characterize these genes, we also profiled the global expression patterns of the cells after treatment with the antifungals and determined the genes and paths that were up- or downregulated. We also cloned Candida albicans homologs of the PDR16, PMP3, MRS3, and TRI1 genes and expressed them in S. cerevisiae Heterologous expression of Candida homologs also provided reduced drug susceptibility to the budding yeast cells. Our analyses suggest the involvement of new genes in antifungal drug resistance.

Fluphenazine antagonizes with fluconazole but synergizes with amphotericin B in the treatment of candidiasis

Candida albicans causes a high mortality rate in immunocompromised individuals, but the increased drug resistance challenges the current antifungal therapeutics. Fluphenazine (FPZ), a commonly used antipsychotic medication, can induce the expression of drug efflux pumps in C. albicans and, thus, may interfere with the therapeutic efficacy of antifungals, such as fluconazole (FLC) and amphotericin B (AmB). Here, we investigated the combined effects of FLC/FPZ and AmB/FPZ against C. albicans in vitro and in a systemic candidiasis mouse model. The antifungal activity of FLC was significantly reduced when supplemented with FPZ. The inhibitory effects of FLC on the expression of the Candida virulence-related genes ALS3 and HWP1 were antagonized by FPZ. However, FPZ enhanced the susceptibility of C. albicans to AmB and further downregulated the expression of ALS3 and HWP1 in a synergistic manner with AmB. FPZ also enhanced the gene expression of ERG11, a key gene of the ergosterol biosynthesis pathway that has been associated with the activities of both FLC and AmB. In our mammalian infection model, mice treated with FLC/FPZ showed notably poor living status and increased fungal burden in their kidneys and brains compared with those treated with FLC alone. Conversely, the combined application of AmB/FPZ significantly improved the survival rate, attenuated the weight loss and reduced the organ fungal burdens of the infected mice. These data suggest that FPZ antagonized the therapeutic efficacy of FLC but enhanced the antifungal activity of AmB in the treatment of candidiasis.

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.

Candida glabrata: A Lot More Than Meets the Eye

Candida glabrata is an opportunistic human fungal pathogen that causes superficial mucosal and life-threatening bloodstream infections in individuals with a compromised immune system. Evolutionarily, it is closer to the non-pathogenic yeast Saccharomyces cerevisiae than to the most prevalent Candida bloodstream pathogen, C. albicans. C. glabrata is a haploid budding yeast that predominantly reproduces clonally. In this review, we summarize interactions of C. glabrata with the host immune, epithelial and endothelial cells, and the ingenious strategies it deploys to acquire iron and phosphate from the external environment. We outline various attributes including cell surface-associated adhesins and aspartyl proteases, biofilm formation and stress response mechanisms, that contribute to the virulence of C. glabrata. We further discuss how, C. glabrata, despite lacking morphological switching and secreted proteolytic activity, is able to disarm macrophage, dampen the host inflammatory immune response and replicate intracellularly.

ERG6 and ERG2 Are Major Targets Conferring Reduced Susceptibility to Amphotericin B in Clinical Candida glabrata Isolates in Kuwait

Candida glabrata is intrinsically less susceptible to azoles, and resistance to echinocandins and reduced susceptibility (RS) to amphotericin B (AMB) have also been detected. The molecular mechanisms of RS to AMB were investigated in C. glabrata strains in Kuwait by sequence analyses of genes involved in ergosterol biosynthesis. A total of 1,646 C. glabrata isolates were tested by Etest, and results for 12 selected isolates were confirmed by reference broth microdilution. PCR sequencing of three genes (ERG2, ERG6, and ERG11) was performed for all isolates with RS to AMB (RS-AMB isolates) and 5 selected wild-type C. glabrata isolates by using gene-specific primers. The total cell sterol content was analyzed by gas chromatography-mass spectrometry. The phylogenetic relationship among the isolates was investigated by multilocus sequence typing. Wild-type isolates contained only synonymous mutations in ERG2, ERG6, or ERG11, and the total sterol content was similar to that of the reference strains. A nonsynonymous ERG6 mutation (AGA48AAA, R48K) was found in both RS-AMB and wild-type isolates. Four RS-AMB isolates contained novel nonsense mutations at Trp286, Tyr192, and Leu341, and 2 isolates contained a nonsynonymous mutation in ERG6 (V126F or C198F); and the sterol content of these isolates was consistent with ERG6 deficiency. Two other RS-AMB isolates contained a novel nonsynonymous ERG2 mutation (G119S or G122S), and their sterol content was consistent with ERG2 deficiency. Of 8 RS-AMB isolates, 1 fluconazole-resistant isolate also contained nonsynonymous Y141H plus L381M mutations, while 7 isolates contained only synonymous mutations in ERG11 All isolates with ERG6, ERG2, and ERG11 mutations were genotypically distinct strains. Our data show that ERG6 and ERG2 are major targets conferring RS-AMB in clinical C. glabrata isolates.

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.

Combination of Posaconazole and Amphotericin B in the Treatment of Candida glabrata Biofilms

Candidemia cases have been increasing, especially among immunosuppressed patients. Candida glabrata is one of the most resistant Candida species, especially to the azole drugs, resulting in a high demand for therapeutic alternatives. The minimum inhibitory concentration (MIC), minimum fungicidal concentration (MFC), and minimum biofilm eradication concentration (MBEC) were determined for posaconazole (Pcz) and amphotericin B (AmB). The drug combinations of both drugs were evaluated on pre-formed biofilms of C. glabrata ATCC 2001, through XTT (2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide) assay, colony forming units (CFU), crystal violet, and the fractional inhibitory concentration index (FICI). C. glabrata revealed higher susceptibility and biofilm reduction in the presence of AmB alone, but both drugs revealed a good capacity in the biomass elimination. In the majority of the tested combinations, the interactions were defined as indifferent (FICI ≤ 4). The combination of the two drugs does not seem to bring a clear advantage in the treatment of biofilms of C. glabrata.

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.

Overexpression or Deletion of Ergosterol Biosynthesis Genes Alters Doubling Time, Response to Stress Agents, and Drug Susceptibility in Saccharomyces cerevisiae

Ergosterol (ERG) is a critical sterol in the cell membranes of fungi, and its biosynthesis is tightly regulated by 25 known enzymes along the ERG production pathway. The effects of changes in expression of each ERG biosynthesis enzyme in Saccharomyces cerevisiae were analyzed by the use of gene deletion or plasmid-borne overexpression constructs. The strains overexpressing the ERG pathway genes were examined for changes in doubling time and responses to a variety of stress agents. In addition, ERG gene overexpression strains and ERG gene deletion strains were tested for alterations in antifungal drug susceptibility. The data show that disruptions in ergosterol biosynthesis regulation can affect a diverse set of cellular processes and can cause numerous phenotypic effects. Some of the phenotypes observed include dramatic increases in doubling times, respiratory deficiencies on glycerol media, cell wall insufficiencies on Congo red media, and disrupted ion homeostasis under iron or calcium starvation conditions. Overexpression or deletion of specific enzymes in the ERG pathway causes altered susceptibilities to a variety of classes of antifungal ergosterol inhibitors, including fluconazole, fenpropimorph, lovastatin, nystatin, amphotericin B, and terbinafine. This analysis of the effect of perturbations to the ERG pathway caused by systematic overexpression of each of the ERG pathway genes contributes significantly to the understanding of the ergosterol biosynthetic pathway and its relationship to stress response and basic biological processes. The data indicate that precise regulation of ERG genes is essential for cellular homeostasis and identify several ERG genes that could be exploited in future antifungal development efforts.IMPORTANCE A common target of antifungal drug treatment is the fungal ergosterol biosynthesis pathway. This report helps to identify ergosterol biosynthesis enzymes that have not previously been appreciated as drug targets. The effects of overexpression of each of the 25 ERG genes in S. cerevisiae were analyzed in the presence of six stress agents that target essential cellular processes (cell wall biosynthesis, protein translation, respiration, osmotic/ionic stress, and iron and calcium homeostasis), as well as six antifungal inhibitors that target ergosterol biosynthesis. The importance of identifying cell perturbations caused by gene overexpression or deletion is emphasized by the prevalence of gene expression alterations in many pathogenic and drug-resistant clinical isolates. Genes whose altered expression causes the most extensive phenotypic alterations in the presence of stressors or inhibitors have the potential to be drug targets.

Emerging multidrug-resistant Candida species

PURPOSE OF REVIEW

To describe the epidemiology, strategies for early detection, and clinical management of infections caused by the most commonly found multidrug-resistant (MDR) Candida spp.

RECENT FINDINGS

Increasing numbers of reports describing invasive infections by MDR Candida auris and Candida glabrata has been reported in medical centers worldwide.

SUMMARY

We checked all papers published along the last 10 years describing epidemiological, diagnostic, and clinical aspects of infections by MDR Candida spp., with emphasis on C. auris and C. glabrata spp. C. auris has been reported in 15 countries and multidrug resistance rates is usually above 30%. Horizontal transmission is a great concern regarding C. auris. C. glabrata ranks the second most reported Candida spp. in deep-seated infections from United States and some European Centers, although multidrug resistance rates above 10% are restricted to some US centers. Candida haemulonii complex isolates with poor susceptibility to azoles and amphotericin B have been isolated in superficial and deep-seated infections, whereas Candida guilliiermondii complex isolates with poor susceptibility to azoles and echinocandins have been recovered from catheter-related bloodstream infections. Other potential MDR Candida species are Candida krusei, Candida lusitaniae, Candida kefyr, Yarrowia (Candida) lypolitica, and Candida rugosa.

Targeting Candida spp. to develop antifungal agents

Invasive fungal infections are a complex challenge throughout the world because of their high incidence, mainly in critically ill patients, and high mortality rates. The antifungal agents currently available are limited; thus, there is a need for the rapid development of new drugs. In silico methods are a modern strategy to explore interactions between new compounds and specific fungal targets, but they depend on precise genetic information. Here, we discuss the main Candida spp. target genes, including information about null mutants, virulence, cytolocalization, co-regulatory genes, and compounds that are related to protein expression. These data will provide a basis for the future in silico development of antifungal drugs.

Liposomal and Deoxycholate Amphotericin B Formulations: Effectiveness against Biofilm Infections of Candida spp

Background: candidiasis is the primary fungal infection encountered in patients undergoing prolonged hospitalization, and the fourth leading cause of nosocomial bloodstream infections. One of the most important Candida spp. virulence factors is the ability to form biofilms, which are extremely refractory to antimicrobial therapy and very difficult to treat with the traditional antifungal therapies. It is known that the prophylaxis or treatment of a systemic candidiasis are recurrently taken without considering the possibility of a Candida spp. biofilm-related infections. Therefore, it is important to assess the effectiveness of the available drugs and which formulations have the best performance in these specific infections. Methods: 24-h-biofilms of four Candida spp. and their response to two amphotericin B (AmB) pharmaceutical formulations (liposomal and deoxycholate) were evaluated. Results: generally, Candida glabrata was the less susceptible yeast species to both AmBs. MBECs revealed that it is therapeutically more appealing to use AmB-L than AmB-Deox for all Candida spp. biofilms, since none of the determined concentrations of AmB-L reached 10% of the maximum daily dose, but both formulations showed a very good capacity in the biomass reduction. Conclusions: the liposomal formulation presents better performance in the eradication of the biofilm cells for all the species in comparison with the deoxycholate formulation.

Multidrug-Resistant Candida: Epidemiology, Molecular Mechanisms, and Treatment

Invasive Candida infections remain an important cause of morbidity and mortality, especially in hospitalized and immunocompromised or critically ill patients. A limited number of antifungal agents from only a few drug classes are available to treat patients with these serious infections. Resistance can be either intrinsic or acquired. Resistance mechanisms are not exchanged between Candida; thus, acquired resistance either emerges in response to an antifungal selection pressure in the individual patient or, more rarely, occur due to horizontal transmission of resistant strains between patients. Although multidrug resistance is uncommon, increasing reports of multidrug resistance to the azoles, echinocandins, and polyenes have occurred in several Candida species, most notably Candida glabrata and more recently Candida auris. Drivers are overall antifungal use, subtherapeutic drug levels at sites of infection/colonization, drug sequestration in the biofilm matrix, and, in the setting of outbreaks, suboptimal infection control. Moreover, recent research suggests that DNA mismatch repair gene mutations may facilitate acquisition of resistance mutations in C. glabrata specifically. Diagnosis of antifungal-resistant Candida infections is critical to the successful management of patients with these infections. Reduction of unnecessary use of antifungals via antifungal stewardship is critical to limit multidrug resistance emergence.

Regulation of ERG3, ERG6, and ERG11 Genes in Antifungal-Resistant isolates of Candida parapsilosis

Background

Candida parapsilosis is one of the five common strains of yeasts involved in invasive candidiasis. The expression analysis of sterol biosynthesis pathway genes, which are associated with resistance, can assist the better understanding of antifungal resistance mechanisms.

Method

The antifungal susceptibility of 120 clinical C. parapsilosis isolates was examined. The changes in the gene expression related to resistance were analyzed.

Results

Eight strains were resistant to fluconazole (FLC), itraconazole (ITC), and amphotericin B (AMB). The regulation variations included increased mRNA levels of ERG3, ERG6, and ERG11 and decreased mRNA levels of ERG3 and ERG6 in response to FLC. ERG11 mRNA level increases in response to ITC and AMB.

Conclusion

The mechanism of resistance to azoles in C. parapsilosis is very similar to C. Albicans. This feature may help to design new treatment strategy for candidiasis.

Candida glabrata's recurrent infections: biofilm formation during Amphotericin B treatment

Candida species are responsible for recurrent human infections, mostly in immunocompromised patients, due to their high vulnerability. Candida glabrata has a major role in systemic candidiasis and Amphotericin B (AmB), a polyene only used in hospitals, is frequently used to treat this disease. Lately, however, clinical evidences of Candida recurrent infections during these treatments are being described, probably due to biofilm (re)formation during this therapy. Thus, this work aims at inferring if C. glabrata biofilms are still being formed during AmB treatment. For that, C. glabrata biofilms were formed in the presence of AmB and analysed by dry weight. Matrix composition was analysed quantifying carbohydrates and, specifically, β-1,3 glucans. Results demonstrated that, although in a lesser extent, C. glabrata is able to develop biofilms in the presence of AmB, with a thick extracellular matrix, with an increase on carbohydrates, especially β-1,3 glucans. Therefore, it is confirmed that complex biofilms of C. glabrata can be formed during an AmB treatment.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows new insights regarding recurrent candidiasis. The authors demonstrated that Amphotericin B did not totally prevent the development of biofilms during Candida glabrata's infection treatment and that the change in the biofilm matrices may have a high responsibility for the fail in the treatment of systemic candidiasis.

Candida glabrata Biofilms: How Far Have We Come?

Infections caused by Candida species have been increasing in the last decades and can result in local or systemic infections, with high morbidity and mortality. After Candida albicans, Candida glabrata is one of the most prevalent pathogenic fungi in humans. In addition to the high antifungal drugs resistance and inability to form hyphae or secret hydrolases, C. glabrata retain many virulence factors that contribute to its extreme aggressiveness and result in a low therapeutic response and serious recurrent candidiasis, particularly biofilm formation ability. For their extraordinary organization, especially regarding the complex structure of the matrix, biofilms are very resistant to antifungal treatments. Thus, new approaches to the treatment of C. glabrata's biofilms are emerging. In this article, the knowledge available on C. glabrata's resistance will be highlighted, with a special focus on biofilms, as well as new therapeutic alternatives to control them.

ERG6 gene deletion modifies Kluyveromyces lactis susceptibility to various growth inhibitors

The ERG6 gene encodes an S-adenosylmethionine dependent sterol C-24 methyltransferase in the ergosterol biosynthetic pathway. In this work we report the results of functional analysis of the Kluyveromyces lactis ERG6 gene. We cloned the KlERG6 gene, which was able to complement the erg6Δ mutation in both K. lactis and Saccharomyces cerevisiae. The lack of ergosterol in the Klerg6 deletion mutant was accompanied by increased expression of genes encoding the last steps of the ergosterol biosynthesis pathway as well as the KlPDR5 gene encoding an ABC transporter. The Klerg6Δ mutation resulted in reduced cell susceptibility to amphotericin B, nystatin and pimaricin and increased susceptibility to azole antifungals, fluphenazine, terbinafine, brefeldin A and caffeine. The susceptibility phenotype was suppressed by the KlPDR16 gene encoding one of the phosphatidylinositol transfer proteins belonging to the Sec14 family. Decreased activity of KlPdr5p in Klerg6Δ mutant (measured as the ability to efflux rhodamine 6G) together with increased amount of KlPDR5 mRNA suggest that the zymosterol which accumulates in the Klerg6Δ mutant may not fully compensate for ergosterol in the membrane targeting of efflux pumps. These results point to the fact that defects in sterol transmethylation appear to cause a multitude of physiological effects in K. lactis cells. Copyright © 2016 John Wiley & Sons, Ltd.

Prevalent mutator genotype identified in fungal pathogen Candida glabrata promotes multi-drug resistance

The fungal pathogen Candida glabrata has emerged as a major health threat since it readily acquires resistance to multiple drug classes, including triazoles and/or echinocandins. Thus far, cellular mechanisms promoting the emergence of resistance to multiple drug classes have not been described in this organism. Here we demonstrate that a mutator phenotype caused by a mismatch repair defect is prevalent in C. glabrata clinical isolates. Strains carrying alterations in mismatch repair gene MSH2 exhibit a higher propensity to breakthrough antifungal treatment in vitro and in mouse models of colonization, and are recovered at a high rate (55% of all C. glabrata recovered) from patients. This genetic mechanism promotes the acquisition of resistance to multiple antifungals, at least partially explaining the elevated rates of triazole and multi-drug resistance associated with C. glabrata. We anticipate that identifying MSH2 defects in infecting strains may influence the management of patients on antifungal drug therapy.

The fungal pathogen Candida glabrata readily acquires resistance to multiple types of antifungal drugs. Here, Healey et al. show that C. glabrata clinical isolates often carry mutations in a gene involved in DNA mismatch repair, and this is associated with increased propensity to develop antifungal resistance.

Candida glabrata, Friend and Foe

Candida glabrata is mostly good, but, at times, it is an opportunistic pathogen. Previously known as Torulopsis glabrata, it enjoyed a good reputation and was even present in starter cultures. Its haploid genome and lack of mating made it an attractive challenge for yeast genetics studies. However, more recently it has become better known due to its character as an emerging cause of candidiasis, and for its resistance to multidrugs that are employed for candidiasis treatment. While now classified as Candida glabrata, it is still not a good fit and tends to stand alone as a very unique yeast. In terms of sequence, it is dissimilar to other Candida yeast and most similar to Saccharomyces cerevisiae.

Tipping the balance both ways: drug resistance and virulence in Candida glabrata

Among existing fungal pathogens, Candida glabrata is outstanding in its capacity to rapidly develop resistance to currently used antifungal agents. Resistance to the class of azoles, which are still widely used agents, varies in proportion (from 5 to 20%) depending on geographical area. Moreover, resistance to the class of echinocandins, which was introduced in the late 1990s, is rising in several institutions. The recent emergence of isolates with acquired resistance to both classes of agents is a major concern since alternative therapeutic options are scarce. Although considered less pathogenic than C. albicans, C. glabrata has still evolved specific virulence traits enabling its survival and propagation in colonized and infected hosts. Development of drug resistance is usually associated with fitness costs, and this notion is documented across several microbial species. Interestingly, azole resistance in C. glabrata has revealed the opposite. Experimental models of infection showed enhanced virulence of azole-resistant isolates. Moreover, azole resistance could be associated with specific changes in adherence properties to epithelial cells or innate immunity cells (macrophages), both of which contribute to virulence changes. Here we will summarize the current knowledge on C. glabrata drug resistance and also discuss the consequences of drug resistance acquisition on the balance between C. glabrata and its hosts.

The Aspergillus fumigatus cell wall integrity signaling pathway: drug target, compensatory pathways, and virulence

Aspergillus fumigatus is the most important airborne fungal pathogen, causing severe infections with invasive growth in immunocompromised patients. The fungal cell wall (CW) prevents the cell from lysing and protects the fungus against environmental stress conditions. Because it is absent in humans and because of its essentiality, the fungal CW is a promising target for antifungal drugs. Nowadays, compounds acting on the CW, i.e., echinocandin derivatives, are used to treat A. fumigatus infections. However, studies demonstrating the clinical effectiveness of echinocandins in comparison with antifungals currently recommended for first-line treatment of invasive aspergillosis are still lacking. Therefore, it is important to elucidate CW biosynthesis pathways and their signal transduction cascades, which potentially compensate the inhibition caused by CW- perturbing compounds. Like in other fungi, the central core of the cell wall integrity (CWI) signaling pathway in A. fumigatus is composed of three mitogen activated protein kinases. Deletion of these genes resulted in severely enhanced sensitivity of the mutants against CW-disturbing compounds and in drastic alterations of the fungal morphology. Additionally, several cross-talk interactions between the CWI pathways and other signaling pathways are emerging, raising the question about their role in the CW compensatory mechanisms. In this review we focused on recent advances in understanding the CWI signaling pathway in A. fumigatus and its role during drug stress response and virulence.