Molecular mechanisms of drug resistance in clinical Candida species isolated from Tunisian hospitals

Vaginal mycobiome characteristics and therapeutic strategies in vulvovaginal candidiasis (VVC): differentiating pathogenic species and microecological features for stratified treatment

SUMMARYVulvovaginal candidiasis (VVC) is a prevalent global health burden, particularly among reproductive-aged women. Recurrent VVC affects a significant proportion of this population, presenting therapeutic challenges. The predominant pathogen, Candida albicans, opportunistically transitions from a commensal organism to a pathogen when microenvironmental conditions become dysregulated. Recently, non-albicans Candida species have gained attention for their reduced antifungal susceptibility and recurrence tendencies. Diagnosis is constrained by the limitations of conventional microbiological techniques, while emerging molecular assays offer enhanced pathogen detection yet lack established thresholds to differentiate between commensal and pathogenic states. Increasing resistance issues are encountered by traditional azole-based antifungals, necessitating innovative approaches that integrate microbiota modulation and precision medicine. Therefore, this review aims to systematically explore the pathogenic diversity, drug resistance mechanisms, and biofilm effects of Candida species. Vaginal microbiota (VMB) alterations associated with VVC were also examined, focusing on the interaction between Lactobacillus spp. and pathogenic fungi, emphasizing the role of microbial dysbiosis in disease progression. Finally, the potential therapeutic approaches for VVC were summarized, with a particular focus on the use of probiotics to modulate the VMB composition and restore a healthy microbial ecosystem as a promising treatment strategy. This review addresses antifungal resistance and adopts a microbiota-centric approach, proposing a comprehensive framework for personalized VVC management to reduce recurrence and improve patient outcomes.

Sub-Therapeutic Concentrations of Hexetidine Induce Virulence Gene Expression in Candida albicans

Purpose

Intraoral concentrations of antimicrobial agents such as hexetidine (HEX) using alternative antimycotics fluctuate significantly due to the dynamics in the oral cavity, which can affect both the virulence capacity and the outcome of antimycotic therapy in the management of denture stomatitis-associated oral candidiasis.

Methods

This study was carried out to evaluate the impact of different sub-therapeutic HEX concentrations (1/100, 1/200) for different times (0.5, 2, 24, 48 h) on the expression levels of the virulence (hwp1, plb1, plb2, sap4, sap5, sap6) and drug-resistance (cdr1, cdr2, mdr1, erg11) genes of Candida albicans SC5314. Total RNA was extracted immediately after antimicrobial exposure. The qRT-PCR data were performed using the comparative 2-ΔΔCT method to calculate the relative expression of the target genes after treatment to HEX concentrations, standardized to the actin housekeeping gene.

Results

Expression levels of the hwp1 and plb1 genes decrease after exposure to HEX to higher concentrations at 0.5 h, while the expression level of the plb2 gene increases. Only the expression of the erg11 gene showed a significant decrease within the resistance genes at 0.5 h. Sap5, plb2, and mdr1 are overexpressed at 48 h. Sub-therapeutic levels of HEX affect the virulence and drug-resistance genes of C. albicans.

Conclusion

These results give an idea of future strategies for developing and using HEX as an adjunctive therapy for oral candidiasis. The innovation and clinical relevance of our research lie in its exploration of how low doses of HEX influence gene expression over varying time periods, potentially paving the way for alternative therapeutic strategies.

Heterogeneity of Candida bloodstream isolates in an academic medical center and affiliated hospitals

Invasive Candida bloodstream infections (candidemia) are a deadly global health threat. Rare Candida species are increasingly important causes of candidemia and phenotypic data, including patterns of antifungal drug resistance, is limited. There is geographic variation in the distribution of Candida species and frequency of antifungal drug resistance, which means that collecting and reporting regional data can have significant clinical value. Here, we report the first survey of species distribution, frequency of antifungal drug resistance, and phenotypic variability of Candida bloodstream isolates from an academic medical center and 5 affiliated hospitals in the Minneapolis-Saint Paul region of Minnesota, collected during an 18-month period from 2019 to 2021. We collected 288 isolates spanning 11 species from 119 patients. C. albicans was the most frequently recovered species, followed by C. glabrata and C. parapsilosis, with 10% of cases representing additional, rare species. We performed antifungal drug susceptibility for the three major drug classes and, concerningly, we identified fluconazole, micafungin and multidrug resistance rates in C. glabrata that were ~ 2 times higher than that reported in other regions of the United States. We report some of the first phenotypic data in rare non-albicans Candida species. Through analysis of serial isolates from individual patients, we identified clinically relevant within-patient differences of MIC values in multiple drug classes. Our results provide valuable clinical data relevant to antifungal stewardship efforts and highlight important areas of future research, including within-patient dynamics of infection and the mechanisms of drug resistance in rare Candida species.

A MALDI-TOF MS-based multiple detection panel of drug resistance-associated multiple single-nucleotide polymorphisms in Candida tropicalis

Candida tropicalis is one of the main causes of invasive candidiasis. Rapid identification of antifungal resistance is crucial for selection of an appropriate antifungal to improve patient outcomes. Mutations at specific loci are strongly correlated with resistance to antifungal agents. In this study, we developed a multi-single-nucleotide polymorphism (SNP) panel to accurately identify 36 mutation sites across seven genes of C. tropicalis that are associated with resistance to azoles and/or echinocandins. Ten isolates were selected to test repeatability, and another 20 isolates of C. tropicalis were selected to validate consistency. Intra-assay and inter-assay repeatability of the panel was 100%, with the loci accuracy being 99.44% (716 of 720). Furthermore, 109 isolates were examined for clinical research, and the most commonly detected mutations were G751A and A866T of UPC2, A491T of TAC1, and A395T and C461T of ERG11. The G751A and A866T mutations of UPC2 as well as the A395T and C461T mutations of ERG11 co-existed. The SNP panel enables identification of specific mutations at critical sites of drug-resistant strains to facilitate the rapid selection of appropriate antifungal agents and efficient monitoring of the regional epidemiological trends of resistance of C. tropicalis.IMPORTANCEC. tropicalis infections pose a growing global public health challenge, with mortality rates approaching 40%. C. tropicalis is one of the top four Candida spp. responsible for candidiasis, particularly in the Asia-Pacific region and Latin America, notably affecting patients with neutropenia and malignancies. The azole resistance rate of C. tropicalis ranges from 0% to 30%. Between 2009 and 2018, the China Hospital Invasive Fungal Surveillance Network reported an increase in fluconazole and voriconazole resistance from 5.7% to ~30%. Although resistance to echinocandins and amphotericin B remains low, multi-resistance to echinocandins and azoles has been observed. Current methods for detecting drug resistance are limited by the long turnaround time of antifungal susceptibility testing, low throughput of Sanger sequence to target resistance mutations, complex data analysis, and high costs of second-generation sequencing. We developed and validated a rapid, high-throughput, and cost-effective panel to detect and monitor drug-resistance mutations of C. tropicalis.

Mechanisms of resistance to cell wall and plasma membrane targeting antifungal drugs in Candida species isolated in Africa

INTRODUCTION

There is a rise in the emergence of multidrug resistant fungal pathogens worldwide, including in Africa.

METHOD

This systematic review summarized the published data on the mechanisms and epidemiology of antifungal resistance in Candida species in Africa between 2000 and early 2024.

RESULT

Seventeen reports from seven African countries were analyzed but due to the paucity of data, the prevalence of antifungal resistant Candida isolates in Africa could not be estimated. However, a total of 1376 (out of 2812) resistant isolates were documented with South Africa reporting the most. Candida auris was the most reported species with multidrug and pandrug resistant strains documented in South Africa. Generally, azoles but not posaconazole or isavuconazole, resistance was reported. Fluconazole resistant isolates harbored Erg11 Y132F, VF125LA and K177A/R/N335S/E343D substitutions, MRR1 gain of function mutations or efflux pump protein over expression. Resistance to members of the echinocandin family was also reported and Fks1 S639P substitution was observed.

CONCLUSION

The data highlight that the increasing Candida species resistance to cell wall and cell membrane active antifungals is a cause for serious concern in Africa. There is need to increase antifungal research capacity and mount epidemiological surveillance to determine the true scale of the problem.

PROSPERO REGISTRATION NUMBER

CRD42024550231.

Acquired amphotericin B resistance leads to fitness trade-offs that can be mitigated by compensatory evolution in Candida auris

Candida auris is a growing concern due to its resistance to antifungal drugs, particularly amphotericin B (AMB), detected in 30 to 60% of clinical isolates. However, the mechanisms of AMB resistance remain poorly understood. Here we investigated 441 in vitro- and in vivo-evolved C. auris lineages from 4 AMB-susceptible clinical strains of different clades. Genetic and sterol analyses revealed four major types of sterol alterations as a result of clinically rare variations in sterol biosynthesis genes ERG6, NCP1, ERG11, ERG3, HMG1, ERG10 and ERG12. In addition, aneuploidies in chromosomes 4 and 6 emerged during resistance evolution. Fitness trade-off phenotyping and mathematical modelling identified diverse strain- and mechanism-dependent fitness trade-offs. Variation in CDC25 rescued fitness trade-offs, thereby increasing the infection capacity. This possibly contributed to therapy-induced acquired AMB resistance in the clinic. Our findings highlight sterol-modulating mechanisms and fitness trade-off compensation as risks for AMB treatment failure in clinical settings.

Collateral sensitivity counteracts the evolution of antifungal drug resistance in Candida auris

Antifungal drug resistance represents a serious global health threat, necessitating new treatment strategies. Here we investigated collateral sensitivity (CS), in which resistance to one drug increases sensitivity to another, and cross-resistance (XR), in which one drug resistance mechanism reduces susceptibility to multiple drugs, since CS and XR dynamics can guide treatment design to impede resistance development, but have not been systematically explored in pathogenic fungi. We used experimental evolution and mathematical modelling of Candida auris population dynamics during cyclic and combined drug exposures and found that especially CS-based drug cycling can effectively prevent the emergence of drug resistance. In addition, we found that a CS-based treatment switch can actively select against or eradicate resistant sub-populations, highlighting the potential to consider CS in therapeutic decision-making upon resistance detection. Furthermore, we show that some CS trends are robust among different strains and resistance mechanisms. Overall, these findings provide a promising direction for improved antifungal treatment approaches.

Adaptation of Candida albicans to specific host environments by gain-of-function mutations in transcription factors

The yeast Candida albicans is usually a harmless member of the normal microbiota in healthy persons but is also a major fungal pathogen that can colonize and infect almost every human tissue. A successful adaptation to environmental changes encountered in different host niches requires an appropriate regulation of gene expression. The zinc cluster transcription factors are the largest family of transcriptional regulators in C. albicans and are involved in the control of virtually all aspects of its biology. Under certain circumstances, mutations in these transcription factors that alter their activity and the expression of their target genes confer a selective advantage, which results in the emergence of phenotypically altered variants that are better adapted to new environmental challenges. This review describes how gain-of-function mutations in different zinc cluster transcription factors enable C. albicans to overcome antifungal therapy and to successfully establish itself in specific host niches.

A First-in-Class Pyrazole-isoxazole Enhanced Antifungal Activity of Voriconazole: Synergy Studies in an Azole-Resistant Candida albicans Strain, Computational Investigation and in Vivo Validation in a Galleria mellonella Fungal Infection Model

The widespread and irrational use of azole antifungal agents has led to an increase of azole-resistant Candida albicans strains with an urgent need for combination drug therapy, enhancing the treatment efficacy. Here, we report the discovery of a first-in-class pyrazole-isoxazole, namely, 5b, that showed remarkable growth inhibition against the C. albicans ATCC 10231 strain in combination with voriconazole, acting as a downregulator of ERG 11 (Cyp51) gene expression with a significant reduction of the yeast-to-hypha morphological transition. Furthermore, C. albicans CYP51 enzyme assay and in-depth molecular docking studies unveiled the unique ability of the combination of 5b and voriconazole to completely fill the CYP51 binding sites. In vivo studies using a Galleria mellonella model confirmed the previously in vitro observed synergistic effect of 5b with voriconazole. Also considering its biocompatibility in a cellular model of human keratinocytes, these results indicate that 5b represents a promising compound for a further optimization campaign.

Geraniol Potentiates the Effect of Fluconazole against Planktonic and Sessile Cells of Azole-Resistant Candida tropicalis: In Vitro and In Vivo Analyses

Candida tropicalis is regarded as an opportunistic pathogen, causing diseases ranging from superficial infections to life-threatening disseminated infections. The ability of this yeast to form biofilms and develop resistance to antifungals represents a significant therapeutic challenge. Herein, the effect of geraniol (GER), alone and combined with fluconazole (FLZ), was evaluated in the planktonic and sessile cells of azole-resistant C. tropicalis. GER showed a time-dependent fungicidal effect on the planktonic cells, impairing the cell membrane integrity. Additionally, GER inhibited the rhodamine 6G efflux, and the molecular docking analyzes supported the binding affinity of GER to the C. tropicalis Cdr1 protein. GER exhibited a synergism with FLZ against the planktonic and sessile cells, inhibiting the adhesion of the yeast cells and the viability of the 48-h biofilms formed on abiotic surfaces. C. tropicalis biofilms treated with GER, alone or combined with FLZ, displayed morphological and ultrastructural alterations, including a decrease in the stacking layers and the presence of wilted cells. Moreover, neither GER alone nor combined with FLZ caused toxicity, and both treatments prolonged the survival of the Galleria mellonella larvae infected with azole-resistant C. tropicalis. These findings indicate that the combination of GER and FLZ may be a promising strategy to control azole-resistant C. tropicalis infections.

cyp51A mutations, protein modeling, and efflux pump gene expression reveals multifactorial complexity towards understanding Aspergillus section Nigri azole resistance mechanism

Black Aspergillus species are the most common etiological agents of otomycosis, and pulmonary aspergillosis. However, limited data is available on their antifungal susceptibility profiles and associated resistance mechanisms. Here, we determined the azole susceptibility profiles of black Aspergillus species isolated from the Indian environment and explored the potential resistance mechanisms through cyp51A gene sequencing, protein homology modeling, and expression analysis of selected genes cyp51A, cyp51B, mdr1, and mfs based on their role in imparting resistance against antifungal drugs. In this study, we have isolated a total of 161 black aspergilli isolates from 174 agricultural soil samples. Isolates had variable resistance towards medical azoles; approximately 11.80%, 3.10%, and 1.24% of isolates were resistant to itraconazole (ITC), posaconazole (POS), and voriconazole (VRC), respectively. Further, cyp51A sequence analysis showed that non-synonymous mutations were present in 20 azole-resistant Aspergillus section Nigri and 10 susceptible isolates. However, Cyp51A homology modeling indicated insignificant protein structural variations because of these mutations. Most of the isolates showed the overexpression of mdr1, and mfs genes. Hence, the study concluded that azole-resistance in section Nigri cannot be attributed exclusively to the cyp51A gene mutation or its overexpression. However, overexpression of mdr1 and mfs genes may have a potential role in drug resistance.

Expression of ERG11, ERG3, MDR1 and CDR1 genes in Candida tropicalis

INTRODUCTION

Drug resistance to azoles is a growing problem in the Candida genus.

OBJECTIVE

To analyze molecularly the genes responsible for fluconazole resistance in Candida tropicalis strains.

MATERIALS AND METHODS

Nineteen strains, with and without exposure to fluconazole, were selected for this study. The expression of MDR1, CDR1, ERG11, and ERG3 genes was analyzed in sensitive, dose-dependent sensitive, and resistant strains exposed to different concentrations of the antifungal drug.

RESULTS

MDR1, ERG11 and ERG3 genes were significantly overexpressed in the different sensitivity groups. CDR1 gene expression was not statistically significant among the studied groups. Seven of the eight fluconazole-resistant strains showed overexpression of one or more of the analyzed genes. In some dose-dependent sensitive strains, we found overexpression of CDR1, ERG11, and ERG3.

CONCLUSION

The frequency of overexpression of ERG11 and ERG3 genes indicates that they are related to resistance. However, the finding of dose-dependent resistant/sensitive strains without overexpression of these genes suggests that they are not exclusive to this phenomenon. More basic research is needed to study other potentially involved genes in the resistance mechanism to fluconazole.

Rapid Evolution of Multidrug Resistance in a Candida lusitaniae Infection during Micafungin Monotherapy

Candida (Clavispora) lusitaniae is a rare, emerging non-albicans Candida species that can cause life-threatening invasive infections, spread within hospital settings, and rapidly acquire antifungal drug resistance, including multidrug resistance. The frequency and spectrum of mutations causing antifungal drug resistance in C. lusitaniae are poorly understood. Analyses of serial clinical isolates of any Candida species are uncommon and often analyze a limited number of samples collected over months of antifungal therapy with multiple drug classes, limiting the ability to understand relationships between drug classes and specific mutations. Here, we performed comparative genomic and phenotypic analysis of 20 serial C. lusitaniae bloodstream isolates collected daily from an individual patient treated with micafungin monotherapy during a single 11-day hospital admission. We identified isolates with decreased micafungin susceptibility 4 days after initiation of antifungal therapy and a single isolate with increased cross-resistance to micafungin and fluconazole, despite no history of azole therapy in this patient. Only 14 unique single nucleotide polymorphisms (SNPs) were identified between all 20 samples, including three different FKS1 alleles among isolates with decreased micafungin susceptibility and an ERG3 missense mutation found only in the isolate with increased cross-resistance to both micafungin and fluconazole. This is the first clinical evidence of an ERG3 mutation in C. lusitaniae that occurred during echinocandin monotherapy and is associated with cross-resistance to multiple drug classes. Overall, the evolution of multidrug resistance in C. lusitaniae is rapid and can emerge during treatment with only first-line antifungal therapy.

The Interkingdom Interaction with Staphylococcus Influences the Antifungal Susceptibility of the Cutaneous Fungus Malassezia

The skin is a dynamic ecosystem on which diverse microbes reside. The interkingdom interaction between microbial species in the skin microbiota is thought to influence the health and disease of the skin although the roles of the intra- and interkingdom interactions remain to be elucidated. In this context, the interactions between Malassezia and Staphylococcus, the most dominant microorganisms in the skin microbiota, have gained attention. This study investigated how the interaction between Malassezia and Staphylococcus affected the antifungal susceptibility of the fungus to the azole antifungal drug ketoconazole. The susceptibility was significantly decreased when Malassezia was co-cultured with Staphylococcus. We found that acidification of the environment by organic acids produced by Staphylococcus influenced the decrease of the ketoconazole susceptibility of M. restricta in the co-culturing condition. Furthermore, our data demonstrated that the significant increased ergosterol content and cell membrane and wall thickness of the M. restricta cells grown in the acidic environment may be the main cause of the altered azole susceptibility of the fungus. Overall, our study suggests that the interaction between Malassezia and Staphylococcus influences the antifungal susceptibility of the fungus and that pH has a critical role in the polymicrobial interaction in the skin environment.

Similarities and Differences among Species Closely Related to Candida albicans: C. tropicalis, C. dubliniensis, and C. auris

Although Candida species are widespread commensals of the microflora of healthy individuals, they are also among the most important human fungal pathogens that under certain conditions can cause diseases (candidiases) of varying severity ranging from mild superficial infections of the mucous membranes to life-threatening systemic infections. So far, the vast majority of research aimed at understanding the molecular basis of pathogenesis has been focused on the most common species—Candida albicans. Meanwhile, other closely related species belonging to the CTG clade, namely, Candida tropicalis and Candida dubliniensis, are becoming more important in clinical practice, as well as a relatively newly identified species, Candida auris. Despite the close relationship of these microorganisms, it seems that in the course of evolution, they have developed distinct biochemical, metabolic, and physiological adaptations, which they use to fit to commensal niches and achieve full virulence. Therefore, in this review, we describe the current knowledge on C. tropicalis, C. dubliniensis, and C. auris virulence factors, the formation of a mixed species biofilm and mutual communication, the environmental stress response and related changes in fungal cell metabolism, and the effect of pathogens on host defense response and susceptibility to antifungal agents used, highlighting differences with respect to C. albicans. Special attention is paid to common diagnostic problems resulting from similarities between these species and the emergence of drug resistance mechanisms. Understanding the different strategies to achieve virulence, used by important opportunistic pathogens of the genus Candida, is essential for proper diagnosis and treatment.

Petrositis caused by fluconazole-resistant candida: case report and literature review

Background

Petrositis is a rare and fatal complication associated with otitis media. It is most likely caused by bacterial infections, but in some cases it is caused by fungal infections.

Case study

The case in this report is associated with fungal petrositis. The clinical symptoms are: ear pain from chronic otitis media, severe headache, peripheral facial palsy and diplopia. The case was finally confirmed through imaging of middle ear, bacterial culture, pathology, and blood Metagenomic next-generation sequencing (mNGS) test. The patient was treated with sensitive antifungal drugs.

Conclusion

Drug treatment is conservative but efficient method in this case. mNGS can provide pathogenic reference, when antibiotic is not efficient enough for fungal infections or drug-resistant fungal infections cases. This allows we to adjust drug use for the treatment.

Mechanisms of azole antifungal resistance in clinical isolates of Candida tropicalis

This study was designed to understand the molecular mechanisms of azole resistance in Candida tropicalis using genetic and bioinformatics approaches. Thirty-two azole-resistant and 10 azole-susceptible (S) clinical isolates of C. tropicalis were subjected to mutation analysis of the azole target genes including ERG11. Inducible expression analysis of 17 other genes potentially associated with azole resistance was also evaluated. Homology modeling and molecular docking analysis were performed to study the effect of amino acid alterations in mediating azole resistance. Of the 32 resistant isolates, 12 (37.5%) showed A395T and C461T mutations in the ERG11 gene. The mean overexpression of CDR1, CDR3, TAC1, ERG1, ERG2, ERG3, ERG11, UPC2, and MKC1 in resistant isolates without mutation (R-WTM) was significantly higher (p<0.05) than those with mutation (R-WM) and the sensitive isolates (3.2-11 vs. 0.2-2.5 and 0.3-2.2 folds, respectively). Although the R-WTM and R-WM had higher (p<0.05) CDR2 and MRR1 expression compared to S isolates, noticeable variation was not seen among the other genes. Protein homology modelling and molecular docking revealed that the mutations in the ERG11 gene were responsible for structural alteration and low binding efficiency between ERG11p and ligands. Isolates with ERG11 mutations also presented A220C in ERG1 and together T503C, G751A mutations in UPC2. Nonsynonymous mutations in the ERG11 gene and coordinated overexpression of various genes including different transporters, ergosterol biosynthesis pathway, transcription factors, and stress-responsive genes are associated with azole resistance in clinical isolates of C. tropicalis.

Triazole Derivatives Target 14α-Demethylase (LDM) Enzyme in Candida albicans Causing Ergosterol Biosynthesis Inhibition

Candida albicans is the most dominant and prevalent cause of fungal infections in humans. Azoles are considered as first-line drugs for the treatment of these infections. However, their prolonged and insistent use has led to multidrug resistance and treatment failures. To overcome this, modification or derivatization of the azole ring has led to the development of new and effective antifungal molecules. In a previous study, we reported on the development of new triazole-based molecules as potential antifungal agents against Candida auris. In this study, the most potent molecules from the previous study were docked and simulated with lanosterol 14-alpha demethylase enzyme. These compounds were further evaluated for in vitro susceptibility testing against C. albicans. In silico results revealed favorable structural dynamics of the compounds, implying that the compounds would be able to effectively bind to the target enzyme, which was further manifested by the strong interaction of the test compounds with the amino acid residues of the target enzyme. In vitro studies targeting quantification of ergosterol content revealed that pta1 was the most active compound and inhibited ergosterol production by >90% in both drug-susceptible and resistant C. albicans isolates. Furthermore, RT-qPCR results revealed downregulation of ERG11 gene when C. albicans cells were treated with the test compound, which aligns with the decreased ergosterol content. In addition, the active triazole derivatives were also found to be potent inhibitors of biofilm formation. Both in silico and in vitro results indicate that these triazole derivatives have the potential to be taken to the next level of antifungal drug development.

Green synthesis of selenium nanoparticles and evaluate their effect on the expression of ERG3, ERG11, and FKS1 antifungal resistance genes in Candida albicans and Candida glabrata

Drug resistance in Candida species has been considerably increased in the last decades. Given the opposition to antifungal agents, toxicity, and interactions of the antimicrobial drugs, identifying new antifungal agents seems essential. This study assessed the antifungal effects of nanoparticles (NPs) on the standard strains of Candida albicans and Candida glabrata and determined the expression genes, including ERG3, ERG11, and FKS1. Selenium nanoparticles (Se-NPs) were biosynthesized with a standard strain of C. albicans and approved by several methods including, UV-Vis spectrophotometer, XRD technique, FTIR analysis, FESEM microscopy, and EDX diagram. The antifungal susceptibility testing performed the minimum inhibitory concentrations (MICs) using the CLSI M27-A3 and M27-S4 broth microdilution method. The expression of the desired genes was examined by the real-time PCR assay between untreated and treated by antifungal drugs and Se-NPs. The MICs of itraconazole, amphotericin B, and anidulafungin against C. albicans and C. glabrata were 64, 16, and 4 µg/ml. In comparison, reduced the MIC values ​​for samples treated with Se-NPs to 1 and 0.5 µg/ml. The results obtained from real-time PCR and analysis of the ∆∆C_q values showed that the expression of ERG3, ERG11, and FKS1 genes was significantly down-regulated in Se-NPs concentrations (P<0.05). This study's evidence implies biosafety Se-NPs have favorable effects on the reducing expression of ERG3, ERG11, and FKS1 antifungal resistance genes in C. albicans and C. glabrata.

Identification of the ergC gene involved in polyene drug sensitivity in the Mucorales species Phycomyces blakesleeanus

BACKGROUND

A strain of Phycomyces blakesleeanus (Mucorales, Mucoromycota) that was previously isolated after ultraviolet mutagenesis has altered responses to polyene antifungal drugs, sterol profiles, and phototropism of its sporangia. In this study, the genetic basis for these changes was sought.

METHODS AND RESULTS

Two base pair substitutions were identified in the mutant within a P. blakelesleeanus gene that is homologous to others characterized from fungi, such as the Saccharomyces cerevisiae ERG3 gene, encoding sterol Δ5,6-desaturase. The polyene resistance and growth reduction phenotypes co-segregated with mutations in the gene in genetic crosses. The P. blakelesleeanus wild type ergC gene complemented a S. cerevisiae deletion strain of ERG3.

CONCLUSIONS

This gene discovery may contribute towards better antifungal use in treating mucormycoses diseases caused by related species in the order Mucorales.

A world-wide analysis of reduced sensitivity to DMI fungicides in the banana pathogen Pseudocercospora fijiensis

BACKGROUND

Pseudocercospora fijiensis is the causal agent of the black leaf streak disease (BLSD) of banana. Bananas are important global export commodities and a major staple food. Their susceptibility to BLSD pushes disease management towards excessive fungicide use, largely relying on multisite inhibitors and sterol demethylation inhibitors (DMIs). These fungicides are ubiquitous in plant disease control, targeting the CYP51 enzyme. We examined sensitivity to DMIs in P. fijiensis field isolates collected from various major banana production zones in Colombia, Costa Rica, Dominican Republic, Ecuador, the Philippines, Guadalupe, Martinique and Cameroon and determined the underlying genetic reasons for the observed phenotypes.

RESULTS

We observed a continuous range of sensitivity towards the DMI fungicides difenoconazole, epoxiconazole and propiconazole with clear cross-sensitivity. Sequence analyses of PfCYP51 in 266 isolates showed 28 independent amino acid substitutions, nine of which correlated with reduced sensitivity to DMIs. In addition to the mutations, we observed up to six insertions in the Pfcyp51 promoter. Such promoter insertions contain repeated elements with a palindromic core and correlate with the enhanced expression of Pfcyp51 and hence with reduced DMI sensitivity. Wild-type isolates from unsprayed bananas fields did not contain any promoter insertions.

CONCLUSION

The presented data significantly contribute to understanding of the evolution and global distribution of DMI resistance mechanisms in P. fijiensis field populations and facilitate the prediction of different DMI efficacy. The overall reduced DMI sensitivity calls for the deployment of a wider range of solutions for sustainable control of this major banana disease. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Molecular and genetic basis of azole antifungal resistance in the opportunistic pathogenic fungus Candida albicans

Candida albicans is an opportunistic yeast and the major human fungal pathogen in the USA, as well as in many other regions of the world. Infections with C. albicans can range from superficial mucosal and dermatological infections to life-threatening infections of the bloodstream and vital organs. The azole antifungals remain an important mainstay treatment of candidiasis and therefore the investigation and understanding of the evolution, frequency and mechanisms of azole resistance are vital to improving treatment strategies against this organism. Here the organism C. albicans and the genetic changes and molecular bases underlying the currently known resistance mechanisms to the azole antifungal class are reviewed, including up-regulated expression of efflux pumps, changes in the expression and amino acid composition of the azole target Erg11 and alterations to the organism's typical sterol biosynthesis pathways. Additionally, we update what is known about activating mutations in the zinc cluster transcription factor (ZCF) genes regulating many of these resistance mechanisms and review azole import as a potential contributor to azole resistance. Lastly, investigations of azole tolerance in C. albicans and its implicated clinical significance are reviewed.

Antifungal susceptibility, genotyping, resistance mechanism, and clinical profile of Candida tropicalis blood isolates

Candida tropicalis is one of the major candidaemia agents, associated with the highest mortality rates among Candida species, and developing resistance to azoles. Little is known about the molecular mechanisms of azole resistance, genotypic diversity, and the clinical background of C. tropicalis infections. Consequently, this study was designed to address those questions. Sixty-four C. tropicalis bloodstream isolates from 62 patients from three cities in Iran (2014–2019) were analyzed. Strain identification, antifungal susceptibility testing, and genotypic diversity analysis were performed by MALDI-TOF MS, CLSI-M27 A3/S4 protocol, and amplified fragment length polymorphism (AFLP) fingerprinting, respectively. Genes related to drug resistance (ERG11, MRR1, TAC1, UPC2, and FKS1 hotspot9s) were sequenced. The overall mortality rate was 59.6% (37/62). Strains were resistant to micafungin [minimum inhibitory concentration (MIC) ≥1 μg/ml, 2/64], itraconazole (MIC > 0.5 μg/ml, 2/64), fluconazole (FLZ; MIC ≥ 8 μg/ml, 4/64), and voriconazole (MIC ≥ 1 μg/ml, 7/64). Pan-azole and FLZ + VRZ resistance were observed in one and two isolates, respectively, while none of the patients were exposed to azoles. MRR1 (T255P, 647S), TAC1 (N164I, R47Q), and UPC2 (T241A, Q340H, T381S) mutations were exclusively identified in FLZ-resistant isolates. AFLP fingerprinting revealed five major and seven minor genotypes; genotype G4 was predominant in all centers. The increasing number of FLZ-R C. tropicalis blood isolates and acquiring FLZ-R in FLZ-naive patients limit the efficiency of FLZ, especially in developing countries. The high mortality rate warrants reaching a consensus regarding the nosocomial mode of C. tropicalis transmission.

Genome-Wide Analysis of Experimentally Evolved Candida auris Reveals Multiple Novel Mechanisms of Multidrug Resistance

Candida auris is globally recognized as an opportunistic fungal pathogen of high concern, due to its extensive multidrug resistance (MDR). Still, molecular mechanisms of MDR are largely unexplored. This is the first account of genome-wide evolution of MDR in C. auris obtained through serial in vitro exposure to azoles, polyenes, and echinocandins. We show the stepwise accumulation of copy number variations and novel mutations in genes both known and unknown in antifungal drug resistance. Echinocandin resistance was accompanied by a codon deletion in FKS1 hot spot 1 and a substitution in FKS1 "novel" hot spot 3. Mutations in ERG3 and CIS2 further increased the echinocandin MIC. Decreased azole susceptibility was linked to a mutation in transcription factor TAC1b and overexpression of the drug efflux pump Cdr1, a segmental duplication of chromosome 1 containing ERG11, and a whole chromosome 5 duplication, which contains TAC1b The latter was associated with increased expression of ERG11, TAC1b, and CDR2 but not CDR1 The simultaneous emergence of nonsense mutations in ERG3 and ERG11 was shown to decrease amphotericin B susceptibility, accompanied with fluconazole cross-resistance. A mutation in MEC3, a gene mainly known for its role in DNA damage homeostasis, further increased the polyene MIC. Overall, this study shows the alarming potential for and diversity of MDR development in C. auris, even in a clade until now not associated with MDR (clade II), stressing its clinical importance and the urge for future research.IMPORTANCECandida auris is a recently discovered human fungal pathogen and has shown an alarming potential for developing multi- and pan-resistance toward all classes of antifungals most commonly used in the clinic. Currently, C. auris has been globally recognized as a nosocomial pathogen of high concern due to this evolutionary potential. So far, this is the first study in which the stepwise progression of multidrug resistance (MDR) in C. auris is monitored in vitro Multiple novel mutations in known resistance genes and genes previously not or vaguely associated with drug resistance reveal rapid MDR evolution in a C. auris clade II isolate. Additionally, this study shows that in vitro experimental evolution can be a powerful tool to discover new drug resistance mechanisms, although it has its limitations.

Rapid detection of ERG11 polymorphism associated azole resistance in Candida tropicalis

Increasing reports of azole resistance in Candida tropicalis, highlight the development of rapid resistance detection techniques. Nonsynonymous mutations in the lanosterol C14 alpha-demethylase (ERG11) gene is one of the predominant mechanisms of azole resistance in C. tropicalis. We evaluated the tetra primer-amplification refractory mutation system-PCR (T-ARMS-PCR), restriction site mutation (RSM), and high-resolution melt (HRM) analysis methods for rapid resistance detection based on ERG11 polymorphism in C. tropicalis. Twelve azole-resistant and 19 susceptible isolates of C. tropicalis were included. DNA sequencing of the isolates was performed to check the ERG11 polymorphism status among resistant and susceptible isolates. Three approaches T-ARMS-PCR, RSM, and HRM were evaluated and validated for the rapid detection of ERG11 mutation. The fluconazole MICs for the 12 resistant and 19 susceptible isolates were 32–256 mg/L and 0.5–1 mg/L, respectively. The resistant isolates showed A339T and C461T mutations in the ERG11 gene. The T-ARMS-PCR and RSM approaches discriminated all the resistant and susceptible isolates, whereas HRM analysis differentiated all except one susceptible isolate. The sensitivity, specificity, analytical sensitivity, time, and cost of analysis suggests that these three methods can be utilized for the rapid detection of ERG11 mutations in C. tropicalis. Additionally, an excellent concordance with DNA sequencing was noted for all three methods. The rapid, sensitive, and inexpensive T-ARMS-PCR, RSM, and HRM approaches are suitable for the detection of azole resistance based on ERG11 polymorphism in C. tropicalis and can be implemented in clinical setups for batter patient management.

Insights on recent approaches in drug discovery strategies and untapped drug targets against drug resistance

BACKGROUND

Despite the various strategies undertaken in the clinical practice, the mortality rate due to antibiotic-resistant microbes has been markedly increasing worldwide. In addition to multidrug-resistant (MDR) microbes, the "ESKAPE" bacteria are also emerging. Of course, the infection caused by ESKAPE cannot be treated even with lethal doses of antibiotics. Now, the drug resistance is also more prevalent in antiviral, anticancer, antimalarial and antifungal chemotherapies.

MAIN BODY

To date, in the literature, the quantum of research reported on the discovery strategies for new antibiotics is remarkable but the milestone is still far away. Considering the need of the updated strategies and drug discovery approaches in the area of drug resistance among researchers, in this communication, we consolidated the insights pertaining to new drug development against drug-resistant microbes. It includes drug discovery void, gene paradox, transposon mutagenesis, vitamin biosynthesis inhibition, use of non-conventional media, host model, target through quorum sensing, genomic-chemical network, synthetic viability to targets, chemical versus biological space, combinational approach, photosensitization, antimicrobial peptides and transcriptome profiling. Furthermore, we optimally briefed about antievolution drugs, nanotheranostics and antimicrobial adjuvants and then followed by twelve selected new feasible drug targets for new drug design against drug resistance. Finally, we have also tabulated the chemical structures of potent molecules against antimicrobial resistance.

CONCLUSION

It is highly recommended to execute the anti-drug resistance research as integrated approach where both molecular and genetic research needs to be as integrative objective of drug discovery. This is time to accelerate new drug discovery research with advanced genetic approaches instead of conventional blind screening.

Detection of ERG11 Overexpression in Candida albicans isolates from environmental sources and clinical isolates treated with inhibitory and subinhibitory concentrations of fluconazole

BACKGROUND

Candida species can cause serious infection in patients with changes in defence mechanisms and/or when anatomical barriers are compromised. Mutations and overexpression in the ERG11 gene are described as molecular mechanisms of azole resistance. Information is limited on these mechanisms in the presence of subinhibitory concentrations of fluconazole.

OBJECTIVES

This study aimed to evaluate the expression of ERG11 gene from Candida albicans isolates, from clinical and hospital environments, in the absence and presence of inhibitory and subinhibitory concentrations of fluconazole.

METHODS

The American Type Culture Collection 10231 strain, five clinical isolates and three isolates from hospital environment colonisation were exposed to inhibitory and subinhibitory concentrations of fluconazole. Susceptibility tests were performed according to EUCAST 7.1 guidelines, and the relative expression analysis of ERG11 was performed by qPCR.

RESULTS

Differences in response to fluconazole concentrations were observed, with the exception only one clinical isolate when treated with 1/4 of the FLU-minimum inhibitory concentration (MIC). All the other isolates, regardless of the isolation source, had an increase in expression. The overexpression occurred in a very broad range, from 1.086 to 126.105 times. In general, treatment with the highest dose of fluconazole (MIC) was the one that most influenced the ERG11 expression, followed by treatments with 1/2 and 1/4 MIC.

CONCLUSIONS

The increased expression of ERG11 by C albicans in the presence of different concentrations of fluconazole is relevant, raising concerns in the care and cleaning of the hospital environment and the prophylactic use of fluconazole that could lead to the selection of potential azole-resistant isolates.

Mutations at sterol 14α-demethylases (CYP51A&B) confer the DMI resistance in Colletotrichum gloeosporioides from grape

BACKGROUND

Grape anthracnose caused by the ascomycete fungus Colletotrichum gloeosporioides has been widely controlled by demethylation inhibitors (DMIs) for decades in China. The resistance status and mechanism of C. gloeosporioides against DMIs is not well understood.

RESULTS

All difenoconazole-resistant (Dfn^R ) isolates from vineyards exhibited decreased fitness. Positive cross-resistance was detected between DMI triazoles. Sequence alignment results from the Dfn^R and Dfn^S isolates revealed that multiple mutations are distributed at CgCYP51A, concomitant with mutations at CgCYP51B. The half maximal effective concentration (EC₅₀ ) values of single deleted and complemented mutants of CgCYP51A and CgCYP51B showed that ΔCgCYP51A became more sensitive to difenoconazole, but not ΔCgCYP51B. Furthermore, all single complemented mutants had a stronger biological fitness than the progenitor strain. All the defectives of ΔCgCYP51A and ΔCgCYP51B could be restored by complementation of the whole corresponding gene from the resistant strains. Relative gene expression of CgCYP51A and CgCYP51B in most of the mutants was greatly upregulated relative to the progenitor isolate when treated with difenoconazole at the same concentration. Moreover, the extension of five amino acids (GNETI) caused by mutation at the stop codon of CgCYP51A, concurrent with other seven amino acid substitutions and the synonymous mutation P10P (CCG → CCT), significantly enhanced DMI resistance.

CONCLUSION

The DMI resistance of C. gloeosporioides selected in vineyards is conferred by mutations at CgCYP51s, and validated by a genetics method. The roles of CgCYP51A and CgCYP51B overlap, and are counter-balanced, but cannot be replaced reciprocally. © 2020 Society of Chemical Industry.

Sphingolipids: Regulators of azole drug resistance and fungal pathogenicity

In recent years, the role of sphingolipids in pathogenic fungi, in terms of pathogenicity and resistance to azole drugs, has been a rapidly growing field. This review describes evidence about the roles of sphingolipids in azole resistance and fungal virulence. Sphingolipids can serve as signaling molecules that contribute to azole resistance through modulation of the expression of drug efflux pumps. They also contribute to azole resistance by participating in various microbial pathways such as the unfolded protein response (UPR), pH-responsive Rim pathway, and pleiotropic drug resistance (PDR) pathway. In addition, sphingolipid signaling and eisosomes also coordinately regulate sphingolipid biosynthesis in response to azole-induced membrane stress. Sphingolipids are important for fungal virulence, playing roles during growth in hosts under stressful conditions, maintenance of cell wall integrity, biofilm formation, and production of various virulence factors. Finally, we discuss the possibility of exploiting fungal sphingolipids for the development of new therapeutic strategies to treat infections caused by pathogenic fungi.

Small-Molecule Inhibitors Targeting Sterol 14α-Demethylase (CYP51): Synthesis, Molecular Modelling and Evaluation Against Candida albicans

Fungal infections are a global issue affecting over 150 million people worldwide annually, with 750 000 of these caused by invasive Candida infections. Azole drugs are the frontline treatment against fungal infections; however, resistance to current azole antifungals in C. albicans poses a threat to public health. Two series of novel azole derivatives, short and extended derivatives, have been designed, synthesised and investigated for CYP51 inhibitory activity, binding affinity and minimum inhibitory concentration (MIC) against C. albicans strains. The short derivatives were more potent against the C. albicans strains (e. g., MIC 2-(4-chlorophenyl)-N-(2,4-dichlorobenzyl)-3-(1H-imidazol-1-yl)propanamide (5 f) <0.03 μg/mL, N-(4-((4-chlorophenyl)sulfonamido)benzyl)-2-phenyl-3-(1H-1,2,4-triazol-1-yl)propanamide (12 c), 1 μg/mL, fluconazole 0.125 μg/mL) but both displayed comparable enzyme binding and inhibition (5 f Kd 62±17 nM, IC50 0.46 μM; 12 c Kd 43±18 nM, IC50 0.33 μM, fluconazole Kd 41±13 nM, IC50 0.31 μM, posaconazole Kd 43±11 nM, IC50 0.2 μM). The short series had poor selectivity for CaCYP51 over the human homologue, whereas the selectivity of the extended series, for example, compound 12 c, was higher (21.5-fold) than posaconazole (4.7-fold) based on Kd values, although posaconazole was more selective (615-fold) than 12 c (461-fold) based on IC50 values. Based on inhibitory activity and selectivity profile, the extended series are the better of the two series for further development.

Dynamics of in vitro development of azole resistance in Candida tropicalis

OBJECTIVES

Increasing incidence of azole resistance in Candida tropicalis, especially to fluconazole, has been seen in Asian countries including India. Limited knowledge is available on the molecular mechanisms associated with the development of azole resistance in C. tropicalis. The present study examined the dynamics of in vitro azole resistance in C. tropicalis after prolonged treatment with fluconazole.

METHODS

Nine fluconazole-susceptible isolates of C. tropicalis were used in this study. Fluconazole resistance was induced experimentally in C. tropicalis isolates. The stability of induced resistance and cross-resistance to other azoles was examined. The molecular mechanisms of azole resistance were assessed by measuring the expression and mutation analysis of different genes.

RESULTS

Varying degrees of resistance [five with minimum inhibitory concentrations (MICs) ≤32 mg/L and four with MICs ≥128 mg/L] were noticed, and the resistance was developed in 3 months. Of the nine resistant isolates, four induced resistant isolates with MICs ≥128 mg/L presented temporal resistance stability up to 10 subcultures. These four isolates presented cross-resistance to other azoles and also an inducible overexpression of transporters (CDR1, CDR2, CDR3 and MDR1), ergosterol biosynthesis pathway genes (ERG1, ERG2, ERG3 and ERG11), transcription factors (TAC1 and UPC2) and stress-responsive genes (HSP90 and MKC1) was noticed. No mutations were seen in any of the four genes (ERG1, ERG3, ERG11 and UPC2) tested.

CONCLUSIONS

Candida tropicalis isolates adapt themselves in the presence of continuous drug exposure and switch back to being susceptible in the absence of the drug. The acquisition of resistance in C. tropicalis is mediated by the overexpression of different resistance-related genes without any molecular alterations.

The yeast, the antifungal, and the wardrobe: a journey into antifungal resistance mechanisms of Candida tropicalis

Candida tropicalis is a prominent non-Candida albicans Candida species involved in cases of candidemia, mainly causing infections in patients in intensive care units and (or) those presenting neutropenia. In recent years, several studies have reported an increase in the recovery rates of azole-resistant C. tropicalis isolates. Understanding C. tropicalis resistance is of great importance, since resistant strains are implicated in persistent or recurrent and breakthrough infections. In this review, we address the main mechanisms underlying C. tropicalis resistance to the major antifungal classes used to treat candidiasis. The main genetic basis involved in C. tropicalis antifungal resistance is discussed. A better understanding of the epidemiology of resistant strains and the mechanisms involved in C. tropicalis resistance can help improve diagnosis and assessment of the antifungal susceptibility of this Candida species to improve clinical management.

Análisis cuantitativo de la expresión de genes de resistencia a fluconazol en cepas de Candida albicans aisladas al ingreso de adultos mayores a una unidad de cuidados intensivos de Manizales, Colombia

Introducción.

Las infecciones oportunistas asociadas con Candida albicans han tenido gran repercusión en la salud pública por la mortalidad que generan en determinados grupos poblacionales. Aunque existen tratamientos farmacológicos disponibles, es evidente el aumento de la resistencia desarrollada por el agente patógeno, por lo que la determinación de los mecanismos de resistencia de las cepas presentes en las áreas hospitalarias es importante, ya que permitiría plantear mejores esquemas de tratamiento.

Objetivo.

Analizar la expresión de los genes ERG11, CDR1 y MDR1 en cepas de C. albicans aisladas de adultos mayores a su ingreso en la unidad de cuidados intensivos del Hospital Santa Sofía de Manizales, Colombia.

Materiales y métodos.

Se seleccionaron 29 muestras (21 resistentes y 8 sensibles) y se conformaron dos grupos de trabajo, uno de muestras con exposición al fluconazol y el otro sin esta. El ARN extraído se cuantificó mediante reacción en cadena de la polimerasa con transcriptasa inversa en tiempo real (RT-qPCR).

Resultados.

Se encontraron diferencias significativas en la expresión del gen MDR1 en el grupo de cepas de C. albicans resistentes. Dos de las cepas resistentes (104 y 62-2) expuestas al antifúngico presentaron valores muy elevados en la expresión de este gen. La expresión del ERG11 y del CDR1 no fue significativa en los grupos estudiados.

Conclusión.

El aumento de sobreexpresión del gen MDR1 indica que este puede ser el responsable de la resistencia; sin embargo, algunas cepas resistentes no sobreexpresaron los genes analizados, lo que indica que puede haber otros genes involucrados en la resistencia de las cepas estudiadas.

In vitro Antibiotic and Modulatory Activity of Mesosphaerum suaveolens (L.) Kuntze against Candida strains

The emergence of fungal resistance to commercial drugs has been a major problem for the WHO. In this context, research with natural products is promising in the discovery of new active substances. Thus, this work evaluated the antifungal effect of a medicinal plant (i.e., Mesosphaerum suaveolens) against strains of the genus Candida, tested the combined effect with the drug fluconazole, and, finally, determined the phenolic constituents present in the species. Initially, aqueous extracts of leaves (AELMs) and aerial parts (AEAPMs) of the species were prepared. For microbiological assays, the minimum fungicidal concentration was determined by broth microdilution, and the combined effect of fluconazole extracts were verified by sub-inhibitory microdilution concentrations (CFM/8) followed by spectrophotometric readings which were used to determine the IC50. HPLC detected the presence of flavonoids and phenolic acids, detecting eight compounds present in the samples of which caffeic acid and quercetin were major components. The AELMs modulated fluconazole activity since it decreased fluconazole's IC50 from 7.8 µg/mL to an IC50 of 4.7 µg/mL (CA LM 77) and from 28.8 µg/mL to 18.26 µg/mL (CA INCQS 40006) for the C. albicans strains. The AEAPMs were able to potentiate the effect of fluconazole more effectively than the AELMs. Such an effect was significant for the 16 µg/mL concentration for CA LM 77 and 32 µg/mL for CA INCQS 40006. The AEAPMs as well as the AELMs presented clinically relevant activities for C. tropicalis strains. For the C. tropicalis LM 23 strain, the AEPMs obtained an IC50 of 25 µg/mL and the AELMs an IC50 of 359.9 µg/mL.

Expression of ERG11 and efflux pump genes CDR1, CDR2 and SNQ2 in voriconazole susceptible and resistant Candida glabrata strains

Candida glabrata causes difficult to treat invasive candidiasis due to its antifungal resistance, mainly to azoles. The aim of the present work was to study the role of the genes ERG11, CDR1, CDR2, and SNQ2 on the resistance to voriconazole (VRC) in a set of C. glabrata strains with known in vitro and in vivo susceptibility to this drug. Eighteen clinical isolates of C. glabrata were exposed in vitro to VRC, and the expression of the cited genes was quantified by real time quantitative polymerase chain reaction (q-PCR). In addition, the ERG11 gene was amplified and sequenced to detect possible mutations. Ten synonymous mutations were found in 15 strains, two of them being reported for the first time; however, no amino acid changes were detected. ERG11 and CDR1 were the most expressed genes in all the strains tested, while the expression of CDR2 and SNQ2 was modest. Our results show that gene expression does not directly correlate with the VRC MIC. In addition, the expression profiles of ERG11 and efflux pump genes did not change consistently after exposure to VRC. Although individual analysis did not result in a clear correlation between MIC and gene expression, we did observe an increase in ERG11 and CDR1 expression in resistant strains. It is of interest that considering both in vitro and in vivo results, the slight increase in such gene expression correlates with the observed resistance to VRC.

Emerging Complexity and the Need for Advanced Drug Delivery in Targeting Candida Species

Background:

Candida species are the important etiologic agents for candidiasis, the most prevalent cause of opportunistic fungal infections. Candida invasion results in mucosal to systemic infections through immune dysfunction and helps in further invasion and proliferation at several sites in the host. The host defence system utilizes a wide array of the cells, proteins and chemical signals that are distributed in blood and tissues which further constitute the innate and adaptive immune system. The lack of antifungal agents and their limited therapeutic effects have led to high mortality and morbidity related to such infections.

Methods:

The necessary information collated on this review has been gathered from various literature published from 1995 to 2019.

Results:

This article sheds light on novel drug delivery approaches to target the immunological axis for several Candida species (C. albicans, C. glabrata, C. parapsilosis, C. tropicalis, C. krusei, C. rugose, C. hemulonii, etc.).

Conclusion:

It is clear that the novel drug delivery approaches include vaccines, adoptive transfer of primed immune cells, recombinant cytokines, therapeutic antibodies, and nanoparticles, which have immunomodulatory effects. Such advancements in targeting various underpinning mechanisms using the concept of novel drug delivery will provide a new dimension to the fungal infection clinic particularly due to Candida species with improved patient compliance and lesser side effects. This advancement in knowledge can also be extended to target various other similar microbial species and infections.

Inhibition of ergosterol synthesis in Candida albicans by novel eugenol tosylate congeners targeting sterol 14α-demethylase (CYP51) enzyme

This study is a continuation and extension of our previous study in which we synthesized seven novel eugenol tosylate congeners (ETC-1 to ETC-7) from a natural compound eugenol and checked their antifungal activity against different isolates of Candida albicans. All these ETCs showed potent antifungal activity to varying degrees. In this study, the aim is to evaluate the effect of most active compounds (ETC-5, ETC-6 and ETC-7) on ergosterol biosynthesis pathway and cellular viability in C. albicans by applying combined approach of in silico and in vitro methodologies. In silico studies were done through all atom molecular mechanics approach and free binding energy estimations, and in vitro study was done by estimating total intracellular sterol content and effect on expression of ERG11 gene. Furthermore, effect on cell viability by these compounds was also tested. Our results demonstrated that these ETCs target ergosterol biosynthesis pathway in C. albicans by inhibiting the lanosterol 14-α demethylase enzyme and also downregulates expression of its related gene ERG11. Furthermore, these ETCs exhibit potent fungicidal effect in cell viability assay, thus overall results advocating the claim that these tosylates have potential to be taken to next level of antifungal drug development.

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.

Pfcyp51 exclusively determines reduced sensitivity to 14α-demethylase inhibitor fungicides in the banana black Sigatoka pathogen Pseudocercospora fijiensis

The haploid fungus Pseudocercospora fijiensis causes black Sigatoka in banana and is chiefly controlled by extensive fungicide applications, threatening occupational health and the environment. The 14α-Demethylase Inhibitors (DMIs) are important disease control fungicides, but they lose sensitivity in a rather gradual fashion, suggesting an underlying polygenic genetic mechanism. In spite of this, evidence found thus far suggests that P. fijiensis cyp51 gene mutations are the main responsible factor for sensitivity loss in the field. To better understand the mechanisms involved in DMI resistance, in this study we constructed a genetic map using DArTseq markers on two F1 populations generated by crossing two different DMI resistant strains with a sensitive strain. Analysis of the inheritance of DMI resistance in the F1 populations revealed two major and discrete DMI-sensitivity groups. This is an indicative of a single major responsible gene. Using the DMI-sensitivity scorings of both F1 populations and the generation of genetic linkage maps, the sensitivity causal factor was located in a single genetic region. Full agreement was found for genetic markers in either population, underlining the robustness of the approach. The two maps indicated a similar genetic region where the Pfcyp51 gene is found. Sequence analyses of the Pfcyp51 gene of the F1 populations also revealed a matching bimodal distribution with the DMI resistant. Amino acid substitutions in P. fijiensis CYP51 enzyme of the resistant progeny were previously correlated with the loss of DMI sensitivity. In addition, the resistant progeny inherited a Pfcyp51 gene promoter insertion, composed of a repeat element with a palindromic core, also previously correlated with increased gene expression. This genetic approach confirms that Pfcyp51 is the single explanatory gene for reduced sensitivity to DMI fungicides in the analysed P. fijiensis strains. Our study is the first genetic analysis to map the underlying genetic factors for reduced DMI efficacy.

Eucalyptal D Enhances the Antifungal Effect of Fluconazole on Fluconazole-Resistant Candida albicans by Competitively Inhibiting Efflux Pump

The frequent emergence of azole-resistant strains has increasingly led azoles to fail in treating candidiasis. Combination with other drugs is a good option to effectively reduce or retard its incidence of resistance. Natural products are a promising synergist source to assist azoles in treating resistant candidiasis. Eucalyptal D (ED), a formyl-phloroglucinol meroterpenoid, is one of the natural synergists, which could significantly enhance the anticandidal activity of fluconazole (FLC) in treating FLC resistant C. albicans. The checkerboard microdilution assay showed their synergistic effect. The agar disk diffusion test illustrated the key role of ED in synergy. The rhodamine 6G (R6G) efflux assay reflected ED could reduce drug efflux, but quantitative reverse transcription PCR analysis revealed the upregulation of CDR1 and CDR2 genes in ED treating group. Efflux pump-deficient strains were hyper-susceptible to ED, thus ED was speculated to be the substrate of efflux pump Cdr1p and Cdr2p to competitively inhibit the excretion of FLC or R6G, which mainly contributed to its synergistic effect.

Fungal Lanosterol 14α-demethylase: A target for next-generation antifungal design

The cytochrome P450 enzyme lanosterol 14α-demethylase (LDM) is the target of the azole antifungals used widely in medicine and agriculture as prophylaxis or treatments of infections or diseases caused by fungal pathogens. These drugs and agrochemicals contain an imidazole, triazole or tetrazole substituent, with one of the nitrogens in the azole ring coordinating as the sixth axial ligand to the LDM heme iron. Structural studies show that this membrane bound enzyme contains a relatively rigid ligand binding pocket comprised of a deeply buried heme-containing active site together with a substrate entry channel and putative product exit channel that reach to the membrane. Within the ligand binding pocket the azole antifungals have additional affinity determining interactions with hydrophobic side-chains, the polypeptide backbone and via water-mediated hydrogen bond networks. This review will describe the tools that can be used to identify and characterise the next generation of antifungals targeting LDM, with the goal of obtaining highly potent broad-spectrum fungicides that will be able to avoid target and drug efflux mediated antifungal resistance.

Evolution of Fluconazole-Resistant Candida albicans Strains by Drug-Induced Mating Competence and Parasexual Recombination

Sexual reproduction is an important mechanism in the evolution of species, since it allows the combination of advantageous traits of individual members in a population. The pathogenic yeast Candida albicans is a diploid organism that normally propagates in a clonal fashion, because heterozygosity at the mating type locus (MTL) inhibits mating between cells. Here we show that C. albicans cells that have acquired drug resistance mutations during treatment with the commonly used antifungal agent fluconazole rapidly develop further increased resistance by genome rearrangements that result in simultaneous loss of heterozygosity for the mutated allele and the mating type locus. This enables the drug-resistant cells of a population to switch to the mating-competent opaque morphology and mate with each other to combine different individually acquired resistance mechanisms. The tetraploid mating products reassort their merged genomes and, under selective pressure by the drug, generate highly resistant progeny that have retained the advantageous mutated alleles. Parasexual propagation, promoted by stress-induced genome rearrangements that result in the acquisition of mating competence in cells with adaptive mutations, may therefore be an important mechanism in the evolution of C. albicans populations.

The clonal population structure of Candida albicans suggests that (para)sexual recombination does not play an important role in the lifestyle of this opportunistic fungal pathogen, an assumption that is strengthened by the fact that most C. albicans strains are heterozygous at the mating type locus (MTL) and therefore mating-incompetent. On the other hand, mating might occur within clonal populations and allow the combination of advantageous traits that were acquired by individual cells to adapt to adverse conditions. We have investigated if parasexual recombination may be involved in the evolution of highly drug-resistant strains exhibiting multiple resistance mechanisms against fluconazole, an antifungal drug that is commonly used to treat infections by C. albicans. Growth of strains that were heterozygous for MTL and different fluconazole resistance mutations in the presence of the drug resulted in the emergence of derivatives that had become homozygous for the mutated allele and the mating type locus and exhibited increased drug resistance. When MTLa/a and MTLα/α cells of these strains were mixed in all possible combinations, we could isolate mating products containing the genetic material from both parents. The initial mating products did not exhibit higher drug resistance than their parental strains, but further propagation under selective pressure resulted in the loss of the wild-type alleles and increased fluconazole resistance. Therefore, fluconazole treatment not only selects for resistance mutations but also promotes genomic alterations that confer mating competence, which allows cells in an originally clonal population to exchange individually acquired resistance mechanisms and generate highly drug-resistant progeny.

Molecular mechanisms of azole resistance in Candida bloodstream isolates

BACKGROUND

Antifungal resistance rates are increasing. We investigated the mechanisms of azole resistance of Candida spp. bloodstream isolates obtained from a surveillance study conducted between 2012 and 2015.

METHODS

Twenty-six azole non-susceptible Candida spp. clinical isolates were investigated. Antifungal susceptibilities were determined using the Sensititre YeastOne® YO10 panel. The ERG11 gene was amplified and sequenced to identify amino acid polymorphisms, while real-time PCR was utilised to investigate the expression levels of ERG11, CDR1, CDR2 and MDR1.

RESULTS

Azole cross-resistance was detected in all except two isolates. Amino acid substitutions (A114S, Y257H, E266D, and V488I) were observed in all four C. albicans tested. Of the 17 C. tropicalis isolates, eight (47%) had ERG11 substitutions, of which concurrent observation of Y132F and S154F was the most common. A novel substitution (I166S) was detected in two of the five C. glabrata isolates. Expression levels of the various genes differed between the species but CDR1 and CDR2 overexpression appeared to be more prominent in C. glabrata.

CONCLUSIONS

There was interplay of various different mechanisms, including mechanisms which were not studied here, responsible for azole resistance in Candida spp in our study.

The Fungal CYP51s: Their Functions, Structures, Related Drug Resistance, and Inhibitors

CYP51 (Erg11) belongs to the cytochrome P450 monooxygenase (CYP) superfamily and mediates a crucial step of the synthesis of ergosterol, which is a fungal-specific sterol. It is also the target of azole drugs in clinical practice. In recent years, researches on fungal CYP51 have stepped into a new stage attributing to the discovery of crystal structures of the homologs in Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus. This review summarizes the functions, structures of fungal CYP51 proteins, and the inhibitors targeting these homologs. In particular, several drug-resistant mechanisms associated with the fungal CYP51s are introduced. The sequences and crystal structures of CYP51 proteins in different fungal species are also compared. These will provide new insights for the advancement of research on antifungal agents.

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.

Chemical composition, antifungal activity and potential anti-virulence evaluation of the Eugenia uniflora essential oil against Candida spp

The development of fungal resistance to antifungal drugs has been worsening over the years and as a result research on new antifungal agents derived from plants has intensified. Eugenia uniflora L. (pitanga) has been studied for its various biological actions. In this study the chemical composition and antifungal effects of the E. uniflora essential oil (EULEO) were investigated against Candida albicans (CA), Candida krusei (CK) and Candida tropicalis (CT) standard strains. The essential oil obtained through hydro-distillation was analyzed by gas chromatography coupled to mass spectrometry (GC-MS). To determine the IC₅₀ of the oil, the cellular viability curve and the inhibitory effects were measured by means of the oil's association with Fluconazole in a broth microdilution assay with spectrophotometric readings. The Minimum Fungicidal Concentration (MFC) was determined by solid medium subculture with the aid of a guide plate while the assays used to verify morphological changes emerging from the action of the fractions were performed in microculture chambers at concentrations based on the microdilution. Two major oil constituents stand out from the chemical analysis: selina-1,3,7(11)-trien-8-one (36.37%) and selina-1,3,7(11)-trien-8-one epoxide (27.32%). The concentration that reduced microorganismal growth was ≥8,192 μg/mL while the IC₅₀ varied, this being between 1892.47 and 12491.80 μg/mL (oil), 10.07 - 80.78 μg/mL (fluconazole) and 18.53 - 295.60 μg/mL (fluconazole + oil). The combined activity (fluconazole + oil) resulted in indifference and antagonism. A MFC of the oil in association with fluconazole was recorded at the concentration of 8,192 μg/mL against CA and CK. The oil caused the inhibition of CA and CT morphological transition. In view of the results obtained, additional research is needed to elucidate the activity of the E. uniflora oil over genetic and biochemical processes regarding its effect on Candida spp. virulence.

The Viscoelastic Properties of the Fungal Cell Wall Allow Traffic of AmBisome as Intact Liposome Vesicles

The fungal cell wall is a critically important structure that represents a permeability barrier and protective shield. We probed Candida albicans and Cryptococcus neoformans with liposomes containing amphotericin B (AmBisome), with or without 15-nm colloidal gold particles. The liposomes have a diameter of 60 to 80 nm, and yet their mode of action requires them to penetrate the fungal cell wall to deliver amphotericin B to the cell membrane, where it binds to ergosterol. Surprisingly, using cryofixation techniques with electron microscopy, we observed that the liposomes remained intact during transit through the cell wall of both yeast species, even though the predicted porosity of the cell wall (pore size, ~5.8 nm) is theoretically too small to allow these liposomes to pass through intact. C. albicans mutants with altered cell wall thickness and composition were similar in both their in vitro AmBisome susceptibility and the ability of liposomes to penetrate the cell wall. AmBisome exposed to ergosterol-deficient C. albicans failed to penetrate beyond the mannoprotein-rich outer cell wall layer. Melanization of C. neoformans and the absence of amphotericin B in the liposomes were also associated with a significant reduction in liposome penetration. Therefore, AmBisome can reach cell membranes intact, implying that fungal cell wall viscoelastic properties are permissive to vesicular structures. The fact that AmBisome can transit through chemically diverse cell wall matrices when these liposomes are larger than the theoretical cell wall porosity suggests that the wall is capable of rapid remodeling, which may also be the mechanism for release of extracellular vesicles.IMPORTANCE AmBisome is a broad-spectrum fungicidal antifungal agent in which the hydrophobic polyene antibiotic amphotericin B is packaged within a 60- to 80-nm liposome. The mode of action involves perturbation of the fungal cell membrane by selectively binding to ergosterol, thereby disrupting membrane function. We report that the AmBisome liposome transits through the cell walls of both Candida albicans and Cryptococcus neoformans intact, despite the fact that the liposome is larger than the theoretical cell wall porosity. This implies that the cell wall has deformable, viscoelastic properties that are permissive to transwall vesicular traffic. These observations help explain the low toxicity of AmBisome, which can deliver its payload directly to the cell membrane without unloading the polyene in the cell wall. In addition, these findings suggest that extracellular vesicles may also be able to pass through the cell wall to deliver soluble and membrane-bound effectors and other molecules to the extracellular space.

An Update on Candida tropicalis Based on Basic and Clinical Approaches

Candida tropicalis has emerged as one of the most important Candida species. It has been widely considered the second most virulent Candida species, only preceded by C. albicans. Besides, this species has been recognized as a very strong biofilm producer, surpassing C. albicans in most of the studies. In addition, it produces a wide range of other virulence factors, including: adhesion to buccal epithelial and endothelial cells; the secretion of lytic enzymes, such as proteinases, phospholipases, and hemolysins, bud-to-hyphae transition (also called morphogenesis) and the phenomenon called phenotypic switching. This is a species very closely related to C. albicans and has been easily identified with both phenotypic and molecular methods. In addition, no cryptic sibling species were yet described in the literature, what is contradictory to some other medically important Candida species. C. tropicalis is a clinically relevant species and may be the second or third etiological agent of candidemia, specifically in Latin American countries and Asia. Antifungal resistance to the azoles, polyenes, and echinocandins has already been described. Apart from all these characteristics, C. tropicalis has been considered an osmotolerant microorganism and this ability to survive to high salt concentration may be important for fungal persistence in saline environments. This physiological characteristic makes this species suitable for use in biotechnology processes. Here we describe an update of C. tropicalis, focusing on all these previously mentioned subjects.

Adaptive Mistranslation Accelerates the Evolution of Fluconazole Resistance and Induces Major Genomic and Gene Expression Alterations in Candida albicans

Regulated erroneous protein translation (adaptive mistranslation) increases proteome diversity and produces advantageous phenotypic variability in the human pathogen Candida albicans. It also increases fitness in the presence of fluconazole, but the underlying molecular mechanism is not understood. To address this question, we evolved hypermistranslating and wild-type strains in the absence and presence of fluconazole and compared their fluconazole tolerance and resistance trajectories during evolution. The data show that mistranslation increases tolerance and accelerates the acquisition of resistance to fluconazole. Genome sequencing, array-based comparative genome analysis, and gene expression profiling revealed that during the course of evolution in fluconazole, the range of mutational and gene deregulation differences was distinctively different and broader in the hypermistranslating strain, including multiple chromosome duplications, partial chromosome deletions, and polyploidy. Especially, the increased accumulation of loss-of-heterozygosity events, aneuploidy, translational and cell surface modifications, and differences in drug efflux seem to mediate more rapid drug resistance acquisition under mistranslation. Our observations support a pivotal role for adaptive mistranslation in the evolution of drug resistance in C. albicans. IMPORTANCE Infectious diseases caused by drug-resistant fungi are an increasing threat to public health because of the high mortality rates and high costs associated with treatment. Thus, understanding of the molecular mechanisms of drug resistance is of crucial interest for the medical community. Here we investigated the role of regulated protein mistranslation, a characteristic mechanism used by C. albicans to diversify its proteome, in the evolution of fluconazole resistance. Such codon ambiguity is usually considered highly deleterious, yet recent studies found that mistranslation can boost adaptation in stressful environments. Our data reveal that CUG ambiguity diversifies the genome in multiple ways and that the full spectrum of drug resistance mechanisms in C. albicans goes beyond the traditional pathways that either regulate drug efflux or alter the interactions of drugs with their targets. The present work opens new avenues to understand the molecular and genetic basis of microbial drug resistance.