Resistance Mechanisms and Clinical Features of Fluconazole-Nonsusceptible Candida tropicalis Isolates Compared with Fluconazole-Less-Susceptible Isolates

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.

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.

Candida tropicalis morphotypes show altered cellular structure and gene expression pre- and post-exposure to fluconazole

A feature of Candida tropicalis is its ability to undergo phenotypic switching that can affect antifungal sensitivity and virulence traits. Here, we investigated the effect of switching on alterations at the cellular structure level of C. tropicalis morphotypes and whether exposure to fluconazole (FLC) in vitro could be associated with these alterations in a morphotype-dependent manner. Candida tropicalis morphotypes included clinical isolate (Parental) and two switch strains (Crepe variant and revertant of Crepe-RC). The minimum inhibitory concentration (MIC50) of fluconazole was determined according to EUCAST. Cell wall porosity, quantification of cell wall components, cell size/complexity, and expression of ERG11 and CDR1 genes in morphotypes pre- and post-exposure to fluconazole were determined. Crepe and RC showed an eightfold higher MIC50 (1 µg/ml) than the Parental (0.125 µg/ml). Exposure to FLC resulted in twofold higher MIC50 for Parental and RC. The Crepe variant exhibited a fourfold higher expression of ERG11, and the RC showed 10-fold higher expression of CDR1 than the clinical isolate. Switch strains showed reduced cell wall porosity compared to Parental, and exposure to FLC resulted in a significant reduction in the porosity of Parental and RC cells. Furthermore, phenotypic switching affected cell wall β-1,3-glucan and chitin contents in a morphotype-dependent manner. Our findings indicate that switching affects cellular structure in C. tropicalis and the occurrence of differential alterations between the clinical isolate and its switched states in response to fluconazole exposure.

Analysis of CDR1 and MDR1 Gene Expression and ERG11 Substitutions in Clinical Candida tropicalis Isolates from Alexandria, Egypt

INTRODUCTION

Candida tropicalis is a common non-albicans Candida (NAC) species that causes numerous fungal infections. Increasing antifungal resistance to azoles in NAC is becoming a major health problem worldwide; however, in Egypt, almost no data is available regarding fluconazole resistance mechanisms in C. tropicalis. The current study aims to investigate two possible important molecular mechanisms involved in fluconazole resistance in C. tropicalis isolates.

MATERIALS

Fifty-four clinical C. tropicalis isolates were included. Identification and antifungal susceptibility profiles of the isolates were carried out using the VITEK 2 compact system. The molecular investigation of fluconazole resistance included the expression of the CDR1 and MDR1 genes by quantitative real-time RT-PCR as well as the sequence analysis of the ERG11 gene.

RESULTS

Antifungal susceptibility testing identified 30 fluconazole-non-susceptible isolates. Statistically, CDR1 gene expression in fluconazole-non-susceptible isolates was significantly higher than that in fluconazole-susceptible isolates, with MDR1 gene expression levels that were similar in both non-susceptible and susceptible isolates. Sequence analysis of the ERG11 gene of 26 fluconazole-resistant isolates identified two missense mutations: A395T (Y132F) and G1390A (G464S).

CONCLUSIONS

This study has highlighted the role of overexpression of the CDR1 gene and ERG11 gene mutations in fluconazole non-susceptibility. Further studies in Egypt are required to investigate other possible molecular mechanisms involved in azole resistance.

Clonal aggregation of fluconazole-resistant Candida tropicalis isolated from sterile body fluid specimens from patients in Hefei, China

Candida tropicalis, a human conditionally pathogenic yeast, is distributed globally, especially in Asia-Pacific. The increasing morbidity and azole resistance of C. tropicalis have made clinical treatment difficult. The correlation between clonality and antifungal susceptibility of clinical C. tropicalis isolates has been reported. To study the putative correlation in C. tropicalis isolated from normally sterile body fluid specimens and explore the distinct clonal complex (CC) in Hefei, 256 clinical C. tropicalis isolates were collected from four teaching hospitals during 2016-2019, of which 30 were fluconazole-resistant (FR). Genetic profiles of 63 isolates, including 30 FR isolates and 33 fluconazole-susceptible (FS) isolates, were characterized using multilocus sequence typing (MLST). Phylogenetic analysis of the data was conducted using UPGMA (unweighted pair group method with arithmetic averages) and the minimum spanning tree algorithm. MLST clonal complexes (CCs) were analyzed using the goeBURST package. Among 35 differentiated diploid sequence types (DSTs), 16 DSTs and 1 genotype were identified as novel. A total of 35 DSTs were assigned to five major CCs based on goeBURST analysis. CC1 (containing DST376, 505, 507, 1221, 1222, 1223, 1226, and 1229) accounted for 86.7% (26/30) of the FR isolates. However, the genetic relationships among the FS isolates were relatively decentralized. The local FR CC1 belongs to a large fluconazole non-susceptible CC8 in global isolates, of which the putative founder genotype was DST225. The putative correlation between MLST types and antifungal susceptibility of clinical C. tropicalis isolates in Hefei showed that DSTs are closely related to FR clones.

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.

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.

Whole Genome Sequencing Shows Genetic Diversity, as Well as Clonal Complex and Gene Polymorphisms Associated with Fluconazole Non-Susceptible Isolates of Candida tropicalis

Resistance to azoles in Candida tropicalis is increasing and may be mediated by genetic characteristics. Using whole genome sequencing (WGS), we examined the genetic diversity of 82 bloodstream C. tropicalis isolates from two countries and one ATCC strain in a global context. Multilocus sequence typing (MLST) and single nucleotide polymorphism (SNP)-based phylogenies were generated. Minimum inhibitory concentrations (MIC) for antifungal agents were determined using Sensititre YeastOne YO10. Eleven (13.2%) isolates were fluconazole-resistant and 17 (20.5%) were classified as fluconazole-non susceptible (FNS). Together with four Canadian isolates, the genomes of 12 fluconazole-resistant (18 FNS) and 69 fluconazole-susceptible strains were examined for gene mutations associated with drug resistance. Fluconazole-resistant isolates contained a mean of 56 non-synonymous SNPs per isolate in contrast to 36 SNPs in fluconazole-susceptible isolates (interquartile range [IQR] 46−59 vs. 31−48 respectively; p < 0.001). Ten of 18 FNS isolates contained missense ERG11 mutations (amino acid substitutions S154F, Y132F, Y257H). Two echinocandin-non susceptible isolates had homozygous FKS1 mutations (S30P). MLST identified high genetic diversity with 61 diploid sequence types (DSTs), including 53 new DSTs. All four isolates in DST 773 were fluconazole-resistant within clonal complex 2. WGS showed high genetic variation in invasive C. tropicalis; azole resistance was distributed across different lineages but with DST 773 associated with in vitro fluconazole resistance.

The emerging threat antifungal-resistant Candida tropicalis in humans, animals, and environment

Antifungal resistance in humans, animals, and the environment is an emerging problem. Among the different fungal species that can develop resistance, Candida tropicalis is ubiquitous and causes infections in animals and humans. In Asia and some Latin American countries, C. tropicalis is among the most common species related to candidemia, and mortality rates are usually above 40%. Fluconazole resistance is especially reported in Asian countries and clonal spread in humans and the environment has been investigated in some studies. In Brazil, high rates of azole resistance have been found in animals and the environment. Multidrug resistance is still rare, but recent reports of clinical multidrug-resistant isolates are worrisome. The molecular apparatus of antifungal resistance has been majorly investigated in clinical C. tropicalis isolates, revealing that this species can develop resistance through the conjunction of different adaptative mechanisms. In this review article, we summarize the main findings regarding antifungal resistance and Candida tropicalis through an "One Health" approach.

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.

Disk Diffusion Susceptibility Testing for the Rapid Detection of Fluconazole Resistance in Candida Isolates

Background

Given the increased fluconazole resistance (FR) among Candida isolates, we assessed the suitability of disk diffusion susceptibility testing (DDT) for the early detection of FR using well-characterized Candida isolates.

Methods

In total, 188 Candida isolates, including 66 C. albicans (seven Erg11 mutants), 69 C. glabrata (33 Pdr1 mutants), 29 C. parapsilosis (15 Erg11 mutants), and 24 C. tropicalis (eight Erg11 mutants) isolates, were tested in this study. FR was assessed using DDT according to the standard CLSI M44-ED3 method, except that two cell suspensions, McFarland 0.5 (standard inoculum) and 2.5 (large inoculum), were used, and the inhibition zones were read at 2-hour intervals from 10 hours to 24 hours.

Results

DDT results for the standard inoculum were readable after 14 hours (C. albicans, C. glabrata, and C. tropicalis) and 20 hours (C. parapsilosis) for >95% of the isolates, whereas the results for the large inoculum were readable after 12 hours (C. glabrata and C. tropicalis), 14 hours (C. albicans), and 16 hours (C. parapsilosis) for >95% of the isolates. Compared with the results produced using the CLSI M27-ED4 broth microdilution method, the first readable results from the DDT method for each isolate exhibited an agreement of 97.0%, 98.6%, 72.4%, and 91.7% for the standard inoculum and 100%, 98.6%, 96.6%, and 95.8% for the large inoculum for C. albicans, C. glabrata, C. parapsilosis, and C. tropicalis, respectively.

Conclusions

DDT using large inoculum may detect FR rapidly and reliably in the four most common Candida species.

Virulence Factors and Azole-Resistant Mechanism of Candida Tropicalis Isolated from Candidemia

BACKGROUND

Limited knowledge exists on the virulence factors of Candida tropicalis and the mechanisms of azole resistance that lead to an intensified pathogenicity and treatment failure. We aimed to evaluate the virulence factors and molecular mechanisms of azole resistance among C. tropicalis isolated from patients with candidemia.

MATERIALS AND METHODS

Several virulence factors, including extracellular enzymatic activities, cell surface hydrophobicity (CSH), and biofilm formation, were evaluated. Antifungal susceptibility pattern and expression level of ERG11, UPC2, MDR1, and CDR1 genes of eight (4 fluconazole resistance and 4 fluconazole susceptible) clinical C. tropicalis isolates were assessed. The correlation between the virulence factors and antifungal susceptibility patterns was analyzed.

RESULTS

During a 4 year study, forty-five C. tropicalis isolates were recovered from candidemia patients. The isolates expressed different frequencies of virulence determinants as follows: coagulase 4 (8.9%), phospholipase 5 (11.1%), proteinase 31 (68.9%), esterase 43 (95.6%), hemolysin 44 (97.8%), biofilm formation 45 (100%) and CSH 45(100%). All the isolates were susceptible to amphotericin B and showed the highest resistance to voriconazole. There was a significant positive correlation between micafungin minimum inhibitory concentrations (MICs) and hemolysin production (r_s = 0.316). However, we found a negative correlation between fluconazole MICs and esterase production (r_s = -0.383). We observed the high expression of ERG11 and UPC2 genes in fluconazole-resistant C. tropicalis isolates.

CONCLUSION

C. tropicalis isolated from candidemia patients extensively displayed capacities for biofilm formation, hemolysis, esterase activity, and hydrophobicity. In addition, the overexpression of ERG11 and UPC2 genes was considered one of the possible mechanisms of azole resistance.

Mechanisms of Azole Resistance and Trailing in Candida tropicalis Bloodstream Isolates

Objectives: Azole-resistant Candida tropicalis has emerged in Asia in the context of its trailing nature, defined by residual growth above minimum inhibitory concentrations (MICs). However, limited investigations in C. tropicalis have focused on the difference of genotypes and molecular mechanisms between these two traits. Methods: Sixty-four non-duplicated C. tropicalis bloodstream isolates collected in 2017 were evaluated for azole MICs by the EUCAST E.def 7.3.1 method, diploid sequence type (DST) by multilocus sequencing typing, and sequences and expression levels of genes encoding ERG11, its transcription factor, UPC2, and efflux pumps (CDR1, CDR2 and MDR1). Results: Isavuconazole showed the highest in vitro activity and trailing against C. tropicalis, followed by voriconazole and fluconazole (geometric mean [GM] MIC, 0.008, 0.090, 1.163 mg/L, respectively; trailing GM, 27.4%, 20.8% and 19.5%, respectively; both overall p < 0.001). Fourteen (21.9%) isolates were non-WT to fluconazole/voriconazole, 12 of which were non-WT to isavuconazole and clustered in clonal complex (CC) 3. Twenty-five (39.1%) isolates were high trailing WT, including all CC2 isolates (44.0%) (containing DST140 and DST98). All azole non-WT isolates carried the ERG11 mutations A395T/W and/or C461T/Y, and most carried the UPC2 mutation T503C/Y. These mutations were not identified in low and high trailing WT isolates. Azole non-WT and high trailing WT isolates exhibited the highest expression levels of ERG11 and MDR1, 3.91- and 2.30-fold, respectively (both overall p < 0.01). Conclusions: Azole resistance and trailing are phenotypically and genotypically different in C. tropicalis. Interference with azole binding and MDR1 up-regulation confer azole resistance and trailing, respectively.

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.

Detection of Cross-Resistance Between Methotrexate and Azoles in Candida albicans and Meyerozyma guilliermondii: An In Vitro Study

In recent years, there has been a rapid increase in the incidence of Candida infections. The different species of the genus Candida vary in their virulence abilities and susceptibility to antifungal agents, depending on several external factors. The result of such modifications may be cross-resistance, which is understood as an acquired resistance to a certain antimicrobial agent after exposure to another drug. The aim of this study was to determine the possibility of cross-resistance between fluconazole, voriconazole, itraconazole, and methotrexate in Candida albicans and Meyerozyma guilliermondii (syn. Candida guilliermondii ). Fifteen strains of M. guilliermondii and eight strains of C. albicans , including the standard strains, were tested. For all strains, the minimum inhibitory concentrations (MICs) for fluconazole, voriconazole, and itraconazole were determined before and after stimulation with methotrexate. The median MICs in M. guilliermondii before and after stimulation were 9.333 and 64 mg/L ( p = 0.005) for fluconazole; 0.917 and 1.667 mg/L ( p = 0.001) for itraconazole, respectively. No significant change in MIC was observed for voriconazole. For C. albicans strains, the median MICs before and after stimulation were 0.917 and 64 mg/L ( p = 0.012) for fluconazole; 0.344 and 1.135 mg/L ( p = 0.018) for voriconazole, respectively. There was no significant change in MIC values for itraconazole. Thus, this study demonstrates the presence of cross-resistance between voriconazole, itraconazole, fluconazole, and methotrexate for the selected strains. Methotrexate exposure induces different responses when certain drugs are used for various species. Therefore, if a patient was previously exposed to methotrexate, there may be a higher risk of treatment failure with fluconazole than with other azoles such as voriconazole for fungemia caused by M. guilliermondii or itraconazole for C. albicans infection.

Azole resistance mechanisms in pathogenic M. furfur

Malassezia are emerging fungal pathogens causing opportunistic skin and severe systemic infection. Nosocomial outbreaks are associated with azole resistance and understanding of the underlying mechanisms are limited to knowledge from other fungal species. Herein, we identified distinct antifungal susceptibility patterns in 26 Malassezia furfur isolates derived from healthy and diseased individuals. A Y67F CYP51 mutation was identified in five isolates of M. furfur However, this mutation alone was insufficient to induce reduce azole susceptibility in the wild type strain. RNA-seq and differential gene analysis of healthy and disease derived strains exposed to clotrimazole in vitro identified several key metabolic pathways and transporter proteins which are involved in reduce azole susceptibility. The pleiotropic drug transporter PDR10 was the single most highly upregulated transporter gene in multiple strains of M. furfur after azole treatment and increased expression of PDR10 is associated with reduced azole susceptibility in some systemic disease isolates of M. furfur Deletion of PDR10 in a pathogenic M. furfur strain with reduced susceptibility reduced MIC values to the level of that in susceptible isolates. The current dearth of antifungal technologies, globally emerging multi-azole resistance, and broad agriculture and consumer care use of azoles means improved understanding of the mechanisms underlying intrinsic and acquired azole resistance in Malassezia is crucial for development of antibiotic stewardship and antifungal treatment strategies.

Candida tropicalis distribution and drug resistance is correlated with ERG11 and UPC2 expression

BACKGROUND

Candida tropicalis (C. tropicalis) is an important opportunistic pathogenic Candida species that can cause nosocomial infection. In this study, we analyzed the distribution and drug susceptibility of C. tropicalis and the relationship between ERG11 and UPC2 expression and resistance to azole antifungal agents.

METHODS

C. tropicalis was cultured and identified by Sabouraud Agar Medium, CHROM Agar Candida and ATB tests (Bio-Mérieux, France). Total RNA was extracted from the collected strains, and the ERG11 and UPC2 mRNA expression levels were analyzed by quantitative real-time PCR.

RESULTS

In total, 2872 clinical isolates of Candida, including 319 strains of C. tropicalis, were analyzed herein; they were mainly obtained from the Departments of Respiratory Medicine and ICU. The strains were predominantly isolated from airway secretion samples, and the detection trend in four years was mainly related to the type of department and specimens. The resistance rates of C. tropicalis to fluconazole, itraconazole and voriconazole had been increasing year by year. The mRNA expression levels of ERG11 and UPC2 in the fluconazole-resistant group were significantly higher than they were in the susceptible group. In addition, there was a significant positive linear correlation between these two genes in the fluconazole-resistant group.

CONCLUSIONS

Overexpression of the ERG11 and UPC2 genes in C. tropicalis could increase resistance to azole antifungal drugs. The routine testing for ERG11 and UPC2 in high-risk patients in key departments would provide a theoretical basis for the rational application of azole antifungal drugs.

Effect of antifungal agents, lysozyme and human antimicrobial peptide LL-37 on clinical Candida isolates with high biofilm production

Introduction. Candida species can form biofilms on tissues and medical devices, making them less susceptible to antifungal agents.Hypothesis/Gap Statement. Antifungal combination may be an effective strategy to fight against Candida biofilm.Aim. In this study, we investigated the in vitro activity of fluconazole, caspofungin and amphotericin B, alone and in combination, against 17 clinical Candida tropicalis and 6 Candida parapsilosis isolates with high biofilm formation. We also tested LL-37 and lysozyme for anti-biofilm activity against a selected C. tropicalis isolate.Methodology. Candida biofilms were prepared using the 96-well plate-based method. The minimum biofilm eradication concentrations were determined for single and combined antifungal drugs. The activity of LL-37 and lysozyme was determined by visual reading for planktonic cells and using the XTT assay for biofilms.Results. Under biofilm conditions, fluconazole plus caspofungin showed synergistic effects against 60.9% (14 of 23) of the tested isolates, including 70.6% of C. tropicalis [fractional inhibitory concentration index (FICI), 0.26-1.03] and 33.3% of C. parapsilosis (FICI, 0.04-2.03) isolates. Using this combination, no antagonism was observed. Amphotericin B plus caspofungin showed no effects against 78.3% (18 of 23) of the tested isolates. Amphotericin B plus fluconazole showed no effects against 65.2% (15 of 23) of the tested isolates and may have led to antagonism against 2 C. tropicalis and 2 C. parapsilosis isolates. LL-37 and lysozyme had no effect on biofilms of the selected C. tropicalis isolate.Conclusions. We found that fluconazole plus caspofungin led to a synergistic effect against C. tropicalis and C. parapsilosis biofilms. The efficacy of the antifungal combination therapies of the proposed schemes against biofilm-associated Candida infections requires careful and constant evaluation.

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.

Antifungal Susceptibility of Non-albicans Candida Species in A Tertiary Care Hospital, Bulgaria

Background: Emerging non-albicans Candida (NAC) species are a major threat because of their intrinsic or acquired resistance to routinely applied antifungal agents. Objectives: The purpose of our study was to reveal in vitro activity of nine antifungal agents against NAC isolates. Methods: A total of 67 NAC (27 Candida glabrata, 10 C. tropicalis, 6 C. krusei, 6 C. parapsilosis, 4 C. lusitaniae, 4 C. lipolytica, etc.) were identified and tested. The antifungal susceptibility was estimated on the basis of minimum inhibitory concentrations (MIC). Results: Overall, 13 species were determined, of which C. glabrata was the most common (40.3%), followed by C. tropicalis (14.9%), C. krusei, and C. parapsilosis (8.9 % each). Forty-nine NAC isolates (73.13%) demonstrated decreased susceptibility to one or more antifungals, and 18 of them were resistant to all azoles. Out of 27 C. glabrata, 12 (44.4%) were resistant to fluconazole with MICs: 32 - >128 µg/mL and 15 (55.6%) were intermediate with MICs: 8 - 16 µg/mL Non-albicans Candida revealed a good susceptibility to echinocandins. Amphotericin B resistance was found in 5.97% of the isolates. Of particular interest was the detection of 6 (8.95%) multidrug-resistant NAC, which expressed resistance to azoles and echinocandins and/or amphotericin B. Conclusions: About one-fourth of the studied NAC were resistant to all azoles. These findings as well as the detection of several multidrug-resistant isolates determine the necessity of susceptibility testing of clinically important yeast isolates and control of the antifungal drugs in our hospital.

Multilocus Sequence Typing Reveals Clonality of Fluconazole-Nonsusceptible Candida tropicalis: A Study From Wuhan to the Global

Candida tropicalis is a globally distributed human pathogenic yeast, and its increasing resistance to azoles makes clinical treatment difficult. In this study, we investigated the clinical features, azole resistance and genetic relatedness of 87 C. tropicalis isolates from central China and combined with the global database to explore the relationship between genetic information and fluconazole susceptibility. Of the 55 diploid sequence types (DSTs) identified by multilocus sequence typing (MLST), 27 DSTs were new to the C. tropicalis MLST database. Fluconazole-nonsusceptible (FNS) isolates were genetically closely related. goeBURST analysis showed that DST225, DST376, DST506, and DST546 formed a distinct and unique FNS clonal complex (CC) in Wuhan. The local FNS CC belongs to the large FNS CC (CC2) in China, in which the putative founder DST225 has been reported from the environment. The three most prevalent types (DST506, DST525, and DST546) in Wuhan had high minimum inhibitory concentrations (MICs) for antifungal azoles, and the six possible nosocomial transmissions we captured were all FNS strains, most of which were from CC2. Unique FNS CCs have been found in Singapore (CC8) and India (CC17) and are close to China's CC2 in the minimum spanning tree. There were no FNS CCs outside Asia. This study is the first to reveal a significant correlation between genetic information and fluconazole susceptibility worldwide and to trace geographical locations, which is of great value for molecular epidemiological surveillance and azole-resistance study of C. tropicalis globally.

Evaluation of Two Commercial Broth Microdilution Methods Using Different Interpretive Criteria for the Detection of Molecular Mechanisms of Acquired Azole and Echinocandin Resistance in Four Common Candida Species

The abilities of the new Vitek 2 AST-YS08 (YS08) and Sensititre YeastOne (SYO) systems to detect the resistances of Candida isolates to azoles and echinocandins were evaluated. In total, 292 isolates, including 28 Candida albicans (6 Erg11 and 2 Fks mutants), 57 Candida parapsilosis (26 Erg11 mutants), 24 Candida tropicalis (10 Erg11 and 1 Fks mutants), and 183 Candida glabrata (39 Pdr1 and 13 Fks mutants) isolates, were tested. The categorical agreements (CAs) between the Clinical and Laboratory Standards Institute (CLSI) method and YS08 fluconazole MICs obtained using clinical breakpoints were 92.4% (C. albicans), 96.5% (C. parapsilosis), and 87.0% (C. tropicalis), and the CAs between the CLSI and SYO MICs were 92.3% (C. albicans), 77.2% (C. parapsilosis), 100% (C. tropicalis), and 98.9% (C. glabrata). For C. glabrata, the CAs with the CLSI micafungin MICs were 92.4% and 55.5% for the YS08 micafungin and caspofungin MICs, respectively; they were 100%, 95.6%, and 98.9% for the SYO micafungin, caspofungin, and anidulafungin MICs, respectively. YS08 does not provide fluconazole data for C. glabrata; the CA with the CLSI fluconazole MIC was 97.8% for the YS08 voriconazole MIC, using an epidemiological cutoff value (ECV) of 0.5 μg/ml. Increased CAs with the CLSI MIC were observed for the YS08 MIC using CLSI ECVs (for fluconazole and C. tropicalis, 100%; for micafungin and C. glabrata, 98.9%) and for the SYO MIC using method-specific ECVs (for fluconazole and C. parapsilosis, 91.2%; for caspofungin and C. glabrata, 98.9%). Therefore, the YS08 and SYO systems may have different abilities to detect mechanisms of azole and echinocandin resistance in four Candida species; the use of method-specific ECVs may improve the performance of both systems.

Recent Increase in the Prevalence of Fluconazole-Non-susceptible Candida tropicalis Blood Isolates in Turkey: Clinical Implication of Azole-Non-susceptible and Fluconazole Tolerant Phenotypes and Genotyping

Candida tropicalis is the fourth leading cause of candidemia in Turkey. Although C. tropicalis isolates from 1997 to 2017 were characterized as fully susceptible to antifungals, the increasing global prevalence of azole-non-susceptible (ANS) C. tropicalis and the association between high fluconazole tolerance (HFT) and fluconazole therapeutic failure (FTF) prompted us to re-evaluate azole susceptibility of C. tropicalis in Turkey. In this study, 161 C. tropicalis blood isolates from seven clinical centers were identified by ITS rDNA sequencing, genotyped by multilocus microsatellite typing, and tested for susceptibility to five azoles, two echinocandins, and amphotericin B (AMB); antifungal resistance mechanisms were assessed by sequencing of ERG11 and FKS1 genes. The results indicated that C. tropicalis isolates, which belonged to 125 genotypes grouped into 11 clusters, were fully susceptible to echinocandins and AMB; however, 18.6% of them had the ANS phenotype but only two carried the ANS-conferring mutation (Y132F). HFT was recorded in 52 isolates, 10 of which were also ANS. Large proportions of patients infected with ANS and HFT isolates (89 and 40.7%, respectively) showed FTF. Patients infected with azole-susceptible or ANS isolates did not differ in mortality, which, however, was significantly lower for those infected with HFT isolates (P = 0.007). There were significant differences in mortality (P = 0.02), ANS (P = 0.012), and HFT (P = 0.007) among genotype clusters. The alarming increase in the prevalence of C. tropicalis blood isolates with ANS and HFT in Turkey and the notable FTF rate should be a matter of public health concern.

Molecular epidemiology of Candida tropicalis isolated from urogenital tract infections

Candida tropicalis is a common human pathogenic yeast, and its molecular typing is important for studying the population structure and epidemiology of this opportunistic yeast, such as epidemic genotype, population dynamics, nosocomial infection, and drug resistance surveillance. In this study, the antifungal susceptibility test and multilocus sequence typing (MLST) analysis were carried out on C. tropicalis from central China. Among 64 urogenital isolates, 45 diploid sequence types (DST) were found, of which 20 DSTs (44.4%) were new to the central database. The goeBURST analysis showed that CC1 (clonal complex) was the only azole‐resistant (100%, 10/10) cluster in Wuhan, which was composed of DST546, DST225, DST376, and DST506, and most of the strains (90%, 9/10) were isolated from the urinary tract. Potential nosocomial infections were mainly caused by CC1 strains. The azole resistance rate of urinary isolates (50.0%, 21/42) was higher than that of vaginal isolates (27.3%, 6/22). The genotype diversity and novelty of vaginal isolates were higher than those of urinary isolates. C. tropicalis population in Wuhan was genetically diverse and divergent from that seen in other countries. In this study, there were significant differences in genotype and azole susceptibility between urine and vaginal strains. The azole‐resistant cluster (CC1) found in urine is of great significance for the clinical treatment and prevention of nosocomial infection. The newly discovered DSTs will contribute to further study the similarity, genetic relationship, and molecular epidemiology of C. tropicalis worldwide.

The Quiet and Underappreciated Rise of Drug-Resistant Invasive Fungal Pathogens

Human fungal pathogens are attributable to a significant economic burden and mortality worldwide. Antifungal treatments, although limited in number, play a pivotal role in decreasing mortality and morbidities posed by invasive fungal infections (IFIs). However, the recent emergence of multidrug-resistant Candida auris and Candida glabrata and acquiring invasive infections due to azole-resistant C. parapsilosis, C. tropicalis, and Aspergillus spp. in azole-naïve patients pose a serious health threat considering the limited number of systemic antifungals available to treat IFIs. Although advancing for major fungal pathogens, the understanding of fungal attributes contributing to antifungal resistance is just emerging for several clinically important MDR fungal pathogens. Further complicating the matter are the distinct differences in antifungal resistance mechanisms among various fungal species in which one or more mechanisms may contribute to the resistance phenotype. In this review, we attempt to summarize the burden of antifungal resistance for selected non-albicansCandida and clinically important Aspergillus species together with their phylogenetic placement on the tree of life. Moreover, we highlight the different molecular mechanisms between antifungal tolerance and resistance, and comprehensively discuss the molecular mechanisms of antifungal resistance in a species level.

Modulation of ERG Genes Expression in Clinical Isolates of Candida tropicalis Susceptible and Resistant to Fluconazole and Itraconazole

Candida tropicalis is a non-albicans Candida specie that causes candidosis in several countries, including Brazil. However, little is known about the mechanisms of drug resistance in C. tropicalis infections. In this study, we used clinical isolates of C. tropicalis susceptible as well as resistant to either Fluconazole or Itraconazole to assess the relationship between drug resistance and the expression of ERG and efflux pump genes. Our results showed that the main mechanism of resistance against both Fluconazole and Itraconazole in this specie is through the up-regulation of ERG rather than that of the efflux pump genes. We demonstrated that, although pre-treatment with azole drugs increases the expression of both ERG6 and ERG11 genes, the resistant or susceptible dose-dependent (SDD) samples are able to maintain high expression levels of these genes for longer periods of time than the susceptible samples.

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.

A comprehensive overview of the medicinal chemistry of antifungal drugs: perspectives and promise

The emergence of new fungal pathogens makes the development of new antifungal drugs a medical imperative that in recent years motivates the talents of numerous investigators across the world. Understanding not only the structural families of these drugs but also their biological targets provides a rational means for evaluating the merits and selectivity of new agents for fungal pathogens and normal cells. An equally important aspect of modern antifungal drug development takes a balanced look at the problems of drug potency and drug resistance. The future development of new antifungal agents will rest with those who employ synthetic and semisynthetic methodology as well as natural product isolation to tackle these problems and with those who possess a clear understanding of fungal cell architecture and drug resistance mechanisms. This review endeavors to provide an introduction to a growing and increasingly important literature, including coverage of the new developments in medicinal chemistry since 2015, and also endeavors to spark the curiosity of investigators who might enter this fascinatingly complex fungal landscape.

A rare clinical entity as large intrarenal abscess in a typeII diabetic patient due to Candida tropicalis: Case report

Background and Purpose:

Fungal renal abscesses are rare entities associated with significant morbidity and mortality. Affected kidneys can have microabscess, pyelonephritis, pyonephrosis, or papillary necrosis.

Case report:

Herein, we reported an unusual case of a large renal abscess cause by Candida tropicalis in a diabetic patient. The entity presented as a lump in the abdomen and later was diagnosed to be an abscess on computed tomography scan. Candida tropicalis was confirmed on the culture of the aspirate. The abscess was successfully treated by percutaneous drainage and administration of amphotericin B deoxycholate.

Conclusion:

Candida tropicalis is now a global concern because of its rising prevalence and high virulence. The growing resistance of this Candida species to azoles, as in our case, calls for a more judicious usage of antifungal agents. Empirical therapy with either amphotericin or echinocandins is an option in case of high azole resistance. This case highlights the importance of timely diagnosis and implementation of aggressive management in cases suffering from fungal abscesses.

An Overview on Conventional and Non-Conventional Therapeutic Approaches for the Treatment of Candidiasis and Underlying Resistance Mechanisms in Clinical Strains

Fungal infections and, in particular, those caused by species of the Candida genus, are growing at an alarming rate and have high associated rates of mortality and morbidity. These infections, generally referred as candidiasis, range from common superficial rushes caused by an overgrowth of the yeasts in mucosal surfaces to life-threatening disseminated mycoses. The success of currently used antifungal drugs to treat candidiasis is being endangered by the continuous emergence of resistant strains, specially among non-albicans Candida species. In this review article, the mechanisms of action of currently used antifungals, with emphasis on the mechanisms of resistance reported in clinical isolates, are reviewed. Novel approaches being taken to successfully inhibit growth of pathogenic Candida species, in particular those based on the exploration of natural or synthetic chemicals or on the activity of live probiotics, are also reviewed. It is expected that these novel approaches, either used alone or in combination with traditional antifungals, may contribute to foster the identification of novel anti-Candida therapies.

Selection and evaluation of appropriate reference genes for RT-qPCR based expression analysis in Candida tropicalis following azole treatment

Candida tropicalis arises as one of the predominant non-Candida albicans Candida (NCAC) species causing invasive candidiasis in Asian countries. A rise in reports of C. tropicalis with a parallel increase in fluconazole resistance has also been observed. The genes and underlying pathways associated with azole antifungal resistance in C. tropicalis is still not properly understood. The RT-qPCR is the most promising approach for expression analysis of target genes to understand the mechanisms of resistance. The reliability and reproducibility of this technique depend on the selection of suitable reference genes for the normalization in expression study. The present study investigated the expression stability levels of ten genes including ACT1, EF1, GAPDH, PGK1, RDN5.8, RDN18, RDN28, SDHA, TUB1, and UBC13 for their suitability in fluconazole treated/untreated C. tropicalis. The stability levels of these genes were examined by the ∆∆CT, ΔCT, Pfaffl methods and five independent software including hkgFinder, geNorm, NormFinder, BestKeeper, and RefFinder software. We report, the EF1 and ACT1 were the most stable reference genes for normalization and can be used for the gene expression analysis in C. tropicalis. To the best of our knowledge, our study is the first to select and validate the reference genes in C. tropicalis for RT-qPCR based expression analysis.

Clonality of Fluconazole-Nonsusceptible Candida tropicalis in Bloodstream Infections, Taiwan, 2011-2017

Candida tropicalis is the leading cause of non-C. albicans candidemia in tropical Asia and Latin America. We evaluated isolates from 344 patients with an initial episode of C. tropicalis candidemia. We found that 58 (16.9%) patients were infected by fluconazole-nonsusceptible (FNS) C. tropicalis with cross resistance to itraconazole, voriconazole, and posaconazole; 55.2% (32/58) of patients were azole-naive. Multilocus sequence typing analysis revealed FNS isolates were genetically closely related, but we did not see time- or place-clustering. Among the diploid sequence types (DSTs), we noted DST225, which has been reported from fruit in Taiwan and hospitals in Beijing, China, as well as DST376 and DST505-7, which also were reported from hospitals in Shanghai, China. Our findings suggest cross-boundary expansion of FNS C. tropicalis and highlight the importance of active surveillance of clinical isolates to detect dissemination of this pathogen and explore potential sources in the community.

Facilitators of adaptation and antifungal resistance mechanisms in clinically relevant fungi

Opportunistic fungal pathogens can cause a diverse range of diseases in humans. The increasing rate of fungal infections caused by strains that are resistant to commonly used antifungals results in difficulty to treat diseases, with accompanying high mortality rates. Existing and newly emerging molecular resistance mechanisms rapidly spread in fungal populations and need to be monitored. Fungi exhibit a diversity of mechanisms to maintain physiological resilience and create genetic variation; processes which eventually lead to the selection and spread of resistant fungal pathogens. To prevent and anticipate this dispersion, the role of evolutionary factors that drive fungal adaptation should be investigated. In this review, we provide an overview of resistance mechanisms against commonly used antifungal compounds in the clinic and for which fungal resistance has been reported. Furthermore, we aim to summarize and elucidate potent generators of genetic variability across the fungal kingdom that aid adaptation to stressful environments. This knowledge can lead to recognizing potential niches that facilitate fast resistance development and can provide leads for new management strategies to battle the emerging resistant populations in the clinic and the environment.

Synergistic Interaction of Fluconazole/Amphotericin B on Inhibition of Enzymes Contributes to the Pathogenesis of Candida Tropicalis

Background: Candidiasis has gained much attention in recent decades due to its increasing prevalence in immunocompromised patients. Usually, antifungals such as fluconazole and amphotricin B are used for treatment of candidiasis, but one of the major clinical problems is the emergence of antifungal resistance. Combination antifungal therapy is one of the most commonly used methods to alleviate the problem of antifungal resistance. Methods: The effect of fluconazole alone and in combination with amphotericin B on C. tropicalis isolates were performed using the Clinical and Laboratory Standards Institute (CLSI) reference method. Eventually hypha formation, time kill study, proteinase and phospholipase activity and expression of PLB and SAP2 genes were carried out to investigate the enzymes inhibitory properties of antifungal tested against C. tropicalis. Results: Results showed the significant synergic effect of fluconazole in combination with amphotericin B in inhibiting the growth of C. tropicalis isolates, with fractional inhibitory concentration indices ranging from 0.06 to 0.5. The combination of fluconazole with amphotericin B reduced the number of yeast form and inhibited the yeast to hyphae transition in C. tropicalis. The antifungals tested were able to show the effect of down regulating expression of the selected genes significantly in fluconazole/amphotericin B ranging from 1.42- to 2.27-fold. Conclusion: Our results demonstrated that the synergistic interaction of fluconazole/amphotericin B would be worth exploring for the management of candidiasis. In addition, PLB and SAP2 genes could be probable molecular targets in the synergistic interaction of fluconazole/amphotericin B in C. tropicalis.

Implications of the EUCAST Trailing Phenomenon in Candida tropicalis for the In Vivo Susceptibility in Invertebrate and Murine Models

Candida tropicalis isolates often display reduced but persistent growth (trailing) over a broad fluconazole concentration range during EUCAST susceptibility testing. Whereas weak trailing (<25% of the positive growth control) is common and found not to impair fluconazole efficacy, we investigated if more pronounced trailing impacted treatment efficacy. Fluconazole efficacy against two weakly (≤25% growth), two moderately (26% to 50% growth), and one heavily (>70% growth) trailing resistant isolate and one resistant (100% growth) isolate were investigated in vitro and in vivo (in a Galleria mellonella survival model and two nonlethal murine models). CDR1 expression levels and ERG11 sequences were characterized. The survival in fluconazole-treated G. mellonella was inversely correlated with the degree of trailing (71% to 9% survival in treatment groups). In mice, resistant and heavily trailing isolates responded poorly to fluconazole treatment. CDR1 expression was significantly higher in trailing and resistant isolates than in wild-type isolates (1.4-fold to 10-fold higher). All isolates exhibited ERG11 wild-type alleles. Heavily trailing isolates were less responsive to fluconazole in all in vivo models, indicating an impact on fluconazole efficacy. CDR1 upregulation may have contributed to the observed differences. Moderately trailing isolates responded less well to fluconazole in larvae only. This confirms clinical data suggesting fluconazole is effective against infections with such isolates in less severely ill patients and supports the current 50% growth endpoint for susceptibility testing. However, it is still unclear if the gradual loss of efficacy observed for moderately trailing isolates in the larva model may be a reason for concern in selected vulnerable patient populations.

Protein kinase A governs growth and virulence in Candida tropicalis

Candida tropicalis is one of the most important human fungal pathogens causing superficial infections in locations such as the oral mucosa and genital tract, as well as systemic infections with high mortality. In its sister species Candida albicans, the cyclic AMP/protein kinase A (cAMP/PKA) pathway regulates fungal adhesion and dimorphism, both of which correlate closely with virulence. CaTpk1 and CaTpk2, the catalytic subunits of PKA, not only share redundant functions in hyphal growth, adhesion, and biofilm formation, but also have distinct roles in stress responses and pathogenesis, respectively. However, studies on PKA in the emerging fungal pathogen C. tropicalis are limited. Our results suggest that Tpk1 is involved in cell wall integrity and drug tolerance. The tpk2/tpk2 mutants, which have no protein kinase A activity, have reduced hyphal growth and adhesion. In addition, the tpk1/tpk1 tpk2/tpk2 double deletion mutant demonstrated delayed growth and impaired hyphal formation. In a murine model of systemic infection, both TPK1 and TPK2 were required for full virulence. We further found that EFG1 and HWP1 expression is regulated by PKA, while BCR1, FLO8, GAL4, and RIM101 are upregulated in the tpk1/tpk1 tpk2/tpk2 mutant. This study demonstrates that Tpk1 is involved in drug tolerance and cell wall integrity, while Tpk2 serves as a key regulator in dimorphism and adhesion. Both Tpk1 and Tpk2 are required for growth and full virulence in C. tropicalis.

Multilocus Sequence Typing (MLST) Genotypes of Candida glabrata Bloodstream Isolates in Korea: Association With Antifungal Resistance, Mutations in Mismatch Repair Gene (Msh2), and Clinical Outcomes

Candida glabrata bloodstream infection (BSI) isolates from a particular geographic area have been reported to comprise a relatively small number of the major sequence types (STs) by multilocus sequence typing (MLST) analysis. Yet little is known about the characteristics of major ST strains of C. glabrata. To address this question in Korea, we investigated antifungal resistance and non-synonymous mutations of the mismatch repair gene (msh2 mutations) in C. glabrata BSI isolates, as well as associated clinical characteristics, and compared the results according to MLST genotype. We assessed a total of 209 C. glabrata BSI isolates from seven hospitals in Korea for 2 years (2009 and 2014). Clinical features of candidemia and their outcomes were analyzed for 185 available cases. According to MLST, ST7 (47.8%) was the most common type, followed by ST3 (22.5%); the remainder represented 28 types of minor STs (29.7%). Fluconazole-resistance (FR) rates for ST7, ST3, and other strains were 9.0% (9/100), 8.5% (4/47), and 4.8% (3/62), respectively, and all were susceptible to amphotericin B and micafungin. All ST7 isolates harbored the V239L mutation in msh2, known to confer hypermutability, while 91.5% of ST3 isolates did not harbor the msh2 mutation. Overall, isolates of the same ST had identical msh2 mutations, with the exception of nine isolates. The msh2 mutations were identified in 68.8% (11/16) of the FR isolates and 67.4% (130/193) of the fluconazole susceptible-dose dependent isolates. There was no significant difference in all clinical characteristics between ST3 and ST7. However, the 30-day mortality of C. glabrata candidemia due to the two major ST (ST3 or ST7) strains was significantly higher than that of candidemia due to other minor ST strains (45.1 vs. 25.0%, p < 0.05). Multivariate logistic regression analysis also showed that two major STs (ST3 and ST7) were independent predictors of 30-day mortality. This study showed for the first time that two STs (ST7 and ST3) were predominant among BSI isolates in Korea, and that C. glabrata BSI isolates belonging to two major MLST genotypes are characterized by higher mortality. In addition, most msh2 mutations align with MLST genotype, irrespective of FR.

Host-Pathogen Interactions Mediated by MDR Transporters in Fungi: As Pleiotropic as it Gets!

Fungal infections caused by Candida, Aspergillus, and Cryptococcus species are an increasing problem worldwide, associated with very high mortality rates. The successful prevalence of these human pathogens is due to their ability to thrive in stressful host niche colonization sites, to tolerate host immune system-induced stress, and to resist antifungal drugs. This review focuses on the key role played by multidrug resistance (MDR) transporters, belonging to the ATP-binding cassette (ABC), and the major facilitator superfamilies (MFS), in mediating fungal resistance to pathogenesis-related stresses. These clearly include the extrusion of antifungal drugs, with C. albicans CDR1 and MDR1 genes, and corresponding homologs in other fungal pathogens, playing a key role in this phenomenon. More recently, however, clues on the transcriptional regulation and physiological roles of MDR transporters, including the transport of lipids, ions, and small metabolites, have emerged, linking these transporters to important pathogenesis features, such as resistance to host niche environments, biofilm formation, immune system evasion, and virulence. The wider view of the activity of MDR transporters provided in this review highlights their relevance beyond drug resistance and the need to develop therapeutic strategies that successfully face the challenges posed by the pleiotropic nature of these transporters.

MDR1 overexpression combined with ERG11 mutations induce high-level fluconazole resistance in Candida tropicalis clinical isolates

Background

Marked increases in fluconazole resistance in Candida tropicalis have been recently reported. In this study, the molecular mechanisms behind fluconazole resistance were investigated.

Methods

Twenty-two C. tropicalis clinical isolates, including 12 fluconazole-resistant isolates and 10 fluconazole-susceptible isolates, were collected from a tertiary care teaching hospital in Beijing between 2013 and 2017. Antifungal susceptibility testing, multilocus sequence typing, ERG11 amplification and sequencing, quantitative real-time reverse transcription-polymerase chain reaction (ERG11, UPC2, MDR1, and CDR1), and clinical data collection were performed for all C. tropicalis isolates.

Results

Multilocus sequence typing revealed that the 10 fluconazole-susceptible isolates and 12 fluconazole-resistant isolates were divided into nine and seven diploid sequence types, respectively. Of the 12 patients with fluconazole-resistant isolates, six had been previously exposed to azole and four had a fatal outcome. Y132F and S154F amino acid substitutions in Erg11p were found in all fluconazole-resistant isolates except one. MDR1 gene overexpression was identified in fluconazole-resistant isolates. In particular, seven high-level fluconazole resistant isolates (minimum inhibitory concentration ≥ 128 mg/L) and three pan-azole resistant isolates were identified. CDR1, ERG11, and UPC2 gene expression levels in fluconazole-resistant isolates were not significantly different from the control isolates (P = 0.262, P = 0.598, P = 0.114, respectively).

Conclusions

This study provides evidence that the combination of MDR1 gene overexpression and ERG11 missense mutations is responsible for high-level fluconazole resistance and pan-azole resistance in C. tropicalis clinical isolates. To the best of our knowledge, this is the first study investigating the relationship between MDR1 gene overexpression and increased fluconazole resistance.

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.

From Lipid Homeostasis to Differentiation: Old and New Functions of the Zinc Cluster Proteins Ecm22, Upc2, Sut1 and Sut2

Zinc cluster proteins are a large family of transcriptional regulators with a wide range of biological functions. The zinc cluster proteins Ecm22, Upc2, Sut1 and Sut2 have initially been identified as regulators of sterol import in the budding yeast Saccharomyces cerevisiae. These proteins also control adaptations to anaerobic growth, sterol biosynthesis as well as filamentation and mating. Orthologs of these zinc cluster proteins have been identified in several species of Candida. Upc2 plays a critical role in antifungal resistance in these important human fungal pathogens. Upc2 is therefore an interesting potential target for novel antifungals. In this review we discuss the functions, mode of actions and regulation of Ecm22, Upc2, Sut1 and Sut2 in budding yeast and Candida.

Azole Antifungal Resistance in Candida albicans and Emerging Non-albicans Candida Species

Within the limited antifungal armamentarium, the azole antifungals are the most frequent class used to treat Candida infections. Azole antifungals such as fluconazole are often preferred treatment for many Candida infections as they are inexpensive, exhibit limited toxicity, and are available for oral administration. There is, however, extensive documentation of intrinsic and developed resistance to azole antifungals among several Candida species. As the frequency of azole resistant Candida isolates in the clinical setting increases, it is essential to elucidate the mechanisms of such resistance in order to both preserve and improve upon the azole class of antifungals for the treatment of Candida infections. This review examines azole resistance in infections caused by C. albicans as well as the emerging non-albicans Candida species C. parapsilosis, C. tropicalis, C. krusei, and C. glabrata and in particular, describes the current understanding of molecular basis of azole resistance in these fungal species.

Antiproliferation of Berberine in Combination with Fluconazole from the Perspectives of Reactive Oxygen Species, Ergosterol and Drug Efflux in a Fluconazole-Resistant Candida tropicalis Isolate

Candida tropicalis has emerged as an important pathogenic fungus in nosocomial infections due to its recalcitrant resistance to conventional antifungal agents, especially to fluconazole (FLC). Berberine (BBR) is a bioactive herbal-originated alkaloids and has been reported to possess antifungal functions against C. albicans. In this paper, we tried to figure out the antifungal mechanisms of BBR and/or FLC in a clinical C. tropicalis isolate 2006. In the microdilution test, the minimum inhibitory concentration (MIC) of BBR was found 16 μg/mL with fractional inhibitory concentration index (FICI) 0.13 in C. tropicalis 2006. The synergism of BBR and FLC was also confirmed microscopically. After the treatments of BBR and/or FLC, the studies revealed that (i) FLC facilitated BBR to increase reactive oxygen species (ROS), (ii) FLC enhanced the intranuclear accumulation of BBR, (iii) BBR decreased the extracellular rhodamine 123 (Rh123) via inhibiting efflux transporters, (iv) FLC assisted BBR to reduce ergosterol content, and (v) BBR in combined with FLC largely downregulated the expressions of Candida drug resistance 1 (CDR1) and CDR2 but impact slightly multidrug resistance 1 (MDR1), and upregulate the expression of ergosterol 11 (ERG11). These results suggested that BBR could become a potent antifungal drug to strengthen FLC efficacy in FLC-resistant C. tropicalis via ROS increase, intracellular BBR accumulation, ergosterol decrease and efflux inhibition.

The Role of UPC2 Gene in Azole-Resistant Candida tropicalis

Azole resistance of Candida tropicalis has, in recent years, become a serious issue in hospitals; however, there is limited knowledge of the mechanisms underlying this resistance. We have previously demonstrated that ERG11 plays a vital role in azole resistance in C. tropicalis. Here, we describe the expression and sequence variation of UPC2, which encodes a transcription factor of ERG11. Quantitative real-time RT-PCR showed that 31 azole-resistant C. tropicalis strains significantly overexpressed UPC2. Those isolates resistant to all three azole antifungals upregulated UPC2 expression to a greater degree than those resistant to only fluconazole or itraconazole. The UPC2 promoter contains mutations -118T-G and -155G-A in azole-resistant strains of C. tropicalis. Meanwhile, the mutation G392E was also detected twice in UPC2 gene in azole-resistant C. tropicalis and was demonstrated to mediate azole antifungal susceptibility by using Saccharomyces cerevisiae as an expression host, particularly for fluconazole and itraconazole.