Participation of the ABC Transporter CDR1 in Azole Resistance of Candida lusitaniae

Modulators of Candida albicans Membrane Drug Transporters: A Lucrative Portfolio for the Development of Effective Antifungals

The escalating prevalence of membrane drug transporters and drug efflux pumps in pathogenic yeast like Candida albicans necessitates a comprehensive understanding of their roles in MDR. The overexpression of drug transporter families, ABC and MFS, implicated in MDR through drug efflux and poses a significant challenge in the diagnosis and treatment of fungal infection. Various mechanisms have been proposed for MDR; however, the upregulation of ABC and MFS superfamily transporters is most noticeable in MDR. The direct inhibition of these transporters seems an efficient strategy to overcome this problem. The goal of the article is to present an overview of the prospect of utilizing these modulators of C. albicans drug transports as effective antifungal molecules against MDR addressing a critical gap in the field. The review tries to address to prevent drug extrusion by modulating the expression of drug transporters of C. albicans. The review discussed the progress in identifying potent, selective, and non-toxic modulators of these transporters to develop some effective antifungals and overcome MDR. We reviewed major studies in this area and found that recent work has shifted toward the exploration of natural compounds as potential modulators to restore drug sensitivity in MDR fungal cells. The focus of this review is to survey and interpret current research information on modulators of C. albicans drug transporters from natural sources emphasizing those compounds that are potent antifungal agents.

Pan-azole-resistant Meyerozyma guilliermondii clonal isolates harbouring a double F126L and L505F mutation in Erg11

BACKGROUND

Meyerozyma guilliermondii is a yeast species responsible for invasive fungal infections. It has high minimum inhibitory concentrations (MICs) to echinocandins, the first-line treatment of candidemia. In this context, azole antifungal agents are frequently used. However, in recent years, a number of azole-resistant strains have been described. Their mechanisms of resistance are currently poorly studied.

OBJECTIVE

The aim of this study was consequently to understand the mechanisms of azole resistance in several clinical isolates of M. guilliermondii.

METHODS

Ten isolates of M. guilliermondii and the ATCC 6260 reference strain were studied. MICs of azoles were determined first. Whole genome sequencing of the isolates was then carried out and the mutations identified in ERG11 were expressed in a CTG clade yeast model (C. lusitaniae). RNA expression of ERG11, MDR1 and CDR1 was evaluated by quantitative PCR. A phylogenic analysis was developed and performed on M. guilliermondii isolates. Lastly, in vitro experiments on fitness cost and virulence were carried out.

RESULTS

Of the ten isolates tested, three showed pan-azole resistance. A combination of F126L and L505F mutations in Erg11 was highlighted in these three isolates. Interestingly, a combination of these two mutations was necessary to confer azole resistance. An overexpression of the Cdr1 efflux pump was also evidenced in one strain. Moreover, the three pan-azole-resistant isolates were shown to be genetically related and not associated with a fitness cost or a lower virulence, suggesting a possible clonal transmission.

CONCLUSION

In conclusion, this study identified an original combination of ERG11 mutations responsible for pan-azole-resistance in M. guilliermondii. Moreover, we proposed a new MLST analysis for M. guilliermondii that identified possible clonal transmission of pan-azole-resistant strains. Future studies are needed to investigate the distribution of this clone in hospital environment and should lead to the reconsideration of the treatment for this species.

Candida parapsilosis Virulence and Antifungal Resistance Mechanisms: A Comprehensive Review of Key Determinants

Candida parapsilosis is the second most common Candida species isolated in Asia, Southern Europe, and Latin America and is often involved in invasive infections that seriously impact human health. This pathogen is part of the psilosis complex, which also includes Candida orthopsilosis and Candida metapsilosis. C. parapsilosis infections are particularly prevalent among neonates with low birth weights, individuals who are immunocompromised, and patients who require prolonged use of a central venous catheter or other indwelling devices, whose surfaces C. parapsilosis exhibits an enhanced capacity to adhere to and form biofilms. Despite this well-acknowledged prevalence, the biology of C. parapsilosis has not been as extensively explored as that of Candida albicans. In this paper, we describe the molecular mechanistic pathways of virulence in C. parapsilosis and show how they differ from those of C. albicans. We also describe the mode of action of antifungal drugs used for the treatment of Candida infections, namely, polyenes, echinocandins, and azoles, as well as the resistance mechanisms developed by C. parapsilosis to overcome them. Finally, we stress the importance of the ongoing search for species-specific features that may aid the development of effective control strategies and thus reduce the burden on patients and healthcare costs.

Myriocin enhances the antifungal activity of fluconazole by blocking the membrane localization of the efflux pump Cdr1

Introduction: Extrusion of azoles from the cell, mediated by an efflux pump Cdr1, is one of the most frequently used strategies for developing azole resistance in pathogenic fungi. The efflux pump Cdr1 is predominantly localized in lipid rafts within the plasma membrane, and its localization is sensitive to changes in the composition of lipid rafts. Our previous study found that the calcineurin signal pathway is important in transferring sphingolipids from the inner to the outer membrane. Methods: We investigated multiple factors that enhance the antifungal activity of fluconazole (FLC) using minimum inhibitory concentration (MIC) assays and disk diffusion assays. We studied the mechanism of action of myriocin through qRT-PCR analysis and confocal microscopy analysis. We tested whether myriocin enhanced the antifungal activity of FLC and held therapeutic potential using a mouse infection model. Results: We found that this signal pathway has no function in the activity of Cdr1. We found that inhibiting sphingolipid biosynthesis by myriocin remarkably increased the antifungal activity of FLC with a broad antifungal spectrum and held therapeutic potential. We further found that myriocin potently enhances the antifungal activity of FLC against C. albicans by blocking membrane localization of the Cdr1 rather than repressing the expression of Cdr1. In addition, we found that myriocin enhanced the antifungal activity of FLC and held therapeutic potential. Discussion: Our study demonstrated that blocking the membrane location and inactivating Cdr1 by inhibiting sphingolipids biogenesis is beneficial for enhancing the antifungal activity of azoles against azole-resistant C. albicans due to Cdr1 activation.

Candida parapsilosis Mdr1B and Cdr1B Are Drivers of Mrr1-Mediated Clinical Fluconazole Resistance

Candida parapsilosis is a common cause of invasive candidiasis worldwide and is the most commonly is7olated Candida species among pediatric and neonatal populations. Previous work has demonstrated that nonsynonymous mutations in the gene encoding the putative transcription factor CpMrr1 can influence fluconazole susceptibility. However, the direct contribution of these mutations and how they influence fluconazole resistance in clinical isolates are poorly understood. We identified 7 nonsynonymous CpMRR1 mutations in 12 isolates from within a collection of 35 fluconazole-resistant clinical isolates. The mutations leading to the A854V, R479K, and I283R substitutions were further examined and found to be activating mutations leading to increased fluconazole resistance. In addition to CpMDR1, we identified two other genes, one encoding a major facilitator superfamily (MFS) transporter (CpMDR1B, CPAR2_603010) and one encoding an ATP-binding cassette (ABC) transporter (CpCDR1B, CPAR2_304370), as being upregulated in isolates carrying CpMRR1-activating mutations. Overexpression of CpMDR1 in a susceptible strain and disruption in resistant clinical isolates that overexpress CpMDR1 had little to no effect on fluconazole susceptibility. Conversely, overexpression of either CpMDR1B or CpCDR1B increased resistance, and disruption in clinical isolates overexpressing these genes decreased fluconazole resistance. Our findings suggest that activating mutations in CpMRR1 represent important genetic determinants of fluconazole resistance in clinical isolates of C. parapsilosis, and unlike what is observed in Candida albicans, this is primarily driven by upregulation of both MFS (CpMdr1B) and ABC (CpCdr1B) transporters.

How Yeast Antifungal Resistance Gene Analysis Is Essential to Validate Antifungal Susceptibility Testing Systems

Objectives

The antifungal susceptibility testing (AFST) of yeast pathogen alerts clinicians about the potential emergence of resistance. In this study, we compared two commercial microdilution AFST methods: Sensititre YeastOne read visually (YO) and MICRONAUT-AM read visually (MN) or spectrophotometrically (MNV), interpreted with Clinical and Laboratory Standards Institute and European Committee on Antimicrobial Susceptibility Testing criteria, respectively.

Methods

Overall, 97 strains from 19 yeast species were measured for nine antifungal drugs including a total of 873 observations. First, the minimal inhibitory concentration (MIC) was compared between YO and MNV, and between MNV and MN, either directly or by assigning them to five susceptibility categories. Those categories were based on the number of MIC dilutions around the breakpoint or epidemiological cut-off reference values (ECOFFs or ECVs). Second, YO and MNV methods were evaluated for their ability to detect the elevation of MICs due to mutation in antifungal resistance genes, thanks to pairs or triplets of isogenic strains isolated from a single patient along a treatment previously analyzed for antifungal resistance gene mutations. Reproducibility measurement was evaluated, thanks to three quality control (QC) strains.

Results

YO and MNV direct MIC comparisons obtained a global agreement of 67%. Performing susceptibility category comparisons, only 22% and 49% of the MICs could be assigned to categories using breakpoints and ECOFFs/ECVs, respectively, and 40% could not be assigned due to the lack of criteria in both consortia. The YO and MN susceptibility categories gave accuracies as low as 50%, revealing the difficulty to implement this method of comparison. In contrast, using the antifungal resistance gene sequences as a gold standard, we demonstrated that both methods (YO and MN) were equally able to detect the acquisition of resistance in the Candida strains, even if MN showed a global lower MIC elevation than YO. Finally, no major differences in reproducibility were observed between the three AFST methods.

Conclusion

This study demonstrates the valuable use of both commercial microdilution AFST methods to detect antifungal resistance due to point mutations in antifungal resistance genes. We highlighted the difficulty to conduct conclusive analyses without antifungal gene sequence data as a gold standard. Indeed, MIC comparisons taking into account the consortia criteria of interpretation remain difficult even after the effort of harmonization.

Deciphering the Mrr1/Mdr1 Pathway in Azole Resistance of Candida auris

Candida auris is an emerging yeast pathogen with a remarkable ability to develop antifungal resistance, in particular to fluconazole and other azoles. Azole resistance in C. auris was shown to result from different mechanisms, such as mutations in the target gene ERG11 or gain-of-function (GOF) mutations in the transcription factor TAC1b and overexpression of the drug transporter Cdr1. The roles of the transcription factor Mrr1 and of the drug transporter Mdr1 in azole resistance is still unclear. Previous works showed that deletion of MRR1 or MDR1 had no or little impact on azole susceptibility of C. auris. However, an amino acid substitution in Mrr1 (N647T) was identified in most C. auris isolates of clade III that were fluconazole resistant. This study aimed at investigating the role of the transcription factor Mrr1 in azole resistance of C. auris. While the MRR1^N647T mutation was always concomitant to hot spot ERG11 mutations, MRR1 deletion in one of these isolates only resulted in a modest decrease of azole MICs. However, introduction of the MRR1^N647T mutation in an azole-susceptible C. auris isolate from another clade with wild-type MRR1 and ERG11 alleles resulted in significant increase of fluconazole and voriconazole MICs. We demonstrated that this MRR1 mutation resulted in reduced azole susceptibility via upregulation of the drug transporter MDR1 and not CDR1. In conclusion, this work demonstrates that the Mrr1-Mdr1 axis may contribute to C. auris azole resistance by mechanisms that are independent from ERG11 mutations and from CDR1 upregulation.