Echinocandin susceptibility testing of Candida spp. Using EUCAST EDef 7.1 and CLSI M27-A3 standard procedures: analysis of the influence of bovine serum albumin supplementation, storage time, and drug lots

Candida auris - Comparison of sensititre YeastOne and Vitek 2 AST systems for antifungal susceptibility testing - A single centre experience

INTRODUCTION

Candida auris is emerging as an important cause of candidemia and deep seated candidal infection. We compared the susceptibility results of bloodstream Candida auris isolates by Vitek 2 with Sensititre YeastOne (SYO) method.

METHODS

Forty-seven C. auris blood stream isolates were simultaneously tested for AFST by Vitek 2 and SYO.

RESULTS

All strains were resistant to Fluconazole. 25.5% isolates showed pan-azole resistance. In comparison with SYO, lower MICs for voriconazole were noted with Vitek 2 (VME rate 76.1%). All strains were sensitive to anidulafungin and micafungin by SYO. For micafungin, Vitek 2 demonstrated higher MICs and an ME rate of 23.5%. Susceptibility interpretation of caspofungin by SYO was challenged by development of 'Eagle effect' resulting in sensitivity of 28.2%. We studied the evolution of caspofungin 'Eagle effect' with SYO by serial hourly MIC readings and noted that paradoxical growth commenced at 21 hrs of incubation. Compared to SYO, Vitek 2 showed higher resistance rate to Amphotericin B with ME rate of 25.6%.

CONCLUSION

Laboratories using commercial AFST systems for Candida auris need to be aware of the possibility of ME and VME for amphotericin B and voriconazole respectively with Vitek 2 and 'Eagle effect' for caspofungin with SYO.

Echinocandin persistence directly impacts the evolution of resistance and survival of the pathogenic fungus Candida glabrata

Recent epidemiological studies documented an alarming increase in the prevalence of echinocandin-resistant (ECR) Candida glabrata blood isolates. ECR isolates are known to arise from a minor subpopulation of a clonal population, termed echinocandin persisters. Although it is believed that isolates with a higher echinocandin persistence (ECP) are more likely to develop ECR, the implication of ECP needs to be better understood. Moreover, replacing laborious and time-consuming traditional approaches to determine ECP levels with rapid, convenient, and reliable tools is imperative to advance our understanding of this emerging concept in clinical practice. Herein, using extensive ex vivo and in vivo systemic infection models, we showed that high ECP isolates are less effectively cleared by micafungin treatment and exclusively give rise to ECR colonies. Additionally, we developed a flow cytometry-based tool that takes advantage of a SYTOX-based assay for the stratification of ECP levels. Once challenged with various collections of echinocandin-susceptible blood isolates, our assay reliably differentiated ECP levels in vitro and predicted ECP levels in real time under ex vivo and in vivo conditions when compared to traditional methods relying on colony-forming unit counting. Given the high and low ECP predictive values of 92.3% and 82.3%, respectively, our assay showed a high agreement with traditional approach. Collectively, our study supports the concept of ECP level determination in clinical settings and provides a robust tool scalable for high-throughput settings. Application of this tool facilitates the interrogation of mutant and drug libraries to further our understanding of persister biology and designing anti-persister therapeutics.

IMPORTANCE

Candida glabrata is a prevalent fungal pathogen able to replicate inside macrophages and rapidly develop resistance against frontline antifungal echinocandins. Multiple studies have shown that echinocandin resistance is fueled by the survival of a small subpopulation of susceptible cells surviving lethal concentrations of echinocandins. Importantly, bacterial pathogens that exhibit high antibiotic persistence also impose a high burden and generate more antibiotic-resistant colonies. Nonetheless, the implications of echinocandin persistence (ECP) among the clinical isolates of C. glabrata have not been defined. Additionally, ECP level determination relies on a laborious and time-consuming method, which is prone to high variation. By exploiting in vivo systemic infection and ex vivo models, we showed that C. glabrata isolates with a higher ECP are associated with a higher burden and more likely develop echinocandin resistance upon micafungin treatment. Additionally, we developed an assay that reliably determines ECP levels in real time. Therefore, our study identified C. glabrata isolates displaying high ECP levels as important entities and provided a reliable and convenient tool for measuring echinocandin persistence, which is extendable to other fungal and bacterial pathogens.

Evaluation of the antidermatophytic activity of potassium salts of N-acylhydrazinecarbodithioates and their aminotriazole-thione derivatives

Nowadays, dermatophyte infections are relatively easy to cure, especially since the introduction of orally administered antifungals such as terbinafine and itraconazole. However, these drugs may cause side effects due to liver damage or their interactions with other therapeutics. Hence, the search for new effective chemotherapeutics showing antidermatophyte activity seems to be the urge of the moment. Potassium salts of N-acylhydrazinecarbodithioates are used commonly as precursors for the synthesis of biologically active compounds. Keeping that in mind, the activity of a series of five potassium N-acylhydrazinecarbodithioates (1a-e) and their aminotriazole-thione derivatives (2a-e) was evaluated against a set of pathogenic, keratinolytic fungi, such as Trichophyton ssp., Microsporum ssp. and Chrysosporium keratinophilum, but also against some Gram-positive and Gram-negative bacteria. All tested compounds were found non-toxic for L-929 and HeLa cells, with the IC30 and IC50 values assessed in the MTT assay above 128 mg/L. The compound 5-amino-3-(naphtalene-1-yl)-4,5-dihydro-1H-1,2,4-triazole-5-thione (2d) was found active against all fungal strains tested. Scanning Electron Microscopy (SEM) revealed inhibition of mycelium development of Trichophyton rubrum cultivated on nail fragments and treated with 2d 24 h after infection with fungal spores. Transmission Electron Microscopy (TEM) observation of mycelium treated with 2d showed ultrastructural changes in the morphology of germinated spores. Finally, the RNA-seq analysis indicated that a broad spectrum of genes responded to stress induced by the 2d compound. In conclusion, the results confirm the potential of N-acylhydrazinecarbodithioate derivatives for future use as promising leads for new antidermatophyte agents development.

EUCAST Ibrexafungerp MICs and Wild-Type Upper Limits for Contemporary Danish Yeast Isolates

Ibrexafungerp is a novel triterpenoid antifungal that inhibits glucan synthase and thus fungal cell wall synthesis. We examined the in vitro activity against contemporary clinical yeast, investigated inter-laboratory and intra-laboratory variability, suggested wild-type upper-limit values (WT-UL), and compared in vitro activity of ibrexafungerp to five licensed antifungals. Susceptibility to ibrexafungerp and comparators was investigated prospectively for 1965 isolates (11,790 MICs) and repetitively for three QC strains (1764 MICs) following the EUCAST E.Def 7.3.2 method. Elevated ibrexafungerp/echinocandin MICs prompted FKS sequencing. Published ibrexafungerp EUCAST MIC-distributions were retrieved and aggregated for WT-UL determinations following EUCAST principles. Ibrexafungerp MICs were ≤2 mg/L except against C. pararugosa, Cryptococcus and some rare yeasts. Modal MICs (mg/L) were 0.06/0.125/0.25/0.5/0.5/0.5/0.5/1/2 for C. albicans/C. dubliniensis/C. glabrata/C. krusei/C. parapsilosis/C. tropicalis/S. cerevisiae/C. guilliermondii/C. lusitaniae and aligned within ±1 dilution with published values. The MIC ranges for QC strains were: 0.06–0.25/0.5–1/0.125–0.5 for CNM-CL-F8555/ATCC6258/ATCC22019. The WT-UL (mg/L) were: 0.25/0.5/1/1/2 for C. albicans/C. glabrata/C. krusei/C. parapsilosis/C. tropicalis. Adopting these, non-wild-type rates were 0.3%/0.6%/0%/8%/3% for C. albicans/C. glabrata/C. krusei/C. parapsilosis/C. tropicalis and overall lower than for comparators except amphotericin B. Five/six non-wild-type C. albicans/C. glabrata were echinocandin and Fks non-wild-type (F641S, F659del or F659L). Eight C. parapsilosis and three C. tropicalis non-wild-type isolates were echinocandin and Fks wild-type. Partial inhibition near 50% in the supra-MIC range may explain variable MICs. Ibrexafungerp EUCAST MIC testing is robust, although the significance of paradoxical growth for some species requires further investigation. The spectrum is broad and will provide an oral option for the growing population with azole refractory infection.

Invasive Candida kefyr infection presenting as pyelonephritis in an ICU hospitalized COVID-19 patient: Case report and review of the literature

Candida kefyr (Kluyveromyces marxianus), an ascomycetous environmental yeast, occasionally isolated from dairy products, represents an uncommon but emerging pathogen in immunocompromised patients. Herein, we present a case of C. kefyr pyelonephritis in a 41-year-old, previously immunocompetent, patient who was hospitalized in an COVID-19 ICU. Pyelonephritis was associated with caliectasis and obstruction due to possible fungus ball formation. Predisposing factors included ICU stay, use of broad spectrum antibiotics and steroids, central venous catheterization, mechanical ventilation and urologic manipulation. Susceptibility testing revealed high MIC values to amphotericin B. Infection was effectively controlled by prolonged administration of fluconazole without further surgical intervention. COVID-19 complicated with invasive candidiasis is an increasingly observed clinical situation that warrants high suspicion index and careful evaluation of laboratory data.

Modifications of antifungal sensibility testing as suggested by CLSI document M27-A4: proposal for using different culture medium and buffer

A common strategy in antifungal susceptibility testing is the utilization of the standardized protocol based on the microbroth dilution assay approach as described by the Clinical Laboratory Standards Institute (CLSI) (M27-A4). One major problem for laboratories in resource-limited countries with this protocol arises from the use of expensive culture media like RPMI-1640 and 3-N-morpholinopropanesulfonic acid (MOPS) buffer. One approach of circumventing this problem in cases of economic need is the evaluation of alternative culture media and buffers. The overall goal of this work was to investigate the influence of modifications in the protocol M27-A4 on diagnostic reliability. We performed univariate analyses evaluating (1) 2 different culture media (YNB and modified SAB); (2) three different buffers (sodium bicarbonate, Tris-HCL, and phosphate), as well as the influence of inoculum concentration (10², 10³, 10⁴, 10⁵ cells/mL), the influence of incubation time, and the influence of the assessment mode (visual, biological dye, and spectrophotometer). Our results suggested that (1) RPMI-1640 may be substituted by modified SAB and (2) MOPS buffer may be substituted by Tris-HCl buffer for defined analyses. By comparing the CLSI protocol and the alternative protocol proposed in the present study (modified SAB and Tris-HCl buffer) for the assessment of fluconazole susceptibility of eighteen yeasts (clinical isolates), similar results with both methodologies were recorded. We feel that this study should stimulate a discussion on the feasibility and evolution of the M27-A4 protocol in order to include pragmatic alternatives for resource-limited settings.

ISO standard 20776-1 or serial 2-fold dilution for antifungal susceptibility plate preparation: that is the question!

BACKGROUND

Since the ISO standard 20776-1 and serial dilution procedures were compared in 2010 for fluconazole and itraconazole, several new antifungals that are hydrophobic and highly potent have been introduced.

OBJECTIVES

To investigate the impact of the number of tip changes during serial dilution, and ISO and serial dilution for nine antifungals.

METHODS

EUCAST E.Def 7.3.2 with serial (0-10 tip changes) and ISO dilution. Candida parapsilosis ATCC 22019, Candida albicans ATCC 64548, C. albicans CNM CL-F8555, Candida krusei ATCC 6258, Aspergillus flavus ATCC 204304 and clinical isolates (n = 5) of C. albicans, Candida dubliniensis, Candida glabrata, C. krusei, A. flavus and Aspergillus terreus were included. GM MICs were compared for ISO and serial dilution and with QC values where available.

RESULTS

Increasing the number of tip changes (0/1/2/10 times) during serial dilution for plate preparation increased the MICs 1 to >2 dilutions for amphotericin B, anidulafungin, micafungin, fluconazole, voriconazole and isavuconazole against C. albicans ATCC 64548 but only isavuconazole MICs against C. parapsilosis ATCC 22019 (3 dilutions). ISO and serial dilution (two tip changes) were compared for eight compounds and four Candida QC strains (352 MICs). Six/41 GM MIC pairs deviated with 1-1.8 dilution (14.6%). Comparing the GM MIC with the QC values, the ISO method GM MIC was closest to the target in 30.8%, the serial dilution in 34.6% and the methods identical in 34.6% of the cases. Finally, ISO and serial dilution MICs were compared for clinical isolates (920 MICs). Five/23 GM MIC pairs (21.7%) deviated 1.0-1.1 dilutions.

CONCLUSIONS

The ISO and serial dilution (two tip changes) method were in acceptable agreement and thus equally applicable for EUCAST testing.

Antifungal Susceptibility Testing and Identification

The requirement for antifungal susceptibility testing is increasing given the availability of new drugs, increasing populations of individuals at risk for fungal infection, and emerging multiresistant fungi. Rapid and accurate fungal identification remains at the forefront of laboratory efforts to guide empiric therapy. Antifungal susceptibility testing methods have greatly improved, but are subject to variation in results between methods. Careful standardization, validation, and extensive training of users is essential to ensure susceptibility results are clinically useful and interpreted appropriately. Interpretive criteria for many drugs and species are still lacking, but this will continue to evolve.

Antimicrobial Susceptibility Testing of Antimicrobial Peptides to Better Predict Efficacy

During the development of antimicrobial peptides (AMP) as potential therapeutics, antimicrobial susceptibility testing (AST) stands as an essential part of the process in identification and optimisation of candidate AMP. Standard methods for AST, developed almost 60 years ago for testing conventional antibiotics, are not necessarily fit for purpose when it comes to determining the susceptibility of microorganisms to AMP. Without careful consideration of the parameters comprising AST there is a risk of failing to identify novel antimicrobials at a time when antimicrobial resistance (AMR) is leading the planet toward a post-antibiotic era. More physiologically/clinically relevant AST will allow better determination of the preclinical activity of drug candidates and allow the identification of lead compounds. An important consideration is the efficacy of AMP in biological matrices replicating sites of infection, e.g., blood/plasma/serum, lung bronchiolar lavage fluid/sputum, urine, biofilms, etc., as this will likely be more predictive of clinical efficacy. Additionally, specific AST for different target microorganisms may help to better predict efficacy of AMP in specific infections. In this manuscript, we describe what we believe are the key considerations for AST of AMP and hope that this information can better guide the preclinical development of AMP toward becoming a new generation of urgently needed antimicrobials.

How to interpret MICs of antifungal compounds according to the revised clinical breakpoints v. 10.0 European committee on antimicrobial susceptibility testing (EUCAST)

BACKGROUND

EUCAST has revised the definition of the susceptibility category I from 'Intermediate' to 'Susceptible, Increased exposure'. This implies that I can be used where the drug concentration at the site of infection is high, either because of dose escalation or through other means to ensure efficacy. Consequently, I is no longer used as a buffer zone to prevent technical factors from causing misclassifications and discrepancies in interpretations. Instead, an Area of Technical Uncertainty (ATU) has been introduced for MICs that cannot be categorized without additional information as a warning to the laboratory that decision on how to act has to be made. To implement these changes, the EUCAST-AFST (Subcommittee on Antifungal Susceptibility Testing) reviewed all, and revised some, clinical antifungal breakpoints.

OBJECTIVES

The aim was to present an overview of the current antifungal breakpoints and supporting evidence behind the changes.

SOURCES

This document is based on the ten recently updated EUCAST rationale documents, clinical breakpoint and breakpoint ECOFF documents.

CONTENT

The following breakpoints (in mg/L) have been revised or established for Candida species: micafungin against C. albicans (ATU = 0.03); amphotericin B (S ≤/> R = 1/1), fluconazole (S ≤/> R = 2/4), itraconazole (S ≤/> R = 0.06/0.06), posaconazole (S ≤/> R = 0.06/0.06) and voriconazole (S ≤/> R = 0.06/0.25) against C. dubliniensis; fluconazole against C. glabrata (S ≤/> R = 0.001/16); and anidulafungin (S ≤/> R = 4/4) and micafungin (S ≤/> R = 2/2) against C. parapsilosis. For Aspergillus, new or revised breakpoints include itraconazole (ATU = 2) and isavuconazole against A. flavus (S ≤/> R = 1/2, ATU = 2); amphotericin B (S ≤/> R = 1/1), isavuconazole (S ≤ /> R = 1/2, ATU = 2), itraconazole (S ≤/> R = 1/1, ATU = 2), posaconazole (ATU = 0.25) and voriconazole (S ≤/> R = 1/1, ATU = 2) against A. fumigatus; itraconazole (S ≤/> R = 1/1, ATU = 2) and voriconazole (S ≤/> R = 1/1, ATU = 2) against A. nidulans; amphotericin B against A. niger (S ≤/> R = 1/1); and itraconazole (S ≤/> R = 1/1, ATU = 2) and posaconazole (ATU = 0.25) against A. terreus.

IMPLICATIONS

EUCAST-AFST has released ten new documents summarizing existing and new breakpoints and MIC ranges for control strains. A failure to adopt the breakpoint changes may lead to misclassifications and suboptimal or inappropriate therapy of patients with fungal infections.

Rezafungin In Vitro Activity against Contemporary Nordic Clinical Candida Isolates and Candida auris Determined by the EUCAST Reference Method

Rezafungin (formerly CD101) is a novel echinocandin in clinical development. EUCAST epidemiological cutoff values (ECOFFs) have not yet been established. We determined the in vitro activity of rezafungin and comparators against 1,293 Nordic yeast isolates and 122 Indian Candida auris isolates and established single-center wild-type upper limits (WT-UL). The isolates (19 Candida spp. and 13 other yeast species) were identified using Chromagar; matrix-assisted laser desorption ionization-time of flight (MALDI-TOF); and, when needed, internal transcribed spacer sequencing. EUCAST E.Def 7.3.1 susceptibility testing included rezafungin, anidulafungin, micafungin, amphotericin B, and fluconazole. WT-UL were established following EUCAST principles for visual and statistical ECOFF setting. fks target genes were sequenced for rezafungin non-wild-type isolates. EUCAST clinical breakpoints for fungi version 9.0 were adopted for susceptibility classification. Rezafungin had species-specific activity similar to that of anidulafungin and micafungin. On a milligram-per-liter basis, rezafungin was overall less active than anidulafungin and micafungin but equally or more active than fluconazole and amphotericin B against the most common Candida species, except C. parapsilosis We identified 37 (3.1%) rezafungin non-wild-type isolates of C. albicans (1.9%), C. glabrata (3.0%), C. tropicalis (2.7%), C. dubliniensis (2.9%), C. krusei (1.2%), and C. auris (14.8%). Alterations in Fks hot spots were found in 26/26 Nordic and 8/18 non-wild-type C. auris isolates. Rezafungin displayed broad in vitro activity against Candida spp., including C. auris Adopting WT-UL established here, few Nordic strains, but a significant proportion of C. auris isolates, had elevated MICs with mutations in fks target genes that conferred echinocandin cross-resistance. fks1 mutations raised rezafungin MICs notably less than anidulafungin and micafungin MICs in C. auris.

EUCAST Determination of Olorofim (F901318) Susceptibility of Mold Species, Method Validation, and MICs

Olorofim is a novel antifungal agent with in vitro activity against Aspergillus and some other molds. Here, we addressed technical aspects for EUCAST olorofim testing and generated contemporary MIC data. EUCAST E.Def 9.3.1 testing was performed comparing two plate preparation methods (serial dilution in medium [serial plates] versus predilution in DMSO [ISO plates]), two lots of olorofim, visual (visual-MIC) versus spectrophotometer (spec-MIC) reading, and four polystyrene plates using 34 to 53 Aspergillus isolates from five genera. Subsequently, olorofim MICs were compared to itraconazole, voriconazole, posaconazole, and amphotericin B MICs for 298 clinical mold isolates (2016 to 2017). Wild-type upper limits (WT-UL) were determined following EUCAST principles for epidemiologic cutoff value (ECOFF) setting. Olorofim median MICs comparing serial plates and ISO plates were identical (25/36 [69%]) or one dilution apart (11/36 [31%]). Interperson agreement for visual-MICs was 92% to 94%/100% for ≤1/≤2 dilutions, respectively. The visual-MIC values across tested microtiter plates and olorofim lots revealed only discrete differences (≤1 dilution lower for treated plates). No single spec-MIC criterion was applicable to all species. Olorofim MICs were low against 275 Aspergillus species isolates (modal MIC, 0.06 mg/liter; MIC range, < 0.004 to 0.25 mg/liter) and three dermatophytes (MICs 0.03 to 0.06 mg/liter). MICs against Fusarium were diverse, with full inhibition of F. proliferatum (MIC, 0.016), 50% growth inhibition of Fusarium solani at 1 to 2 mg/liter, and no inhibition of F. dimerum Olorofim displayed potent in vitro activity against most mold isolates and was associated with limited variation in EUCAST susceptibility testing.

Fungemia Surveillance in Denmark Demonstrates Emergence of Non-albicans Candida Species and Higher Antifungal Usage and Resistance Rates than in Other Nations

Recent changes in the occurrence of fungal species and the difficulties in performing reference antifungal susceptibility testing highlight the importance of surveillance of fungal organisms and antifungal resistance rates. K. M. T. Astvad et al. report results from recent (2012 to 2015) fungemia surveillance in Denmark and compare the results to previous data (2004 to 2011), showing a decrease in Candida albicans infections accompanied by an increase in C. glabrata and C. dubliniensis infections (J Clin Microbiol 56:e01564-17, 2018, https://doi.org/10.1128/JCM.01564-17). Azole resistance among C. tropicalis and C. parapsilosis isolates and echinocandin resistance in C. krusei isolates were higher in Denmark than in other regions. Interestingly, the usage of antifungals is higher in Denmark than in other Nordic countries.

Multicentre determination of rezafungin (CD101) susceptibility of Candida species by the EUCAST method

OBJECTIVES

Rezafungin (CD101) is a new long-acting echinocandin allowing weekly dosing, currently undergoing phase-II clinical trials for invasive candidiasis. The aim of this study was to assess rezafungin's in vitro activity against the most frequent Candida species following the EUCAST methodology.

METHODS

The susceptibility of 2018 clinical Candida isolates was determined at four European laboratories. In parallel, six control strains were repeatedly tested. Wild-type upper limits (WT-ULs), defined as the MIC value where the wild-type distribution ends, were determined following the principles for EUCAST ECOFF-setting.

RESULTS

The lowest rezafungin MICs (geometric MIC (GM-MIC), MIC range (mg/L)) were observed for C. albicans (0.016, 0.002-0.125) and the highest for C. parapsilosis (1.657, 0.063->4). MICs for the remaining species were in between these values (GM-MICs 0.048-0.055). Visual and statistical WT-ULs were identical for C. glabrata (0.125), C. krusei (0.125), C. parapsilosis (4), and C. tropicalis (0.25). If adopting these WT-ULs for classification into WT and non-WT populations, 1/413 C. glabrata, 1/402 C. krusei, 1/398 C. parapsilosis, and 1/402 C. tropicalis isolates were categorized as non-WT, all of which derived from Laboratory 1. For C. albicans unexplained laboratory variation was observed (WT-UL: 0.063-0.125 in Laboratories 1 and 2 versus 0.016 in Laboratories 3 and 4). A similar systematic difference was observed comparing the MICs for the three C. albicans QC strains, specifically, obtained in Laboratories 1and 2 with those in Laboratories 3 and 4.

DISCUSSION

Rezafungin displayed species-specific activity similar to other echinocandins. Interlaboratory variation was observed for the most susceptible species C. albicans clinical and QC strains, an observation that warrants further investigation.

Culture-Independent Molecular Methods for Detection of Antifungal Resistance Mechanisms and Fungal Identification

Resistance to azoles and echinocandins has emerged as a significant factor affecting the clinical management of patients with invasive fungal infections. Immunosuppressed patients at high risk for invasive fungal infections often have prolonged or repeated exposure to antifungals resulting in either the well-documented selection of naturally occurring, less susceptible fungal species, or the in situ development of specific resistance mechanisms. Nucleic acid-based molecular diagnostics are particularly well suited for the rapid detection of low-abundance fungal pathogens and identification of the infecting pathogen to the genus and species levels, as well as assessment of resistance mechanisms. A wide range of molecular probing technologies involving real-time polymerase chain reaction (PCR) assays that facilitate direct analysis of a single infecting genome in a sterile blood specimen are available and have recently been commercialized (eg, Roche LightCycler SeptiFast and T2 Biosystems T2Candida). One of the exciting applications of molecular technology is the direct detection of specific resistance mechanisms that evolve during therapy. In principle, the detection of resistance mechanisms that have been independently validated to cause resistance provides a culture-independent biomarker for potential therapeutic failure. The emergence of real-time PCR assays utilizing allele-specific molecular detection technology that is highly sensitive, robust, and high-throughput has the potential to improve patient care by providing faster detection of drug-resistant infecting strains and to help inform therapeutic management.

Yeasts

Yeasts are unicellular organisms that reproduce mostly by budding and less often by fission. Most medically important yeasts originate from Ascomycota or Basidiomycota. Here, we review taxonomy, epidemiology, disease spectrum, antifungal drug susceptibility patterns of medically important yeast, laboratory diagnosis, and diagnostic strategies.

Echinocandin Resistance in Candida Species Isolates from Liver Transplant Recipients

Liver transplant recipients are at risk of invasive fungal infections, especially candidiasis. Echinocandin is recommended as prophylactic treatment but is increasingly associated with resistance. Our aim was to assess echinocandin drug resistance in Candida spp. isolated from liver transplant recipients treated with this antifungal class. For this, all liver-transplanted patients in a University Hospital (Créteil, France) between January and June of 2013 and 2015 were included. Susceptibilities of Candida isolates to echinocandins were tested by Etest and the EUCAST reference method. Isolates were analyzed by FKS sequencing and genotyped based on microsatellites or multilocus sequence typing (MLST) profiles. Ninety-four patients were included, and 39 patients were colonized or infected and treated with echinocandin. Echinocandin resistance appeared in 3 (8%) of the treated patients within 1 month of treatment. One patient was colonized by resistant Candida glabrata, one by resistant Candida dubliniensis, and one by resistant Candida albicans. Molecular analysis found three mutations in FKS2 HS1 (F659S, S663A, and D666E) for C. glabrata and one mutation in FKS1 HS1 (S645P) for C. dubliniensis and C. albicans. Susceptible and resistant isolates belonged to the same genotype. To our knowledge, this is the first study on echinocandin resistance in Candida spp. in a liver transplant population. Most resistant isolates were found around/in digestive sites, perhaps due to lower diffusion of echinocandin in these sites. This work documents the risk of emergence of resistance to echinocandin, even after short-term treatment.

Inhibitory effects of carvacrol on the expression of secreted aspartyl proteinases 1-3 in fluconazole-resistant Candida albicans isolates

BACKGROUND AND OBJECTIVES

Secreted Aspartyl Proteinase (SAP) is one of the main virulence factors in the pathogenesis of Candida. This enzyme is encoded by a family of at least ten genes. Among these genes, the role of SAP1-3 in mucosal infections is evident. This study aimed to investigate the expression of SAP1-3 genes of Candida albicans isolates after treatment with Echinophora platyloba extract, carvacrol and caspofungin drug.

MATERIALS AND METHODS

Vaginal samples of 68 women with suspected vaginitis were obtained and cultured. Canida albicans species were identified using phenotypic and genotyping methods. Spectrophotometry was used to investigate the presence of SAP protein in the vaginal samples, and SDS-PAGE was used to confirm its protein composition. Real-time PCR was performed to ascertain the effects of subinhibitory concentrations of Echinophora platyloba extract, carvacrol and caspofungin on the expression of SAP1-3 genes before and after treatment.

RESULTS

C. albicans was found as the abundant species (59.6%), and different amounts of SAP were present in all vaginal samples, which were higher than Candida krusei strain. The protein composition of SAP in C. albicans samples was estimated with the approximate molecular weight of 45 kDa. mRNA levels of total SAP in FLU-resistant isolates (P=0.01) were more than those of FLU-susceptible isolates (P=0.07). The findings indicated that carvacrol is effective in reduction of SAP1-3 expression with a particular effect against FLU-resistant isolates.

CONCLUSION

Carvacrol contains an essential oil (carvacrol); therefore, it can be considered as an alternative effective antifungal compound.

Echinocandin Resistance in Candida Species: a Review of Recent Developments

The echinocandins are important agents for the treatment of invasive fungal infections, especially those caused by Candida species. However, as with other antimicrobial agents, microbiologic resistance to this class of antifungal agents has emerged and can result in clinical failure. Several studies have recently reported an increase in echinocandin resistance in Candida glabrata isolates at various medical centers in different geographic regions of the USA. Recent studies have also reported that many of these isolates may also be fluconazole resistant, leaving few treatment options available for clinicians to use in patients with invasive candidiasis caused by this species. Our understanding of the clinical relevance of specific point mutations within the FKS genes that cause echinocandin resistance and risk factors for the development of microbiologic resistance and clinical failure have also increased. The purpose of this review is to discuss echinocandin resistance in Candida species and recent reports that have increased our understanding of this growing clinical problem.

Multicenter study of epidemiological cutoff values and detection of resistance in Candida spp. to anidulafungin, caspofungin, and micafungin using the Sensititre YeastOne colorimetric method

Neither breakpoints (BPs) nor epidemiological cutoff values (ECVs) have been established for Candida spp. with anidulafungin, caspofungin, and micafungin when using the Sensititre YeastOne (SYO) broth dilution colorimetric method. In addition, reference caspofungin MICs have so far proven to be unreliable. Candida species wild-type (WT) MIC distributions (for microorganisms in a species/drug combination with no detectable phenotypic resistance) were established for 6,007 Candida albicans, 186 C. dubliniensis, 3,188 C. glabrata complex, 119 C. guilliermondii, 493 C. krusei, 205 C. lusitaniae, 3,136 C. parapsilosis complex, and 1,016 C. tropicalis isolates. SYO MIC data gathered from 38 laboratories in Australia, Canada, Europe, Mexico, New Zealand, South Africa, and the United States were pooled to statistically define SYO ECVs. ECVs for anidulafungin, caspofungin, and micafungin encompassing ≥97.5% of the statistically modeled population were, respectively, 0.12, 0.25, and 0.06 μg/ml for C. albicans, 0.12, 0.25, and 0.03 μg/ml for C. glabrata complex, 4, 2, and 4 μg/ml for C. parapsilosis complex, 0.5, 0.25, and 0.06 μg/ml for C. tropicalis, 0.25, 1, and 0.25 μg/ml for C. krusei, 0.25, 1, and 0.12 μg/ml for C. lusitaniae, 4, 2, and 2 μg/ml for C. guilliermondii, and 0.25, 0.25, and 0.12 μg/ml for C. dubliniensis. Species-specific SYO ECVs for anidulafungin, caspofungin, and micafungin correctly classified 72 (88.9%), 74 (91.4%), 76 (93.8%), respectively, of 81 Candida isolates with identified fks mutations. SYO ECVs may aid in detecting non-WT isolates with reduced susceptibility to anidulafungin, micafungin, and especially caspofungin, since testing the susceptibilities of Candida spp. to caspofungin by reference methodologies is not recommended.

Echinocandin resistance: an emerging clinical problem?

PURPOSE OF REVIEW

Echinocandin resistance in Candida is a great concern, as the echinocandin drugs are recommended as first-line therapy for patients with invasive candidiasis. Here, we review recent advances in our understanding of the epidemiology, underlying mechanisms, methods for detection and clinical implications.

RECENT FINDINGS

Echinocandin resistance has emerged over the recent years. It has been found in most clinically relevant Candida spp., but is most common in C. glabrata with rates exceeding 10% at selected institutions. It is most commonly detected after 3-4 weeks of treatment and is associated with a dismal outcome. An extensive list of mutations in hot spot regions of the genes encoding the target has been characterized and associated with species and drug-specific loss of susceptibility. The updated antifungal susceptibility testing reference methods identify echinocandin-resistant isolates reliably, although the performance of commercial tests is somewhat more variable. Alternative technologies are being developed, including molecular detection and matrix-assisted laser desorption ionization-time of flight.

SUMMARY

Echinocandin resistance is increasingly encountered and its occurrence makes susceptibility testing essential, particularly in patients with prior exposure. The further development of rapid and user-friendly commercially available susceptibility platforms is warranted. Antifungal stewardship is important in order to minimize unnecessary selection pressure.

Use of anidulafungin as a surrogate marker to predict susceptibility and resistance to caspofungin among 4,290 clinical isolates of Candida by using CLSI methods and interpretive criteria

This study addressed the application of anidulafungin as a surrogate marker to predict the susceptibility of Candida to caspofungin due to unacceptably high interlaboratory variation of caspofungin MIC values. CLSI reference broth microdilution methods and species-specific interpretive criteria were used to test 4,290 strains of Candida (eight species), including 71 strains with documented fks mutations. Caspofungin MIC values were compared with those of anidulafungin to determine the percentage of categorical agreement (CA) and very major (VME), major (ME), and minor error rates, as well as the ability to detect fks mutants. For all 4,290 isolates the CA was 97.1% (0.2% VME and ME, 2.5% minor errors) using anidulafungin as the surrogate. Among the 62 isolates of Candida albicans (4 isolates), C. tropicalis (5 isolates), C. krusei (4 isolates), C. kefyr (2 isolates), and C. glabrata (47 isolates) that were nonsusceptible (NS; either intermediate [I] or resistant [R]) to both caspofungin and anidulafungin, 52 (83.8%) contained a mutation in fks1 or fks2. Eight mutants of C. glabrata, two of C. albicans, and one each of C. tropicalis and C. krusei were classified as susceptible (S) to both antifungal agents. The remaining 7 mutants (2 C. albicans and 5 C. glabrata) were susceptible to one of the agents and either intermediate or resistant to the other. Using the epidemiological cutoff value (ECV) of 0.12 μg/ml for both caspofungin and anidulafungin to differentiate wild-type (WT) from non-WT strains of C. glabrata, 42 of the 55 (76.4%) C. glabrata mutants were non-WT and 8 of the 55 (14.5%) were WT for both agents (90.9% concordance). Anidulafungin can accurately serve as a surrogate marker to predict S and R of Candida to caspofungin.

Antifungal resistance to fluconazole and echinocandins is not emerging in yeast isolates causing fungemia in a Spanish tertiary care center

Accurate knowledge of fungemia epidemiology requires identification of strains to the molecular level. Various studies have shown that the rate of resistance to fluconazole ranges from 2.5% to 9% in Candida spp. isolated from blood samples. However, trends in antifungal resistance have received little attention and have been studied only using CLSI M27-A3 methodology. We assessed the fungemia epidemiology in a large tertiary care institution in Madrid, Spain, by identifying isolates to the molecular level and performing antifungal susceptibility testing according to the updated breakpoints of European Committee for Antimicrobial Susceptibility Testing (EUCAST) definitive document (EDef) 7.2. We studied 613 isolates causing 598 episodes of fungemia in 544 patients admitted to our hospital (January 2007 to December 2013). Strains were identified after amplification and sequencing of the ITS1-5.8S-ITS2 region and further tested for in vitro susceptibility to amphotericin B, fluconazole, posaconazole, voriconazole, micafungin, and anidulafungin. Resistance was defined using EUCAST species-specific breakpoints, and epidemiological cutoff values (ECOFFs) were applied as tentative breakpoints. Most episodes were caused by Candida albicans (46%), Candida parapsilosis (28.7%), Candida glabrata (9.8%), and Candida tropicalis (8%). Molecular identification enabled us to better detect cryptic species of Candida guilliermondii and C. parapsilosis complexes and episodes of polyfungal fungemia. The overall percentage of fluconazole-resistant isolates was 5%, although it was higher in C. glabrata (8.6%) and non-Candida yeast isolates (47.4%). The rate of resistance to echinocandins was 4.4% and was mainly due to the presence of intrinsically resistant non-Candida species. Resistance mainly affected non-Candida yeasts. The rate of resistance to fluconazole and echinocandins did not change considerably during the study period.

Comparison of EUCAST and CLSI broth microdilution methods for the susceptibility testing of 10 systemically active antifungal agents when tested against Candida spp

The antifungal broth microdilution (BMD) method of the European Committee on Antimicrobial Susceptibility Testing (EUCAST) was compared with Clinical and Laboratory Standards Institute (CLSI) BMD method M27-A3 for amphotericin B, flucytosine, anidulafungin, caspofungin, micafungin, fluconazole, isavuconazole, itraconazole, posaconazole, and voriconazole susceptibility testing of 357 isolates of Candida. The isolates were selected from global surveillance collections to represent both wild-type (WT) and non-WT MIC results for the azoles (12% of fluconazole and voriconazole results were non-WT) and the echinocandins (6% of anidulafungin and micafungin results were non-WT). The study collection included 114 isolates of Candida albicans, 73 of C. glabrata, 76 of C. parapsilosis, 60 of C. tropicalis, and 34 of C. krusei. The overall essential agreement (EA) between EUCAST and CLSI results ranged from 78.9% (posaconazole) to 99.6% (flucytosine). The categorical agreement (CA) between methods and species of Candida was assessed using previously determined CLSI epidemiological cutoff values. The overall CA between methods was 95.0% with 2.5% very major (VM) and major (M) discrepancies. The CA was >93% for all antifungal agents with the exception of caspofungin (84.6%), where 10% of the results were categorized as non-WT by the EUCAST method and WT by the CLSI method. Problem areas with low EA or CA include testing of amphotericin B, anidulafungin, and isavuconazole against C. glabrata, itraconazole, and posaconazole against most species, and caspofungin against C. parapsilosis, C. tropicalis, and C. krusei. We confirm high level EA and CA (>90%) between the 2 methods for testing fluconazole, voriconazole, and micafungin against all 5 species. The results indicate that the EUCAST and CLSI methods produce comparable results for testing the systemically active antifungal agents against the 5 most common species of Candida; however, there are several areas where additional steps toward harmonization are warranted.

Antifungal drug resistance mechanisms in pathogenic fungi: from bench to bedside

The phenotypic methods for identification of antifungal resistance are reliable procedures, and MIC determination by reference techniques is the gold standard to detect resistant clinical isolates. In recent years, progress has been made towards the description of resistance mechanisms at molecular level. There are methods of detection that can be useful for clinical laboratories, but lack of standardization precludes their full and effective integration in the routine daily practice. The molecular detection of Candida resistance to azoles and to echinocandins and of Aspergillus resistance to triazoles can be clinically relevant and could help to design more efficient prevention and control strategies. This text reviews the present state of the detection of mechanisms of resistance at the molecular level in Candida spp. and Aspergillus spp. and its relevance to clinical practice.

Breakpoints for antifungal agents: an update from EUCAST focussing on echinocandins against Candida spp. and triazoles against Aspergillus spp

Candida and Aspergillus infections have emerged as significant pathogens in recent decades. During this same time, broad spectrum triazole and echinocandin antifungal agents have been developed and increasingly used. One consequence of widespread use is leading to the emergence of mutants with acquired resistance mutations. Therefore, accurate susceptibility testing and appropriate clinical breakpoints for the interpretation of susceptibility results have become increasingly important. Here we review the underlying methodology by which breakpoints have been selected by EUCAST (European Committee on Antimicrobial Susceptibility Testing). Five parameters are evaluated: dosing regimens used; EUCAST MIC distributions from multiple laboratories, species and compound specific epidemiological cut off values (upper MIC limits of wild type isolates or ECOFFs), pharmacokinetic/pharmacodynamic relationships and targets associated with outcome and finally clinical data by species and MIC when available. The general principles are reviewed followed by a detailed review of the individual aspects for Candida species and the three echinocandins and for Aspergillus and the three mould-active azoles. This review provides an update of the subcommittee on antifungal susceptibility testing (AFST) of the EUCAST methodology and summarises the current EUCAST breakpoints for Candida and Aspergillus. Recommendations about applicability of antifungal susceptibility testing in the routine setting are also included.

Use of micafungin as a surrogate marker to predict susceptibility and resistance to caspofungin among 3,764 clinical isolates of Candida by use of CLSI methods and interpretive criteria

Due to unacceptably high interlaboratory variation in caspofungin MIC values, we evaluated the use of micafungin as a surrogate marker to predict the susceptibility of Candida spp. to caspofungin using reference methods and species-specific interpretive criteria. The MIC results for 3,764 strains of Candida (eight species), including 73 strains with fks mutations, were used. Caspofungin MIC values and species-specific interpretive criteria were compared with those of micafungin to determine the percent categorical agreement (%CA) and very major error (VME), major error (ME), and minor error rates as well as their ability to detect fks mutant strains of Candida albicans (11 mutants), Candida tropicalis (4 mutants), Candida krusei (3 mutants), and Candida glabrata (55 mutants). Overall, the %CA was 98.8% (0.2% VMEs and MEs, 0.8% minor errors) using micafungin as the surrogate marker. Among the 60 isolates of C. albicans (9 isolates), C. tropicalis (5 isolates), C. krusei (2 isolates), and C. glabrata (44 isolates) that were nonsusceptible (either intermediate or resistant) to both caspofungin and micafungin, 54 (90.0%) contained a mutation in fks1 or fks2. An additional 10 C. glabrata mutants, two C. albicans mutants, and one mutant each of C. tropicalis and C. krusei were classified as susceptible to both antifungal agents. Using the epidemiological cutoff values (ECVs) of 0.12 μg/ml for caspofungin and 0.03 μg/ml for micafungin to differentiate wild-type (WT) from non-WT strains of C. glabrata, 80% of the C. glabrata mutants were non-WT for both agents (96% concordance). Micafungin may serve as an acceptable surrogate marker for the prediction of susceptibility and resistance of Candida to caspofungin.

Molecular identification and antifungal susceptibility of yeast isolates causing fungemia collected in a population-based study in Spain in 2010 and 2011

We report the molecular identifications and antifungal susceptibilities of the isolates causing fungemia collected in the CANDIPOP population-based study conducted in 29 Spanish hospitals. A total of 781 isolates (from 767 patients, 14 of them having mixed fungemia) were collected. The species found most frequently were Candida albicans (44.6%), Candida parapsilosis (24.5%), Candida glabrata (13.2%), Candida tropicalis (7.6%), Candida krusei (1.9%), Candida guilliermondii (1.7%), and Candida lusitaniae (1.3%). Other Candida and non-Candida species accounted for approximately 5% of the isolates. The presence of cryptic species was low. Compared to findings of previous studies conducted in Spain, the frequency of C. glabrata has increased. Antifungal susceptibility testing was performed by using EUCAST and CLSI M27-A3 reference procedures; the two methods were comparable. The rate of fluconazole-susceptible isolates was 80%, which appears to be a decrease compared to findings of previous studies, explained mainly by the higher frequency of C. glabrata. Using the species-specific breakpoints and epidemiological cutoff values, the rate of voriconazole and posaconazole in vitro resistance was low (<2%). In the case of C. tropicalis, using the EUCAST procedure, the rate of azole resistance was around 20%. There was a correlation between the previous use of azoles and the presence of fluconazole-resistant isolates. Resistance to echinocandins was very rare (2%), and resistance to amphotericin B also was very uncommon. The sequencing of the hot spot (HS) regions from FKS1 or FKS2 genes in echinocandin-resistant isolates revealed previously described point mutations. The decrease in the susceptibility to fluconazole in Spanish isolates should be closely monitored in future studies.

Frequency of fks mutations among Candida glabrata isolates from a 10-year global collection of bloodstream infection isolates

Among 119 echinocandin non-wild-type (non-WT) Candida glabrata strains from two global surveys, mutations in fks hot spots (HSs) were detected in 28 (from 7 countries and 8 U.S. states): 24 strains (85.7%) had non-WT MICs for micafungin, 22 (78.6%) for anidulafungin, and 25 (89.3%) for caspofungin. The most common FKS substitutions among non-WT strains were at positions F659 (n = 7) and S663 (n = 7). Three isolates displaying WT MIC results had F625Y, L630I, and D632Y substitutions or non-HS mutations. Mutations that have been reported to decrease the echinocandin binding to the 1,3-β-d-glucan synthase were categorized as resistant by applying the new CLSI breakpoint criteria for all three echinocandins.

Interlaboratory variability of Caspofungin MICs for Candida spp. Using CLSI and EUCAST methods: should the clinical laboratory be testing this agent?

Although Clinical and Laboratory Standards Institute (CLSI) clinical breakpoints (CBPs) are available for interpreting echinocandin MICs for Candida spp., epidemiologic cutoff values (ECVs) based on collective MIC data from multiple laboratories have not been defined. While collating CLSI caspofungin MICs for 145 to 11,550 Candida isolates from 17 laboratories (Brazil, Canada, Europe, Mexico, Peru, and the United States), we observed an extraordinary amount of modal variability (wide ranges) among laboratories as well as truncated and bimodal MIC distributions. The species-specific modes across different laboratories ranged from 0.016 to 0.5 μg/ml for C. albicans and C. tropicalis, 0.031 to 0.5 μg/ml for C. glabrata, and 0.063 to 1 μg/ml for C. krusei. Variability was also similar among MIC distributions for C. dubliniensis and C. lusitaniae. The exceptions were C. parapsilosis and C. guilliermondii MIC distributions, where most modes were within one 2-fold dilution of each other. These findings were consistent with available data from the European Committee on Antimicrobial Susceptibility Testing (EUCAST) (403 to 2,556 MICs) for C. albicans, C. glabrata, C. krusei, and C. tropicalis. Although many factors (caspofungin powder source, stock solution solvent, powder storage time length and temperature, and MIC determination testing parameters) were examined as a potential cause of such unprecedented variability, a single specific cause was not identified. Therefore, it seems highly likely that the use of the CLSI species-specific caspofungin CBPs could lead to reporting an excessive number of wild-type (WT) isolates (e.g., C. glabrata and C. krusei) as either non-WT or resistant isolates. Until this problem is resolved, routine testing or reporting of CLSI caspofungin MICs for Candida is not recommended; micafungin or anidulafungin data could be used instead.

Caspofungin MICs correlate with treatment outcomes among patients with Candida glabrata invasive candidiasis and prior echinocandin exposure

Mutations in Candida glabrata FKS genes, which encode the echinocandin target enzyme, are independent risk factors for treatment failures during invasive candidiasis. We retrospectively compared the ability of caspofungin susceptibility testing methods to identify C. glabrata FKS mutant isolates and predict outcomes among patients at our center. Eight percent (10/120) of sterile-site C. glabrata isolates harbored FKS1 (n = 3) or FKS2 (n = 7) mutations, including 32% (10/32) recovered from patients with prior echinocandin exposure. Median echinocandin exposures for mutant and nonmutant isolates were 55 (range, 7 to 188) and 13 (3 to 84) days, respectively (P = 0.004). Sensitivity and specificity of the CLSI caspofungin resistance breakpoint MIC (>0.12 μg/ml by broth microdilution using RPMI medium [BMD-RPMI]) were 90% (9/10) and 3% (3/110), respectively, for identifying FKS mutants. Sensitivity and specificity of receiver-operator characteristic (ROC) curve-derived breakpoints by BMD-RPMI, BMD-antibiotic medium 3, Etest, and YeastOne ranged from 70 to 100% and 89 to 95%, respectively; susceptibility rates varied from 83 to 90%. The 14-day echinocandin treatment success rate was 67% (44/66); failure was more likely with prior echinocandin exposure (P = 0.002) or infection with an FKS mutant (P = 0.0001) or echinocandin-resistant isolates by BMD-AM3, Etest, and YeastOne (P ≤ 0.03). The failure rate among patients with prior exposure and infection with a resistant isolate was 91% (10/11); it was 22% (12/55) among others (P < 0.0001). In conclusion, ROC-derived caspofungin MIC breakpoints by several methods were sensitive and specific for identifying C. glabrata FKS mutant isolates. Mutations were seen exclusively among patients with prior echinocandin exposure. A paradigm that considers prior echinocandin exposure and caspofungin MICs accurately classified treatment outcomes for C. glabrata invasive candidiasis.

Stepwise development of a homozygous S80P substitution in Fks1p, conferring echinocandin resistance in Candida tropicalis

Three Candida tropicalis isolates were obtained from a patient with acute lymphoblastic leukemia. The first isolate was susceptible to all drug classes, while isolates 2 and 3, obtained after 8 and 8.5 weeks of caspofungin treatment, respectively, were resistant to the three echinocandins. Multilocus sequence genotyping suggested a clonal relation among all isolates. FKS1 sequencing revealed a stepwise development of a heterozygous and finally a homozygous mutation, leading to S80S/P and S80P amino acid substitutions.

Evaluation of caspofungin susceptibility testing by the new Vitek 2 AST-YS06 yeast card using a unique collection of FKS wild-type and hot spot mutant isolates, including the five most common candida species

FKS mutant isolates associated with breakthrough or failure cases are emerging in clinical settings. Discrimination of these from wild-type (wt) isolates in a routine laboratory setting is complicated. We evaluated the ability of caspofungin MIC determination using the new Vitek 2 AST-Y06 yeast susceptibility card to correctly identify the fks mutants from wt isolates and compared the performance to those of the CLSI and EUCAST reference methods. A collection of 98 Candida isolates, including 31 fks hot spot mutants, were included. Performance was evaluated using the FKS genotype as the "gold standard" and compared to those of the CLSI and EUCAST methodologies. The categorical agreement for Vitek 2 was 93.9%, compared to 88.4% for the CLSI method and 98.7% for the EUCAST method. Vitek 2 misclassified 19.4% (6/31) of the fks mutant isolates as susceptible, in contrast to <4% for each of the reference methods. The overall essential agreement between the CLSI method and Vitek 2 MICs was 92.6% (88/95) but was substantially lower for fks mutant isolates (78.6% [22/28]). Correct discrimination between susceptible and intermediate Candida glabrata isolates was not possible, as the revised species-specific susceptibility breakpoint was not included in the Vitek 2 detection range (MIC of ≤0.250 to ≥4 mg/liter). In conclusion, the Vitek 2 allowed correct categorization of all wt isolates as susceptible. However, despite an acceptable categorical agreement, it failed to reliably classify isolates harboring fks hot spot mutations as intermediate or resistant, which was in part due to the fact that the detection range did not span the susceptibility breakpoint for C. glabrata.

Effect of 50% human serum on the killing activity of micafungin against eight Candida species using time-kill methodology

Micafungin activity was determined against 24 wild-type clinical isolates and 5 American Type Culture Collection strains belonging to 8 Candida species in RPMI-1640 with and without 50% serum using broth microdilution and time-kill methodology. MIC values increased from 4- to 128-folds in 50% serum for all Candida species. Micafungin was not fungicidal against C. albicans, C. tropicalis, and against 2 of 3 C. metapsilosis at ≥0.25, 1, and 1 μg/mL, respectively, after 48 h with 50% serum, showing good fungistatic activity. Fungicidal activity at ≥2, 4, and 32 μg/mL was noticed against C. glabrata, C. inconspicua, and C. krusei isolates, respectively. Micafungin at 8-32 μg/mL showed fungistatic activity against C. parapsilosis and C. orthopsilosis. Serum decreased the in vitro activity of micafungin. With serum binding of echinocandins taken into account, safely fungistatic or fungicidal concentrations seem to require elevated doses against some Candida species, including C. parapsilosis, C. orthopsilosis, and C. krusei.

EUCAST technical note on the EUCAST definitive document EDef 7.2: method for the determination of broth dilution minimum inhibitory concentrations of antifungal agents for yeasts EDef 7.2 (EUCAST-AFST)

The European Committee on Antimicrobial Susceptibility Testing-Subcommittee on Antifungal Susceptibility Testing (EUCAST-AFST) has revised the EDef 7.1 document on the method for the determination of broth dilution minimum inhibitory concentrations of antifungal agents for fermentative yeasts. Changes are: dimethylsulphoxide is now the recommended solvent for caspofungin, micafungin and fluconazole; the shelf-life of plates containing the echinocandins prepared from stock solutions in dimethylsulphoxide is extended to 6 months at -80°C; testing of amphotericin and Cryptococcus has been incorporated; and minimum inhibitory concentration ranges for quality control strains and anidulafungin are included.

Caspofungin Etest susceptibility testing of Candida species: risk of misclassification of susceptible isolates of C. glabrata and C. krusei when adopting the revised CLSI caspofungin breakpoints

The purpose of this study was to evaluate the performance of caspofungin Etest and the recently revised CLSI breakpoints. A total of 497 blood isolates, of which 496 were wild-type isolates, were included. A total of 65/496 susceptible isolates (13.1%) were misclassified as intermediate (I) or resistant (R). Such misclassifications were most commonly observed for Candida krusei (73.1%) and Candida glabrata (33.1%). The revised breakpoints cannot be safely adopted for these two species.

Comparison of dimethyl sulfoxide and water as solvents for echinocandin susceptibility testing by the EUCAST methodology

Ninety-six strains of Candida, including 29 resistant and 67 susceptible isolates with mutations in the FKS1 and FKS2 genes were tested by the European Committee on Antibiotic Susceptibility Testing EDef 7.1 and 7.2 methodologies to determine the impact on the MIC when water was replaced with dimethyl sulfoxide (DMSO) as the solvent for caspofungin and micafungin. The MICs were significantly lower and the MIC ranges were narrower when DMSO was used as the solvent. The use of DMSO may help to better discriminate between susceptible and resistant populations.

Differential in vivo activities of anidulafungin, caspofungin, and micafungin against Candida glabrata isolates with and without FKS resistance mutations

We recently observed that the micafungin MICs for some Candida glabrata fks hot spot mutant isolates are less elevated than those for the other echinocandins, suggesting that the efficacy of micafungin may be differentially dependent on such mutations. Three clinical C. glabrata isolates with or without (S3) fks hot spot mutations R83 (Fks2p-S663F) and RR24 (Fks1p-S629P) and low, medium, and high echinocandin MICs, respectively, were evaluated to assess the in vivo efficacy in an immunocompetent mouse model using three doses of each echinocandin. Drug concentrations were determined in plasma and kidneys by high-performance liquid chromatography (HPLC). A pharmacokinetic-pharmacodynamic mathematical model was used to define the area under the concentration-time curve (AUC) that produced half- and near-maximal activity. Micafungin was equally efficacious against the S3 and R83 isolates. The estimates for the AUCs of each echinocandin that induced half-maximal effect (E(50)s) were 194.2 and 53.99 mg · h/liter, respectively. In contrast, the maximum effect (E(max)) for caspofungin was higher against S3 than R83, but the estimates for E(50) were similar (187.1 and 203.5 mg · h/liter, respectively). Anidulafungin failed to induce a ≥1-log reduction for any of the isolates (AUC range, 139 to 557 mg · h/liter). None of the echinocandins were efficacious in mice challenged with the RR24 isolate despite lower virulence (reduced maximal growth, prolonged lag phase, and lower kidney burden). The AUC associated with half-maximal effect was higher than the average human exposure for all drug-dose-bug combinations except micafungin and the R83 isolate. In conclusion, differences in micafungin MICs are associated with differential antifungal activities in the animal model. This study may have implications for clinical practice and echinocandin breakpoint determination, and further studies are warranted.

Comparative effects of micafungin, caspofungin, and anidulafungin against a difficult-to-treat fungal opportunistic pathogen, Candida glabrata

The aim of this study was to compare the in vitro and in vivo activities of micafungin, caspofungin, and anidulafungin against Candida glabrata. The MICs against 28 clinical isolates showed that the overall susceptibilities to caspofungin and to micafungin were not statistically different in the absence of human serum, whereas the isolates were less susceptible to micafungin than to caspofungin in its presence. Minimum fungicidal concentrations, as well as time-kill experiments, showed that caspofungin was more active than anidulafungin, while micafungin was superior to either caspofungin or anidulafungin without serum; its addition rendered caspofungin and micafungin equally effective. A murine model of systemic candidiasis against a C. glabrata-susceptible isolate was performed to study the effects of all three echinocandins, and kidney burden counts showed that caspofungin, micafungin, and anidulafungin were active starting from 0.25, 1, and 5 mg/kg of body weight/day, respectively. Two echinocandin-resistant strains of C. glabrata were selected: C. glabrata 30, a laboratory strain harboring the mutation Fks2p-P667T, and C. glabrata 51, a clinical isolate harboring the mutation Fks2p-D666G. Micafungin activity was shown to be as effective as or more effective than that of caspofungin or anidulafungin in terms of MICs. In vivo studies against these resistant strains showed that micafungin was active starting from 1 mg/kg/day, while caspofungin was effective only when administrated at higher doses of 5 or 10 mg/kg/day. Although a trend toward colony reduction was observed with the highest doses of anidulafungin, a significant statistical difference was never reached.

Antifungal susceptibility profiles of Candida isolates from a prospective survey of invasive fungal infections in Italian intensive care units

The antifungal susceptibility pattern of 302 Candida isolates collected during an Italian survey on invasive fungal infections in an intensive care setting was investigated. The results were correlated with some epidemiological data and compared with the antifungal profiles obtained in a previous survey. No resistance to echinocandins was detected. The overall resistance levels to fluconazole, posaconazole and voriconazole were 12.6, 6.0 and 7.1 %, respectively. Candida tropicalis and Candida parapsilosis accounted for more than half of all the fluconazole resistant isolates. Reduced susceptibility to fluconazole is not uncommon among isolates (12.3 %) and appears to be increasing, particularly among C. parapsilosis isolates, which showed an increase in resistant isolates from 2 % in the 1990s to 25.8 % in the present study. Routine antifungal susceptibility testing of this species is therefore recommended.

Current perspectives on echinocandin class drugs

It has been nearly a decade since caspofungin was approved for clinical use as the first echinocandin class antifungal agent, followed by micafungin and anidulafungin. The echinocandin drugs target the fungal cell wall by inhibiting the synthesis of β-1,3-D-glucan, a critical cell wall component of many pathogenic fungi. They are fungicidal for Candida spp. and fungistatic for moulds, such as Aspergillus fumigatus, where they induce abnormal morphology and growth properties. The echinocandins have a limited antifungal spectrum but are highly active against most Candida spp., including azole-resistant strains and biofilms. As they target glucan synthase, an enzyme absent in mammalian cells, the echinocandins have a favorable safety profile. They show potent MIC and epidemiological cutoff values against susceptible Candida and Aspergillus isolates, and the frequency of resistance is low. When clinical breakthrough occurs, it is associated with high MIC values and mutations in Fks subunits of glucan synthase, which can reduce the sensitivity of the enzyme to the drug by several thousand-fold. Such strains were not adequately captured by an early clinical breakpoint for susceptibility prompting a revised lower value, which addresses the FKS resistance mechanism and new pharmacokinetic/pharmacodynamic studies. Elevated MIC values unlinked to therapeutic failure can occur and result from adaptive cell behavior, which is FKS-independent and involves the molecular chaperone Hsp90 and the calcineurin pathway. Mutations in FKS1 and/or FKS2 alter the kinetic properties of glucan synthase, which reduces the relative fitness of mutant strains causing them to be less pathogenic. The echinocandin drugs also modify the cell wall architecture exposing buried glucans, which in turn induce a variety of important host immune responses. Finally, the future for glucan synthase inhibitors looks bright with the development of new orally active compounds.