The trailing end point phenotype in antifungal susceptibility testing is pH dependent

The impact of the fluconazole trailing effect on the persistence of Candida albicans bloodstream infection when treated with fluconazole

OBJECTIVES

The trailing effect of Candida species is a phenomenon characterized by reduced but persistent growth at antifungal concentrations above the MIC. We assessed the impact of trailing growth on the persistence of Candida albicans candidemia in patients receiving fluconazole (FLC) therapy.

METHODS

We retrospectively investigated candidemia isolates at three hospitals in southern Taiwan between 2013 and 2020. Patients treated with FLC for FLC-susceptible C. albicans candidemia were enrolled. The degree of trailing was determined as the average growth above the MIC divided by the measured growth at the lowest drug concentration using the EUCAST method and classified into four categories: residual (0.1-5%), slight (6-10%), moderate (11-15%), and heavy trailers (>15%).

RESULTS

Among isolates from 190 patients, the proportions of heavy trailers at 24 hours, 48 hours, and 72 hours were 63.7% (121/190), 63.2% (120/190), and 74.7% (142/190), respectively. Persistent candidemia was observed in 17 (8.9 %) patients. The proportion of persistent C. albicans candidemia in heavy trailing isolates at 48 hours was higher than in isolates without heavy trailing (13.3% [16/120] vs. 1.4% [1/70], p = 0.007). A multivariate analysis showed that immunosuppression (OR = 7.92; 95% CI: 2.38-26.39, p = 0.001), hospitalization days after the index date of C. albicans identification (OR = 1.03; 95% CI: 1.01-1.05, p = 0.011), and heavy trailing isolates at 48 hours (OR = 10.04; 95% CI: 1.27-79.88, p = 0.029) were independent factors for persistent candidemia.

DISCUSSION

The current study revealed that heavy trailing in C. albicans isolates is associated with persistent candidemia in patients receiving FLC treatment.

Novel antifungals and treatment approaches to tackle resistance and improve outcomes of invasive fungal disease

SUMMARYFungal infections are on the rise, driven by a growing population at risk and climate change. Currently available antifungals include only five classes, and their utility and efficacy in antifungal treatment are limited by one or more of innate or acquired resistance in some fungi, poor penetration into "sequestered" sites, and agent-specific side effect which require frequent patient reassessment and monitoring. Agents with novel mechanisms, favorable pharmacokinetic (PK) profiles including good oral bioavailability, and fungicidal mechanism(s) are urgently needed. Here, we provide a comprehensive review of novel antifungal agents, with both improved known mechanisms of actions and new antifungal classes, currently in clinical development for treating invasive yeast, mold (filamentous fungi), Pneumocystis jirovecii infections, and dimorphic fungi (endemic mycoses). We further focus on inhaled antifungals and the role of immunotherapy in tackling fungal infections, and the specific PK/pharmacodynamic profiles, tissue distributions as well as drug-drug interactions of novel antifungals. Finally, we review antifungal resistance mechanisms, the role of use of antifungal pesticides in agriculture as drivers of drug resistance, and detail detection methods for antifungal resistance.

Approaches for identifying and measuring heteroresistance in azole-susceptible Candida isolates

Heteroresistance to antifungal agents poses a significant challenge in the treatment of fungal infections. Currently, the absence of established methods for detecting and measuring heteroresistance impedes progress in understanding this phenomenon in fungal pathogens. In response to this gap, we present a comprehensive set of new and optimized methods designed to detect and quantify azole heteroresistance in Candida albicans. Here, we define two primary assays for measuring heteroresistance: population analysis profiling, based on growth on solid medium, and single-cell assays, based on growth in liquid culture. We observe good correlations between the measurements obtained with liquid and solid assays, validating their utility for studying azole heteroresistance. We also highlight that disk diffusion assays could serve as an additional tool for the rapid detection of heteroresistance. These methods collectively provide a versatile toolkit for researchers seeking to assess heteroresistance in C. albicans. They also serve as a critical step forward in the characterization of antifungal heteroresistance, providing a framework for investigating this phenomenon in diverse fungal species and in the context of other antifungal agents. Ultimately, these advancements will enhance our ability to effectively measure antifungal drug responses and combat fungal infections.IMPORTANCEHeteroresistance involves varying antimicrobial susceptibility within a clonal population. This phenomenon allows the survival of rare resistant subpopulations during drug treatment, significantly complicating the effective management of infections. However, the absence of established detection methods hampers progress in understanding this phenomenon in human fungal pathogens. We propose a comprehensive toolkit to address this gap in the yeast Candida albicans, encompassing population analysis profiling, single-cell assays, and disk diffusion assays. By providing robust and correlated measurements through both solid and liquid assays, this work will provide a framework for broader applications across clinically relevant Candida species. These methods will enhance our ability to understand this phenomenon and the failure of antifungal therapy.

Understanding fluconazole tolerance in Candida albicans: implications for effective treatment of candidiasis and combating invasive fungal infections

OBJECTIVES

Fluconazole (FLC) tolerant phenotypes in Candida species contribute to persistent candidemia and the emergence of FLC resistance. Therefore, making FLC fungicidal and eliminating FLC tolerance are important for treating invasive fungal diseases (IFDs) caused by Candida species. However, the mechanisms of FLC tolerance in Candida species remain to be fully explored.

METHODS

This review discusses the high incidence of FLC tolerance in Candida species and the importance of successfully clearing FLC tolerance in treating candidiasis. We further define and characterize FLC tolerance in C. albicans.

RESULTS

This review identifies global factors affecting FLC tolerance and suggest that FLC tolerance is a strategy of C. albicans response to FLC damage whose mechanism differs from FLC resistance.

CONCLUSIONS

This review highlights the significance of the cell membrane and cell wall integrity in FLC tolerance, guiding approaches to combat IFDs caused by Candida species..

Candida parapsilosis sensu stricto Antifungal Resistance Mechanisms and Associated Epidemiology

Fungal diseases cause millions of deaths per year worldwide. Antifungal resistance has become a matter of great concern in public health. In recent years rates of non-albicans species have risen dramatically. Candida parapsilosis is now reported to be the second most frequent species causing candidemia in several countries in Europe, Latin America, South Africa and Asia. Rates of acquired azole resistance are reaching a worrisome threshold from multiple reports as in vitro susceptibility testing is now starting also to explore tolerance and heteroresistance to antifungal compounds. With this review, the authors seek to evaluate known antifungal resistance mechanisms and their worldwide distribution in Candida species infections with a specific focus on C. parapsilosis.

Resistance to Fluconazole of Candida albicans in Vaginal Isolates: a 10-Year Study in a Clinical Referral Center

Review of vaginal isolates of Candida albicans that caused clinical failure in a 10-year collection of vaginal C. albicans specimens obtained in a university vaginitis referral clinic indicated an increase in fluconazole resistance. Factors contributing to azole resistance are discussed, including treatment choice associated with fluconazole-resistant C. albicans vaginal infection.

A longitudinal study on fluconazole resistance in Candida albicans vaginal isolates

BACKGROUND

Recurrent vulvovaginal candidiasis (RVVC), despite its worldwide prevalence, has limited treatment options; and a long-term prophylactic regimen utilising fluconazole is the dominant choice.

OBJECTIVES

An increase in fluconazole resistance is reported, and little information is available about the reversibility of resistance status following the withdrawal of fluconazole.

METHODS

Repeated antifungal susceptibility tests (ASTs) for fluconazole at a median interval of 3 months between them were evaluated in women with refractory or recurrent VVC patients at the Vaginitis clinic from 2012 to 2021 (10 years) and performed at pH 7 and pH 4.5 using the broth microdilution tests based on the CLSI M27-A4 reference method.

RESULTS

Of 38 patients with long-term follow-up with repeat ASTs, 13 patients (13/38, 34.2%) tested at pH 7.0 remained susceptible to fluconazole with MIC ≤2 μg/mL. Nineteen patients (19/38, 50%) remained resistant to fluconazole with MIC ≥8 μg/mL, whereas four (4/38, 10.5%) changed from susceptible to resistant and two (2/38, 5.2%) changed from resistant to susceptible over time. At pH 4.5, among the 37 patients with repeated MIC values, nine (9/37, 24.3%) remained susceptible to fluconazole and 22 (22/37, 59.5%) remained resistant. Three isolates (3/37, 8.1%) changed from susceptible to resistant, while 3 (3/37, 8.1%) changed from resistant to susceptible over time.

CONCLUSION

Fluconazole susceptibility in Candida albicans vaginal isolates obtained longitudinally in women with RVVC remains stable with rare reversal of resistance despite azole avoidance.

A Small Molecule Inhibitor of Erg251 Makes Fluconazole Fungicidal by Inhibiting the Synthesis of the 14α-Methylsterols

Fluconazole (FLC) is widely used to prevent and treat invasive fungal infections. However, FLC is a fungistatic agent, allowing clinical FLC-susceptible isolates to tolerate FLC. Making FLC fungicidal in combination with adjuvants is a promising strategy to avoid FLC resistance and eliminate the persistence and recurrence of fungal infections. Here, we identify a new small molecule compound, CZ66, that can make FLC fungicidal. The mechanism of action of CZ66 is targeting the C-4 sterol methyl oxidase, encoded by the ERG251 gene, resulting in decreased content of sterols with the 14α-methyl group and ultimately eliminating FLC tolerance of Candida albicans. CZ66 most likely interacts with Erg251 through residues Glu195, Gly206, and Arg241. Establishing Erg251 as a synergistic lethal target protein of FLC should direct research to identify specific small molecule inhibitors of 14α-methylsterol synthesis and open the way to abolishing fungal FLC tolerance. IMPORTANCE Fluconazole (FLC) tolerance increases the frequency of acquired FLC resistance, and a high FLC tolerance level is associated with persistent candidemia. Multiple functional proteins, such as calcineurin, heat shock protein 90 (Hsp90), and ADP ribosylation factor, are essential for the survival of C. albicans exposed to FLC, but how these factors increase the fungicidal activity of FLC remains to be determined. In this study, we found that 14α-methylsterols replace ergosterol to allow C. albicans to survive FLC, but Erg251 inactivated by CZ66 results in loss of 14α-methylsterol synthesis and cell death of C. albicans treated with FLC. Establishing Erg251 as a synergistic lethal target protein of FLC should direct research to identify specific small molecule inhibitors of 14α-methylsterol synthesis and open the way to abolishing fungal FLC tolerance.

A computer vision chemometric-assisted approach to access pH and glucose influence on susceptibility of Candida pathogenic strains

Microorganisms adapt to environmental conditions as a survival strategy for different interactions with the environment. The adaptive capacity of fungi allows them to cause disease at various sites of infection in humans. In this study, we propose digital images as responses of a complete factorial 2³. Furthermore, we compared two experimental approaches: the experimental design (3D) and the checkerboard assay (2D) to know the influence of pH, glucose, and fluconazole concentration on different strains of the genus Candida. The digital images obtained from the factorial 2³ were used as input in the PCA-ANOVA to analyze the results of this experimental design. pH modification in the culture medium modifies the susceptibility in some species less adapted to this type of modification. For the first time, to the best of our knowledge, digital images were used as input to PCA-ANOVA to obtain information on Candida spp.. Therefore, a higher concentration of antifungals is needed to inhibit the same strain at a lower pH. In short, we present an alternative with less use of reagents and time. In addition, the use of digital images allows obtaining information about fungal susceptibility with three or more factors.

Determining Susceptibility in Candida Vaginal Isolates

Antifungal drug susceptibility tests (AST) for Candida albicans are increasingly demanded for women with refractory or recurrent Candida vaginitis due to fluconazole resistance. Given reduced activity of azole drugs at pH levels found in women with Candida vaginitis, it is proposed that AST be performed at pH 4.5, since testing at only the recommended pH 7.0 is likely to miss a significant number of clinically relevant azole-resistant C. albicans vaginal isolates.

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.

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

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

Candida albicans targets that potentially synergize with fluconazole

Fluconazole has characteristics that make it widely used in the clinical treatment of C. albicans infections. However, fluconazole has only a fungistatic activity in C. albicans, therefore, in the long-term treatment of C. albicans infection with fluconazole, C. albicans has the potential to acquire fluconazole resistance. A promising approach to increase fluconazole's efficacy is identifying potential targets of drugs that can enhance the antifungal effect of fluconazole, or even make the drug fungicidal. In this review, we systematically provide a global overview of potential targets of drugs synergistic with fluconazole in C. albicans, identify new avenues for research on fluconazole potentiation, and highlight the promise of combinatorial strategies with fluconazole in combatting C. albicans infections.

Identification and Characterization of Mediators of Fluconazole Tolerance in Candida albicans

Candida albicans is an important human pathogen and a major concern in intensive care units around the world. C. albicans infections are associated with a high mortality despite the use of antifungal treatments. One of the causes of therapeutic failures is the acquisition of antifungal resistance by mutations in the C. albicans genome. Fluconazole (FLC) is one of the most widely used antifungal and mechanisms of FLC resistance occurring by mutations have been extensively investigated. However, some clinical isolates are known to be able to survive at high FLC concentrations without acquiring resistance mutations, a phenotype known as tolerance. Mechanisms behind FLC tolerance are not well studied, mainly due to the lack of a proper way to identify and quantify tolerance in clinical isolates. We proposed here culture conditions to investigate FLC tolerance as well as an easy and efficient method to identity and quantify tolerance to FLC. The screening of C. albicans strain collections revealed that FLC tolerance is pH- and strain-dependent, suggesting the involvement of multiple mechanisms. Here, we addressed the identification of FLC tolerance mediators in C. albicans by an overexpression strategy focusing on 572 C. albicans genes. This strategy led to the identification of two transcription factors, CRZ1 and GZF3. CRZ1 is a C2H2-type transcription factor that is part of the calcineurin-dependent pathway in C. albicans, while GZF3 is a GATA-type transcription factor of unknown function in C. albicans. Overexpression of each gene resulted in an increase of FLC tolerance, however, only the deletion of CRZ1 in clinical FLC-tolerant strains consistently decreased their FLC tolerance. Transcription profiling of clinical isolates with variable levels of FLC tolerance confirmed a calcineurin-dependent signature in these isolates when exposed to FLC.

Impact of tolerance to fluconazole on treatment response in Candida albicans bloodstream infection

BACKGROUND

Treatment of Candida albicans bloodstream infection with fluconazole is associated with significant mortality despite in vitro susceptibility to the drug.

OBJECTIVES

We sought to determine whether tolerance to fluconazole is predictive of treatment failure.

METHODS

We reviewed patients with monomicrobial C albicans bloodstream infection who received primary monotherapy with fluconazole. Tolerance to fluconazole, defined as the fraction of growth above the MIC, was quantified using the disc diffusion assay and digital image analyses. Survival analyses were performed with host and treatment factors as predictive variables.

RESULTS

Among 44 patients included in the study, all-cause mortality was 29.5% at 30 days and 43.1% at 12 weeks. Forty-one isolates (93%) were susceptible to fluconazole (MIC50, 0.5 mg/L). Fluconazole tolerance was strongly associated with death for patients treated with fluconazole within 24 h of candidemia onset (33.3% vs 0%; p = .007), but not among patients whose treatment was started later. MIC did not correlate with survival, regardless of treatment delay. A Cox regression model including time to treatment, tolerance to fluconazole, fluconazole exposure and Pitt bacteraemia score provided good prediction of treatment outcome (area under the receiver-operator curve, 0.82).

CONCLUSIONS

In patients with C albicans bloodstream infection, tolerance testing was predictive of fluconazole efficacy if the drug was started early. Further study is required to validate the utility of this metric to guide treatment choices.

The prevalence and clinical significance of microcolonies when tested according to contemporary interpretive breakpoints for fluconazole against Candida species using E-test

Changes in the interpretive-breakpoints for antifungals against various Candida species have raised the need to examine the significance of the phenomenon of the growth of microcolonies in agar diffusion inhibition zones, which has generally been considered negligible. The objective was to determine the incidence of cases in which microcolonies demonstrate fluconazole resistance according to current interpretive-breakpoints and whether their growth is associated with therapeutic failure. The fluconazole minimum inhibitory concentrations (MICs) of 100 blood culture isolates of Candida were performed by E-test on Roswell Park Memorial Institute (RPMI) agar and examined for the appearance of microcolonies. Fluconazole MICs of microcolonies were then determined over three generations. The significance of the phenomenon of microcolonies was determined according to clinical data retrieved from electronic files. Microcolonies were a common phenomenon among Candida isolates following incubation on RPMI agar, with a higher frequency among C. albicans isolates as compared to non-albicans Candida across generations (57-93% vs 31-93%, respectively) and a similar fluconazole susceptibility rate over three generations. The rate of microcolonies was similar in both patients with successful and unsuccessful outcome (41% vs 42%, respectively). Microcolonies are a common phenomenon. No increase in MIC was demonstrated throughout three generations of microcolony inoculation on RPMI, and no difference in clinical outcome was observed.

Adapting to survive: How Candida overcomes host-imposed constraints during human colonization

Successful human colonizers such as Candida pathogens have evolved distinct strategies to survive and proliferate within the human host. These include sophisticated mechanisms to evade immune surveillance and adapt to constantly changing host microenvironments where nutrient limitation, pH fluctuations, oxygen deprivation, changes in temperature, or exposure to oxidative, nitrosative, and cationic stresses may occur. Here, we review the current knowledge and recent findings highlighting the remarkable ability of medically important Candida species to overcome a broad range of host-imposed constraints and how this directly affects their physiology and pathogenicity. We also consider the impact of these adaptation mechanisms on immune recognition, biofilm formation, and antifungal drug resistance, as these pathogens often exploit specific host constraints to establish a successful infection. Recent studies of adaptive responses to physiological niches have improved our understanding of the mechanisms established by fungal pathogens to evade the immune system and colonize the host, which may facilitate the design of innovative diagnostic tests and therapeutic approaches for Candida infections.

Antifungal Susceptibility Testing: Current Approaches

Although not as ubiquitous as antibacterial susceptibility testing, antifungal susceptibility testing (AFST) is a tool of increasing importance in clinical microbiology laboratories. The goal of AFST is to reliably produce MIC values that may be used to guide patient therapy, inform epidemiological studies, and track rates of antifungal drug resistance. There are three methods that have been standardized by standards development organizations: broth dilution, disk diffusion, and azole agar screening for Aspergillus Other commonly used methods include gradient diffusion and the use of rapid automated instruments. Novel methodologies for susceptibility testing are in development. It is important for laboratories to consider not only the method of testing but also the interpretation (or lack thereof) of in vitro data.

Drug resistance and tolerance in fungi

Systemic fungal infections pose a serious clinical problem. Treatment options are limited, and antifungal drug resistance is increasing. In addition, a substantial proportion of patients do not respond to therapy despite being infected with fungi that are susceptible to the drug. The discordance between overall treatment outcome and low levels of clinical resistance may be attributable to antifungal drug tolerance. In this Review, we define and distinguish resistance and tolerance and discuss the current understanding of the molecular, genetic and physiological mechanisms that contribute to those phenomena. Distinguishing tolerance from resistance might provide important insights into the reasons for treatment failure in some settings.

New Antifungal Susceptibility Test Based on Chitin Detection by Image Cytometry

The antifungal susceptibility tests used in clinical laboratories have several limitations. We developed a new test, SensiFONG, based on the detection of chitin levels after exposure to antifungal drugs. The optimal culture conditions were 30°C for 6 h for yeast strains and 26°C for 16 h for molds. The strains were exposed to a range of echinocandin or azole concentrations. Chitin was stained with calcofluor white. The percentage of fungal cells with high chitin levels was determined with an automatic epifluorescence microscope. The SensiFONG results were compared to those with the EUCAST method. Image acquisition and analysis were performed with ScanR software. Fifty-nine strains (28 Candida albicans, 17 Candida glabrata, and 14 Aspergillus fumigatus) were analyzed. Thresholds for the classification of strains as resistant or susceptible were determined for each fungal species. The strains displaying an increase in chitin content of ≥32% for C. albicans, ≥6% for C. glabrata, and ≥17% for A. fumigatus were considered susceptible. The application of these thresholds to all 59 strains resulted in a sensitivity of 0.87, 0.93, and 1.00 and a specificity of 0.93, 0.84, and 0.82 for C. albicans, C. glabrata, and A. fumigatus, respectively. The correlation between the results obtained in the SensiFONG and EUCAST assays was excellent. We developed a new test, SensiFONG, based on a new concept. While current assays assess growth inhibition, our test detects changes in chitin levels after exposure to antifungal drugs. Here, we present preliminary results and we propose a proof of concept of this methodology.

Minimal Inhibitory Concentration (MIC)-Phenomena in Candida albicans and Their Impact on the Diagnosis of Antifungal Resistance

Antifungal susceptibility testing (AFST) of clinical isolates is a tool in routine diagnostics to facilitate decision making on optimal antifungal therapy. The minimal inhibitory concentration (MIC)-phenomena (trailing and paradoxical effects (PXE)) observed in AFST complicate the unambiguous and reproducible determination of MICs and the impact of these phenomena on in vivo outcome are not fully understood. We aimed to link the MIC-phenomena with in vivo treatment response using the alternative infection model Galleria mellonella. We found that Candida albicans strains exhibiting PXE for caspofungin (CAS) had variable treatment outcomes in the Galleria model. In contrast, C. albicans strains showing trailing for voriconazole failed to respond in vivo. Caspofungin- and voriconazole-susceptible C. albicans strains responded to the respective antifungal therapy in vivo. In conclusion, MIC data and subsequent susceptibility interpretation of strains exhibiting PXE and/or trailing should be carried out with caution, as both effects are linked to drug adaptation and treatment response is uncertain to predict.

Copper potentiates azole antifungal activity in a way that does not involve complex formation

To survive, fungal pathogens must acquire nutrient metals that are restricted by the host while also tolerating mechanisms of metal toxicity that are induced by the host. Given this dual vulnerability, we hypothesized that a pathogen's access to and control of essential yet potentially dangerous metal ions would affect fungal tolerance to antifungal drug stress. Here, we show that Candida albicans becomes sensitized to both Cu limitation and Cu elevation during exposure in liquid culture to the antifungal drug fluconazole, a widely prescribed antifungal agent. Spectroscopic data confirm that while fluconazole forms a complex with Cu(ii) in water, interactions of fluconazole with neither Cu(ii) nor Cu(i) are observed in the cell culture media used for the cellular assays. This result is further supported by growth assays in deletion strains that lack Cu import machinery. Overall, we establish that increases in Cu levels by as little as 40 nM over basal levels in the growth medium reduce tolerance of C. albicans to fluconazole in a way that does not require formation of a Cu-fluconazole complex. Rather, our data point to a more complex relationship between drug stress and Cu availability that gives rise to metal-mediated outcomes of drug treatment.

A Novel 1,3-Beta-d-Glucan Inhibitor, Ibrexafungerp (Formerly SCY-078), Shows Potent Activity in the Lower pH Environment of Vulvovaginitis

Ibrexafungerp (IBX) (formerly SCY-078) is a novel glucan synthase inhibitor whose oral availability is being evaluated for efficacy against vulvovaginal candidiasis (VVC). Bioavailability and in vitro activity are important efficacy indicators, but accepted susceptibility methods do not always accurately predict activity in an acidic environment, such as the vagina. Studies were 3-fold, as follows: (i) pharmacokinetic study following oral administration in a murine model; (ii) susceptibility testing of isolates from a phase 2 VVC clinical trial by CLSI M27-A4 methodology; and (iii) susceptibility testing of Candida albicans and Candida glabrata isolates obtained from this trial group in RPMI 1640 adjusted to 3 different pH values, 7.0, 5.72, and 4.5, compared to susceptibility testing for micafungin and fluconazole. IBX readily accumulated in vaginal tissues and secretions following oral administration. Potent in vitro activity was demonstrated against Candida strains obtained at baseline and end of study visits. Moreover, the geometric mean (GM) values for IBX at pH 4.5 were dramatically lower than those at pH 7.0 and 5.72. The MIC₉₀ values of micafungin remained the same regardless of pH value, while those of fluconazole tended to increase with lower pH values. IBX is able to reach target tissues following oral administration at pharmacologically meaningful levels. IBX demonstrated potent in vitro activity, with no development of resistance, following repeated exposure over the course of the clinical trial. Importantly, activity of IBX in an acidic medium suggests a therapeutic advantage of this novel antifungal in the treatment of vaginal Candida infections.

Loss of C-5 Sterol Desaturase Activity in Candida albicans: Azole Resistance or Merely Trailing Growth?

Increased expression of drug efflux pumps and changes in the target enzyme Erg11p are known to contribute to azole resistance in Candida albicans, one of the most prevalent fungal pathogens. Mutations that inactivate ERG3, which encodes sterol Δ^5,6-desaturase, also confer in vitro azole resistance. However, it is unclear whether the loss of Erg3p activity is sufficient to confer resistance within the mammalian host, and relatively few erg3 mutants have been reported among azole-resistant clinical isolates. Trailing growth (residual growth in the presence of the azoles) is a phenotype observed with many C. albicans isolates and, in its extreme form, can be mistaken for resistance. The purpose of this study was to determine whether the growth of Erg3p-deficient C. albicans mutants in the presence of the azoles possesses the characteristics of azole resistance or of an exaggerated form of trailing growth. Our results demonstrate that, similar to trailing isolates, the capacity of an erg3Δ/Δ mutant to endure the consequences of azole exposure is at least partly dependent on both temperature and pH. This contrasts with true azole resistance that results from enhanced drug efflux and/or changes in the target enzyme. The erg3Δ/Δ mutant and trailing isolates also appear to sustain significant membrane damage upon azole treatment, further distinguishing them from resistant isolates. However, the insensitivity of the erg3Δ/Δ mutant to azoles is unaffected by the calcineurin inhibitor cyclosporin A, distinguishing it from trailing isolates. In conclusion, the erg3 mutant phenotype is qualitatively and quantitatively distinct from both azole resistance and trailing growth.

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.

Antifungal tolerance is a subpopulation effect distinct from resistance and is associated with persistent candidemia

Tolerance to antifungal drug concentrations above the minimal inhibitory concentration (MIC) is rarely quantified, and current clinical recommendations suggest it should be ignored. Here, we quantify antifungal tolerance in Candida albicans isolates as the fraction of growth above the MIC, and find that it is distinct from susceptibility/resistance. Instead, tolerance is due to the slow growth of subpopulations of cells that overcome drug stress more efficiently than the rest of the population, and correlates inversely with intracellular drug accumulation. Many adjuvant drugs used in combination with fluconazole, a widely used fungistatic drug, reduce tolerance without affecting resistance. Accordingly, in an invertebrate infection model, adjuvant combination therapy is more effective than fluconazole in treating infections with highly tolerant isolates and does not affect infections with low tolerance isolates. Furthermore, isolates recovered from immunocompetent patients with persistent candidemia display higher tolerance than isolates readily cleared by fluconazole. Thus, tolerance correlates with, and may help predict, patient responses to fluconazole therapy.

The authors show that antifungal tolerance, defined as the fraction of growth of a fungal pathogen above the minimal inhibitory concentration, is due to the slow growth of subpopulations of cells that overcome drug stress, and that high tolerance is often associated with persistent infections.

The Rim Pathway Mediates Antifungal Tolerance in Candida albicans through Newly Identified Rim101 Transcriptional Targets, Including Hsp90 and Ipt1

Invasive candidiasis (IC) is a major cause of morbidity and mortality despite antifungal treatment. Azoles and echinocandins are used as first-line therapies for IC. However, their efficacy is limited by yeast tolerance and the emergence of acquired resistance. Tolerance is a reversible stage created due to the yeast's capacity to counter antifungal drug exposure, leading to persistent growth. For Candida albicans, multiple stress signaling pathways have been shown to contribute to this adaptation. Among them, the pH-responsive Rim pathway, through its transcription factor Rim101p, was shown to mediate azole and echinocandin tolerance. The Rim pathway is fungus specific, is conserved among the members of the fungal kingdom, and plays a key role in pathogenesis and virulence. The present study aimed at confirming the role of Rim101p and investigating the implication of the other Rim proteins in antifungal tolerance in C. albicans, as well as the mechanisms underlying it. Time-kill curve experiments and colony formation tests showed that genetic inhibition of all the Rim factors enhances echinocandin and azole antifungal activity. Through RNA sequencing analysis of a rim101^-/- mutant, a strain constitutively overexpressing RIM101, and control strains, we discovered novel Rim-dependent genes involved in tolerance, including HSP90, encoding a major molecular chaperone, and IPT1, involved in sphingolipid biosynthesis. Rim mutants were also hypersensitive to pharmacological inhibition of Hsp90. Taken together, these data suggest that Rim101 acts upstream of Hsp90 and that targeting the Rim pathway in combination with existing antifungal drugs may represent a promising antifungal strategy to indirectly but specifically target Hsp90 in yeasts.

In vitro activity of the novel echinocandin CD101 at pH 7 and 4 against Candida spp. isolates from patients with vulvovaginal candidiasis

Background: The novel echinocandin CD101 has stability properties amenable to topical formulation for use in the treatment of acute vulvovaginal candidiasis (VVC) and recurrent VVC (RVVC). CD101 has demonstrated potent antifungal activity at pH 7, but assessment of its activity at the physiological pH of the vaginal environment is needed.

Objectives: To evaluate the antifungal activity of CD101 against clinical VVC isolates of Candida spp., including azole-resistant strains, at pH 4.

Methods: MIC values of CD101 and comparators (fluconazole, itraconazole, micafungin, caspofungin and anidulafungin) were assessed via broth microdilution. MIC assays were conducted at pH 7 and 4 after 24 and 48 h against a 108 VVC isolate panel of Candida spp., including Candida albicans (n = 60), Candida glabrata (n = 21), Candida parapsilosis (n = 14) and Candida tropicalis (n = 13).

Results: Overall, MIC values of all drugs were slightly higher at pH 4 versus 7 and at 48 versus 24 h of incubation. CD101 MIC values typically exhibited ∼4-fold shifts at pH 4 and were not affected by azole susceptibility. C. parapsilosis susceptibility was the least affected at pH 4 and did not increase for most drugs.

Conclusions: CD101 had potent activity against all Candida isolates tested, including azole-resistant strains. Although there was some reduction in activity at pH 4 versus 7, the resulting MIC values were still well below the intravaginal CD101 drug concentrations anticipated to be present following topical administration. These results support continued development of topical CD101 for the treatment of VVC/RVVC.

pH-Dependant Antifungal Activity of Valproic Acid against the Human Fungal Pathogen Candida albicans

Current antifungal drugs suffer from limitations including toxicity, the emergence of resistance and decreased efficacy at low pH that are typical of human vaginal surfaces. Here, we have shown that the antipsychotic drug valproic acid (VPA) exhibited a strong antifungal activity against both sensitive and resistant Candida albicans in pH condition similar to that encountered in vagina. VPA exerted a strong anti-biofilm activity and attenuated damage of vaginal epithelial cells caused by C. albicans. We also showed that VPA synergizes with the allylamine antifungal, Terbinafine. We undertook a chemogenetic screen to delineate biological processes that underlies VPA-sensitivity in C. albicans and found that vacuole-related genes were required to tolerate VPA. Confocal fluorescence live-cell imaging revealed that VPA alters vacuole integrity and support a model where alteration of vacuoles contributes to the antifungal activity. Taken together, this study suggests that VPA could be used as an effective antifungal against vulvovaginal candidiasis.

[Candiduria! What now? : Therapy of urinary tract infections with Candida]

BACKGROUND

Yeasts are found in urine specimens relatively often, especially in the elderly and patients under treatment with broad spectrum antibiotics, i. e. especially in intensive care unit (ICU) patients. In some cases, the number of pathogens is very high, i. e. >10⁵/ml. The clinical relevance of detecting Candida in urine is difficult to assess. In the German S3 guidelines it is apodictically stated that an ascending infection of the urinary tract by yeasts does not occur but this may undoubtedly happen in certain instances in patients at risk, for example in the elderly, in diabetic persons and in the case of foreign bodies in the urinary tract. A hematogenous spread of yeasts can lead to pyelonephritis, which accompanies candiduria. In rare cases this can be induced by prostatitis and epididymitis. Therapy is indicated in all cases when a urological manipulation is planned, particularly those with injury to the mucosal barrier, in order to prevent an intraoperative spread of pathogens.

AIM

The antimicrobial agents suitable for therapy of candiduria are limited, namely flucytosine, amphotericin B, which is also used for irrigation and fluconazole.

MATERIAL AND METHODS

The in vitro effect of nitroxoline on 100 isolates of yeasts from urine was tested by an agar diffusion test.

RESULTS

Nitroxoline exerted a good activity against all yeast isolates.

DISCUSSION

The antibiotic nitroxoline has a good antifungal activity. It achieves high concentrations in urine and in addition, it is effective at low pH as well as against pathogens in biofilms, which most antimycotics cannot achieve. Hence, nitroxoline is suitable for termination of candiduria. Foreign bodies in the urinary tract, on which biofilms are formed, should be removed whenever possible.

The Etest Performed Directly on Blood Culture Bottles Is a Reliable Tool for Detection of Fluconazole-Resistant Candida albicans Isolates

We assessed the ability of the Etest performed directly on positive blood cultures (ET_DIR) to detect fluconazole susceptibility in 6 fluconazole-resistant and 12 fluconazole-susceptible Candida albicans isolates, according to CLSI M27-A3 and EUCAST EDef 7.2 procedures. Categorical agreement between ET_DIR and broth microdilution was 100% when the trays were incubated at 25°C and trailing effect was ruled out. ET_DIR is a reliable procedure when screening for the presence of fluconazole resistance in C. albicans.

Evaluation of Virulence Factors In vitro, Resistance to Osmotic Stress and Antifungal Susceptibility of Candida tropicalis Isolated from the Coastal Environment of Northeast Brazil

Several studies have been developed regarding human health risks associated with the recreational use of beaches contaminated with domestic sewage. These wastes contain various micro-organisms, including Candida tropicalis. In this context, the objective of this study was to characterize C. tropicalis isolates from the sandy beach of Ponta Negra, Natal, Rio Grande do Norte, Brazil, regarding the expression of in vitro virulence factors, adaptation to osmotic stress and susceptibility to antifungal drugs. We analyzed 62 environmental isolates and observed a great variation among them for the various virulence factors evaluated. In general, environmental isolates were more adherent to human buccal epithelial cells (HBEC) than C. tropicalis ATCC13803 reference strain, and they also showed increased biofilm production. Most of the isolates presented wrinkled phenotypes on Spider medium (34 isolates, 54.8%). The majority of the isolates also showed higher proteinase production than control strains, but low phospholipase activity. In addition, 35 isolates (56.4%) had high hemolytic activity (hemolysis index > 0.55). With regard to C. tropicalis resistance to osmotic stress, 85.4% of the isolates were able to grow in a liquid medium containing 15% sodium chloride. The strains were highly resistant to the azoles tested (fluconazole, voriconazole and itraconazole). Fifteen strains were resistant to the three azoles tested (24.2%). Some strains were also resistant to amphotericin B (14 isolates; 22.6%), while all of them were susceptible for the echinocandins tested, except for a single strain of intermediate susceptibility to micafungin. Our results demonstrate that C. tropicalis isolated from the sand can fully express virulence attributes and showed a high persistence capacity on the coastal environment; in addition of showing high minimal inhibitory concentrations to several antifungal drugs used in current clinical practice, demonstrating that environmental isolates may have pathogenic potential.

Evaluation of the possible influence of trailing and paradoxical effects on the clinical outcome of patients with candidemia

OBJECTIVE

Paradoxical growth (PG) and trailing effect (TE) are frequently observed during antifungal susceptibility testing (AFST). These two phenomena interfere with the determination of the minimal inhibitory concentration (MIC). The aim of this study was to assess the clinical impact of TE and PG.

METHODS

We analysed the frequency of TE and PG of 690 Candida isolates collected from a population-based study performed in Spain (CANDIPOP) and correlated the results with clinical outcome of the patients.

RESULTS

Around 70% (484/690) of the isolates exhibited TE to azoles. Candida tropicalis showed the highest presence of TE (39/53 isolates exhibited residual growth >25% of control). No TE was seen in most of the isolates from the psilosis complex. PG was mainly associated with echinocandins. In patients treated with fluconazole within the first 48 hours after blood sampling (n = 221), the presence of TE to azoles tended to be associated with lower 30-day mortality (odds ratio (OR) 0.55, 95% confidence interval (CI) 0.25-1.00) but not with clinical failure (OR 0.85, 95% CI 0.45-1.54). In the subgroup of 117 patients treated with echinocandins, the presence of PG was not associated with patient's response to antifungal treatment (OR for 30-day mortality 1.63, 95% CI 0.76-4.03; OR for clinical failure 1.17, 95% CI 0.53-2.70).

CONCLUSIONS

TE or PG are widely expressed among Candida spp., although they do not seem to influence clinical outcome.

The RTA3 Gene, Encoding a Putative Lipid Translocase, Influences the Susceptibility of Candida albicans to Fluconazole

The RTA3 gene, coding for a member of the Rta1p-like lipid-translocating exporter family, is coordinately upregulated with the ATP-binding cassette transporter genes CDR1 and CDR2 in azole-resistant clinical isolates of Candida albicans that carry activating mutations in the transcription factor Tac1p. We show here that deleting RTA3 in an azole-resistant clinical isolate carrying a Tac1p-activating mutation lowered fluconazole resistance by 2-fold, while overexpressing RTA3 in an azole-susceptible clinical isolate resulted in enhanced fluconazole tolerance associated with trailing growth in a liquid microtiter plate assay. We also demonstrate that an Rta3p-green fluorescent protein (GFP) fusion protein localizes predominantly to the plasma membrane, consistent with a putative function for Rta3p as a lipid translocase.

A Combination Fluorescence Assay Demonstrates Increased Efflux Pump Activity as a Resistance Mechanism in Azole-Resistant Vaginal Candida albicans Isolates

Candida albicans is a pathogenic fungus causing vulvovaginal candidiasis (VVC). Azole drugs, such as fluconazole, are the most common treatment for these infections. Recently, azole-resistant vaginal C. albicans isolates have been detected in patients with recurring and refractory vaginal infections. However, the mechanisms of resistance in vaginal C. albicans isolates have not been studied in detail. In oral and systemic resistant isolates, overexpression of the ABC transporters Cdr1p and Cdr2p and the major facilitator transporter Mdr1p is associated with resistance. Sixteen fluconazole-susceptible and 22 fluconazole-resistant vaginal C. albicans isolates were obtained, including six matched sets containing a susceptible and a resistant isolate, from individual patients. Using quantitative real-time reverse transcriptase PCR (qRT-PCR), 16 of 22 resistant isolates showed overexpression of at least one efflux pump gene, while only 1 of 16 susceptible isolates showed such overexpression. To evaluate the pump activity associated with overexpression, an assay that combined data from two separate fluorescent assays using rhodamine 6G and alanine β-naphthylamide was developed. The qRT-PCR results and activity assay results were in good agreement. This combination of two fluorescent assays can be used to study efflux pumps as resistance mechanisms in clinical isolates. These results demonstrate that efflux pumps are a significant resistance mechanism in vaginal C. albicans isolates.

In Vitro Susceptibility and Trailing Growth Effect of Clinical Isolates of Candida Species to Azole Drugs

Background:

Emergence of resistance to respective antifungal drugs is a primary concern for the treatment of candidiasis. Hence, determining antifungal susceptibility of the isolated yeasts is of special importance for effective therapy. For this purpose, the clinical laboratory standard institute (CLSI) has introduced a broth microdilution method to determine minimum inhibitory concentration (MIC). However, the so-called “Trailing effect” phenomenon might sometimes pose ambiguity in the interpretation of the results.

Objectives:

The present study aimed to determine the in vitro susceptibility of clinical isolates of Candida against azoles and the frequency of the Trailing effect.

Materials and Methods:

A total of 193 Candida isolates were prospectively collected and identified through the polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Using a broth microdilution test, according to the guidelines of CLSI M27-A3, antifungal susceptibilities of the isolated yeasts against Fluconazole (FLU), Itraconazole (ITR), Ketoconazole (KET) and Voriconazole (VOR) were assessed. Moreover, trailing growth was determined when a susceptible MIC was incubated for 24 hours, and turned into a resistant one after 48 hours of incubation.

Results:

Among the tested antifungal drugs in this study, the highest rate of resistance was observed against ITR (28.5%) followed by VOR (26.4%), FLU (20.8%) and KET (1.5%). The trailing effect was induced in 27 isolates (14.0%) by VOR, in 26 isolates (13.5%) by ITR, in 24 isolates (12.4%) by FLU, and in 19 isolates (9.8%) by KET.

Conclusions:

The monitoring of antifungal susceptibilities of Candida species isolated from clinical sources is highly recommended for the efficient management of patients. Moreover, the trailing effect should be taken into consideration once the interpretation of the results is intended.

Defining the frontiers between antifungal resistance, tolerance and the concept of persistence

A restricted number of antifungal agents are available for the therapy of fungal diseases. With the introduction of epidemiological cut-off values for each agent in important fungal pathogens based on the distribution of minimal inhibitory concentration (MIC), the distinction between wild type and drug-resistant populations has been facilitated. Antifungal resistance has been described for all currently available antifungal agents in several pathogens and most of the associated resistance mechanisms have been deciphered at the molecular level. Clinical breakpoints for some agents have been proposed and can have predictive value for the success or failure of therapy. Tolerance to antifungals has been a much more ignored area. By definition, tolerance operates at antifungal concentrations above individual intrinsic inhibitory values. Important is that tolerance to antifungal agents favours the emergence of persister cells, which are able to survive antifungal therapy and can cause relapses. Here we will review the current knowledge on antifungal tolerance, its potential mechanisms and also evaluate the role of antifungal tolerance in the efficacy of drug treatments.

Trafficking through the late endosome significantly impacts Candida albicans tolerance of the azole antifungals

The azole antifungals block ergosterol biosynthesis by inhibiting lanosterol demethylase (Erg11p). The resulting depletion of cellular ergosterol and the accumulation of "toxic" sterol intermediates are both thought to compromise plasma membrane function. However, the effects of ergosterol depletion upon the function of intracellular membranes and organelles are not well described. The purpose of this study was to characterize the effects of azole treatment upon the integrity of the Candida albicans vacuole and to determine whether, in turn, vacuolar trafficking influences azole susceptibility. Profound fragmentation of the C. albicans vacuole can be observed as an early consequence of azole treatment, and it precedes significant growth inhibition. In addition, a C. albicans vps21Δ/Δ mutant, blocked in membrane trafficking through the late endosomal prevacuolar compartment (PVC), is able to grow significantly more than the wild type in the presence of several azole antifungals under standard susceptibility testing conditions. Furthermore, the vps21Δ/Δ mutant is able to grow despite the depletion of cellular ergosterol. This phenotype resembles an exaggerated form of "trailing growth" that has been described for some clinical isolates. In contrast, the vps21Δ/Δ mutant is hypersensitive to drugs that block alternate steps in ergosterol biosynthesis. On the basis of these results, we propose that endosomal trafficking defects may lead to the cellular "redistribution" of the sterol intermediates that accumulate following inhibition of ergosterol biosynthesis. Furthermore, the destination of these intermediates, or the precise cellular compartments in which they accumulate, may be an important determinant of their toxicity and thus ultimately antifungal efficacy.

pH signaling in human fungal pathogens: a new target for antifungal strategies

Fungi are exposed to broadly fluctuating environmental conditions, to which adaptation is crucial for their survival. An ability to respond to a wide pH range, in particular, allows them to cope with rapid changes in their extracellular settings. PacC/Rim signaling elicits the primary pH response in both model and pathogenic fungi and has been studied in multiple fungal species. In the predominant human pathogenic fungi, namely, Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans, this pathway is required for many functions associated with pathogenesis and virulence. Aspects of this pathway are fungus specific and do not exist in mammalian cells. In this review, we highlight recent advances in our understanding of PacC/Rim-mediated functions and discuss the growing interest in this cascade and its factors as potential drug targets for antifungal strategies. We focus on both conserved and distinctive features in model and pathogenic fungi, highlighting the specificities of PacC/Rim signaling in C. albicans, A. fumigatus, and C. neoformans. We consider the role of this pathway in fungal virulence, including modulation of the host immune response. Finally, as now recognized for other signaling cascades, we highlight the role of pH in adaptation to antifungal drug pressure. By acting on the PacC/Rim pathway, it may therefore be possible (i) to ensure fungal specificity and to limit the side effects of drugs, (ii) to ensure broad-spectrum efficacy, (iii) to attenuate fungal virulence, (iv) to obtain additive or synergistic effects with existing antifungal drugs through tolerance inhibition, and (v) to slow the emergence of resistant mutants.

In VitroSusceptibilities ofCandidaspp. to Panomycocin, a Novel Exo-β-1,3-Glucanase Isolated fromPichia anomalaNCYC 434

Panomycocin, the killer toxin of Pichia anomala NCYC 434 (K5), is a 49 kDa monomeric glycoprotein with exo-beta-1,3-glucanase activity (patent pending). In this study we evaluated the in vitro activity of panomycocin against a panel of 109 human isolates of seven different pathogenic Candida spp. using microdilution and time-kill methods. Panomycocin was most active against C. tropicalis, C. pseudotropicalis and C. glabrata with MIC(90) values of 1 microg/ml. It displayed significant activity against C. albicans and C. parapsilosis with MIC(90) values of 4 and 2 microg/ml, respectively. For C. krusei, the MIC(90) value was 8 microg/ml. Panomycocin was fungicidal against all the tested Candida spp. The MFC values were only one or 2 dilutions higher than the MICs with the exception of C. krusei isolates with MFCs greater than or equal to 4xMIC. Results of this study indicated that panomycocin could be considered as a natural antifungal agent against Candida infections and has significant potential for further investigation.

Fluconazole-resistant Candida albicans vulvovaginitis

OBJECTIVE

As a result of high recurrence rates of Candida albicans vaginitis, successful suppressive fluconazole is widely used, and drug resistance is considered rare. We report increased occurrence of secondary fluconazole resistance, analysis of risk factors thereof, and describe management of fluconazole-refractory vaginitis.

METHODS

Patients referred to the Vaginitis Clinic at Wayne State University with clinically refractory fluconazole-resistant (minimum inhibitory concentration [MIC] 2 micrograms/mL or greater) C albicans vaginitis from 2000 to 2010 were enrolled. Patients completed a questionnaire pertaining to demographics, comorbidities, behavioral characteristics, exposure to antimicrobials and antifungals, fluconazole consumption in defined daily doses in the previous 6 months, management received, and outcomes. With patients not located, data were extracted from charts. Susceptibilities to antifungals were determined by broth microdilution.

RESULTS

Twenty-five women with fluconazole-resistant recurrent C albicans vaginitis were identified, and 16 returned filled questionnaires. Study cohort consisted mainly of married, insured white women with more than 12 years of formal education and average or above average socioeconomic status. Median fluconazole MIC was 8 micrograms/mL (range 2-128 micrograms/mL). Risk factors for mycologic failure included increased fluconazole consumption (P=.03) with 16 of 25 women exposed to low-dose weekly fluconazole maintenance therapy. All patients were clinically controlled successfully, although treatment was difficult and often prolonged.

CONCLUSION

Fluconazole-resistant C albicans vaginitis was previously considered rare. We report 25 cases over an 11-year period, indicating an emerging problem. All patients had fluconazole consumption in the previous 6 months. Management of fluconazole refractory disease is extremely difficult with limited options, and new therapeutic modalities are needed.

Effect of pH on in vitro susceptibility of Candida glabrata and Candida albicans to 11 antifungal agents and implications for clinical use

The treatment of vulvovaginal candidiasis (VVC) due to Candida glabrata is challenging, with limited therapeutic options. Unexplained disappointing clinical efficacy has been reported with systemic and topical azole antifungal agents in spite of in vitro susceptibility. Given that the vaginal pH of patients with VVC is unchanged at 4 to 4.5, we studied the effect of pH on the in vitro activity of 11 antifungal agents against 40 C. glabrata isolates and compared activity against 15 fluconazole-sensitive and 10 reduced-fluconazole-susceptibility C. albicans strains. In vitro susceptibility to flucytosine, fluconazole, voriconazole, posaconazole, itraconazole, ketoconazole, clotrimazole, miconazole, ciclopirox olamine, amphotericin B, and caspofungin was determined using the CLSI method for yeast susceptibility testing. Test media were buffered to pHs of 7, 6, 5, and 4. Under conditions of reduced pH, C. glabrata isolates remained susceptible to caspofungin and flucytosine; however, there was a dramatic increase in the MIC(90) for amphotericin B and every azole drug tested. Although susceptible to other azole drugs tested at pH 7, C. albicans strains with reduced fluconazole susceptibility also demonstrated reduced susceptibility to amphotericin B and all azoles at pH 4. In contrast, fluconazole-sensitive C. albicans isolates remained susceptible at low pH to azoles, in keeping with clinical observations. In selecting agents for treatment of recurrent C. glabrata vaginitis, clinicians should recognize the limitations of in vitro susceptibility testing utilizing pH 7.0.

Deletion of Cryptococcus neoformans AIF ortholog promotes chromosome aneuploidy and fluconazole-resistance in a metacaspase-independent manner

Apoptosis is a form of programmed cell death critical for development and homeostasis in multicellular organisms. Apoptosis-like cell death (ALCD) has been described in several fungi, including the opportunistic human pathogen Cryptococcus neoformans. In addition, capsular polysaccharides of C. neoformans are known to induce apoptosis in host immune cells, thereby contributing to its virulence. Our goals were to characterize the apoptotic signaling cascade in C. neoformans as well as its unique features compared to the host machinery to exploit the endogenous fungal apoptotic pathways as a novel antifungal strategy in the future. The dissection of apoptotic pathways revealed that apoptosis-inducing factor (Aif1) and metacaspases (Mca1 and Mca2) are independently required for ALCD in C. neoformans. We show that the apoptotic pathways are required for cell fusion and sporulation during mating, indicating that apoptosis may occur during sexual development. Previous studies showed that antifungal drugs induce ALCD in fungi and that C. neoformans adapts to high concentrations of the antifungal fluconazole (FLC) by acquisition of aneuploidy, especially duplication of chromosome 1 (Chr1). Disruption of aif1, but not the metacaspases, stimulates the emergence of aneuploid subpopulations with Chr1 disomy that are resistant to fluconazole (FLC^R) in vitro and in vivo. FLC^R isolates in the aif1 background are stable in the absence of the drug, while those in the wild-type background readily revert to FLC sensitivity. We propose that apoptosis orchestrated by Aif1 might eliminate aneuploid cells from the population and defects in this pathway contribute to the selection of aneuploid FLC^R subpopulations during treatment. Aneuploid clinical isolates with disomies for chromosomes other than Chr1 exhibit reduced AIF1 expression, suggesting that inactivation of Aif1 might be a novel aneuploidy-tolerating mechanism in fungi that facilitates the selection of antifungal drug resistance.

Fungal pathogens can cause life-threatening diseases, and the infections that they cause are notoriously difficult to treat. Despite the availability of antifungal drugs, most inhibit fungal growth but do not consistently or efficiently eliminate the pathogen. In addition, fungal cells are very similar to human cells, and therefore, many of the available antifungal agents have toxic side effects. Thus, more efficient drugs with less adverse effects are clearly needed. We investigated apoptosis, a process in which cells become programmed to commit suicide, in the pathogenic fungus Cryptococcus neoformans. We studied genes that regulate apoptosis in C. neoformans and, after inactivating three genes involved in this pathway, we observed defects in sexual reproduction. Such mating defects decrease the production of spores, which are inhaled and cause cryptococcal disease. We also showed that the absence of one investigated apoptotic gene, aif1, resulted in the selection of antifungal-resistant pathogens (when the fungal cells no longer respond to the drug), which makes treatment of the disease more difficult. The discovery of drugs that kill fungal cells specifically without affecting the cells of the patient being treated holds great potential. Therefore, triggering apoptosis should be further investigated as a new approach to treat fungal pathogens.

Cryptic species and azole resistance in the Aspergillus niger complex

Aspergillus niger is a common clinical isolate. Multiple species comprise the Aspergillus section Nigri and are separable using sequence data. The antifungal susceptibility of these cryptic species is not known. We determined the azole MICs of 50 black aspergilli, 45 from clinical specimens, using modified EUCAST (mEUCAST) and Etest methods. Phylogenetic trees were prepared using the internal transcribed spacer, beta-tubulin, and calmodulin sequences to identify strains to species level and the results were compared with those obtained with cyp51A sequences. We attempted to correlate cyp51A mutations with azole resistance. Etest MICs were significantly different from mEUCAST MICs (P < 0.001), with geometric means of 0.77 and 2.79 mg/liter, respectively. Twenty-six of 50 (52%) isolates were itraconazole resistant by mEUCAST (MICs > 8 mg/liter), with limited cross-resistance to other azoles. Using combined beta-tubulin/calmodulin sequences, the 45 clinical isolates grouped into 5 clades, A. awamori (55.6%), A. tubingensis (17.8%), A. niger (13.3%), A. acidus (6.7%), and an unknown group (6.7%), none of which were morphologically distinguishable. Itraconazole resistance was found in 36% of the isolates in the A. awamori group, 90% of the A. tubingensis group, 33% of the A. niger group, 100% of the A. acidus group, and 67% of the unknown group. These data suggest that cyp51A mutations in section Nigri may not play as important a role in azole resistance as in A. fumigatus, although some mutations (G427S, K97T) warrant further study. Numerous cryptic species are found in clinical isolates of the Aspergillus section Nigri and are best reported as "A. niger complex" by clinical laboratories. Itraconazole resistance was common in this data set, but azole cross-resistance was unusual. The mechanism of resistance remains obscure.