Amino acid substitutions at the major insertion loop of Candida albicans sterol 14alpha-demethylase are involved in fluconazole resistance

Molecular Mechanisms of Azole Resistance in Four Clinical Candida albicans Isolates

Azole resistance constitutes a serious clinical problem in the management of infections caused by Candida albicans. This study aimed to explore azole-resistant mechanisms in clinical C. albicans isolates collected in Jinan, Shandong, China. In total, 22 samples were collected and analyzed. Among these, four isolates (28A, 28D, 28I, and 28J) exhibited high level of pan-azole-resistance that was Hsp90 dependent. Gene sequencing revealed that the four Hsp90-dependent strains contained different ERG3 mutations that led to four novel amino acid substitutions (S265Y, N322D, N324S, and E355D) in Erg3. The role of these substitutions in azole resistance development was determined by constructing one copy of the mutated ERG3 from the 28A, 28D, and 28I strains into C. albicans CAI4, respectively. The minimum inhibitory concentration value of fluconazole (FLC) against C. albicans CAI4-ERG3^28I increased fourfold compared with the wild-type C. albicans strain, suggesting that the novel combination of substitutions S265Y, N322D, and N324S played an important role in mediating azole resistance in 28I. Besides, we identified several different mechanisms in other three isolates. Strains 28A and 28D displayed increased efflux ability and overexpression of MDR1. Strain 28J showed high level of ERG11 expression, but no mutation in its regulator Upc2 was observed. Our study revealed that multiple factors confer azole resistance in clinical C. albicans isolates and combination therapy should be conducted clinically.

Computational insights into fluconazole resistance by the suspected mutations in lanosterol 14α-demethylase (Erg11p) of Candida albicans

Mutations in the ergosterol biosynthesis gene 11 (ERG11) of Candida albicans have been frequently reported in fluconazole-resistant clinical isolates. Exploring the mutations and their effect could provide new insights into the underlying mechanism of fluconazole resistance. Erg11p_Threonine285Alanine (Erg11p_THR285ALA), Erg11p_Leucine321Phenylalanine (Erg11p_LEU321PHE) and Erg11p_Serine457Proline (Erg11p_SER457PRO) are three fluconazole-resistant suspected mutations reported in clinical isolates of C. albicans. Therefore, our study aims to investigate the role of these suspected mutations in fluconazole resistance using in-silico methods. Molecular dynamics simulation (MDS) analysis of apo-protein for 25ns (nanosecond) showed that suspected mutant proteins underwent slight conformational changes in the tertiary structure. Molecular docking with fluconazole followed by binding free energy analysis showed reduced non-bonded interactions with loss of heme interaction and the least binding affinity for Erg11p_SER457PRO mutation. MDS of suspected mutant proteins-fluconazole complexes for 50ns revealed that Erg11p_SER457PRO and Erg11p_LEU321PHE have clear differences in the interaction pattern and loss or reduced heme interaction compared to wild type Erg11p-fluconazole complex. MDS and binding free energy analysis of Erg11p_SER457PRO-fluconazole complex showed the least binding similar to verified mutation Erg11p_TYR447HIS-fluconazole complex. Taken together, our study concludes that suspected mutation Erg11p_THR285ALA may not have any role whereas Erg11p_LEU321PHE could have a moderate role. However, Erg11p_SER457PRO mutation has a strong possibility to play an active role in fluconazole resistance of C. albicans.

In Vitro and In Silico Analysis of Ascorbic Acid Towards Lanosterol 14-α-Demethylase Enzyme of Fluconazole-Resistant Candida albicans

Antibiotic resistance is one of the major concerns and the biggest threats to the world population. The incidents of antibiotic resistance in Candida spp. were frequently recorded. In the present investigation, antifungal potential of ascorbic acid (AA) was evaluated. According to the in vitro analysis, the zone of inhibition of AA (24.75 ± 0.35 mm) against C. albicans was greater as compared to other vitamins tested. AA significantly modulate the growth of C. albicans at 25 mg/ml. The highest percentage (94.67%) of cell viability was observed in untreated cells, and low cell viability (29.36%) was observed in cells treated with 50 mg/ml of AA (2 × MIC). Further, AO/EB (acridine orange/ethidium bromide), propidium iodide staining, and real-time qPCR confirmed the loss of membrane integrity due to membrane lesions that caused cell death. Lanosterol 14-α-demethylase (L-14α-DM) is the product of ERG11 and acted as superior drug target of C. albicans. Molecular docking analysis confirmed that active interaction of ascorbic acid with L-14α-DM. Based on the present investigation, the efficiency of AA was effectively proved through the in vitro and in silico analysis. This finding has evidenced the effectiveness of AA as a potential candidate against C. albicans.

Biological exploration of a novel 1,2,4-triazole-indole hybrid molecule as antifungal agent

(2-(2,4-Dichlorophenyl)-3-(1H-indol-1-yl)-1-(1,2,4-1H-triazol-1-yl)propan-2-ol (8 g), a new 1,2,4-triazole-indole hybrid molecule, showed a broad-spectrum activity against Candida, particularly against low fluconazole-susceptible species. Its activity was higher than fluconazole and similar to voriconazole on C. glabrata (MIC₉₀ = 0.25, 64 and 1 µg/mL, respectively), C. krusei (MIC₉₀ = 0.125, 64 and 0.125 µg/mL, respectively) and C. albicans (MIC₉₀ = 0.5, 8 and 0.25 µg/mL, respectively). The action mechanisms of 8 g were also identified as inhibition of ergosterol biosynthesis and phospholipase A2-like activity. At concentration as low as 4 ng/mL, 8g inhibited ergosterol production by 82% and induced production of 14a-methyl sterols, that is comparable to the results obtained with fluconazole at higher concentration. 8 g demonstrated moderate inhibitory effect on phospholipase A2-like activity being a putative virulence factor. Due to a low MRC5 cytotoxicity, this compound presents a high therapeutic index. These results pointed out that 8 g is a new lead antifungal candidate with potent ergosterol biosynthesis inhibition.

Precise genome editing using a CRISPR-Cas9 method highlights the role of CoERG11 amino acid substitutions in azole resistance in Candida orthopsilosis

Background

Azoles are one of the main antifungal classes for the treatment of candidiasis. In the current context of emerging drug resistance, most studies have focused on Candida albicans, Candida glabrata or Candida auris but, so far, less is known about the underlying mechanisms of resistance in other species, including Candida orthopsilosis.

Objectives

We investigated azole resistance in a C. orthopsilosis clinical isolate recovered from a patient with haematological malignancy receiving fluconazole prophylaxis.

Methods

Antifungal susceptibility to fluconazole was determined in vitro (CLSI M27-A3) and in vivo (in a Galleria mellonella model of invasive candidiasis). The CoERG11 gene was then sequenced and amino acid substitutions identified were mapped on the predicted 3D structure of CoErg11p. A clustered regularly interspaced short palindromic repeat-Cas9 (CRISPR-Cas9) genome-editing strategy was used to introduce relevant mutations into a fluconazole-susceptible C. orthopsilosis isolate.

Results

Compared with unrelated C. orthopsilosis isolates, the clinical isolate exhibited both in vitro and in vivo fluconazole resistance. Sequencing of the CoERG11 gene identified several amino acid substitutions, including two possibly involved in fluconazole resistance (L376I and G458S). Both mutations mapped close to the active site of CoErg11p. Engineering these mutations in a different genetic background using CRISPR-Cas9 demonstrated that G458S, but not L376I, confers resistance to fluconazole and voriconazole.

Conclusions

Our data show that the G458S amino acid substitution in CoERG11p, but not L376I, contributes to azole resistance in C. orthopsilosis. In addition to highlighting the potential of CRISPR-Cas9 technology for precise genome editing in the field of antifungal resistance, we discuss some points that are critical to improving its efficiency.

A new mechanism for reduced sensitivity to demethylation-inhibitor fungicides in the fungal banana black Sigatoka pathogen Pseudocercospora fijiensis

The Dothideomycete Pseudocercospora fijiensis, previously Mycosphaerella fijiensis, is the causal agent of black Sigatoka, one of the most destructive diseases of bananas and plantains. Disease management depends on fungicide applications, with a major contribution from sterol demethylation-inhibitors (DMIs). The continued use of DMIs places considerable selection pressure on natural P. fijiensis populations, enabling the selection of novel genotypes with reduced sensitivity. The hitherto explanatory mechanism for this reduced sensitivity was the presence of non-synonymous point mutations in the target gene Pfcyp51, encoding the sterol 14α-demethylase enzyme. Here, we demonstrate a second mechanism involved in DMI sensitivity of P. fijiensis. We identified a 19-bp element in the wild-type (wt) Pfcyp51 promoter that concatenates in strains with reduced DMI sensitivity. A polymerase chain reaction (PCR) assay identified up to six Pfcyp51 promoter repeats in four field populations of P. fijiensis in Costa Rica. We used transformation experiments to swap the wt promoter of a sensitive field isolate with a promoter from a strain with reduced DMI sensitivity that comprised multiple insertions. Comparative in vivo phenotyping showed a functional and proportional up-regulation of Pfcyp51, which consequently decreased DMI sensitivity. Our data demonstrate that point mutations in the Pfcyp51 coding domain, as well as promoter inserts, contribute to the reduced DMI sensitivity of P. fijiensis. These results provide new insights into the importance of the appropriate use of DMIs and the need for the discovery of new molecules for black Sigatoka management.

A newly identified amino acid substitution T123I in the 14α-demethylase (Erg11p) of Candida albicans confers azole resistance

The increasing prevalence of azole resistance in Candida albicans poses a growing problem for clinical treatment. Amino acid substitution of the 14α-demethylase (Erg11p) encoded by the ERG11 gene is one of the most common mechanisms involved in azole resistance. Although amino acid substitutions of Erg11p have been observed in many clinical isolates, only a few amino acid substitutions have been confirmed to be related to azole resistance. In this study, by amplifying and sequencing the open reading frame of the ERG11 gene from 55 clinical isolates, we identified 27 fluconazole-resistant isolates that harbor a novel amino acid substitution, T123I, in Erg11p, in addition to the previously described homozygous substitution Y132H. We investigated both the contribution of this novel substitution T123I and its synergistic effect with substitution Y132H to azole resistance by heterogeneously expressing the C. albicans Erg11p with different substitution forms in Saccharomyces cerevisiae. Results showed that S. cerevisiae cells harboring the substitution T123I displayed higher (4-fold) minimum inhibitory concentration values to both fluconazole and voriconazole than the cells expressing the wild-type version of C. albicans Erg11p, but this was not true for itraconazolele. More importantly, a synergistic effect of substitutions T123I and Y132H was observed in an assay of voriconazole resistance. These results indicate that amino acid substitutions of Erg11p are prevalent among azole-resistant isolates and that the substitution T123I confers resistance to both fluconazole and voriconazole.

Resistance Surveillance in Candida albicans: A Five-Year Antifungal Susceptibility Evaluation in a Brazilian University Hospital

Candida albicans caused 44% of the overall candidemia episodes from 2006 to 2010 in our university tertiary care hospital. As different antifungal agents are used in therapy and also immunocompromised patients receive fluconazole prophylaxis in our institution, this study aimed to perform an antifungal susceptibility surveillance with the C.albicans bloodstream isolates and to characterize the fluconazole resistance in 2 non-blood C.albicans isolates by sequencing ERG11 gene. The study included 147 C. albicans bloodstream samples and 2 fluconazole resistant isolates: one from oral cavity (LIF 12560 fluconazole MIC: 8μg/mL) and one from esophageal cavity (LIF-E10 fluconazole MIC: 64μg/mL) of two different patients previously treated with oral fluconazole. The in vitro antifungal susceptibility to amphotericin B (AMB), 5-flucytosine (5FC), fluconazole (FLC), itraconazole (ITC), voriconazole (VRC), caspofungin (CASP) was performed by broth microdilution methodology recommended by the Clinical and Laboratory Standards Institute documents (M27-A3 and M27-S4, CLSI). All blood isolates were classified as susceptible according to CLSI guidelines for all evaluated antifungal agents (MIC range: 0,125–1.00 μg/mL for AMB, ≤0.125–1.00 μg/mL for 5FC, ≤0.125–0.5 μg/mL for FLC, ≤0.015–0.125 μg/mL for ITC, ≤0.015–0.06 μg/mL for VRC and ≤0.015–0.125 μg/mL for CASP). In this study, we also amplified and sequenced the ERG11 gene of LIF 12560 and LIF-E10 C.albicans isolates. Six mutations encoding distinct amino acid substitutions were found (E116D, T128K, E266D, A298V, G448V and G464S) and these mutations were previously described as associated with fluconazole resistance. Despite the large consumption of antifungals in our institution, resistant blood isolates were not found over the trial period. Further studies should be conducted, but it may be that the very prolonged direct contact with the oral antifungal agent administered to the patient from which was isolated LIF E-10, may have contributed to the development of resistance.

The amino acid substitution N136Y in Candida albicans sterol 14alpha-demethylase is involved in fluconazole resistance

Resistance to fluconazole antifungal is an ongoing impediment to a successful treatment of Candida albicans infections. One of the most prevalent mechanisms leading to azole resistance is genetic alterations of the 14α-demethylase, the target of azole antifungals, through point mutations. Site-directed mutagenesis and molecular modeling of 14α-demethylase rationalize biological data about the role of protein substitutions in the azole treatment failure. In this work, we investigated the role of N136Y substitution by site-directed mutagenesis into Pichia pastoris guided by structural analysis. Single amino acid substitutions were created by site-directed mutagenesis into P. pastoris with C. albicans ERG11 gene as template. In vitro susceptibility of P. pastoris transformants expressing wild-type and mutants to azole compounds was determined by CLSI M27-A2 and spot agar methods. The fluconazole effect on ergosterol biosynthesis was analyzed by gas chromatography-mass spectrometry. By microdilution and spot tests, N136Y transformants showed a reduced in vitro susceptibility to fluconazole compared to wild-type controls. As expected, ergosterol/lanosterol ratios were higher in N136Y transformants compared to the wild-type controls after treatment with fluconazole. Molecular modeling suggests that residue Asn136 located within the first mutation hot spot, could play a role during heme and azole binding. These results provide new insights into the structural basis for 14α-demethylase-azole interaction and could guide the design of novel azole antifungals.

Contribution of clinically derived mutations in ERG11 to azole resistance in Candida albicans

In Candida albicans, the ERG11 gene encodes lanosterol demethylase, the target of the azole antifungals. Mutations in ERG11 that result in an amino acid substitution alter the abilities of the azoles to bind to and inhibit Erg11, resulting in resistance. Although ERG11 mutations have been observed in clinical isolates, the specific contributions of individual ERG11 mutations to azole resistance in C. albicans have not been widely explored. We sequenced ERG11 in 63 fluconazole (FLC)-resistant clinical isolates. Fifty-five isolates carried at least one mutation in ERG11, and we observed 26 distinct positions in which amino acid substitutions occurred. We mapped the 26 distinct variant positions in these alleles to four regions in the predicted structure for Erg11, including its predicted catalytic site, extended fungus-specific external loop, proximal surface, and proximal surface-to-heme region. In total, 31 distinct ERG11 alleles were recovered, with 10 ERG11 alleles containing a single amino acid substitution. We then characterized 19 distinct ERG11 alleles by introducing them into the wild-type azole-susceptible C. albicans SC5314 strain and testing them for susceptibilities to FLC, itraconazole (ITC), and voriconazole (VRC). The strains that were homozygous for the single amino acid substitutions Y132F, K143R, F145L, S405F, D446E, G448E, F449V, G450E, and G464S had a ≥ 4-fold increase in FLC MIC. The strains that were homozygous for several double amino acid substitutions had decreased azole susceptibilities beyond those conferred by any single amino acid substitution. These findings indicate that mutations in ERG11 are prevalent among azole-resistant clinical isolates and that most mutations result in appreciable changes in FLC and VRC susceptibilities.

Resistance to antifungals that target CYP51

Fungal diseases are an increasing global burden. Fungi are now recognised to kill more people annually than malaria, whilst in agriculture, fungi threaten crop yields and food security. Azole resistance, mediated by several mechanisms including point mutations in the target enzyme (CYP51), is increasing through selection pressure as a result of widespread use of triazole fungicides in agriculture and triazole antifungal drugs in the clinic. Mutations similar to those seen in clinical isolates as long ago as the 1990s in Candida albicans and later in Aspergillus fumigatus have been identified in agriculturally important fungal species and also wider combinations of point mutations. Recently, evidence that mutations originate in the field and now appear in clinical infections has been suggested. This situation is likely to increase in prevalence as triazole fungicide use continues to rise. Here, we review the progress made in understanding azole resistance found amongst clinically and agriculturally important fungal species focussing on resistance mechanisms associated with CYP51. Biochemical characterisation of wild-type and mutant CYP51 enzymes through ligand binding studies and azole IC50 determinations is an important tool for understanding azole susceptibility and can be used in conjunction with microbiological methods (MIC50 values), molecular biological studies (site-directed mutagenesis) and protein modelling studies to inform future antifungal development with increased specificity for the target enzyme over the host homologue.

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

Antifungal resistance of Candida species is a clinical problem in the management of diseases caused by these pathogens. In this study we identified from a collection of 423 clinical samples taken from Tunisian hospitals two clinical Candida species (Candida albicans JEY355 and Candida tropicalis JEY162) with decreased susceptibility to azoles and polyenes. For JEY355, the fluconazole (FLC) MIC was 8 μg/ml. Azole resistance in C. albicans JEY355 was mainly caused by overexpression of a multidrug efflux pump of the major facilitator superfamily, Mdr1. The regulator of Mdr1, MRR1, contained a yet-unknown gain-of-function mutation (V877F) causing MDR1 overexpression. The C. tropicalis JEY162 isolate demonstrated cross-resistance between FLC (MIC > 128 μg/ml), voriconazole (MIC > 16 μg/ml), and amphotericin B (MIC > 32 μg/ml). Sterol analysis using gas chromatography-mass spectrometry revealed that ergosterol was undetectable in JEY162 and that it accumulated 14α-methyl fecosterol, thus indicating a perturbation in the function of at least two main ergosterol biosynthesis proteins (Erg11 and Erg3). Sequence analyses of C. tropicalis ERG11 (CtERG11) and CtERG3 from JEY162 revealed a deletion of 132 nucleotides and a single amino acid substitution (S258F), respectively. These two alleles were demonstrated to be nonfunctional and thus are consistent with previous studies showing that ERG11 mutants can only survive in combination with other ERG3 mutations. CtERG3 and CtERG11 wild-type alleles were replaced by the defective genes in a wild-type C. tropicalis strain, resulting in a drug resistance phenotype identical to that of JEY162. This genetic evidence demonstrated that CtERG3 and CtERG11 mutations participated in drug resistance. During reconstitution of the drug resistance in C. tropicalis, a strain was obtained harboring only defective Cterg11 allele and containing as a major sterol the toxic metabolite 14α-methyl-ergosta-8,24(28)-dien-3α,6β-diol, suggesting that ERG3 was still functional. This strain therefore challenged the current belief that ERG11 mutations cannot be viable unless accompanied by compensatory mutations. In conclusion, this study, in addition to identifying a novel MRR1 mutation in C. albicans, constitutes the first report on a clinical C. tropicalis with defective activity of sterol 14α-demethylase and sterol Δ(5,6)-desaturase leading to azole-polyene cross-resistance.

Update on mechanisms of azole resistance in Mycosphaerella graminicola and implications for future control

This review summarises recent investigations into the molecular mechanisms responsible for the decline in sensitivity to azole (imidazole and triazole) fungicides in European populations of the Septoria leaf blotch pathogen, Mycosphaerella graminicola. The complex recent evolution of the azole target sterol 14α-demethylase (MgCYP51) enzyme in response to selection by the sequential introduction of progressively more effective azoles is described, and the contribution of individual MgCYP51 amino acid alterations and their combinations to azole resistance phenotypes and intrinsic enzyme activity is discussed. In addition, the recent identification of mechanisms independent of changes in MgCYP51 structure correlated with novel azole cross-resistant phenotypes suggests that the further evolution of M. graminicola under continued selection by azole fungicides could involve multiple mechanisms. The prospects for azole fungicides in controlling European M. graminicola populations in the future are discussed in the context of these new findings.

Association of T916C (Y257H) mutation in Candida albicans ERG11 with fluconazole resistance

Repeated and prolonged use of fluconazole in treating candidosis leads to drug resistance. The aim of this study was to confirm the relationship between T916C (Y257H) mutation in Candida albicans ERG11 and fluconazole resistance. We replaced one copy of ERG11 with ERG11 containing the T916C mutation in C. albicans CAI4 and expressed ERG11 with the T916C mutation in Saccharomyces cerevisiae INVSc1. The MIC values were two- to four-fold greater in CAI4 transformants with than without the T916C mutation and 128 and 32 μg ml(-1) for S. cerevisiae INVSc1-containing ERG11 with and without the T916C mutation. T916C mutation may be associated with fluconazole resistance in C. albicans.

First human model of in vitro Candida albicans persistence within granuloma for the reliable study of host-fungi interactions

Backgound

The balance between human innate immune system and Candida albicans virulence signaling mechanisms ultimately dictates the outcome of fungal invasiveness and its pathology. To better understand the pathophysiology and to identify fungal virulence-associated factors in the context of persistence in humans, complex models are indispensable. Although fungal virulence factors have been extensively studied in vitro and in vivo using different immune cell subsets and cell lines, it is unclear how C. albicans survives inside complex tissue granulomas.

Methodology/Principal Finding

We developed an original model of in vitro human granuloma, reproducing the natural granulomatous response to C. albicans. Persistent granulomas were obtained when the ratio of phagocytes to fungi was high. This in vitro fungal granuloma mimics natural granulomas, with infected macrophages surrounded by helper and cytotoxic T lymphocytes. A small proportion of granulomas exhibited C. albicans hyphae. Histological and time-lapse analysis showed that C. albicans blastoconidia were located within the granulomas before hyphae formation. Using staining techniques, fungal load calculations, as well as confocal and scanning electron microscopy, we describe the kinetics of fungal granuloma formation. We provide the first direct evidence that C. albicans are not eliminated by immunocompetent cells inside in vitro human granulomas. In fact, after an initial candicidal period, the remaining yeast proliferate and persist under very complex immune responses.

Conclusions/Significance

Using an original in vitro model of human fungal granuloma, we herein present the evidence that C. albicans persist and grow into immunocompetent granulomatous structures. These results will guide us towards a better understanding of fungal invasiveness and, henceforth, will also help in the development of better strategies for its control in human physiological conditions.

Fungal cytochrome P450 sterol 14α-demethylase (CYP51) and azole resistance in plant and human pathogens

Azoles have been applied widely to combat pathogenic fungi in medicine and agriculture and, consequently, loss of efficacy has occurred in populations of some species. Often, but not always, resistance was found to result from amino acid substitutions in the molecular target of azoles, 14α-sterol demethylase (CYP51 syn. ERG11). This review summarizes CYP51 function, evolution, and structure. Furthermore, we compare the occurrence and contribution of CYP51 substitutions to azole resistance in clinical and field isolates of important fungal pathogens. Although no crystal structure is available yet for any fungal CYP51, homology modeling using structures from other origins as template allowed deducing models for fungal orthologs. These models served to map amino acid changes known from clinical and field isolates. We conclude with describing the potential consequences of these changes on the topology of the protein to explain CYP51-based azole resistance. Knowledge gained from molecular modeling and resistance research will help to develop novel azole structures.

Amino acid substitutions in the Candida albicans sterol Δ5,6-desaturase (Erg3p) confer azole resistance: characterization of two novel mutants with impaired virulence

OBJECTIVES

To determine the mechanisms responsible for fluconazole resistance in two Candida albicans isolates (CAAL2 and CAAL76) recovered from two hospitalized patients after fluconazole prophylaxis.

METHODS

MICs of fluconazole and voriconazole were determined by the broth microdilution method (CLSI M27-A3), and by Etest(®) for amphotericin B. RNA expression levels of CDR1, MDR1 and ERG11 were determined by RT-PCR. Mutations in ERG11 and ERG3 were investigated by amplification and sequencing. Sterol membrane profiles were determined by gas chromatography-mass spectrometry (GC-MS). In vivo virulence was determined in a murine model of invasive candidiasis.

RESULTS

Both isolates displayed azole cross-resistance and reduced susceptibility to amphotericin B, and are novel Δ(5,6)-desaturase (Erg3p) mutants. CAAL2 harbours a new amino acid substitution (L193R), whereas a 13 bp deletion leading to a truncated Erg3p (Δ366-378) was found in CAAL76. Both genetic alterations impaired Erg3p function as shown by GC-MS in these isolates (ergosterol content below 10%, and accumulation of ergosta-7,22-dienol above 40%). In vivo, in a murine model of invasive candidiasis, both CAAL2 and CAAL76 exhibited a significant trend toward reduced virulence, which seems to be linked to a reduced capacity for hyphal growth.

CONCLUSIONS

These findings demonstrate the critical role of residue 193 in Erg3p function and azole resistance. We suggest that this attenuated in vivo virulence phenotype could be linked to lower potential for hyphal growth. Taken together, our findings highlight the fact that erg3 mutants must be considered in future studies aiming at investigating azole antifungal drug resistance.

The cellular and molecular defense mechanisms of the Candida yeasts against azole antifungal drugs

The molecular mechanisms supporting resistance to azole antifungals have attracted a great interest during the last decades because of the emergence of clinical resistance to the treatment of fungal infections. The availability of genome sequencing data, of molecular biology tools, and of a large set of clinical and laboratory azole-resistant strains, made the yeasts Candida the biological material of choice to decipher azole resistance mechanisms. The yeast Candida albicans has several cellular ways to resist to azole drugs: decreased affinity of the target protein Erg11p for the drugs, increased biosynthesis of Erg11p, and efflux of the drugs outside the fungal cells. At the molecular level, two main mechanisms are operating: point mutation in the target gene or in transcriptional activator factors, eventually associated to a loss of heterozygosity, and gene duplication that results from the extraordinary plasticity of the genome. This review proposes to explore the different molecular strategies that are used by Candida yeasts to fight azole antifungals.

The effect of biomaterials and antifungals on biofilm formation by Candida species: a review

Candida albicans, C. glabrata, C. parapsilosis, and C. tropicalis are able to form biofilms on virtually any biomaterial implanted in a human host. Biofilms are a primary cause of mortality in immunocompromised and hospitalized patients, as they cause recurrent and invasive candidiasis, which is difficult to eradicate. This is due to the fact that the biofilm cells show high resistance to antifungal treatments and the host defense mechanisms, and exhibit an excellent ability to adhere to biomaterials. Elucidation of the mechanisms of antifungal resistance in Candida biofilms is of unquestionable importance; therefore, this review analyzes both the chemical composition of biomaterials used to fabricate the medical devices, as well as the Candida genes and proteins that confer drug resistance.

S279 point mutations in Candida albicans Sterol 14-α demethylase (CYP51) reduce in vitro inhibition by fluconazole

The effects of S279F and S279Y point mutations in Candida albicans CYP51 (CaCYP51) on protein activity and on substrate (lanosterol) and azole antifungal binding were investigated. Both S279F and S279Y mutants bound lanosterol with 2-fold increased affinities (K(s), 7.1 and 8.0 μM, respectively) compared to the wild-type CaCYP51 protein (K(s), 13.5 μM). The S279F and S279Y mutants and the wild-type CaCYP51 protein bound fluconazole, voriconazole, and itraconazole tightly, producing typical type II binding spectra. However, the S279F and S279Y mutants had 4- to 5-fold lower affinities for fluconazole, 3.5-fold lower affinities for voriconazole, and 3.5- to 4-fold lower affinities for itraconazole than the wild-type CaCYP51 protein. The S279F and S279Y mutants gave 2.3- and 2.8-fold higher 50% inhibitory concentrations (IC₅₀s) for fluconazole in a CYP51 reconstitution assay than the wild-type protein did. The increased fluconazole resistance conferred by the S279F and S279Y point mutations appeared to be mediated through a combination of a higher affinity for substrate and a lower affinity for fluconazole. In addition, lanosterol displaced fluconazole from the S279F and S279Y mutants but not from the wild-type protein. Molecular modeling of the wild-type protein indicated that the oxygen atom of S507 interacts with the second triazole ring of fluconazole, assisting in orientating fluconazole so that a more favorable binding conformation to heme is achieved. In contrast, in the two S279 mutant proteins, this S507-fluconazole interaction is absent, providing an explanation for the higher K(d) values observed.