Polyene Resistance Research Articles

Tracking Candidemia Trends and Antifungal Resistance Patterns across Europe: An In-Depth Analysis of Surveillance Systems and Surveillance Studies.

The WHO fungal priority list classifies Candida species as critical and high-priority pathogens, and the WHO GLASS fungi initiative seeks to establish a standardised global framework for antifungal resistance monitoring. We aimed to review resistance rates and antifungal resistance patterns across European surveillance systems and studies in response to these recent calls for action. A systematic review of national and international surveillance systems and peer-reviewed surveillance studies available up to June 2024 was conducted. Descriptive and trend analyses were performed on surveillance data reporting resistance to different antifungals in Candida spp. In total, 6 national surveillance systems and 28 studies from 13 countries provided candidemia resistance data, mostly about the C. albicans, C. glabrata and C. parapsilosis complex. Azole resistance was most frequently reported (6/6 surveillance systems and 27/28 studies) with the highest resistance rate, especially for C. glabrata, in Croatia (100%, 28/28 isolates) and Slovenia (85.7%, 82/96) and C. parapsilosis in Croatia (80.6%, 54/67) and Italy (72.6%, 106/146). Echinocandin and polyene resistance rates were nearly zero. The number of isolates included in the surveillance systems increased over the years, particularly for C. albicans (+40-60 isolates/year), C. glabrata, and C. parapsilosis (+15-30 isolates/year). No surveillance system or study reported resistance data for C. auris. Pooled data from national surveillance revealed a decreasing trend in azole resistance in C. albicans and C. glabrata. The increasing azole-resistance trend in C. parapsilosis disappeared after adjusting for between-country heterogeneity. Overall, echinocandin and polyene resistance trends appeared relatively stable. Awareness of antifungal resistance is growing, but further actions are needed to strengthen surveillance capacity and knowledge-sharing networks across Europe.

P021 Comparative transcriptomic analysis of environmental Candida auris showing variable azole susceptibility

Poster session 1, September 21, 2022, 12:30 PM - 1:30 PM Objective Candida auris is a multidrug-resistant pathogen that presents a serious global threat to human health. The U.S. Centers for Disease Control and Prevention has classified C. auris as an urgent threat to public health due to its clinical and economic impact and future projections of new infections over the next 10 years. Candida auris infections are difficult to treat since many isolates display high levels of resistance to fluconazole and exhibit variable resistance to amphotericin B and echinocandins. In this study, we performed comparative transcriptomics to understand the molecular mechanisms associated with azole-resistance in C. auris environmental isolates.Material and MethodsTwo sets of environmental isolates including azole-resistant (n = 2) and azole susceptible (n = 1) isolates were used for RNA-Seq analysis. Pair-wise comparisons in edgeR were used for comparing the number of differentially expressed genes (DEGs) between the azole susceptible and resistant isolates. GO term enrichment analysis was performed using the ‘enrichGO’ function from the cluster Profiler package. Only GO categories with a q-value <0.05 were considered significant.ResultsOur data show significant enrichment of ergosterol biosynthesis genes, drug transport, MAPK pathway as well as chromatin remodeling genes in azole-resistant strains compared to susceptible isolates. A total of 468 and 564 differentially expressed genes were identified in two azole-resistant isolates compared with the susceptible strain. A large number of multidrug transporter genes (CDR1, MDR1, HGT2, HGT7, HGT13, HGT17, and NGT1) were differentially expressed between the two sets of strains. Interestingly, the overexpression of ERG11 (azole target gene), and CDR1 (drug transporter) genes was observed in resistant isolates as compared with susceptible strain. Furthermore, resistant strain has two copies of ERG11 while susceptible isolate has single copy of ERG11. Notably, 8/21 genes involved in the ergosterol biosynthesis pathway were found to be induced in azole resistant isolates. These include HMG1, ERG1, ERG2, ERG3, ERG6, ERG10, ERG13, and ERG25. Furthermore, other multidrug transporters MDR1 and SNQ2 responsible for azole resistance in other Candida species like C. glabrata also showed significant expression changes between the two sets of isolates. Furthermore, HGT7 (glucose transporter) and NGT1, (N-acetyl glucosamine transporter) genes associated with azole and polyene resistance were found to be upregulated in the resistant isolate as compared with susceptible strain. Additionally, a Glycophosphatidylinositol (GPI)-anchored protein unique for C. auris, PGA7 was found to be overexpressed in resistant isolate. Importantly, we also identified several secreted aspartic proteases (SAP3, SAP5, SAP8, and SAP9) to be downregulated between the two sets.ConclusionThe present study identifies several gene families that are differentially expressed in azole resistant vs susceptible C. auris strains. These findings suggest that azole-resistance in C. auris environmental isolates is influenced by changes in cell wall, lipid, and ergosterol biosynthesis. Overall, these data provide a framework for the mechanistic understanding of azole resistance mechanisms in C. auris environmental isolates.

S2.5d Exploring multidrug resistance, fitness compensation, and collateral sensitivity in Candida auris : Fight fire with fire?

S2.5 Rare yeasts, September 21, 2022, 3:00 PM - 4:30 PM Candida auris (C. auris) is a recently emerged human fungal pathogen of growing concern due to its ability to acquire extensive multidrug resistance (MDR) to all four antifungal drug classes. The unprecedented extent of MDR in C. auris, suggests accelerated resistance evolution, novel mechanisms of resistance, and/or potential fitness compensation. Despite being the first fungus to be officially considered an urgent antimicrobial resistance threat by the CDC (US), insights into the resistance mechanisms and evolutionary dynamics of C. auris are still scarce. By using high-throughput in vitro experimental evolution with various antifungal drugs, we have obtained a library of resistant strains from four different clades. Through both genome and targeted sequencing, we have discovered novel mutations, especially for polyene resistance, which indicate new mechanisms of resistance and fitness compensation. For the validation of mutations, we have optimized a recyclable CRISPR/Cas9 tool for C. auris based on the C. albicans HIS-FLP system. By mapping drug susceptibility responses of evolved strains across a library of several antifungals and repurposed drugs, we have discovered trends of cross-resistance and collateral sensitivity. Both phenomena have been extensively studied in tumors and bacteria but remain unexplored in fungi. In the light of these observations, we explore novel treatment schemes that prevent antifungal drug resistance development in C. auris and other pathogenic fungi.

Identification of the ergC gene involved in polyene drug sensitivity in the Mucorales species Phycomyces blakesleeanus.

A strain of Phycomyces blakesleeanus (Mucorales, Mucoromycota) that was previously isolated after ultraviolet mutagenesis has altered responses to polyene antifungal drugs, sterol profiles, and phototropism of its sporangia. In this study, the genetic basis for these changes was sought. Two base pair substitutions were identified in the mutant within a P. blakelesleeanus gene that is homologous to others characterized from fungi, such as the Saccharomyces cerevisiae ERG3 gene, encoding sterol Δ5,6-desaturase. The polyene resistance and growth reduction phenotypes co-segregated with mutations in the gene in genetic crosses. The P. blakelesleeanus wild type ergC gene complemented a S. cerevisiae deletion strain of ERG3. This gene discovery may contribute towards better antifungal use in treating mucormycoses diseases caused by related species in the order Mucorales.

Amphotericin B and Other Polyenes-Discovery, Clinical Use, Mode of Action and Drug Resistance.

Although polyenes were the first broad spectrum antifungal drugs on the market, after 70 years they are still the gold standard to treat a variety of fungal infections. Polyenes such as amphotericin B have a controversial image. They are the antifungal drug class with the broadest spectrum, resistance development is still relatively rare and fungicidal properties are extensive. Yet, they come with a significant host toxicity that limits their use. Relatively recently, the mode of action of polyenes has been revised, new mechanisms of drug resistance were discovered and emergent polyene resistant species such as Candida auris entered the picture. This review provides a short description of the history and clinical use of polyenes, and focusses on the ongoing debate concerning their mode of action, the diversity of resistance mechanisms discovered to date and the most recent trends in polyene resistance development.

Genotype Analysis of Clinical Candida albicans Isolates Using PCRs Targeting 25S rDNA and ALT Repeat Sequences of the RPS and Antifungal Susceptibility in Ouagadougou (Burkina Faso).

ObjectiveCandida albicans is a yeast with multiple genotypes. It’s a commensal fungus colonizing various sites. However, when the host’s immune system weakens, it becomes pathogenic and is responsible for various lesions. In Burkina Faso, antifungal drugs are frequently used, particularly fluconazole, the most used systemic antifungal. This antifungal drug and other antifungal drugs are often used for self-medication or prescribed outside of antifungal susceptibility test results. These situations led to the emergence of Candida albicans strains resistant to antifungal drugs commonly used in Burkina Faso. The aim of this study was to determine the types of Candida albicans using PCRs targeting 25S rDNA and ALT repeat sequences of the RPS and to establish their azoles and polyenes susceptibility profile.Material and methodsAntifungal susceptibility testing by disk diffusion method was performed in accordance with CLSI document M44-A for yeasts and the manufacturer’s instructions. Candida albicans isolates were genotyped using specific PCR primers of the rDNA and RPS genes.ResultsTen (10) RPS types of Candida albicans were found in our study: The most common RPS types are A3 (40.6%), A2 (24.0%) and A2/3 (14.6%) for genotype A, B2/3 (5.2%) for genotype B and C2 (3.2%) for genotype C. The Azole resistance, especially fluconazole (74.4%), was the most common with genotype A, including A3 (36.6%), A2 (18. 3%). Polyene resistance was rare with nystatin, only A3 (1.2%) resistant isolate to nystatin was observed. For amphotericin B, the highest observed resistance rates were A3 (11.0%) and A2/3 (8.5%) for the genotype A and B2 (10.0%), B3 (10.0%) and B2/3 (10.0%) for genotype B.ConclusionOur study showed that Candida albicans resistance to azoles, especially to fluconazole, is an important phenomenon in Ouagadougou, and several genotypes RPS types are involved. Thus, fluconazole would not be an antifungal agent for first-line prescribing for treatment of candidiasis in Ouagadougou. This study will be continued to determine the molecular mechanisms involved in these antifungal resistances, for further research of new molecules with different action targets.

Aspergillus terreus: Novel lessons learned on amphotericin B resistance.

The polyene antifungal amphotericin B (AmB) exerts a powerful and broad activity against a vast array of fungi and in general displays a remarkably low rate of antimicrobial resistance. Aspergillus terreus holds an exceptional position among the Aspergilli due to its intrinsic AmB resistance, in vivo and in vitro. Until now, the underlying mechanisms of polyene resistance were not well understood. This review will highlight the molecular basis of A. terreus and AmB resistance recently gained and will display novel data on the mode of action of AmB. A main focus is set on fundamental stress response pathways covering the heat shock proteins (Hsp) 90/Hsp70 axis, as well as reactive oxygen species detoxifying enzymes in response to AmB. The effect on main cellular functions such as fungal respiration will be addressed in detail and resistance mechanisms will be highlighted. Based on these novel findings we will discuss new molecular targets for alternative options in the treatment of invasive infections due to A. terreus.

Does the use of antifungal agents in agriculture and food foster polyene resistance development? A reason for concern.

Environmental fungicides are used in agriculture to reduce fungal spoilage of crops to a minimum, and the polyene macrolide natamycin is used as a food preservative. The use of natamycin in yoghurt has recently been authorised in the USA and some other countries. However, resistance development is a serious risk associated with the use of antimicrobials as food additives and environmental fungicides. Cross-resistance between agricultural and medical azoles and between azoles and amphotericin B (AMB) not being used in agriculture has been demonstrated in clinical and environmental isolates. Polyene resistance can be elicited in vitro by the use of subinhibitory polyene concentrations and a large number of transfers. This condition may mirror the exposure of faecal Candida spp. to natamycin following consumption of natamycin-containing food. A large number of environmental and clinical isolates are resistant to AMB, and strong evidence linking farm antibiotic use and multidrug resistance, including AMB resistance, in human infections has been provided. In contrast to the acquisition of resistant environmental strains, consumption of natamycin-containing food may expose the gastrointestinal fungal flora directly to resistance selective pressure. So far, whether natamycin itself may cause the emergence of polyene resistance in gastrointestinal fungal flora and/or may act as an AMB resistance selector is probable but speculative. Use of any anti-infective agent as a food preservative should be limited to an absolute minimum as the clinical efficacy of anti-infectives used to treat serious life-threatening infections has to be preserved.

Molecular bases of antifungal resistance in filamentous fungi

Invasive mould infections due to Aspergillus spp., Fusarium spp., Scedosporium spp. and other filamentous fungi remain a significant cause of morbidity and mortality in immunocompromised patients. Antifungal therapy is required for successful patient management; however, limited antifungal drugs as well as the emergence of drug resistance pose a challenge to clinicians for effective management of these diseases. The evolution of multidrug-resistant strains owing to selective pressure as well as intrinsically-resistant pathogenic species are a major concern. Thus, it is necessary to gain a better insight into the antifungal resistance mechanisms and their clinical impact. So far, the resistance mechanisms responsible for acquired azole resistance include alteration of the drug target and its overexpression, biofilm formation and efflux pump upregulation. However, mechanisms responsible for polyene and echinocandin resistance are less understood in filamentous moulds, primarily due to incomplete correlation between in vitro susceptibility and clinical response to treatment. The present review gives an insight into the mechanisms implicated in resistance to different classes of antifungal agents both in human and plant pathogenic filamentous fungi. Furthermore, the spectrum of antifungal resistance in these filamentous fungi is highlighted.

In vitro and in vivo role of heat shock protein 90 in Amphotericin B resistance of Aspergillus terreus

Aspergillus terreus (A. terreus) is of serious concern because of a high propensity to dissemination and in vitro and in vivo resistance to Amphotericin B (AmB). The underlying molecular mechanism of AmB is not known yet and here we want to explore whether fungal heat shock protein 90 (HSP90) is involved in polyene resistance in A. terreus. AmB-susceptible (ATS) and AmB-resistant (ATR) A. terreus and AmB-susceptible Aspergillus fumigatus (AFS) were investigated in response to AmB with a special focus on HSP90. HSP90 inhibitors resulted in significant improvement of AmB activity against ATR as minimum inhibitory concentrations (MIC) decreased from 32 to 0.38 mg/L. Gene expression profiling showed a greater basal amount of HSP90 levels in ATR and ATS when compared with AFS. HSP90 blockers in combination with AmB were evaluated in a murine model of disseminated aspergillosis. HSP90 inhibitors were not beneficial for mice infected with ATR, and neither mono- nor combination treatment with AmB yielded clinical improvement. HSP90 inhibition with 17-allylamino-17-demethoxygeldanamycin (17-AAG) was harmful. HSP90 seems to play a vital role in antifungal stress response in all aspergilli tested, whereas HSP90 does not substantiate the origin of AmB resistance in ATR.

Treating Resistant Filamentous Fungi Infections

Resistance to treatment can be caused by intrinsic resistance to antifungal drugs, and also by the angioinvasive nature of some of these mycoses, which causes distal pulmonary infarction. As blood flow is blocked, antifungal drug penetraiton is decreased. Both considerations must be addressed. With the development of infarction, there is decreased penetration of polyenes and presumably all antifungal drugs into tissue. Inadequate delivery of antifungal drugs is one reason which causes in vitro susceptible pathogens, like Aspergillus species (triazoles) or zygomycetes (polyenes) to progress in the presence of antifungal therapy which “should be effective”. Management of clinical resistance in these patient can be improved at the outset, by accelerating the speed of diagnosis and initiating treatment more promptly. Tools for this include a) rapidlly identifying patients at high risk and b) intensive surveillance with serum (and now bronchoalveolar lavage) galactomannan, beta-D-glucan, and PCR. The former allows identification of groups for antifungal prophylaxis, and the latter allows for identifcation of patients ever earlier in the course of disease. There is room for considerable improvement in rapid diagnosis. Intrinsically high resistance to antifungals can be predicted for many (not all) isolates simply by identifying the fungal species. This is straightforward for polyene resistance of Aspergillus terreus, or almost pan-antifungal resistance of Scedosporium prolificans. Others, such as Fusarium or Paecilomyces species, are very challenging, but may respond to high levels of certain antifungals. For these patients animal studies and small clinical series may provde guidance. There is great temptation to use combinations of antifungal drugs, especially when synergy is suggested by in vitro studies. This area is complicated by the rarity of such isolates and inability to collect large series of infected patients. For some, recommendations may need to be individualls tailored. Abstracts for SupplementInternational Journal of Infectious DiseasesVol. 14Preview Full-Text PDF Open Archive

A Nonsense Mutation in the ERG6 Gene Leads to Reduced Susceptibility to Polyenes in a Clinical Isolate of Candida glabrata

Unlike the molecular mechanisms that lead to azole drug resistance, the molecular mechanisms that lead to polyene resistance are poorly documented, especially in pathogenic yeasts. We investigated the molecular mechanisms responsible for the reduced susceptibility to polyenes of a clinical isolate of Candida glabrata. Sterol content was analyzed by gas-phase chromatography, and we determined the sequences and levels of expression of several genes involved in ergosterol biosynthesis. We also investigated the effects of the mutation harbored by this isolate on the morphology and ultrastructure of the cell, cell viability, and vitality and susceptibility to cell wall-perturbing agents. The isolate had a lower ergosterol content in its membranes than the wild type, and the lower ergosterol content was found to be associated with a nonsense mutation in the ERG6 gene and induction of the ergosterol biosynthesis pathway. Modifications of the cell wall were also seen, accompanied by increased susceptibility to cell wall-perturbing agents. Finally, this mutation, which resulted in a marked fitness cost, was associated with a higher rate of cell mortality. Wild-type properties were restored by complementation of the isolate with a centromeric plasmid containing a wild-type copy of the ERG6 gene. In conclusion, we have identified the molecular event responsible for decreased susceptibility to polyenes in a clinical isolate of C. glabrata. The nonsense mutation detected in the ERG6 gene of this isolate led to a decrease in ergosterol content. This isolate may constitute a useful tool for analysis of the relevance of protein trafficking in the phenomena of azole resistance and pseudohyphal growth.

Resistance to Antifungal Agents: Mechanisms and Clinical Impact

Despite advances in preventive, diagnostic, and therapeutic interventions, invasive fungal infections cause significant morbidity and mortality in immunocompromised patients. The burden of antifungal resistance in such high-risk patients is becoming a major concern. A better understanding of the mechanisms and clinical impact of antifungal resistance is essential to the prompt and efficient treatment of patients with invasive mycoses and to improving the outcome of such infections. Although recent guidelines have attempted to standardize antifungal susceptibility testing, limitations still exist as a result of the incomplete correlation between in vitro susceptibility and clinical response to treatment. Four major mechanisms of resistance to azoles have been identified, all of which rely on altered gene expression. Mechanisms responsible for polyene and echinocandin resistance are less well understood. In addition to discussing the molecular mechanisms of antifungal resistance, this article elaborates on the concept of clinical resistance, which is critical to the understanding of treatment failure in patients with invasive fungal infections.

Antimicrobial resistance prevention initiative—an update: Proceedings of an expert panel on resistance

Antimicrobial resistance is a growing problem that complicates the treatment of important nosocomial and community-acquired infections. It is a worldwide problem that spans the range of human pathogens, including bacteria, fungi, and viruses. This update from the Antimicrobial Resistance Prevention Initiative (ARPI) provides a review of some important trends in antibiotic, antifungal, and antiviral resistance. Areas of focus include multidrug-resistant bacteria in the hospital setting; the growing problem of community-acquired methicillin-resistant Staphylococcus aureus; triazole and polyene resistance in nosocomial infections caused by non-Candida albicans or Aspergillus species, and the utility of in vitro susceptibility testing for these fungal infections; antiviral resistance in alpha- or beta-herpesviruses causing genital herpes or cytomegalovirus infection in immunocompromised hosts; and concerns about a possible pandemic involving avian influenza A and the importance of minimizing emergence of resistant strains of this highly pathogenic virus. The challenges in each area are different, but the general keys to addressing the growing problem of antimicrobial resistance continue to be responsible antimicrobial stewardship and the development of newer antimicrobial agents.

Antimicrobial Resistance Prevention Initiative—An Update: Proceedings of an Expert Panel on Resistance

Antimicrobial resistance is a growing problem that complicates the treatment of important nosocomial and community-acquired infections. It is a worldwide problem that spans the range of human pathogens, including bacteria, fungi, and viruses. This update from the Antimicrobial Resistance Prevention Initiative (ARPI) provides a review of some important trends in antibiotic, antifungal, and antiviral resistance. Areas of focus include multidrug-resistant bacteria in the hospital setting; the growing problem of community-acquired methicillin-resistant Staphylococcus aureus; triazole and polyene resistance in nosocomial infections caused by non– Candida albicans or Aspergillus species, and the utility of in vitro susceptibility testing for these fungal infections; antiviral resistance in α- or β-herpesviruses causing genital herpes or cytomegalovirus infection in immunocompromised hosts; and concerns about a possible pandemic involving avian influenza A and the importance of minimizing emergence of resistant strains of this highly pathogenic virus. The challenges in each area are different, but the general keys to addressing the growing problem of antimicrobial resistance continue to be responsible antimicrobial stewardship and the development of newer antimicrobial agents.

Reduced Susceptibility to Polyenes Associated with a Missense Mutation in the ERG6 Gene in a Clinical Isolate of Candida glabrata with Pseudohyphal Growth

Little information is available about the molecular mechanisms responsible for polyene resistance in pathogenic yeasts. A clinical isolate of Candida glabrata with a poor susceptibility to polyenes, as determined by disk diffusion method and confirmed by determination of MIC, was recovered from a patient treated with amphotericin B. Quantitative analysis of sterols revealed a lack of ergosterol and an accumulation of late sterol intermediates, suggesting a defect in the final steps of the ergosterol pathway. Sequencing of CgERG11, CgERG6, CgERG5, and CgERG4 genes revealed exclusively a unique missense mutation in CgERG6 leading to the substitution of a cysteine by a phenylalanine in the corresponding protein. In addition, real-time reverse transcription-PCR demonstrated an overexpression of genes encoding enzymes involved in late steps of the ergosterol pathway. Moreover, this isolate exhibited a pseudohyphal growth whatever the culture medium used, and ultrastructural changes of the cell wall of blastoconidia were seen consisting in a thinner inner layer. Cell wall alterations were also suggested by the higher susceptibility of growing cells to Calcofluor white. Additionally, complementation of this isolate with a wild-type copy of the CgERG6 gene restored susceptibility to polyenes and a classical morphology. Together, these results demonstrated that mutation in the CgERG6 gene may lead to a reduced susceptibility to polyenes and to a pseudohyphal growth due to the subsequent changes in sterol content of the plasma membrane.

Nonsterol related resistance in Ustilago maydis to the polyene antifungals, amphotericin B and nystatin

Two amphotericin B resistant mutants of Ustilago maydis were isolated following direct selection from a wild-type population. Each mutant was demonstrated to be cross-resistant to nystatin yet remained sensitive to azoles. Sterol analysis indicated a sterol profile similar to the parent strain, precluding the involvement of an alteration in ergosterol biosynthesis as the cause of polyene resistance.

Protein hypersecretory Trichoderma reesei mutant RUT C-30 displays increased ethanol and polyene resistance

Addition of low amounts of ethanol (0.5–2%, v/v) or of polyene antibiotics (nystatine, amphotericin B 0.5–2 mg l −1, respectively) to cultures of T. reesei QM 9414 in the early phase of mycelial growth, arrested the secretion of three cellulases (cellobiohydrolase I and II, endoglucanase I) as well as of a xylanase. Enzyme formation by the enzyme-hyperproducing strain T. reesei RUT C-30, in contrast, was less sensitive to these agents. By the use of [U- 14C]- l-valine addition, this blockage of enzyme formation was shown to be due to a general inhibition of protein secretion by ethanol. Ethanol and amphotericin B also arrested the formation of intracellular precursers of cellobiohydrolase I, and the formation of the cbh1-mRNA, hence proving that blockage occurs at a pre-translational level. The higher sensitivity of the lower producing mutant T. reesei QM 9414 correlated with an increased synthesis of a 36 kDa stress protein. The data are discussed with respect to a plasma membrane located resistance to ethanol or polyenes in the hyperproducing mutant T. reesei RUT C-30, probably caused by the use of bile acids as colony restrictor in the course of mutant selection.

Sterols in Candida albicans mutants resistant to polyene or azole antifungals, and of a double mutant C. albicans 6.4.

Investigations of resistant mutants could help resolve differences and similarities in the action of azole and polyene antifungals whose modes of action are related; both disrupt membrane properties, such as permeability, by interfering with membrane sterols--polyenes by direct binding and azoles by inhibiting their synthesis. Studies of laboratory-derived mutants of Candida albicans which have an altered sterol content and/or an altered sterol composition do not provide evidence for a unified mechanism of polyene resistance. Clinical isolates of azole-resistant C. albicans have an increased or unaltered content of ergosterol and are impermeable to azoles. C. albicans 6.4, a laboratory-derived mutant resistant to both polyenes and azoles, is impermeable to azoles and has an increased content of methylated sterols. This unusual sterol composition resembles that of sensitive strains grown in the presence of azoles and may prevent polyene binding.

Incidence of polyene-resistant yeasts recovered from clinical specimens.

The development of resistance to amphotericin B and nystatin in yeast isolates was determined. Organisms recovered from patients on the oncology service, undergoing extensive chemotherapy for acute leukemia and bone marrow transplantation, were compared with yeasts recovered from patients on other services in the same hospital over a 7-month period. An agar dilution method was used to assay the susceptibility for each antibiotic; resistance was defined as a minimal inhibitory concentration of greater than or equal to 2 micrograms/ml for amphotericin B and greater than or equal to 16 micrograms/ml for nystatin. None of 625 isolates from 238 patients on non-oncology services demonstrated polyene resistance. Resistance only occurred in a subpopulation of oncology patients, in which 55 isolates (7.4%) from six patients (8.6%) exhibited polyene resistance. Resistance yeasts included Candida albicans (three strains), Candida tropicalis (one strain), and Torulopsis glabrata (two strains). All of the patients from whom resistant yeasts were recovered had experienced extensive chemotherapy with cytotoxic agents, granulocytopenia, and long-term treatment with both antibacterial and polyene antibiotics. Resistance to 2 micrograms of amphotericin B per ml and to 16 micrograms of nystatin per ml was associated with loss or marked depression of ergosterol in the cell membrane as measured by ultraviolet spectra. A significant incidence of polyene resistance in an oncology subpopulation was documented, suggesting a need for susceptibility testing in patients who are at high risk for development of drug-resistant fungal pathogens.