Amphotericin B Resistance in Aspergillus terreus Is Overpowered by Coapplication of Pro-oxidants

Specificity of Membrane-Associated J-Domain Protein, Caj1, in Amphotericin B Tolerance in Budding Yeast

Hsp70:J-domain protein (JDP) machineries play pivotal roles in maintaining cellular proteostasis and governing various aspects of fungal physiology. While Hsp70 is known for its involvement in conferring tolerance to diverse antifungal drugs, the specific contribution of JDPs remains unclear. In this study, we examined the sensitivity of cytosolic JDP deletion strains of budding yeast to amphotericin B (AmB), a polyene antifungal agent widely utilized in fungal disease treatment due to its ability to disrupt the fungal plasma membrane (PM). Deleting Caj1, a PM-associated class II JDP, heightened susceptibility to AmB, and the protection conferred by Caj1 against AmB necessitated both its N-terminal J-domain and C-terminal lipid binding domain. Moreover, Caj1 deficiency compromised PM integrity as evidenced by increased phosphate efflux and exacerbated AmB sensitivity, particularly at elevated temperatures. Notably, phytosphingosine (PHS) addition as well as overexpression of PMP3, a positive PM integrity regulator, significantly rescued AmB sensitivity of caj1Δ cells. Our results align with the notion that Caj1 associates with the PM and cooperates with Hsp70 to regulate PM proteostasis, thereby influencing PM integrity in budding yeast. Loss of Caj1 function at the PM compromises PM protein quality control, thereby rendering yeast cells more susceptible to AmB.

Inorganic Nanoparticles: Tools to Emphasize the Janus Face of Amphotericin B

Amphotericin B is the oldest antifungal molecule which is still currently widely used in clinical practice, in particular for the treatment of invasive diseases, even though it is not devoid of side effects (particularly nephrotoxicity). Recently, its redox properties (i.e., both prooxidant and antioxidant) have been highlighted in the literature as mechanisms involved in both its activity and its toxicity. Interestingly, similar properties can be described for inorganic nanoparticles. In the first part of the present review, the redox properties of Amphotericin B and inorganic nanoparticles are discussed. Then, in the second part, inorganic nanoparticles as carriers of the drug are described. A special emphasis is given to their combined redox properties acting either as a prooxidant or as an antioxidant and their connection to the activity against pathogens (i.e., fungi, parasites, and yeasts) and to their toxicity. In a majority of the published studies, inorganic nanoparticles carrying Amphotericin B are described as having a synergistic activity directly related to the rupture of the redox homeostasis of the pathogen. Due to the unique properties of inorganic nanoparticles (e.g., magnetism, intrinsic anti-infectious properties, stimuli-triggered responses, etc.), these nanomaterials may represent a new generation of medicine that can synergistically enhance the antimicrobial properties of Amphotericin B.

Decoding the antifungal resistance mechanisms in biofilms of emerging, ubiquitous and multidrug-resistant species belonging to the Scedosporium/Lomentospora genera

The opportunistic filamentous fungi belonging to the Scedosporium and Lomentospora genera are highly tolerant to all classes of available antifungal drugs. Moreover, the mature biofilm formed by these fungi presents higher antifungal resistance when compared to planktonic cells. Nevertheless, the resistance mechanisms developed by the biofilm lifestyle are not completely elucidated. In the current study, we have investigated the mainly known resistance mechanisms to azoles (voriconazole and fluconazole) and polyenes (amphotericin B - AMB) in S. apiospermum, S. minutisporum, S. aurantiacum, and L. prolificans (formerly S. prolificans) biofilms. Both classes of antifungals can physically bind to the extracellular matrix of mature biofilms, preventing the drugs from reaching their targets on biofilm-forming cells, which precludes their activity and toxicity. In addition, the activity of efflux pumps, measured by Rhodamine 6 G, was increased along the maturation of the biofilm. The efflux pump's inhibition by L-Phe-L-Arg-β-naphthylamide culminated in a 2- to 16-fold increase in azole susceptibility in conidial cells, but not in mature biofilms. Finally, we demonstrated by using specific inhibitors that in conidia, but not in biofilms, AMB induced the production of reactive oxygen species through the activity of the oxidative phosphorylation system (complex I to IV and alternative oxidases). However, the cellular redox imbalance caused by AMB was well coped with the high activity of antioxidative enzymes, such as superoxide dismutase and catalase. Altogether, our results revealed that Scedosporium/Lomentospora biofilm resistance occurs through various mechanisms that operate concomitantly, which could explain the huge challenge in the clinical treatment of scedosporiosis/lomentosporiosis.

Adaptive Role of Cell Death in Yeast Communities Stressed with Macrolide Antifungals

Microorganisms cooperate with each other to protect themselves from environmental stressors. An extreme case of such cooperation is regulated cell death for the benefit of other cells. Dying cells can provide surviving cells with nutrients or induce their stress response by transmitting an alarm signal; however, the role of dead cells in microbial communities is unclear. Here, we searched for types of stressors the protection from which can be achieved by death of a subpopulation of cells. Thus, we compared the survival of Saccharomyces cerevisiae cells upon exposure to various stressors in the presence of additionally supplemented living versus dead cells. We found that dead cells contribute to yeast community resistance against macrolide antifungals (e.g., amphotericin B [AmB] and filipin) to a greater extent than living cells. Dead yeast cells absorbed more macrolide filipin than control cells because they exposed intracellular sterol-rich membranes. We also showed that, upon the addition of lethal concentrations of AmB, supplementation with AmB-sensitive cells but not with AmB-resistant cells enabled the survival of wild-type cells. Together, our data suggest that cell-to-cell heterogeneity in sensitivity to AmB can be an adaptive mechanism helping yeast communities to resist macrolides, which are naturally occurring antifungal agents.

IMPORTANCE Eukaryotic microorganisms harbor elements of programmed cell death (PCD) mechanisms that are homologous to the PCD of multicellular metazoa. However, it is still debated whether microbial PCD has an adaptive role or whether the processes of cell death are an aimless operation in self-regulating molecular mechanisms. Here, we demonstrated that dying yeast cells provide an instant benefit for their community by absorbing macrolides, which are bacterium-derived antifungals. Our results illustrate the principle that the death of a microorganism can contribute to the survival of its kin and suggest that early plasma membrane permeabilization improves community-level protection. The latter makes a striking contrast to the manifestations of apoptosis in higher eukaryotes, the process by which plasma membranes maintain integrity.

The Elevated Endogenous Reactive Oxygen Species Contribute to the Sensitivity of the Amphotericin B-Resistant Isolate of Aspergillus flavus to Triazoles and Echinocandins

Aspergillus flavus has been frequently reported as the second cause of invasive aspergillosis (IA), as well as the leading cause in certain tropical countries. Amphotericin B (AMB) is a clinically important therapy option for a range of invasive fungal infections including invasive aspergillosis, and in vitro resistance to AMB was associated with poor outcomes in IA patients treated with AMB. Compared with the AMB-susceptible isolates of A. terreus, the AMB-resistant isolates of A. terreus showed a lower level of AMB-induced endogenous reactive oxygen species (ROS), which was an important cause of AMB resistance. In this study, we obtained one AMB-resistant isolate of A. flavus, with an AMB MIC of 32 μg/mL, which was sensitive to triazoles and echinocandins. This isolate presented elevated endogenous ROS levels, which strongly suggested that no contribution of decreased AMB-induced endogenous ROS for AMB-resistance, opposite to those observed in A. terreus. Further, we confirmed that the elevated endogenous ROS contributed to the sensitivity of the AMB-resistant A. flavus isolate to triazoles and echinocandins. Further investigation is needed to elucidate the causes of elevated endogenous ROS and the resistance mechanism to AMB in A. flavus.

A 20-Year Antifungal Susceptibility Surveillance (From 1999 to 2019) for Aspergillus spp. and Proposed Epidemiological Cutoff Values for Aspergillus fumigatus and Aspergillus flavus: A Study in a Tertiary Hospital in China

The emergence of resistant Aspergillus spp. is increasing worldwide. Long-term susceptibility surveillance for clinically isolated Aspergillus spp. strains is warranted for understanding the dynamic change in susceptibility and monitoring the emergence of resistance. Additionally, neither clinical breakpoints (CBPs) nor epidemiological cutoff values (ECVs) for Aspergillus spp. in China have been established. In this study, we performed a 20-year antifungal susceptibility surveillance for 706 isolates of Aspergillus spp. in a clinical laboratory at Peking University First Hospital from 1999 to 2019; and in vitro antifungal susceptibility to triazoles, caspofungin, and amphotericin B was determined by the Clinical and Laboratory Standards Institute (CLSI) broth microdilution method. It was observed that Aspergillus fumigatus was the most common species, followed by Aspergillus flavus and Aspergillus terreus. Forty isolates (5.7%), including A. fumigatus, A. flavus, A. terreus, Aspergillus niger, and Aspergillus nidulans, were classified as non-wild type (non-WT). Importantly, multidrug resistance was observed among A. flavus, A. terreus, and A. niger isolates. Cyp51A mutations were characterized for 19 non-WT A. fumigatus isolates, and TR34/L98H/S297T/F495I was the most prevalent mutation during the 20-year surveillance period. The overall resistance trend of A. fumigatus increased over 20 years in China. Furthermore, based on ECV establishment principles, proposed ECVs for A. fumigatus and A. flavus were established using gathered minimum inhibitory concentration (MIC)/minimum effective concentration (MEC) data. Consequently, all the proposed ECVs were identical to the CLSI ECVs, with the exception of itraconazole against A. flavus, resulting in a decrease in the non-WT rate from 6.0 to 0.6%.

Aspergillus terreus Species Complex

Infections due to Aspergillus species are an acute threat to human health; members of the Aspergillus section Fumigati are the most frequently occurring agents, but depending on the local epidemiology, representatives of section Terrei or section Flavi are the second or third most important. Aspergillus terreus species complex is of great interest, as it is usually amphotericin B resistant and displays notable differences in immune interactions in comparison to Aspergillus fumigatus. The latest epidemiological surveys show an increased incidence of A. terreus as well as an expanding clinical spectrum (chronic infections) and new groups of at-risk patients being affected. Hallmarks of these non-Aspergillus fumigatus invasive mold infections are high potential for tissue invasion, dissemination, and possible morbidity due to mycotoxin production. We seek to review the microbiology, epidemiology, and pathogenesis of A. terreus species complex, address clinical characteristics, and highlight the underlying mechanisms of amphotericin B resistance. Selected topics will contrast key elements of A. terreus with A. fumigatus. We provide a comprehensive resource for clinicians dealing with fungal infections and researchers working on A. terreus pathogenesis, aiming to bridge the emerging translational knowledge and future therapeutic challenges on this opportunistic pathogen.

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.

Unmasking the Amphotericin B Resistance Mechanisms in Candida haemulonii Species Complex

The polyene amphotericin B (AMB) exerts a powerful and broad antifungal activity. AMB acts by (i) binding to ergosterol, leading to pore formation at the fungal plasma membrane with subsequent ion leakage, and (ii) inducing the intracellular accumulation of reactive oxygen species (ROS). Herein, we have deciphered the AMB resistance mechanisms in clinical isolates of Candida haemulonii complex (C. haemulonii, C. duobushaemulonii, C. haemulonii var. vulnera) in comparison to other clinically relevant non-albicans Candida species. Membrane gas chromatography-mass spectrometry analysis revealed that the vast majority of sterols were composed of ergosterol pathway intermediates, evidencing the absence of AMB target. Supporting this data, C. haemulonii species complex demonstrated poor membrane permeability after AMB treatment. Regarding the oxidative burst, AMB induced the formation of ROS in all species tested; however, this phenomenon was slightly seen in C. haemulonii complex isolates. Our results indicated that these isolates displayed altered respiratory status, as revealed by their poor growth in nonfermented carbon sources, low consumption of oxygen, and derisive mitochondrial membrane potential. The use of specific inhibitors of mitochondrial respiratory chain (complex I-IV) revealed no effects on the yeast growth, highlighting the metabolic shift to fermentative pathway in C. haemulonii strains. Also, C. haemulonii complex proved to be highly resistant to oxidative burst agents, which can be correlated with a high activity of antioxidant enzymes. Our data demonstrated primary evidence suggesting that ergosterol content, mitochondrial function, and fungal redox homeostasis are involved in AMB fungicidal effects and might explain the resistance presented in this multidrug-resistant, emergent, and opportunistic fungal complex.

Antifungal resistance in Aspergillus terreus: A current scenario

Invasive aspergillosis caused by intrinsically resistant non-fumigatus Aspergillus species displays a poor outcome in immunocompromised patients. The polyene antifungal amphotericin B (AmB) remains to be "gold standard" in the treatment of invasive fungal infections. Aspergillus terreus is innately resistant to AmB, in vivo and in vitro. Till now, the exact mode of action in polyene resistance is not well understood. This review highlights the underlying molecular basis of AmB resistance in A. terreus, displaying data obtained from AmB susceptible A. terreus and AmB resistant A. terreus strains. The effect of AmB on main cellular and molecular functions such as fungal respiration and stress response pathways will be discussed in detail and resistance mechanisms will be highlighted. The fungal stress response machinery seems to be a major player in the onset of AmB resistance in A. terreus.

Stress-Induced Changes in the Lipid Microenvironment of β-(1,3)-d-Glucan Synthase Cause Clinically Important Echinocandin Resistance in Aspergillus fumigatus

Resistance to first-line triazole antifungal agents among Aspergillus species has prompted the use of second-line therapy with echinocandins. As the number of Aspergillus-infected patients treated with echinocandins is rising, clinical observations of drug resistance are also increasing, indicating an emerging global health threat. Our knowledge regarding the development of clinical echinocandin resistance is largely derived from Candida spp., while little is known about resistance in Aspergillus. Therefore, it is important to understand the specific cellular responses raised by A. fumigatus against echinocandins. We discovered a new mechanism of resistance in A. fumigatus that is independent of the well-characterized FKS mutation mechanism observed in Candida. This study identified an off-target effect of CAS, i.e., ROS production, and integrated oxidative stress and sphingolipid alterations into a novel mechanism of resistance. This stress-induced response has implications for drug resistance and/or tolerance mechanisms in other fungal pathogens.

Aspergillus fumigatus is a leading cause of invasive fungal infections. Resistance to first-line triazole antifungals has led to therapy with echinocandin drugs. Recently, we identified several high-minimum-effective-concentration (MEC) A. fumigatus clinical isolates from patients failing echinocandin therapy. Echinocandin resistance is known to arise from amino acid substitutions in β-(1,3)-d-glucan synthase encoded by the fks1 gene. Yet these clinical isolates did not contain mutations in fks1, indicating an undefined resistance mechanism. To explore this new mechanism, we used a laboratory-derived strain, RG101, with a nearly identical caspofungin (CAS) susceptibility phenotype that also does not contain fks1 mutations. Glucan synthase isolated from RG101 was fully sensitive to echinocandins. Yet exposure of RG101 to CAS during growth yielded a modified enzyme that was drug insensitive (4 log orders) in kinetic inhibition assays, and this insensitivity was also observed for enzymes isolated from clinical isolates. To understand this alteration, we analyzed whole-enzyme posttranslational modifications (PTMs) but found none linked to resistance. However, analysis of the lipid microenvironment of the enzyme with resistance induced by CAS revealed a prominent increase in the abundances of dihydrosphingosine (DhSph) and phytosphingosine (PhSph). Exogenous addition of DhSph and PhSph to the sensitive enzyme recapitulated the drug insensitivity of the CAS-derived enzyme. Further analysis demonstrated that CAS induces mitochondrion-derived reactive oxygen species (ROS) and that dampening ROS formation by antimycin A or thiourea eliminated drug-induced resistance. We conclude that CAS induces cellular stress, promoting formation of ROS and triggering an alteration in the composition of plasma membrane lipids surrounding glucan synthase, rendering it insensitive to echinocandins.

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.

Treatment of Infections Due to Aspergillus terreus Species Complex

The Aspergillus terreus species complex is found in a wide variety of habitats, and the spectrum of diseases caused covers allergic bronchopulmonary aspergillosis, Aspergillus bronchitis and/or tracheobronchitis, and invasive and disseminated aspergillosis. Invasive infections are a significant cause of morbidity and mortality mainly in patients with hematological malignancy. The section Terrei covers a total of 16 accepted species of which most are amphotericin B resistant. Triazoles are the preferred agents for treatment and prevention of invasive aspergillosis. Poor prognosis in patients with invasive A. terreus infections seems to be independent of anti-Aspergillus azole-based treatment.

Impact of Morphological Sectors on Antifungal Susceptibility Testing and Virulence Studies

Morphological heterogeneity of Aspergillus terreus cultures was observed during continued cultivation of amphotericin B (AMB)-resistant isolates on drug-free medium. Outgrowth leads to the emergence of multiple sectors that might result from increased growth rates at drug-free conditions. We evaluated the differences in AMB susceptibility and virulence between sector subcultures (ATSec), AMB-resistant (ATR) strains, and AMB-susceptible (ATS) strains. By comparing A. terreus AMB-resistant (ATR) strains and A. terreus sector (ATSec) cultures we observed a highly significant reduction of AMB MICs in ATSec (ATR MIC, 2 to 32 μg/ml; ATSec MIC, 0.12 to 5 μg/ml). Furthermore, Galleria mellonella survival studies revealed an enhanced virulence of ATSec, which was comparable with that of AMB-sensitive Aspergillus terreus strains (median survival rates for ATS isolates, 72 h; for ATSec isolate ATSec^G1, 84 h; for ATR isolates, 144 h). Our findings clearly demonstrate that spontaneous culture degeneration occurs in A. terreus and, most importantly, crucially impacts drug efficacy and virulence.

Induction of Mitochondrial Reactive Oxygen Species Production by Itraconazole, Terbinafine, and Amphotericin B as a Mode of Action against Aspergillus fumigatus

Drug resistance in fungal pathogens is of incredible importance to global health, yet the mechanisms of drug action remain only loosely defined. Antifungal compounds have been shown to trigger the intracellular accumulation of reactive oxygen species (ROS) in human-pathogenic yeasts, but the source of those ROS remained unknown. In the present study, we examined the role of endogenous ROS for the antifungal activity of the three different antifungal substances itraconazole, terbinafine, and amphotericin B, which all target the fungal cell membrane. All three antifungals had an impact on fungal redox homeostasis by causing increased intracellular ROS production. Interestingly, the elevated ROS levels induced by antifungals were abolished by inhibition of the mitochondrial respiratory complex I with rotenone. Further, evaluation of lipid peroxidation using the thiobarbituric acid assay revealed that rotenone pretreatment decreased ROS-induced lipid peroxidation during incubation of Aspergillus fumigatus with itraconazole and terbinafine. By applying the mitochondrion-specific lipid peroxidation probe MitoPerOx, we also confirmed that ROS are induced in mitochondria and subsequently cause significant oxidation of mitochondrial membrane in the presence of terbinafine and amphotericin B. To summarize, our study suggests that the induction of ROS production contributes to the ability of antifungal compounds to inhibit fungal growth. Moreover, mitochondrial complex I is the main source of deleterious ROS production in A. fumigatus challenged with antifungal compounds.

Increasing the Fungicidal Action of Amphotericin B by Inhibiting the Nitric Oxide-Dependent Tolerance Pathway

Amphotericin B (AmB) induces oxidative and nitrosative stresses, characterized by production of reactive oxygen and nitrogen species, in fungi. Yet, how these toxic species contribute to AmB-induced fungal cell death is unclear. We investigated the role of superoxide and nitric oxide radicals in AmB's fungicidal activity in Saccharomyces cerevisiae, using a digital microfluidic platform, which enabled monitoring individual cells at a spatiotemporal resolution, and plating assays. The nitric oxide synthase inhibitor L-NAME was used to interfere with nitric oxide radical production. L-NAME increased and accelerated AmB-induced accumulation of superoxide radicals, membrane permeabilization, and loss of proliferative capacity in S. cerevisiae. In contrast, the nitric oxide donor S-nitrosoglutathione inhibited AmB's action. Hence, superoxide radicals were important for AmB's fungicidal action, whereas nitric oxide radicals mediated tolerance towards AmB. Finally, also the human pathogens Candida albicans and Candida glabrata were more susceptible to AmB in the presence of L-NAME, pointing to the potential of AmB-L-NAME combination therapy to treat fungal infections.

Oxidative Stress Response Tips the Balance in Aspergillus terreus Amphotericin B Resistance

In this study, we characterize the impact of antioxidative enzymes in amphotericin B (AmB)-resistant (ATR) and rare AmB-susceptible (ATS) clinical Aspergillus terreus isolates. We elucidate expression profiles of superoxide dismutase (SOD)- and catalase (CAT)-encoding genes, enzymatic activities of SODs, and superoxide anion production and signaling pathways involved in the oxidative stress response (OSR) in ATS and ATR strains under AmB treatment conditions. We show that ATR strains possess almost doubled basal SOD activity compared to that of ATS strains and that ATR strains exhibit an enhanced OSR, with significantly higher sod2 mRNA levels and significantly increased cat transcripts in ATR strains upon AmB treatment. In particular, inhibition of SOD and CAT proteins renders resistant isolates considerably susceptible to the drug in vitro In conclusion, this study shows that SODs and CATs are crucial for AmB resistance in A. terreus and that targeting the OSR might offer new treatment perspectives for resistant species.

Amphotericin B-copper(II) complex shows improved therapeutic index in vitro

The AmB-Cu(II) complex has recently been reported as an antifungal agent with reduced aggregation of AmB in aqueous solutions, increased anti C. albicans activity and lower toxicity against human cells in vitro. In the present work, investigations of the activity of the AmB-Cu (II) complex against fungal pathogens with varying susceptibility, including C. albicans and C. parapsilosis strains and intrinsically resistant A. niger, and cytotoxicity in normal human dermal fibroblasts (NHDF) in vitro were performed. For better understanding of the mechanism of reduced cytotoxicity and increased fungicidal activity, the influence of the AmB-Cu (II) complex on membrane integrity and accumulation of cellular reactive oxygen species (ROS) and mitochondrial superoxide was compared with that of conventional AmB. In the sensitive C. albicans and C. parapsilosis strains, the AmB-Cu(II) complex showed higher fungicidal activity (the MIC value was 0.35-0.7μg/ml for the AmB-Cu (II) complex, and 0.45-0.9μg/ml for Fungizone) due to increased induction of oxidative damage with rapid inhibition of the ability to reduce tetrazolium dye (MTT). In the NHDF cell line, the CC₅₀ value was 30.13±1.53μg/ml for the AmB-Cu(II) complex and 17.46±1.24μg/ml for (Fungizone), therefore, the therapeutic index (CC₅₀/MIC₉₀) determined in vitro was 86.09-43.04 for the AmB-Cu(II) complex and 38.80-19.40 for Fungizone. The lower cytotoxicity of the AmB-Cu(II) complex in human cells resulted from lower accumulation of cellular and mitochondrial reactive oxygen species. This phenomenon was probably caused by the induction of successful antioxidant defense of the cells. The mechanism of the reduced cytotoxicity of the AmB-Cu(II) complex needs further investigation, but the preliminary results are very promising.

The Role of Signaling via Aqueous Pore Formation in Resistance Responses to Amphotericin B

Drug resistance studies have played an important role in the validation of antibiotic targets. In the case of the polyene antibiotic amphotericin B (AmB), such studies have demonstrated the essential role that depletion of ergosterol plays in the development of AmB-resistant (AmB-R) organisms. However, AmB-R strains also occur in fungi and parasitic protozoa that maintain a normal level of ergosterol at the plasma membrane. Here, I review evidence that shows not only that there is increased protection against the deleterious consequences of AmB-induced ion leakage across the membrane in these resistant pathogens but also that a set of events are activated that block the cell signaling responses that trigger the oxidative damage produced by the antibiotic. Such signaling events appear to be the consequence of a membrane-thinning effect that is exerted upon lipid-anchored Ras proteins by the aqueous pores formed by AmB. A similar membrane disturbance effect may also explain the activity of AmB on mammalian cells containing Toll-like receptors. These resistance mechanisms expand our current understanding of the role that the formation of AmB aqueous pores plays in triggering signal transduction responses in both pathogens and host immune cells.