The clinical stakes are considerable. Invasive fungal infections affect millions of people worldwide each year, and severe fungal disease is associated with a substantial global mortality burden. Clinicians typically rely on antifungal susceptibility testing to guide therapy, choosing drugs based on whether a laboratory isolate appears susceptible at standard drug concentrations. Heteroresistance undermines this logic. Because the resistant subpopulation represents only a fraction of the total cells, standard tests that measure the behavior of the bulk population report a susceptible result, even though a tiny contingent of cells is fully capable of surviving therapeutic drug exposure. Once the susceptible majority is killed by treatment, the resistant minority can repopulate the infection, producing relapse or breakthrough disease that appears inexplicable on paper.

The concept is not new, but its application to fungi is far less mature than its bacterial counterpart. Heteroresistance was first described in bacteria in the mid-twentieth century, with classic studies on streptomycin-resistant variants of Haemophilus influenzae and on methicillin-resistant staphylococci, and it has since been extensively characterized in problems such as heterogeneous vancomycin-intermediate Staphylococcus aureus and colistin heteroresistance in Acinetobacter baumannii. In fungi, however, the review notes that definitions, diagnostic criteria, and clinical implications remain incompletely understood. Fungal heteroresistance also overlaps with related adaptive phenomena, including antifungal tolerance, persistence, and transient stress responses, which complicates both its recognition and its interpretation.

At the molecular level, the mechanisms underlying fungal heteroresistance are diverse and often reversible, which distinguishes them from the stable, mutation-driven resistance that is more familiar in the clinic. A central theme is genomic plasticity. Many pathogenic fungi tolerate aneuploidy, the gain or loss of whole chromosomes, far better than most human cells do. Exposure to azole drugs such as fluconazole can select for cells that have duplicated specific chromosomes carrying resistance determinants. In Candida albicans, for example, formation of an isochromosome containing the ERG11 gene, which encodes the lanosterol 14-alpha-demethylase targeted by azoles, increases gene dosage and drives fluconazole resistance. In Cryptococcus neoformans, disomy of specific chromosomes has been shown to confer azole tolerance, and dynamic ploidy changes have been documented in human cryptococcal meningitis cases. Similar whole-chromosome duplications have been implicated in azole responses in Aspergillus fumigatus and Aspergillus flavus.

Crucially, aneuploid-based resistance is often unstable. When drug pressure is removed, the extra chromosomes can be lost, and the population reverts to susceptibility. This reversibility means that a patient’s isolate may test susceptible before therapy, harbor a hidden aneuploid minority during therapy, and present as resistant only after treatment has failed. Other mechanisms contribute as well, including overexpression of efflux pumps such as the ABC transporters Cdr1 and Cdr2, which actively export azole drugs from the cell, alterations in the drug target enzymes themselves, elevated chitin content that buffers cells against echinocandin attack, and calcineurin-dependent stress adaptation pathways that have been linked to caspofungin heteroresistance in Candida glabrata.

The review highlights several clinically alarming examples. In Candida auris, a globally emerging pathogen notorious for multidrug resistance, transcriptomic studies have revealed stress-response programs associated with amphotericin B tolerance, and pan-drug-resistant isolates resistant to all four major antifungal classes have been documented. Perhaps most striking is recent work on Candida parapsilosis, where antifungal heteroresistance was shown to cause prophylaxis failure and facilitate breakthrough infections in patients, demonstrating that the phenomenon is not merely a laboratory curiosity but a direct cause of therapeutic failure. In Cryptococcus neoformans, fluconazole heteroresistance appears to be an intrinsic property of some strains and has been associated with virulence, and epidemiological surveys have documented its prevalence among clinical isolates.

Diagnosis is the weakest link in the chain. Standard broth microdilution and disk diffusion methods report a single minimum inhibitory concentration that reflects the dominant population and is blind to minority subpopulations. Detecting heteroresistance instead requires specialized approaches, such as plating large inocula onto agar containing drug concentrations several-fold above the MIC and quantifying the frequency of surviving colonies, population analysis profiles that plot colony counts across a range of drug concentrations, and time-kill assays that reveal regrowth at inhibitory concentrations. Even these methods lack standardization, and the review emphasizes that the absence of agreed diagnostic criteria, combined with the overlap between heteroresistance, tolerance, and persistence, makes it difficult to compare studies or translate findings into clinical practice.

The therapeutic implications are profound. If a resistant subpopulation exists at a frequency of one in a million cells, an infection carrying a large fungal burden contains an enormous absolute number of pre-resistant cells, and monotherapy at standard dosing may simply enrich for them. This reasoning has already influenced thinking about cryptococcal meningitis, where fluconazole monotherapy has been shown to be a suboptimal initial option partly because of the emergence of resistance. Combination therapy, higher drug doses, therapeutic drug monitoring, and rapid escalation strategies may all need to be reconsidered in light of heteroresistance, particularly for immunocompromised patients who depend on prophylaxis and have little immunological reserve to compensate for a failing drug.

The review also points to the broader evolutionary context. Fungal genomes are remarkably dynamic, with polyploidy, aneuploidy, and copy-number variation serving as rapid, reversible strategies for adaptation to stress. Aneuploidy is a double-edged sword: it imposes fitness costs in drug-free conditions but provides swift solutions under selection, and it can generate cross-adaptation, as seen when fluconazole adaptation via aneuploidy in Cryptococcus also conferred tolerance to amphotericin B and flucytosine. Understanding these genome dynamics is therefore essential not only for explaining individual treatment failures but also for anticipating how the widespread use of antifungals in human medicine and agriculture shapes resistance in fungal populations globally.

The authors conclude that fungal heteroresistance deserves far greater attention from clinicians, diagnostic laboratories, and researchers alike. Incorporating heteroresistance detection into antifungal susceptibility workflows, developing standardized definitions and protocols, and integrating genomic surveillance of ploidy changes into clinical microbiology could close a dangerous gap between what the laboratory reports and what actually happens inside the patient. As antifungal resistance continues to spread worldwide and the pipeline of new antifungal drugs remains thin, recognizing the resistant minorities that conventional tests cannot see may prove to be a critical step in preserving the effectiveness of the drugs that medicine currently has.

Subject of Research: Molecular mechanisms, detection, and clinical implications of fungal heteroresistance in antifungal treatment failure

Article Title: Beyond antifungal susceptibility: molecular mechanisms, detection, and clinical implications of fungal heteroresistance

Article References: Taş, E., Çetin, B., Yılmazer, A. N., & Işık, T. (2026). Beyond antifungal susceptibility: molecular mechanisms, detection, and clinical implications of fungal heteroresistance. Molecular Biology Reports, 53(1), Article 1546. https://doi.org/10.1007/s11033-026-12737-3

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12737-3

Keywords: fungal heteroresistance, antifungal resistance, Candida, Cryptococcus, Aspergillus, aneuploidy, antifungal susceptibility testing, treatment failure, efflux pumps, Candida auris, diagnostic challenges, molecular mechanisms

The research team, led by Ahmed Rezk, Heba S. Said, Sherin M. Elfeky, Mohammed El-Mowafy, and Ramadan Hassan of Mansoura University, began by surveying the local resistance landscape. They collected 64 clinical isolates of C. albicans from hospitals in Dakhlya governorate, drawn from urine samples, vaginal swabs, blood cultures, and other clinical sources between May and July 2022. Using standard disk diffusion and broth microdilution methods aligned with Clinical and Laboratory Standards Institute guidelines, they found that 16 isolates, a full 25 percent of the collection, were resistant to fluconazole. Minimum inhibitory concentrations among these resistant strains ranged from 64 to 2048 micrograms per milliliter, levels far above what fluconazole can achieve safely in patient tissue.

That resistance is not accidental. Fluconazole works by inhibiting the fungal enzyme lanosterol demethylase, encoded by the ERG11 gene, which is essential for building ergosterol, the structural backbone of the fungal cell membrane. Resistant fungi fight back in several ways: they mutate ERG11 so the drug binds poorly, they overproduce the target enzyme, and, most importantly for this study, they crank up expression of efflux pumps that simply eject the drug from the cell before it can do damage. The dominant player among these pumps is Cdr1, an ATP-binding cassette transporter embedded in the plasma membrane. Cdr1 operates like a molecular bilge pump: azole drugs drift into the cytoplasm, slip into a hydrophobic cavity in the transporter’s transmembrane domains, and are then expelled outward when ATP binding and hydrolysis at the nucleotide-binding domains drive a conformational flip from an inward-facing to an outward-facing state.

Disulfiram, chemically known as tetraethylthiuram disulfide, has a long and curious pharmacological history. Approved for aversion therapy in alcohol use disorder, it inhibits aldehyde dehydrogenase in the liver, causing acetaldehyde to accumulate and producing the flushing, nausea, and headaches that discourage drinking. But over the past two decades it has attracted attention as a repurposing candidate in oncology and infectious disease. Previous work had already hinted at antifungal potential: an earlier study showed disulfiram directly modulates Cdr1p, and a non-toxic concentration of the drug enhanced the sensitivity of yeast cells expressing the pump to fluconazole and miconazole. It has also shown activity against methicillin-resistant Staphylococcus aureus, Helicobacter pylori, and metallo-beta-lactamase enzymes.

To test whether that promise held up against real clinical resistance, the Mansoura team measured disulfiram’s own minimum inhibitory concentration against six representative fluconazole-resistant isolates, obtaining values between 10 and 20 micrograms per milliliter. They then repeated fluconazole susceptibility testing in the presence of a subinhibitory dose of disulfiram, one too low to inhibit fungal growth on its own. The results were striking: fluconazole’s MIC dropped by factors ranging from 2-fold to as much as 128-fold across the resistant isolates. In other words, a dose of disulfiram that by itself could not harm the fungus dramatically re-sensitized it to a drug it had previously shrugged off.

Checkerboard assays, in which both drugs are titrated in two dimensions across a microtiter plate, confirmed the synergy quantitatively. Using the fractional inhibitory concentration index, the team classified the combination as fully synergistic in three isolates, partially synergistic in two, and additive in one. No antagonism was observed. Under the conventional interpretation scheme, a FICI of 0.5 or below indicates synergy, meaning the drugs together accomplish more than the sum of their individual effects, a hallmark of a genuine resistance-reversing interaction rather than simple additive killing.

The molecular mechanism behind this synergy emerged from two complementary lines of evidence. First, quantitative reverse-transcription PCR on two resistant isolates, Ca15 and Ca39, showed that Cdr1 was overexpressed when the fungi were exposed to fluconazole alone, consistent with the pump being the engine of resistance. When disulfiram was added at subinhibitory concentrations, either alone or in combination with fluconazole, Cdr1 expression was downregulated relative to untreated controls. Second, molecular docking simulations using the recently solved cryo-EM structure of Cdr1 bound to fluconazole revealed that disulfiram can occupy two strategic positions on the transporter. At the hydrophobic substrate cavity in the transmembrane domain, disulfiram binds in a mode nearly identical to fluconazole, forming a hydrogen bond with Asn1240 and hydrophobic contacts with residues including Phe552, Leu555, Phe559, and Ile1237, with a calculated binding affinity of minus 6.46 kilocalories per mole, slightly stronger than fluconazole’s minus 6.07. At the ATP-binding site in the nucleotide-binding domain, disulfiram mimics ATP itself, hydrogen bonding with Thr195, Cys193, Gly190, Ser194, and Gly192, residues essential for ATP binding and hydrolysis. By competing at both sites simultaneously, disulfiram appears to jam the pump’s fuel supply and its cargo door at once, allowing fluconazole to accumulate in the cytoplasm and attack its target enzyme.

Beyond reversing resistance, disulfiram delivered a second blow to the fungus: it interfered with biofilm formation. Biofilms, the organized communities of cells encased in a protective matrix that adhere to catheters, mucosal surfaces, and medical devices, are notoriously tolerant of antifungals and a major driver of persistent infection. Using a tissue culture plate assay with crystal violet staining, the researchers showed that disulfiram reduced biofilm formation in a dose-dependent manner at sub-MIC concentrations of one-half, one-quarter, one-eighth, and one-sixteenth of its MIC. Remarkably, even at the weakest dose tested, one-sixteenth of the MIC, the drug still achieved roughly 14 to 37 percent inhibition of biofilm formation and significant eradication of mature, 24-hour-old biofilms. This dual activity, chemosensitization plus anti-biofilm action, suggests disulfiram could undermine two of the most stubborn survival strategies C. albicans deploys in the human host.

The study’s authors are careful to frame these results as a foundation rather than a finished therapy. All experiments were conducted in vitro, and the researchers explicitly call for follow-up work including site-directed mutagenesis, functional binding assays, and investigation of other resistance mechanisms such as ERG11 point mutations and the efflux genes Cdr2 and Mdr1. In vivo efficacy and pharmacodynamic studies will be essential before any clinical conclusions can be drawn. Still, the appeal of the strategy is considerable. Drug repurposing bypasses the years of safety profiling required for entirely new molecules, and combination therapy can lower effective doses of both agents, potentially reducing toxicity and slowing the further evolution of resistance. With fluconazole resistance rates in Egyptian bloodstream isolates recently reported as high as 58.3 percent, and with the researchers highlighting the infrequent use of routine antifungal susceptibility testing in the region, the need for such adjuvant approaches is urgent. If subsequent animal and clinical studies validate what the Mansoura team observed in the laboratory, a fifty-year-old sobriety drug could become an unexpected ally in preserving the usefulness of modern antifungal medicine.

Subject of Research: Reversal of fluconazole resistance in Candida albicans by disulfiram through Cdr1 efflux pump inhibition and biofilm disruption

Article Title: Disulfiram reverses fluconazole-resistance mediated by Cdr1 in Candida albicans and interferes with biofilm formation

Article References: Rezk, A., Said, H. S., Elfeky, S. M., El-Mowafy, M., & Hassan, R. (2026). Disulfiram reverses fluconazole-resistance mediated by Cdr1 in Candida albicans and interferes with biofilm formation. Applied Microbiology and Biotechnology, 110(1), Article 269. https://doi.org/10.1007/s00253-026-13977-w

Image Credits: AI Generated

DOI: 10.1007/s00253-026-13977-w

Keywords: Candida albicans, fluconazole resistance, disulfiram, Cdr1 efflux pump, drug repurposing, antifungal synergy, biofilm inhibition, ATP-binding cassette transporter, molecular docking, candidiasis, antimicrobial resistance, combination therapy