Hidden Resistant Subpopulations in Fungi May Explain Why Antifungal Drugs Fail

A new review published in Molecular Biology Reports argues that one of the most underappreciated drivers of antifungal treatment failure may be hiding in plain sight inside routine laboratory results. Fungal heteroresistance, the phenomenon in which a small subpopulation of cells within an otherwise drug-susceptible fungal population can survive concentrations of antifungal agents far above the reported minimum inhibitory concentration, is emerging as a serious diagnostic blind spot. The review, authored by Esra Taş of Samsun University together with colleagues at Karadeniz Technical University in Türkiye, synthesizes evidence from clinically important fungal pathogens including Candida, Cryptococcus, and Aspergillus species and warns that conventional susceptibility testing systematically misses these dangerous minorities.

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