Chitosan is produced by deacetylating chitin, the structural polymer that fortifies crustacean shells and fungal cell walls. What makes the molecule chemically interesting is its behavior at acidic pH: in that range, abundant amino groups along its backbone grab protons from solution, converting the neutral polymer into a polycation studded with positive charges. Yeast cells, by contrast, present a largely negative face to the world, with phosphorylated mannans, glucans, and other acidic components decorating their cell walls and membranes. Opposite charges attract, and that simple electrostatic logic underlies everything chitosan does to a fungal cell. When the polycation docks onto the negatively charged cell periphery, it disrupts the carefully regulated barrier function of the envelope, allowing small molecules to leak in and out and, at sufficient concentrations, halting growth altogether.

Exactly how this interaction plays out, however, varies dramatically from one fungal species to another, and that variability is where the new study makes its mark. The Mexican team compared three yeasts with very different lifestyles and clinical relevance: Saccharomyces cerevisiae, the brewer’s and baker’s yeast that serves as the workhorse of eukaryotic cell biology; Candida albicans, a common human commensal and opportunistic pathogen; and Debaryomyces hansenii, a salt-tolerant yeast famous for colonizing cheese rinds and other extreme food environments. By tracking growth in the presence of chitosan across these species, the researchers established a clear hierarchy of susceptibility. Debaryomyces hansenii emerged as the most vulnerable of the three, its growth strongly inhibited by the polymer, while the other species displayed comparatively milder responses. The result underscores a point that has been emerging from the antifungal literature for years: chitosan is not a blunt instrument but a strain-dependent agent whose potency depends on the fine chemistry of each organism’s surface.

To find out why, the team needed to watch chitosan in action rather than merely tally its consequences. They turned to a fluorescent version of the molecule, conjugating chitosan to fluorescein isothiocyanate, a dye that glows green under the microscope. This allowed them to monitor where the polymer goes and how firmly it sticks. The binding assays showed that chitosan associates with the cell periphery of all three species, becoming incorporated at the cell surface over time, but the extent of that association tracked with each species’ sensitivity. The cells that bound the polymer most avidly and internalized it most readily were the ones whose growth suffered most. In other words, the antifungal effect is not some diffuse, indirect consequence of chitosan’s presence in the medium; it is written in the physical contact between polymer and cell.

Transmission electron microscopy provided the structural confirmation. Ultrastructural examination of treated cells revealed the damage that chitosan inflicts on the yeast envelope, consistent with its role as a permeabilizing agent. Where the polymer had acted, the orderly architecture of the cell wall and plasma membrane was compromised, and the cells showed the kinds of lesions expected when a barrier designed to hold contents in and keep the environment out begins to fail. Together, the fluorescence binding data and the electron micrographs built a coherent mechanistic picture: chitosan’s positive charges latch onto negatively charged cell-surface components, the interaction destabilizes the envelope, and permeability rises as a direct result.

What elevates the study beyond a mechanistic curiosity is the application the researchers built on top of it. If chitosan can open yeast cells in a controlled, tunable way, then those permeabilized cells become analytical tools in their own right. The team demonstrated exactly that, using chitosan-induced permeabilization to measure the activity of selected enzymes in situ, meaning inside cells that remain structurally intact rather than in messy homogenates. In situ measurements of this kind have long been a goal in enzymology because they preserve the native context of the cell: substrates and cofactors diffuse in through the permeabilized envelope, reactions proceed within the crowded, compartmentalized interior, and the activities read out reflect the enzymes as they actually exist in the organism, not as they behave after extraction and purification.

The practical implications ripple outward in several directions. For basic researchers probing fungal physiology, permeabilized yeast offers a faster and arguably more faithful route to enzyme kinetics than classical cell-free assays, which require breaking cells open, clarifying lysates, and accepting that much of the native organization is lost along the way. For the biotechnology sector, where yeasts are engineered to produce everything from bioethanol to pharmaceutical proteins, a simple chemical treatment that exposes intracellular biochemistry without destroying the cells could accelerate screening and process monitoring. And for those studying antifungal action itself, the species-dependent results provide a comparative framework: understanding why Debaryomyces hansenii is exquisitely sensitive while its relatives are more resistant may point toward the surface features, wall composition, or membrane properties that determine chitosan susceptibility in fungi more broadly.

The work also carries a certain elegant irony. Chitosan has been marketed for decades as a weight-loss supplement and wound dressing, and its antimicrobial properties have been exploited in food packaging and agriculture, yet the molecular details of its interaction with fungal cells have remained only partially resolved. By combining growth analysis, fluorescent binding assays, electron microscopy, and functional biochemistry in a single coherent experimental program, the UNAM team has connected those dots across three yeast species, showing that a single electrostatic mechanism, tuned by each species’ surface chemistry, explains both the antifungal effect and the analytical utility of the molecule. The same property that makes chitosan lethal at high doses makes it a precise instrument at controlled ones.

There are, of course, caveats and open questions. The study examined three species, and the authors themselves frame chitosan’s effects as strain-dependent, which means extrapolation to other fungi, particularly filamentous molds or clinically resistant Candida strains, will require further comparative work. The precise molecular targets of chitosan at the cell periphery, and the structural thresholds that separate reversible permeabilization from irreversible damage, remain fertile ground for future investigation. The researchers also note that their article was shared early as a citable, peer-reviewed accepted version subject to further editorial processing, so some details may be refined in the final version of record.

Even so, the central achievement stands. A team working with modest resources, supported by grants from UNAM’s Dirección General de Asuntos del Personal Académico, has taken a molecule familiar from seafood waste streams and turned it into a dual-purpose reagent: an antifungal agent whose species-specific action illuminates the chemistry of the fungal envelope, and a permeabilization tool that lets enzymologists watch metabolism from the inside. As interest in fungal biology grows, driven by rising rates of invasive fungal infections and by the central role of yeasts in industrial fermentation, tools that bridge the gap between living cells and laboratory measurement are likely to be in steady demand. Chitosan, it turns out, was sitting on the shelf all along.

Subject of Research: Chitosan-induced permeabilization of yeast cell envelopes and its application to in situ enzymatic measurements

Article Title: Chitosan-induced permeabilization in yeast and its use for in situ enzymatic measurements

Article References: Araiza-Villanueva, M., Sánchez, N. S., Calahorra, M., Padilla-Garfias, F., & Peña, A. (2026). Chitosan-induced permeabilization in yeast and its use for in situ enzymatic measurements. Applied Microbiology and Biotechnology. https://doi.org/10.1007/s00253-026-14027-1

Image Credits: AI Generated

DOI: 10.1007/s00253-026-14027-1

Keywords: chitosan, yeast, cell permeabilization, Saccharomyces cerevisiae, Candida albicans, Debaryomyces hansenii, in situ enzyme activity, electrostatic interaction, antifungal agents, fungal physiology, transmission electron microscopy, fluorescein isothiocyanate