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

The problem the researchers set out to solve is a familiar one. Seafood is densely packed with nutrients, and even when it is chilled or frozen, endogenous enzymes and microorganisms gradually break down proteins and other compounds, releasing volatile amines such as ammonia and biogenic amines. These gases are the key contributors to total volatile basic nitrogen, or TVB-N, the gold-standard chemical indicator of seafood freshness. Conventional laboratory methods for measuring TVB-N, including chemical-microbiological assays, chromatography, mass spectrometry, and spectroscopy, are accurate but slow, destructive to the sample, and dependent on expensive instrumentation and skilled technicians. They cannot tell a shopper whether the shrimp in the display case is fresh, and they cannot track freshness continuously inside a sealed package.

Fluorescent sensing tags have emerged as a promising alternative because they can be placed inside packaging and photographed with a smartphone. Early versions, however, relied on a single fluorescence signal that switched on or off, which made them vulnerable to environmental interference and difficult to judge by eye. The field responded with ratiometric probes, which carry two well-resolved emission bands so that one signal serves as an internal reference for the other. But many dual-emission probes built from dyes or quantum dots suffer from aggregation-caused quenching, in which the fluorescent signal collapses when the molecules cluster together on a solid surface. Aggregation-induced emission, or AIE, flips that behavior, glowing brighter when aggregated, and it has become a favored design principle for solid-state sensors.

The team, led by Wenyang Zhang with co-authors You Tian, Yanwu Chen, and Min Wei, built their tag around a relatively unexplored class of AIE materials: bimetallic nanoclusters. Using a microwave-assisted synthesis run at 90 degrees Celsius for just 150 seconds, they prepared silver-copper nanoclusters capped with D-penicillamine, abbreviated DPA-AgCuNCs. Transmission electron microscopy showed dense aggregates of particles averaging about 3 nanometers in diameter, and energy-dispersive X-ray mapping confirmed a homogeneous distribution of silver, copper, sulfur, carbon, and nitrogen. Spectroscopic characterization revealed a strong orange emission at 585 nanometers when excited at 360 nanometers, a large Stokes shift of 225 nanometers, and a long photoluminescence lifetime of about 4 microseconds, all hallmarks of a ligand-to-metal charge transfer from the sulfur atom of the capping ligand to the metal core.

Exposure to ammonia vapor quenched the nanocluster fluorescence in a linear fashion across concentrations from 10 to 120 parts per million, with a detection limit of 2.95 parts per million. Mechanistic experiments showed that the quenching is a synergistic combination of static and dynamic pathways. Ultraviolet-visible spectroscopy revealed two new absorption bands after ammonia exposure, evidence of a non-fluorescent ground-state complex, while fluorescence lifetime measurements showed the average lifetime plummeting from 4.023 microseconds to 3.27 nanoseconds, a signature of dynamic quenching. X-ray photoelectron spectroscopy showed the nitrogen content of the material rising after exposure and shifts in the copper oxidation-state features, and electron microscopy captured the physical disaggregation of the nanocluster aggregates. The researchers attribute the dominant pathway to static quenching through formation of an ammonia-coordinated copper-silver complex.

To convert this single-signal quenching into a visually readable ratiometric response, the team combined the orange-emitting nanoclusters with fluorescein isothiocyanate, or FITC, a dye that behaves in the opposite way. In the presence of the alkaline environment created by ammonia, the lactone ring of FITC opens to its carboxylate form and its green fluorescence at 510 nanometers brightens. Drop-casting the two probes together onto circular cellulose fiber paper produced a tag that glows orange-yellow under 365 nanometer ultraviolet light but shifts progressively toward green as amine concentrations rise. Scanning electron microscopy confirmed that both components, sheet-like FITC aggregates and petal-shaped nanocluster particles, coexisted on the paper fibers. After optimizing the volume ratio of nanoclusters to FITC at 4 to 0.5, the tag displayed a clean, graduated orange-to-yellow-to-green progression.

The practical test came with fresh sand shrimp stored at 4 degrees Celsius, with a tag affixed inside the lid of a sealed Petri dish where it never touched the sample. Independent measurements of TVB-N and total viable counts classified the shrimp as fresh at days zero to one, less fresh at days two to three, and spoiled at day four. The tag’s color changes tracked these grades precisely: bright orange when fresh, pale yellow when declining, and distinctly green when spoiled. Based on this behavior, the team built a fluorescence color card allowing anyone to grade freshness at a glance. The tag also proved rugged, retaining 94.47 percent of its initial fluorescence after 30 days at 4 degrees Celsius, more than 91 percent after a month across relative humidities from roughly 33 to 90 percent, and 95.47 percent after five days of continuous ultraviolet exposure.

Visual color cards, however, remain hostage to human subjectivity, since individual differences in color perception can produce inconsistent judgments. To eliminate that variability, the researchers trained five convolutional neural network architectures on a dataset of 1,842 labeled fluorescence images spanning the three freshness categories, augmented with flipping, rotation, scaling, cropping, translation, and Gaussian noise. ResNet-34 decisively outperformed the alternatives, achieving 98.39 percent accuracy, 98.48 percent precision, 98.41 percent recall, a 98.41 percent F1-score, and an area under the receiver operating characteristic curve of 99.02 percent on held-out data. VGGNet-16 and DenseNet managed only about 75 to 76 percent accuracy, GoogLeNet 53 percent, and AlexNet a dismal 36 percent. Stratified five-fold cross-validation confirmed ResNet-34’s robustness, with accuracy of 96.81 plus or minus 0.92 percent across folds, and on an independent test set of 122 unseen images the model achieved perfect precision for fresh and spoiled samples and 95.24 percent for the intermediate category.

The optimized network was then packaged into a proof-of-concept smartphone application called FreshSense, in which a user photographs the illuminated tag and the app displays an automatic freshness prediction, with inference currently handled on a remote server. The system generalized well beyond shrimp: applied to salmon, cod, and grass carp stored at 4 degrees Celsius, it achieved classification accuracies of 93.52, 90.74, and 92.59 percent, respectively, showing that the sensing chemistry is not tied to a single species. The authors note that future work will target model compression and on-device deployment so the tool can function fully offline, along with systematic evaluation of computational costs. Selectivity testing against thirteen volatile substances produced during spoilage, including methanol, hydrogen sulfide, formaldehyde, and acetic acid, showed strong responses only to ammonia, trimethylamine, putrescine, and a mixed-gas cocktail, precisely the compounds that matter for spoilage, suggesting the platform could become a practical, intelligent fixture in cold chains, retail displays, and eventually home refrigerators.

Subject of Research: A CNN-integrated ratiometric fluorescent nanocluster tag for rapid seafood freshness monitoring at refrigerated temperatures

Article Title: CNN-integrated ratiometric fluorescent tag for rapid monitoring of seafood freshness at refrigerated temperatures

Article References: Zhang, W., Tian, Y., Chen, Y., & Wei, M. (2026). CNN-integrated ratiometric fluorescent tag for rapid monitoring of seafood freshness at refrigerated temperatures. Current Research in Food Science, 13, Article 101560. https://doi.org/10.1016/j.crfs.2026.101560

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101560

Keywords: seafood freshness, ratiometric fluorescent tag, aggregation-induced emission, silver-copper nanoclusters, volatile amines, ammonia detection, convolutional neural network, ResNet-34, fluorescein isothiocyanate, TVB-N, smartphone sensing, refrigerated storage