Polymers have become indispensable materials in modern industries due to their versatile physicochemical properties and tunable functionality. These macromolecules, composed of linked repeating monomer units, enable the design of materials with tailored mechanical, thermal, and chemical characteristics. Their applications span critical sectors, including biomedical engineering (e.g., drug delivery systems, biomaterials), precision agriculture (e.g., controlled-release fertilizers), and sustainable packaging. However, the dominance of synthetic polymers, primarily petroleum-based plastics that are largely non-degradable and difficult to recycle, poses significant environmental challenges. These challenges include persistent ecosystem contamination from microplastic accumulation and the leaching of toxic compounds (Vicente et al., 2024).

Consequently, natural polymers (e.g., proteins, starch, cellulose, collagen, alginates, hyaluronic acid) have garnered significant interest in recent decades due to their biocompatibility, biodegradability, and simpler recycling potential. Among these biopolymers, the nitrogen-containing polysaccharide chitin and its derivative, chitosan, are among the most extensively studied and applied. Moreover, in recent years, these natural polymers have begun to be considered as promising biomaterials to be used in various fields of medicine, pharmaceutics and cosmetics, due to their unique molecular structure, complex of physicochemical properties, biological activity and high biocompatibility, low toxicity and biodegradability (Zargar et al., 2015).

Also, the results of current investigations show that chitosan and its derivatives are applied for solving different cosmetic and dermatological problems. The positive charge of chitosan active amino groups contributes to it's antibacterial and antifungal properties, making it a promising candidate for use in biomedical fields, treatment of skin diseases and supporting a normal skin microbiome.

Recent research has revealed the critical role of skin microbiota in both maintaining cutaneous homeostasis and contributing to dermatopathological mechanisms (Egert et al., 2017, Gill et al., 2006). These findings caused growing interest in developing innovative microbiome-targeted interventions, including advanced biomaterials, bioactive formulations, and next-generation cosmeceuticals designed to modulate skin microbial communities for therapeutic and protective benefits.

PubMed search results for “chitosan in cosmetics” (Fig. 1 A) and “ human skin microbiome” (Fig. 1 B) show a constantly growing interest of researchers in these areas in the last 15 years.

This review systematically examines recent advances in the application of chitosan and its derivatives in dermatology and cosmetics, focusing on their antimicrobial properties, mechanisms of action against pathogenic microbes, and modulatory effects on microbial pathogenesis and the human microbiome. We critically evaluate chitosan dual functionality as both a selective antimicrobial agent and a potential prebiotic, highlighting its capacity to disrupt harmful pathogens while supporting beneficial microbiota – a rather unique advantage for developing microbiome-targeted skincare therapeutics. By analyzing structure-activity relationships across molecular weights, degrees of deacetylation, and chemical modifications, we demonstrate how chitosan tunable properties can be optimized for diverse dermatological applications, from treating acne and atopic dermatitis to maintaining cutaneous microbial homeostasis.

Diseases of the skin and subcutaneous tissue are increasingly prevalent in the modern world, significantly impacting patients' quality of life by contributing to low self-esteem, psychological stress, anxiety, and insomnia (Chowdhury et al., 2022). These diseases are highly varied in their presentation and etiology, with strong associations to genetic predisposition, aging, diet, allergic reactions, and exposure to environmental factors such as solar radiation and toxic chemicals. At the present stage, an urgent task is to develop measures and improve the prevention of chronic skin diseases such as atopic dermatitis, psoriasis, acne, and contact dermatitis, which are widespread throughout the world (Lapsley, 2000). But the most dangerous problem, which occupies one of the leading positions in oncological diseases of the male and female population, is skin cancer (Hasan et al., 2023).

An indispensable symptom of various types of dermatological skin problems is its dryness (xerosis - dry skin). Xerosis of the skin shows not only a decrease in the water content in the dermis, but also changes in the functioning of the stratum corneum (Pons-Guiraud, 2007). Lipids of various classes (ceramides, fatty acids, cholesterol, etc.) that form the stratum corneum play a major role in the permeation function through the skin barrier for various chemical substances (Weerheim and Ponec, 2001). In healthy skin, all of these types of lipids are in approximately equal a molar ratio, which is not seen in the composition of atopic skin. In atopic skin (dry skin), there is an increase in water loss and a decrease in the level of ceramides and fatty acids (van Smeden et al., 2014). Regular treatment of xerosis includes the application of moisturizers, occlusives, or emollients to restore the integrity of the dermal barrier.

One of the most common skin diseases is atopic dermatitis. It is a systemic multifactorial genetically determined inflammatory skin disease with signs of polyorgan pathology, characterized by itching, chronic relapsing course, age-related features of localization and morphology of lesions. Atopic dermatitis is one of the most common diseases (from 20 % to 40 % in the structure of skin diseases), occurring in all countries, in people of both sexes and in different age groups. The prevalence of atopic dermatitis among children is up to 20 %, among the adult population - 2–8 % (Silverberg and Jonathan, 2021, Barbarot et al., 2018). Atopic dermatitis manifests itself differently on various skin types. For people with lighter skin tones, atopic dermatitis appears as erythematous spots and plaques, while on darker skin types the disease often manifests itself as stains of ash-gray or dark brown color (Sangha, 2021). The occurrence of atopic dermatitis is associated with disturbances in the composition of the gastrointestinal tract microbiota: a decrease in the total number of bifido- and lactobacilli and an increase in the proportion of Staphylococcus aureus and Escherichia coli microorganisms are detected in this case. A direct correlation between the number of Helicobacter pylori bacteria in the stomach and the severity of atopic dermatitis is also reported (Kim and Kim, 2019). Treatment of patients with atopic dermatitis includes a set of measures aimed at restoring the skin barrier and in anti-inflammatory, antimicrobial and antipruritic treatment, and is aimed at prolonging remissions and preventing relapses. Patients are prescribed antihistamines, immunomodulators, vitamins, external glucocorticoids (Mazur et al., 2023). But by now, such types of treatment for atopic dermatitis are considered to be quite conservative. There is increasing interest in new, safer and more versatile therapeutic approaches using highly effective delivery systems based on biopolymers, such as chitosan, for new generation antimicrobial preparations (LePoidevin et al., 2019)

Another common skin disease, acne is a polymorphic multifactorial disease of the hair follicles and sebaceous glands, which occurs in 80 % of adolescents and young adults. Among the various clinical varieties of acne, acne vulgaris is the most common. Up to 35 % of male adolescents and 23 % of female adolescents suffer from this dermatitis. Only after the age of 24 does this figure drop to 10 % and below. Emerson and Straus in 1972, having examined more than 1000 students aged 15–18, found acne in 80 % of them, and the disease was detected equally often in both boys and girls (Heng and Chew, 2020). The key factors in the development of acne are sebum dysregulation (lipid imbalance), polymicrobial dysbiosis, dominated by Cutibacterium acnes but also involving Staphylococcus epidermidis, Malassezia spp., and Staphylococcus aureus, which collectively disrupt skin homeostasis, sebaceous gland activity, impaired circulation of some hormones, hereditary predisposition, follicular hyperkeratosis and impaired keratinization processes (Xu et al., 2025). Recent studies have demonstrated that acne development involves early vascular endothelial cell activation and inflammatory responses (Jeremy et al., 2003), with immune system dysregulation playing a key role in pathogenesis (Jin et al., 2023). Modern treatment strategies consider topical retinoids as the gold standard for their triple-action benefits (comedolytic, sebostatic, and anti-inflammatory), while also incorporating adjunctive therapies including antimicrobial agents, sebum production inhibitors, topical probiotics, and antiandrogens. Emerging approaches, such as immunomodulatory vaccination strategies (Li et al., 2024), highlight the evolving understanding of acne as a multifactorial condition requiring targeted interventions.

Finally, to the date, psoriasis is considered to be another frequent skin disease. Depending on the severity and clinical features, psoriasis may manifest itself in various ways. Psoriasis is a chronic inflammatory skin disease characterized by the appearance of red, thick, and scaly patches on the skin, and its origins and mechanism has not been fully studied yet. However, it is believed that the state of the immune system, genetic characteristics, and environmental influences are among the main factors in its development. This progressive dermatoid decease can affect any area of the human skin, but usually affects the elbows, knees, scalp, lower back, and nails (Rachakonda et al., 2014). Traditional methods of treating psoriasis were based on topical application of biologically active substances isolated from medicinal plants. Up to date mainstream medical treatments primarily rely on topical corticosteroids, vitamin D analogs, phototherapy, and systemic immunomodulators. Modern advancements in psoriasis treatment have introduced innovative therapeutic approaches resulting in cutting-edge dosage forms and drug delivery systems. These developments provide diverse solutions for managing psoriasis by enhancing drug efficacy, improving targeted delivery, and minimizing side effects. For instance, researchers have engineered hyaluronic acid-coated chitosan nanoparticles loaded with folic acid to boost therapeutic outcomes. The positively charged chitosan facilitates superior transdermal drug penetration, establishing an efficient localized delivery system for psoriasis treatment (Sheikh et al., 2023). Another breakthrough involves the development of chitosan/hyaluronan nanogels co-delivering methotrexate and 5-aminolevulinic acid for combined chemo-photodynamic therapy. This dual-action strategy enhances treatment effectiveness while reducing systemic toxicity. The nanogels exhibit improved skin accumulation and retention, offering a safer and more efficient therapeutic approach for psoriasis management (Wang et al., 2022). These innovations exemplify how advanced drug delivery systems, such as nanoparticles and nanogels, are revolutionizing psoriasis treatment by optimizing drug performance, minimizing adverse effects, and enabling targeted therapy.

Thus, we can highlight the growing prevalence and psychosocial impact of common skin diseases like atopic dermatitis, acne, and psoriasis. These conditions share underlying mechanisms of immune dysregulation and skin barrier dysfunction. Atopic dermatitis involves microbiome imbalances and ceramide deficiency, treated with emollients and emerging biopolymer-based therapies (Kim and Kim, 2019, Mazur et al., 2023, LePoidevin et al., 2019). Acne vulgaris stems from sebaceous hyperactivity and C. acnes proliferation, managed via retinoids and investigational vaccines (Xu et al., 2025, Jeremy et al., 2003, Jin et al., 2023, Li et al., 2024). Psoriasis, driven by genetic and immune factors, now utilizes corticosteroid creams, light therapy and biologic drugs as standard interventions (Rachakonda et al., 2014, Sheikh et al., 2023, Wang et al., 2022). Collectively, these conditions underscore the shift toward precision dermatology, combining barrier repair, microbiome maintenance, immunomodulation, and advanced biomaterials and targeted nanotechnologies to enhance drug delivery to address their multifactorial origins.

At the same time, regardless of the skin disease type, an imbalance of the skin microbiota is observed mostly in all cases. However, at present, there is still insufficient data to understand the cause-and-effect relationships between the disease and key types of microorganisms. In the future, the treatment of skin diseases should not be limited to the use of anti-inflammatory drugs, local corticosteroids, laser therapy and other traditional methods. The possibilities of modulating the skin microbiota with a healthy diet, a combination of probiotics and prebiotics, and skin microbiota transplantation are being considered. Thus, the skin microbiome modulation may become a powerful tool for the treatment of skin diseases in the future (Byrd et al., 2018).

Chitin is a linear polysaccharide, which consists of elementary units of 2-acetamido-2-deoxy-D-glucose connected by a 1,4-β-glycosidic bond (Boymirzaev, 2023). Chitin is found in high concentrations in the cells of crustaceans, insects and fungi. It is obtained as a result of alternating reactions of deproteination and demineralization of the feedstock. The number of stages of deproteination and demineralization, as well as the sequence of their implementation, are determined by the requirements for the quality of the final product (Madduma-Bandarage et al., 2022).

Chitosan is a product of chitin deacetylation (Fig. 2). Its production is based on the reaction of complete or partial elimination of the acetyl group from the structural unit of chitin. When this reaction occurs, some of the glycosidic bonds are broken, resulting in a mixture of shorter polymer units with a lower molecular weight than the original chitin. In most cases, technical chitosan contains up to 5–15 % residual acetamide groups and has molecular weight (MW) ∼ 200 kDa. To obtain low molecular weight water-soluble chitosan and oligomers with higher biological activity, both chemical and enzymatic methods are used (Azmana et al., 2021, Yeritsyan et al., 2020, Wang et al., 2020).

Chitosan finds application in various spheres of human live activity, for example, in agriculture and food industry, in the textile, paper, nuclear and mining industries (Madduma-Bandarage et al., 2022, Vergara-Castañeda et al., 2020). The increased solubility of low molecular weight chitosans makes it possible to use them as polymers for medical and biological applications. Chitosan has also found widespread use in the production of different cosmetic products as moisture absorption and retention agent, emulsion stabilizer, antimicrobial agent, antioxidant agent or delivery system (Fig. 2). For these purposes it is included in lotions, gels, toothpastes, shampoos and creams (Kim et al., 2019).

The origins of the chitosan antimicrobial activity have not been precisely established and explained yet. The possible mechanisms of the chitosan antimicrobial activity will be discussed further in this review. Briefly, it is believed that the interaction of positively charged chitosan macromolecules with negatively charged structures on the cell surface leads to membrane dysfunction and cell lysis. And at higher concentrations, chitosan can cover the cell surface with a dense layer and thereby prevent the entry of nutrients into the cell and the removal of the products of cell metabolism (Helander et al., 2001, Hosseinnejad and Jafari, 2016, Chung et al., 2004, Côté et al., 2000, Raafat and Sahl, 2009, Lim and Hudson, 2004, Choi et al., 2001). There is evidence that chitosan can penetrate into the cell and interact with various structural components of the cell, thereby disrupting their normal functioning (Fei Liu et al., 2001, Moon et al., 2007), which leads to cell death (Barbosa et al., 2019).

As a fact, it is shown that chitosan exhibits antibacterial, antifungal, and antiviral activities (Roller and Covill, 1999, Vadivel and Dhamodaran, 2016, Thaya et al., 2016, Yilmaz Atay, 2019, Kochkina and Chirkov, 2001). However, its antimicrobial activity depends on many both endogenous and exogenous factors (Fig. 3).

It is noted that the antimicrobial activity of chitosan depends on the type of raw material from which it is obtained. For example, chitosan obtained from crab shells exhibited higher antibacterial activity than chitosan obtained from crab legs (Byun et al., 2013). Shell chitin has higher mineral content (CaCO3) and more complex protein matrices than leg chitin, which may lead to more extensive deacetylation during processing (higher positive charge density) and different crystalline structures after demineralization (altered solubility/bioactivity).

One of the most important factors affecting the antimicrobial activity of chitosan is the acidity of the environment. It is well known that native chitosan is soluble in organic acids at pH below 6, but insoluble in water, organic solvents and alkaline media. Most studies have shown that at an environmental pH below pKa, the polymer has an increased antimicrobial effect (Lim and Hudson, 2004, Kulikov et al., 2006), which probably is due to increased adsorption of chitosan on the cell surface (Chung et al., 2004, Meng et al., 2012, Kumar et al., 2004), impaired permeability of cell membranes, which ultimately leads to cell death (Hosseinnejad and Jafari, 2016, Chung et al., 2004, Rasola and Bernardi, 2011, Matica et al., 2019).

low molecular weight (LMw) chitosan, also called “oligochitosan” or “short chain chitosan” (molecular weight <50 kDa);

medium molecular weight (MMw) chitosan with a molecular weight from 50 kDa to 250 kDa;

high molecular weight (HMw) chitosan with a molecular weight > 250 kDa (Lim and Hudson, 2004, Farias et al., 2019).

Here, it should be noted that the data we found in different studies on the effect of the molecular weight of chitosan on its antimicrobial activity are very contradictory.

Thus, for example, in various studies on a number of different bacteria, such as Staphylococcus aureus, Bacillus cereus, Klebsiella pneumoniae and Escherichia coli, it was shown that the lower the molecular weight of chitosan, the higher the antibacterial effect it has (Verlee et al., 2017). This is explained by the size and conformation of chitosan particles, which appear to play an important role in understanding the effectiveness of low molecular weight chitosan. The mobility, attraction and ionic interaction of small chains is easier than that of large ones (Vishu Kumar et al., 2004, Vishu Kumar et al., 2005). In addition, it is noted that low molecular weight chitosan with a molecular weight of no more than 5000 Da can penetrate the bacterial cell wall and form complexes with DNA (Xing et al., 2009). At the same time, there are studies in which, on the contrary, it is noted that chitosan with a higher molecular weight has excellent antibacterial properties against skin pathogens such as Staphylococcus aureus and Propionibacterium acnes. Moreover, the antibacterial effect increases with decreasing pH and completely disappears at pH > 7.5 (Champer et al., 2013).

The study of Hong Kyoon No et al. provides a comparative assessment of the bactericidal and antifungal activity of chitosan with different molecular weights (from 3 to 150 kDa) and with a constant degree of deacetylation (85 %). It was shown that chitosan with a MW from 5 to 50 kDa showed higher antibacterial activity against gram-negative bacteria Pseudomonas syringae, Erwinia carotovora and the gram-positive species Bacillus polymyxa compared to chitosan with a lower MW (3 kDa) and a higher MW (150 kDa). Moreover, all chitosan samples inhibited the growth of the mycelium of the fungi Fusarium oxysporum and the fungus Sclerotinia sclerotiorum. Reducing the MW of the polymer to 3 kDa reduced the inhibitory effect of chitosan on the growth of the fungus Fusarium oxysporum, which confirms the dependence of the antifungal activity of chitosan on its MW (No et al., 2002).

Such a contradiction in the results can be probably explained by the fact that the antimicrobial activity of chitosan is determined not only by its molecular weight (degree of polymerization), but by the entire complex of its physicochemical characteristics (degree of deacetylation, for example), as well as by their combination.

The degree of polymerization and the degree of deacetylation are two intrinsic parameters that directly affect the structure of chitosan polymeric chain. These are the main physicochemical parameters that determine the use of chitosan for various purposes. During polymerization process, the molecular weight of chitosan is gradually increased, which in turn affects its viscosity and solubility in aqueous media. Studies have shown that a higher degree of polymerization improves the antibacterial activity of chitosan by increasing the size of the molecule and improving interactions with bacterial cells (Lim and Hudson, 2004, Younes et al., 2014). The degree of polymerization of chitosan also affects the reproduction of non-cellular organisms, in particular bacteriophage 1–97 A in the culture of Bacillus thuringiensis galleriae. High-molecular weight chitosan polymer caused more pronounced destruction of phage particles than a chitosan oligomer with a degree of polymerization of 15 (Kochkina and Chirkov, 2001).

Another important parameter determining the biological activity of chitosan is its degree of deacetylation (Byun et al., 2013). The degree of deacetylation of chitosan is a parameter that indicates the number of acetyl groups removed from the chitosan molecule, which affects the charge of the molecule and its solubility in water. A decrease in the degree of deacetylation and an increase in the number of acetyl groups in the chitosan molecule leads to a decrease in the positive charge of chitosan, a decrease in its interaction with the negatively charged surface of the microbial cell, and thereby a decrease in its antimicrobial activity. As the degree of acetylation of chitosan increases, its solubility in water decreases, which limits the ability of chitosan to penetrate bacterial cell membranes and lowers its bioavailability [62]; Vinsova and Vavrikova, (2011); Sahariah and Másson, (2017); Martins et al., (2014)).

Thus, in a study conducted by Karpova et al., the influence of the main physicochemical characteristics of chitosan (molecular weight, degree of deacetylation and polydispersity) on the growth of the fungus Botrytis cinerea was investigated. It was found that the greatest inhibitory effect on fungal growth (conidial germination index less than 50 %) was exerted by chitosans with a molecular weight of 2–13 kDa, a degree of deacetylation of 85–98 %, and a polydispersity of 2–2.5. In this case, the maximum effect was found at a chitosan MW of 13 kDa, a degree of deacetylation of 98 %, at a chitosan concentration in the medium of 0.938 mg/ml (Karpova et al., 2019).

Studies carried out by Younes et al (Younes et al., 2014). examined the effect of chitosan on the growth of gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumonia and Salmonella typhimurium) and gram-positive bacteria (Staphylococcus aureus, Bacillus cereus, Enterococcus faecalis and Micrococcus luteus). At the same time, the combination of such parameters of chitosan as MW and the degree of acetylation at different pH values was assessed. As a result of the research, it was found that chitosans with a low degree of acetylation and at lower pH value exhibited greater antimicrobial activity against most of the studied bacterial species. At the same time, a decrease in the MW of chitosan contributed to an increase in antimicrobial activity against gram-negative bacteria, followed by simultaneous decrease of chitosan antimicrobial activity against gram-positive bacteria under the same conditions.

Continuing this topic, it should be noted that, on the other hand, the sensitivity of microorganisms to chitosan depends both on the taxonomic position of the microorganism in the classification system and on its species and strain characteristics, as well as the age of cell culture. For example, there are data, that the minimum inhibitory concentration (MIC) of chitosan against bacteria and yeast is 0.9–3.0 mg/ml, and the MIC of chitosan against the Aspergillus niger filamentous fungi is more than 5 mg/ml (Seo et al., 2002).

For the Staphylococcus aureus CCRC 12657 g-positive bacteria, it was found that cells in the late exponential phase (after 12 h of cultivation) were more sensitive to the water-soluble chitosan derivative containing lactose, compared to cells in the stationary phase (after 24 h of cultivation). At the same time the cells in the middle of the exponential phase (after 6 h of cultivation) turned out to be the most resistant to the action of chitosan (Chen and Chou, 2005).

In studies conducted by Kulikov et al. it was shown that the antifungal effect of low molecular weight chitosan against Candida albicans depends on the strain peculiarities of the yeast. The authors note that the collection and clinical strains of Candida albicans had different sensitivities to low molecular weight chitosan. In the presence of chitosan, the cells of the collection strain of the fungus, in contrast to the clinical isolate, underwent significant morphological changes, and died at a polymer concentration in the medium of 200 μg/ml. The morphology of the cells of the clinical isolate of the fungus remained almost unchanged, and the cells themselves continued to multiply at a given concentration of chitosan in the medium. At the same time, the clinical strain ceased to form growth tubes and pseudomycelium, which are one of the most important indicators of its high virulence (Kulikov et al., 2014). In other work, for low molecular weight chitosan with a molecular weight of 96.5 kDa, complete destruction of the cell membrane and lysis of Candida albicans yeast cells was detected at higher concentrations in the medium (1 mg/ml) (Peña et al., 2013).

Going further, the antimicrobial activity of chitosan is also affected by the presence of substituents in its molecule. A number of studies have shown that chitosan derivatives are active against gram-positive and gram-negative bacteria only at a low degree of substitution. For this purpose, chitosan derivatives should have a larger number of protonated amino groups (Varvaresou et al., 2009, Gerasimenko et al., 2004, Lunkov et al., 2018). In the study of Cai Zhaosheng et al. the increase in antimicrobial activity of carboxyethyl chitosan and its quaternized analogue is noted against Escherichia coli and Staphylococcus aureus cultures (Cai et al., 2009). The addition of Cu2 + and Mn2+ ions to chitosan increases its antibacterial activity against Staphylococcus aureus, Escherichia coli and Salmonella bacteria (Severino et al., 2015, Marambio-Jones and Hoek, 2010). And the inclusion of silver nanoparticles in chitosan provides an antimicrobial effect not only against Staphylococcus aureus and Escherichia coli, but also against Bacillus subtilis spore-forming species (Marambio-Jones and Hoek, 2010, Kalwar et al., 2017).

Currently, a major challenge in antimicrobial therapy is the ability of microorganisms to form biofilms on biotic and abiotic surfaces. More than 65 % of all infectious diseases are caused by microorganisms existing in the form of biofilms (highly ordered communities of microorganisms inside the polysaccharide matrix they form) (Polygach et al., 2018). It has been reliably established that in the human body, biofilms are formed by both representatives of normal microbiome and pathogens, which plays a crucial role in the development of the pathological process (Stoodley et al., 2002, Costerton et al., 1999). The formation of biofilms by microorganisms significantly increases the tolerance of microorganisms located in its matrix to antimicrobial agents, which easily destroy the same microorganisms in the planktonic state (Costerton et al., 2003). In this regard, studies that have demonstrated the inhibitory effect of chitosan on biofilms formed by the bacteria Streptococcus mutans, one of the main species capable of adhesion to the teeth surface and the formation of biofilms on them are of great interest to get acquainted with. It has been shown that chitosan suppresses the formation of biofilms by this type of bacteria both on the stage of cell adhesion to the surface of teeth and on the growth of mature biofilms (Kawakita et al., 2019a, Pasquantonio et al., 2008). As there are only few published works devoted to chitosan interaction with biofilms and influence on biofilms formation and growth, it is an interesting and promising area for further investigations.

In general, we can conclude that the data on the antimicrobial activity of chitosan presented in the literature are quite contradictory, since antimicrobial activity depends on many very different factors, as well as on their combination, which, on the other hand, makes it possible to choose or obtain chitosan with the antimicrobial activity adjusted for the specific research purposes and expands the range of its possible applications.

Recently, chitosan has been used quite often, either independently or as part of combined preparations, as an antimicrobial agent. The two main approaches are used to enhance chitosan antimicrobial properties. In the first case, chitosan is chemically modified and various bilologically active chitosan derivatives are synthesized. For example, it was shown that water-soluble chitosan derivatives (3-aminopyridine and 3-amino-4-methylpyridine) possessed a higher antifungal effect compared to the original chitosan against some phytopathogenic fungi such as Phytophthora capsici, Rhizoctonia solani, Fusarium oxysporum and Fusarium solani (Liu et al., 2018). Moreover, it was found that chitosan modified with quaternary amines showed greater antimicrobial activity against gram-positive bacteria Staphylococcus aureus (Zhou et al., 2013). Another way to increase chitosan antimicrobial activity is preparing various colloid systems such as micro- and nanoparticles and gels on its basis. Comparative testing revealed that chitosan nanoparticles obtained by ionic gelation have greater antibacterial activity than chitosan and chitin itself, due to the spherical shape of such particles, their small size and larger surface area, which allowed them interacting more effectively with cells (Hamedi et al., 2018).

As we can see recently, researchers actively study chitosan and its derivatives exhibiting potent antimicrobial activity, supported by in vitro and in vivo studies (Liu et al., 2018, Zhou et al., 2013, Hamedi et al., 2018). However, despite the great interest in chitosan, the mechanism of its antimicrobial action has not been sufficiently studied to the present moment. Several mechanisms of the antimicrobial effect of chitosan are described, which can be divided into extracellular effects, intracellular effects, or combined effects, depending on the initial characteristics of this biopolymer (Fig. 4).

The most widely recognized hypothesis is the destruction of the cell membrane and wall of microorganisms due to the ionic interaction of positively charged amino groups of chitosan with negatively charged surface molecules of microorganisms (with teichoic acids of gram-positive bacteria, with lipopolysaccharides of gram-negative bacteria or with phospholipids of fungi) under acidic conditions, which leads to the destruction of the cell membrane, leakage of intracellular compounds and cell death (Yan et al., 2021, Xing et al., 2018). Even in this case, different mechanisms of microbial cell membrane and wall disruption can occur (Zheng et al., 2022). Thus, chitosan nanoparticles were shown to disrupt the cell wall of Escherichia coli O157:H7 at neutral pH by interacting with the major outer membrane porin protein (OmpA). In contrast, positively charged plain chitosan disrupts the cell wall of Escherichia coli O157:H7 at acidic pH by binding to negatively charged lipopolysaccharide molecules (Divya et al., 2017).

Another possible alternative mechanism is that chitosan acts as a chelating agent that selectively binds to trace metal elements, causing the production of toxins and inhibiting the growth of microorganisms (Divya et al., 2017). And this mechanism is primarily specific for high molecular weight chitosan. Since high molecular weight chitosan due to larger molecules size is generally unable to penetrate the cell wall and cell membrane, its potential antimicrobial effect is that it acts as a chelator of essential metals, preventing extracellular uptake of nutrients from cells and altering cell permeability (Dutta et al., 2009, Rabea et al., 2003, Wang et al., 2005). Thus, during the discussion of the antimicrobial mechanism and molecular structure of chitosan-metal complexes, it was concluded that chitosan-metal complexes exhibit higher antimicrobial activity due to the stronger positive charge after complex formation (Wang et al., 2005). More chelated metal ions, stronger bonds, higher molecular weight and degree of deacetylation of chitosan and lower pH of the environment contribute to obtaining a higher positive charge, resulting in better antimicrobial activity.

Finally, the proposed mechanism for chitosan's antibacterial activity involves a sequential process: it first binds to the negatively charged bacterial cell wall, causing disruption and altered membrane permeability. It then attaches to DNA, inhibiting replication and leading to cell death (Nasaj et al., 2024).

The exact mechanism of chitosan antibacterial activity remains to be fully elucidated. The antimicrobial activity of chitosan is influenced by numerous factors of both biological and structural origin. Physicochemical factors such as pH, temperature, ionic strength, presence of divalent metal cations, and the solvent used, play a decisive role in determining the efficacy of the biopolymer (Jeon et al., 2014). The characteristics of chitosan also matters, as low molecular weight chitosan has not only extracellular antimicrobial activity, but also intracellular antimicrobial activity, due to more effective penetration into the cell and the effect on the synthesis of RNA, proteins and mitochondrial function (Sudarshan et al., 1992, Roy et al., 1999). In addition, the mode of antimicrobial action of chitosan largely depends on the type of target microorganism (Ke et al., 2021).

The chitosan influence on microbial pathogenesis is another interesting topic to discuss in the context of growing number of attempts to use chitosan for antimicrobial preparations and products. This influence is in great dependence on the structure and properties of specific microorganism.

Thus it is of interest to study the effect of chitosan and its derivatives on Staphylococcus aureus culture, since in some skin diseases, in particular atopic dermatitis, an increase in the proportion of this species in the microbiota is noted (Fölster-Holst, 2022). In the review (Geoghegan et al., 2018), the authors provide generalized data on the pathogenesis of Staphylococcus aureus in atopic dermatitis, which is caused by both the direct effect of bacteria due to the production of toxins and dysbiosis of the skin microbiota, and the effect on immune responses leading to a weakening of the skin barrier, an increase in its sensitivity to allergens and a deepening of inflammation. In addition, active reproduction of Staphylococcus aureus is accompanied by the formation of biofilms resistant to antibiotics (Craft et al., 2019). The study (Potara et al., 2011) showed that a bionanocomposite of silver nanoparticles and chitosan causes significant changes in the morphology of Staphylococcus aureus cells due to disruption of the cell wall integrity. At the same time, the minimum inhibitory and minimum bactericidal concentrations of such obtained bionanocomposite are significantly lower than for other types of nanoparticles and pure chitosan. The study (Chung and Chen, 2008) showed that chitosan reacts with both the cell wall of Staphylococcus aureus and the membrane, affecting the structure of the lipid bilayer. As a result of this interaction, the permeability of the cell membrane changed, which was accompanied by a leakage of enzymes and nucleotides.

In another group of studies, it was found that the active reproduction of Propionebacterium acnes commensal skin bacteria, which is observed in acne disease together with Staphylococcus epidermidis, Malassezia spp., and Staphylococcus aureus, contributes to inflammation of the follicles and hyperkeratinization in patients. In this case, Propionebacterium acnes bacteria form stable biofilms, which complicates treatment with antimicrobial drugs (Liu et al., 2015). Nanoparticles synthesized from chitosan with alginate demonstrated antimicrobial activity against Propionibacterium acnes, which are found on the skin in acne. Electron microscopy showed that chitosan-alginate nanoparticles destroy the cell membrane. It was also found that the resulting nanoparticles inhibited the production of inflammatory cytokines in human monocytes and keratinocytes induced by Propionibacterium acnes, thereby exerting an anti-inflammatory effect on the skin (Friedman et al., 2013).

Finally, a study (Fahle´n et al., 2012) showed difference in the composition of the skin microbiota in individuals with normal skin and those with psoriasis. One of the most frequently detected microorganisms on the skin of patients with psoriasis, especially guttate psoriasis, is Streptococcus pyogenes (Weisenseel and Prinz, 2005). A study (Baker et al., 2003) showed that streptococcal cell wall proteins are the cause of an increased immune response in the skin in psoriasis.

Also, the relationship between intestinal microbiota and various skin diseases has been actively studied recently. The review (Zou et al., 2024) presents research data showing that intestinal dysbiosis causes activation of the immune system and thus can affect immune-mediated skin diseases. Therefore, in addition to traditional therapy for skin diseases, restoration of intestinal microbiota can be considered as a promising direction in the treatment of inflammatory skin diseases. Prebiotics and probiotics play an important role in the process of normalizing intestinal microbiota. Prebiotics are non-digestible compounds that selectively promote beneficial gut microbiota growth, while probiotics are live microorganisms that confer health benefits when administered in adequate amounts, collectively supporting microbiome balance and host well-being. Emerging research has demonstrated the prebiotic potential of chitosan and its derivatives. Guan and Feng (Guan and Feng, 2022a) recently characterized chitosan's ability to selectively modulate gut microbiota, while Guan et al (Guan et al., 2019). documented that chitooligosaccharide supplementation in piglets significantly increased beneficial Bifidobacterium and Lactobacillus populations in both ileal and colonic regions while reducing potentially pathogenic Escherichia coli colonization in the colon. Ma and Kim (Ma and Kim, 2016) investigated the activity of chitosan microparticles in simulated rumen gastrointestinal fluids of cows, which is the environment the particles encounter during oral administration. It was found that chitosan microparticles do not disrupt digestibility, pH and total production of volatile fatty acids, which indicates that chitosan microparticles do not affect the functionality of the rumen. The results of in vitro studies on representatives of the human intestinal microbiota demonstrated selective inhibition of the growth of individual bacterial species, which gives grounds for considering it as a prebiotic for normalizing intestinal microbiome in the future (Šimůnek et al., 2012).

Chitosan and its derivatives have demonstrated significant antibiofilm activity (ability of biologically active substance to disrupt established microbial biofilms and/or inhibit their formation) against clinically relevant microorganisms such as Streptococcus mutans (Kawakita et al., 2019b, Chávez de Paz et al., 2011) and Pseudomonas aeruginosa (Reighard et al., 2015). Notably, chitosan exhibits potent antibiofilm effects against Staphylococcus aureus, a key colonizer in atopic dermatitis. A 2022 study (Rubini et al., 2018) revealed that chitosan derived from Portunus sanguinolentus crab shells effectively disrupted mature biofilms of both standard and clinical methicillin-resistant Staphylococcus aureus strains. Importantly, this marine-derived chitosan outperformed commercial chitosan in degrading the dense exopolysaccharide matrix, suggesting enhanced therapeutic potential for resistant infections. Peng et al. (2011) demonstrated that coating polystyrene surfaces with chitosan or its quaternized derivative significantly inhibited Staphylococcus epidermidis and Staphylococcus aureus biofilm formation, with efficacy increasing proportionally with substitution degree and concentration (Peng et al., 2011). Importantly, chitosan derivatives also disrupted pre-existing biofilms and suppressed icaA-mediated polysaccharide production, suggesting chitosan potential as an effective anti-biofilm coating for medical implants to prevent device-related infections.

A number of studies have also been devoted to the investigation of the immunological activity of the chitosan biopolymer. The work (Ivanushko et al., 2007) demonstrated the immunological activity of a number of synthesized chitosan derivatives (N-3-hydroxymyristoyl chitosan, carboxymethyl chitosan, carboxyethyl chitosan, carboxypropyl chitosan, etc.). A number of reviews summarized modern concepts of the immunological activity of chitosan in vitro. The effect of chitosan on individual links of innate and adaptive immunity was reported. It was shown that chitosan affects the maturation, activation, production of cytokines and polarization of dendritic cells and macrophages (Ghattas et al., 2025).

The immune response induced by chitosan involves several signaling pathways, including cGAS–STING, STAT-1, and NLRP3 inflammasomes. In human and mouse macrophage primary models and in models derived from these cell lines, it has been shown that, depending on the immune response pathway, chitosan can exhibit both anti-inflammatory and pro-inflammatory activity. The type of activity depends not only on the characteristics of chitosan, but also largely on its dose. However, pro-inflammatory responses are not always “harmful”, depending on the type and degree of inflammation, but in vivo studies are needed to obtain reliable data on the immunological activity of chitosan (Fong and Hoemann, 2017).

The presence of endotoxin in chitosan has a significant effect on its immunoreactivity. In a study (Ravindranathan et al., 2016), chitosan solutions from various sources were treated with murine and human antigen-presenting cells: macrophages and/or dendritic cells. Immunoreactivity was assessed by the amount of tumor necrosis factor-α (TNF-α) secreted by the cells. Studies have shown that chitosan is virtually inert at endotoxin levels below 0.01 IU mg/ml. Thus, chitosan purification can reduce the undesirable proinflammatory effects of chitosan and expand the boundaries of its clinical application. Based on the above, it can be assumed that the anti-inflammatory activity of chitosan can be useful for solving dermatological problems caused by impaired immune responses in the human body.

Chitosan application in cosmetics and dermatology

The current great interest in chitosan in the cosmetics industry and dermatology is due to the fact that this polymer has many useful properties: it is a good structure-former, has high biocompatibility with skin cells, exhibits moisturizing, antioxidant, anti-inflammatory and antimicrobial properties, it is non-toxic and biodegradable (Aranaz et al., 2009).

Chitosan is actively used as an ingredient in many cosmetic products. It is added to creams, shampoos, deodorants, shower gels, liquid soaps, gel toothpastes, hair styling and curling products, as well as lotions, where it is used as a cleansing, conditioning, softening and moisturizing component, and a surfactant, emulsifier, stabilizer and viscosity imparting component (Table 1) (Resende et al., 2019, Berthalon et al., 2024, Desbrieres et al., 2010, Chen and Heh, 2000). The antioxidant activity of chitosan and its derivatives provides skin protection from oxidative stress, as well as protection from oxidation of the components of the product itself (Sun et al., 2007, Sánchez et al., 2017). And the ability of chitosan to absorb UV in the region below 400 nm makes it possible to use it as a sunscreen agent (Gomaa et al., 2010).

The unique properties of chitosan make it possible to use it not only as an ingredient in cosmetics, but also as delivery systems (carriers) of biologically active substances beneficial to the skin, as well as medicines for dermal use in the treatment of skin diseases (topical preparations) (Bernkop-Schnürch and Dünnhaupt, 2012). Various chitosan-based systems including hydrogels, films, micro- and nanoparticles, and chitosan fabrics are capable of absorbing huge amounts of water and biological fluids, which is of great interest for loading drugs from solutions and colloids or for absorbing skin exudates and, therefore, they present excellent base for dermal applications in biomedical areas (Table 2) (Almoshari, 2022). However, the moisture-absorbing and moisture-holding abilities of chitosan depend on its physical and chemical properties such as molecular weight and the degree of deacetylation of the polymer. For example, in a study conducted by Qin et al., of all tested samples, the best water-retaining and moisture-absorbing properties were found for chitosan with a molecular weight of 0.45 × 104 kDa and a degree of deacetylation of 0.1 (Qin et al., 2002). In work (Chipovskaya et al., 2018), a chitosan-containing hydrogel for cosmetic skin care is proposed, consisting of low molecular weight chitosan, lactic acid, water, and which, according to the invention, additionally contains glycolic acid. Such gel helps to increase skin moisture and reduce the depth of wrinkles.

Chitosan as a drug carrier is being actively studied for the topical treatment of atopic dermatitis, one of the common skin diseases, the symptoms of which are observed among 10–28 % of children and 1–3 % of adults (Mehta and Fulmali, 2022). Chitosan gels with liposomes of 220–350 nm size containing corticosteroids (betamethasone valerate/diflucortolone valerate), which are traditionally used for the treatment of atopic dermatitis, are described (Eroğlu et al., 2016). Studies have shown that the resulting chitosan gel with liposomes turned out to be more effective in model experiments in vitro and ex vivo in comparison to several existing commercial products and can be considered as a valuable alternative for the local treatment of this disease. Chitosan-glycerin gel, obtained by simple mixing of the components, during in vitro experiments effectively reduced the transdermal absorption of certain metal ions, which makes it possible to use it as a barrier preparation for contact dermatitis in metallurgical, chemical industries, construction and agriculture, where there is an increased content of ions metals in the air (Ramesan and Jain, 2019). A chitosan gel (cross-linked with glutaraldehyde) with introduced solid lipid nanoparticles loaded with an ebastine antihistamine drug is also described for the treatment of allergic skin dermatoses, in particular idiopathic chronic urticaria. The gel provided sustained release of the drug for 24 h (Kazim et al., 2021). Chitosan gels containing anti-inflammatory drugs, silver nanoparticles and/or antioxidants have shown positive results in restoring skin in the treatment of wounds and burns (Soriano-Ruiz et al., 2020, Popova et al., 2023, Guadarrama-Escobar et al., 2023).

Chitosan, as a drug carrier, is also used in the form of microcapsules, nanoparticles, films and nanofibers [142]; Nagpal et al., (2010); Taokaew and Chuenkaek, (2024); Satchanska et al., (2024); Sarmento et al., (2007); Abadi et al., (2022); Russo et al., (2014); Rengifo et al., (2019); Sapra et al., (2008)). The size of chitosan nanoparticles in the range of 100–250 nm ensures their easy penetration into the intercellular space, and chitosan itself provides more dense contact with skin cells and thereby enhances the penetration of active components into the skin (Kou et al., 2022, Feng et al., 2021, Jeon et al., 2015). The work (Janes et al., 2001) describe a mucoadhesive composition based on thiolated chitosan (TCS) – lithocholic acid (LA) nanomicelles enriched with β-carotene, the average size of which is < 300 nm. Studies in mice have demonstrated a reduction in skin cancer growth, which has been confirmed by morphological and biochemical studies. In study (Baidamshina et al., 2021) wound healing preparation has been developed, which contained powdered chitosan (65 %), ascorbic acid (5 %), trimecaine (0.75 %), pepsin and collagenase (32 %). The dosage form for obtained composition was presented in the form of a powder, ointment or gel.

Thus, the use of chitosan as a base natural polymer makes it possible to obtain a great number and diversity of products for the treatment of acute and chronic skin diseases. It can be noted that the most common skin diseases with all their diversity, have two indicators in common – a disruption in the functioning of the immune system and a change in the composition of the skin microbiota, connected with increased colonization of the skin by certain types of microorganisms. In this regard, the antimicrobial properties of chitosan are of great interest, since they allow it to be used additionally as a natural preservative to protect finished products from microbial contamination, as well as an antiseptic agent in the treatment of wounds and burns of the skin.

This section is devoted to a review of scientific research that allows us to evaluate the prospects for using chitosan as a beneficial ingredient for the microbiota of human skin.

The number of microorganisms living on the surface of the human skin can exceed 1 million per 1 cm2 (Grice and Segre, 2011). They perform a number of functions important for skin health: they have a beneficial modulating effect on the immune response, prevent the colonization of the skin by opportunistic or pathogenic microorganisms, and also promote skin regeneration in case of damage (Moskovicz et al., 2020, Swaney and Kalan, 2021, Li et al., 2019, Flowers and Grice, 2020). There is a balanced equilibrium between skin cells and microbiota, the disruption of which changes the composition of skin microbial communities and its barrier function leading to dysbiosis (Dréno et al., (2016); [162]. Today, it is a proven and generally accepted fact that microorganisms inhabiting skin are involved in maintaining its health, as well as in the development of various pathological conditions, including atopic dermatitis, psoriasis, acne, and contact dermatitis. During skin diseases, there is a decrease in microbial diversity of microbiota and an increase in the number of pathogenic microorganisms, which further aggravates skin problems (Salava and Lauerma, 2014, Fitz-Gibbon et al., 2013, Jugé et al., 2018).

The study of the microbiome of human skin is a new and rapidly developing area of research. The skin is an ecosystem abundantly populated by microorganisms, the number and diversity of which is the most important criterion for determining skin health. A huge number of different bacteria can be found on the skin, including four dominating representatives types: Actinobacteria (51.8 %), Firmicutes (24.4 %), Proteobacteria (16.5 %) and Bacteroidetes (6.3 %). The predominant microorganisms of the skin are Staphylococcus epidermis, Staphylococcus aureus, Micrococcus spp., Sarсina spp., Propionibacterium spp., representatives of Corynebacterium spp., Brevibacterium spp. The composition of each person’s microbiota is unique and subject to changes under the influence of various endogenous and exogenous factors: age, health, medication, climate, eating habits, hygiene, etc (Grice et al., 2009).

In addition to bacterial microbiome, fungi, archaea, viruses, protozoa and Demodex mites also live on the surface of the skin. Thanks to advances in the study of the fungal microbiome of the skin (or mycobiome), fungi of the genera Malassezia, Rhodotorula, Aspergillus, Debaromyces, Cryptococcus and Candida have been discovered in the skin microbiota. But in general, it is believed that the main representatives of a healthy human skin mycobiome during life are representatives of the genera Saccharomyces, Malassezia and Candida (Guého et al., 1996, Sugita et al., 2001).

The archaeome and virome of the skin are currently poorly studied and also represents an interesting area for further investigations. Beyond bacteria, emerging research highlights the importance of the human skin archaeome (dominated by methanogens like Methanobrevibacter spp.) (Moissl-Eichinger, 2018) and virome (primarily bacteriophages) (Hannigan et al., 2015) in modulating skin health and microbial balance. The skin virome, particularly phage predation (Natarelli et al., 2023) of pathogens like Staphylococcus aureus, may synergize with chitosan antimicrobial effects, offering novel combinatorial strategies for microbiome-targeted therapies.

Advances in skin microbiome science have differentiated three therapeutic approaches: prebiotics (microbiome nutrients), probiotics (beneficial live microbes), and postbiotics (microbial metabolites). Clinical studies show their combined use restores microbial diversity, that is critical for managing conditions like acne (via C. acnes strain balancing) and aging (through S. epidermidis derived ceramide modulation) (Habeebuddin et al., 2022, Petrov et al., 2022, Sharma et al., 2016, Lanzalaco et al., 2012). Therefore, maintaining the qualitative and quantitative composition of the skin microbiota and restoring its biodiversity is currently one of the actively developing areas of the cosmetic industry, and is also becoming an important part of the treatment of skin diseases and the correction of age-related skin changes.

Nowadays, lysates of such microorganisms as Bifidobacterium, Lactobacillus, Vitreoscilla filiformis and Saccharomyces yeast (Mahe et al., 2013, Seité et al., 2016, Seité et al., 2017, Durmusoglu et al., 2021) are the most widely used in cosmetics are postbiotics. According to realized studies results, topical products containing lysates of probiotic strains of bacteria promote wound healing and improve the course of a number of common dermatoses (Habeebuddin et al., 2022, Petrov et al., 2022); reduce skin sensitivity, as well as irritation after chemical peels and minimally invasive hardware procedures (Sharma et al., 2016, Lanzalaco et al., 2012). There are studies indicating a slowdown in the process of photoaging, a decrease in oxidative stress, and an improvement in skin barrier function under the influence of topical probiotics (Lanzalaco et al., 2012).

Along with postbiotics, prebiotics are also actively introduced into the composition of cosmetic products. Prebiotics are substances of non-microbial origin that selectively stimulate the growth and/or activity of one or more types of beneficial bacteria. The main prebiotic substances are various oligosugars, which are interconnected by β-glycosidic bonds. The most famous of them are inulin and its derivatives (Davani-Davari et al., 2019).

The effect of galactooligosaccharides on Staphylococcus epidermidis and Staphylococcus aureus, the main “good” and main “bad” bacteria of the skin, was studied. In vitro results showed that galacto-oligosaccharides at a concentration of 5 % (w/v) had a strong stimulatory effect on Staphylococcus epidermidis, while simultaneously exhibiting an inhibitory effect on Staphylococcus aureus, which confirmed the effectiveness of prebiotics in maintaining a healthy balance of the skin microbiome (Petrov et al., 2022).

In this regard, the main object of current review, chitosan is a natural cationic aminopolysaccharide consisting of elementary units of 2-acetamido-2-deoxy-D-glucose connected by a 1,4-β-glycosidic bond, the biological activity of which depends on its physicochemical properties, primarily on molecular weight and degree of deacetylation (Zargar et al., 2015). This made it possible to consider chitosan and its derivative oligosaccharides as promising prebiotics for the intestinal microbiota (Guan and Feng, 2022b). By regulating and maintaining the balance of its main species (lactobacteria, bifidobacteria, Roseburia spp., Faecalibacterium spp. and Akkermansia spp.), an improvement in the function of the gastrointestinal mucosa and the host immune system is achieved (Sanders et al., 2019). The prebiotic potential of chitosan oligosaccharide with a degree of polymerization of 2–8, obtained by enzymatic hydrolysis of a completely deacetylated chitosan polymer, was studied. As a result of research, it was found that this oligosaccharide showed a selective stimulating effect on the studied species of bifidobacteria and lactobacilli, like the well-known prebiotic fructooligosaccharide (Lee et al., 2002).

Preclinical studies demonstrate that dietary chitosan and its oligosaccharides modulate gut microbiota composition in animal models. In weaned mini-piglets, chitooligosaccharide supplementation significantly increased the abundance of beneficial commensals like Lactobacillus spp. while reducing Escherichia coli populations, correlating with improved intestinal health markers (Kong et al., 2014). Similarly, in high-fat diet-fed mice, low molecular weight chitosan oligosaccharides attenuated obesity-related metabolic disorders by enriching Akkermansia and Gammaproteobacteria, which are associated with improved gut barrier function and reduced systemic inflammation (He et al., 2020). Furthermore, chitosan supplementation in immunosuppressed mice enhanced resistance to Cryptosporidium parvum infection by restoring gut microbial diversity and activating TLR4/STAT1-mediated immune responses, particularly through increased Bacteroidetes/Bacteroides, and decreased Proteobacteria, Tenericutes, Defferribacteres and Firmicutes abundance (Rahman et al., 2021). These studies collectively demonstrate that chitosan derivatives promote a favorable gut microbiota profile associated with enhanced immunity and metabolic health.

The effect of insect chitin on the gastrointestinal microbiota is being actively studied, since the latter are considered as alternative types of food products as a source of protein and vitamins (Stull and Weir, 2023). For example, chitosan obtained from crickets of the Scapsipedusicipe species, in concentrations up to 20 % in vitro, stimulated the growth of probiotic bacteria Lactobacillus fermentum, Bifidobacterium adolescentis, Lactobacillus acidophilus and inhibited the growth of the pathogenic species Salmonella typhi (Kipkoech et al., 2021). The positive effect of chitosan and its derivatives on the microbiota of the gastrointestinal tract allows suggesting a positive effect of these compounds on the microbiota of the skin. Due to the unique properties of chitosan, the huge number of its derivatives, and the various forms of its application, different options for its use for the skin microbiota control and effect are theoretically possible.

It is known that the most active chitinolytics, capable of destroying glycosidic bonds in the chitosan molecule, are representatives of actinobacteria (Bai et al., 2016, Lacombe-Harvey et al., 2018). At the same time, actinobacteria are one of the dominant groups of bacteria in the skin microbiota, the number of which constitutes more than 50 % of all inhabitants of the human skin (Grice et al., 2009). It can be assumed that actinobacteria of the skin will be able to effectively hydrolyze the polymer to monomeric components, thereby providing sources of carbon and energy for the skin microbiome. However, at present time there are only a few studies in this area and we hope that this new interesting direction of research will gain interest and obtained results in near future. As an example, the patent (Günter et al., 2020) describes a liquid composition containing chitosan with a degree of acetylation of 20 % or less, which, when applied to the skin, forms a membrane on its surface and stimulates the growth of beneficial species of microorganisms compared to pathogenic species of the microbiome, and also helps to increase the biodiversity of the skin microbiota. Depending on the tasks at hand: treatment of dysbacteriosis, restoration of microbiota, or restoration of the skin after external local damage caused by skin peeling, laser procedures, plasma procedures, tattoos and their removal, the authors suggest introducing various active ingredients into the composition.

Chitosan as a carrier can be used to deliver to the skin various proven biologically active components (postbiotics, prebiotics, probiotics) that will help to restore and maintain the skin microbiota. When chitosan is applied to the skin as a carrier of biologically active substances in the form of a hydrogel, due to the formation of a film on the surface of the skin, additional conditions will be created for better penetration of the active components into the deep layers of the skin. The presence of the “intestinal-skin” axis makes it possible to orally use chitosan-based preparations with pro- and prebiotics to improve the c