Abstract
Postbiotic and paraprobiotic preparations are increasingly investigated as non-viable microbial-derived components for topical formulation development, offering an alternative to systems containing live microorganisms. However, their physicochemical and biological performance may depend strongly on the polymeric carrier, and direct comparisons of postbiotic and paraprobiotic fractions derived from the same microbial source in different gel matrices remain limited. This study aimed to develop and comparatively evaluate Carbopol®– and chitosan-based pharmabiotic gel-serums containing Lacticaseibacillus casei-derived postbiotic (PB) and paraprobiotic (PPB) fractions.
The formulations were characterized in terms of macroscopic appearance, pH, spreadability, and rheological behavior, and their antimicrobial activity, effects on preformed biofilm biomass, and DPPH radical-scavenging activity were evaluated. Blank Carbopol® and chitosan formulations and free PB and PPB fractions were included as controls to distinguish formulation-matrix-associated effects from those observed for the complete formulations. The developed gel-serums exhibited homogeneous initial macroscopic characteristics, pH values ranging from 4.22 to 5.66, and pseudoplastic shear-thinning behavior. Among the PB-containing systems, CA-CS-PB exhibited pronounced antimicrobial activity, with inhibition-zone diameters of 18.00 mm against Candida albicans and 19.93 mm against Candida krusei, and MIC values of 1156 μg/mL against both species. CA-CS-PB also reduced preformed Staphylococcus aureus and Pseudomonas aeruginosa biofilm biomass by 92.89% and 87.63%, respectively, and exhibited the highest DPPH radical-scavenging activity (73.50%). Among the PPB-containing systems, CA-C-PPB produced reductions of 94.63% and 93.71% in preformed S. aureus and P. aeruginosa biofilm biomass, respectively.
Blank formulations also exhibited measurable biological responses, indicating that the activities of the complete formulations should be interpreted as formulation-level effects rather than being attributed exclusively to the incorporated PB or PPB fractions. Because PB and PPB were incorporated at different concentrations (5% and 1% w/w, respectively), these findings should not be interpreted as an equivalent-dose comparison of their intrinsic biological potency. Overall, the findings demonstrate the feasibility of incorporating L. casei-derived PB and PPB fractions into different polymeric gel-serum systems and highlight the influence of both the pharmabiotic fraction and carrier matrix on formulation performance. These results support further investigation of these systems as topical formulation platforms; however, storage stability, release behavior, compositional characterization, and skin-relevant safety and performance require further evaluation before dermocosmetic or dermatological applicability can be established.
Introduction
The skin is the largest organ of the human body and serves as the first line of defense against physical, chemical, and biological insults while simultaneously regulating immune responses, preventing excessive transepidermal water loss, and maintaining tissue homeostasis [1,2]. As the outermost interface between the host and the external environment, healthy skin harbors a complex and dynamic microbial ecosystem that plays a fundamental role in maintaining barrier integrity and cutaneous immune function.
Disturbance of this balanced microbiota or impairment of the skin barrier facilitates colonization by opportunistic microorganisms, including Staphylococcus aureus, Pseudomonas aeruginosa, and Candida spp., thereby increasing susceptibility to acute and chronic skin infections. A major challenge associated with these pathogens is their ability to establish highly organized biofilms, in which microbial cells are embedded within an extracellular polymeric matrix that markedly enhances resistance to antimicrobial agents and host immune defenses. Consequently, biofilm-associated infections frequently delay wound healing, reduce therapeutic efficacy, and contribute to the increasing burden of antimicrobial resistance, emphasizing the urgent need for safe and multifunctional topical alternatives capable of simultaneously protecting the skin barrier and controlling microbial colonization [3]. In recent years, advances in cosmetic science have transformed the concept of topical skin care from products intended solely for cleansing or aesthetic improvement into biologically active formulations capable of supporting skin physiology and microbiome homeostasis. This transition has accelerated the development of dermocosmetic and pharmabiotic products containing functional bioactive compounds with antioxidant, antimicrobial, anti-inflammatory, and barrier-protective properties [4,5,6]. Among available topical dosage forms, gel-serums have attracted considerable attention because of their lightweight texture, rapid absorption, high water content, excellent spreadability, and pleasant sensory characteristics. Furthermore, gel-serum systems enable efficient incorporation of hydrophilic bioactive substances while ensuring homogeneous distribution and prolonged residence time on the skin surface, making them particularly attractive carriers for microbiome-derived active ingredients [7].
Probiotic microorganisms and their derived bioactive products have emerged as promising candidates for next-generation dermocosmetic formulations owing to their broad spectrum of biological activities. Numerous studies have demonstrated that probiotic-derived compounds exhibit antimicrobial, antioxidant, anti-inflammatory, and immunomodulatory effects that may contribute to the maintenance of skin health and restoration of microbial homeostasis [8,9,10]. Nevertheless, the incorporation of viable probiotic microorganisms into topical products remains challenging because their biological activity is highly dependent on storage conditions, formulation composition, and environmental factors. In addition, the presence of live microorganisms may raise safety concerns, particularly for immunocompromised individuals or patients with impaired skin barriers [8].
These limitations have encouraged increasing interest in non-viable probiotic-derived preparations that retain biological activity while offering superior stability and improved formulation compatibility. Among these emerging pharmabiotic ingredients, postbiotics and paraprobiotics have gained particular attention as safe and stable alternatives to conventional probiotics. According to the International Scientific Association for Probiotics and Prebiotics (ISAPP), postbiotics are defined as preparations of inanimate microorganisms and/or their components that confer health benefits to the host [8,11]. These preparations comprise a complex mixture of biologically active compounds, including organic acids, bacteriocins, enzymes, peptides, amino acids, exopolysaccharides, short-chain fatty acids, vitamins, and other microbial metabolites capable of exerting antimicrobial, antioxidant, anti-inflammatory, and immunomodulatory activities [9,10]. In contrast, paraprobiotics consist of metabolically inactive microbial cells that retain structural integrity following controlled inactivation processes. Their biological activity is primarily associated with preserved cell-wall components, including peptidoglycans, lipoteichoic acids, and surface proteins, which interact with host cells and modulate immune responses without requiring microbial viability [12,13].
Compared with conventional probiotics, both postbiotics and paraprobiotics exhibit greater physicochemical stability, longer shelf life, easier manufacturing and storage, and a lower risk of microbial translocation or infection, making them particularly attractive active ingredients for topical pharmabiotic formulations [8,9,11]. Probiotic bacteria, Lacticaseibacillus casei is one of the most extensively investigated species because of its remarkable capacity to produce a diverse repertoire of biologically active metabolites with broad therapeutic potential. Previous studies have demonstrated that L. casei-derived postbiotics inhibit pathogenic microorganisms through the production of organic acids, bacteriocins, biosurfactants, and other antimicrobial metabolites, while simultaneously reducing oxidative stress and suppressing biofilm formation [9,14].
Similarly, L. casei paraprobiotics have been reported to exert beneficial biological effects through preserved cell-wall components, including peptidoglycans, lipoteichoic acids, and surface-associated proteins, which contribute to immune modulation, maintenance of epithelial barrier integrity, and regulation of host–microbe interactions [8,12,13]. Owing to these multifunctional biological properties, L. casei-derived postbiotics and paraprobiotics have emerged as promising pharmabiotic ingredients for topical formulations intended to support skin health while minimizing the limitations associated with viable probiotics [15,16,17].
The biological performance of topical pharmabiotic formulations, however, depends not only on the incorporated active ingredient but also on the physicochemical characteristics of the carrier system. An ideal topical vehicle should provide suitable rheological behavior, ease of application, prolonged residence time on the skin surface, and adequate stability while preserving the biological activity of incorporated compounds. Among the available polymeric carriers, Carbopol® and chitosan represent two of the most widely investigated materials because of their distinct physicochemical and functional properties [18,19]. Carbopol® is a synthetic cross-linked poly(acrylic acid) polymer capable of forming transparent, highly viscous hydrogels at relatively low concentrations. Its excellent rheological characteristics, pH stability, high water-retention capacity, and formulation versatility have resulted in widespread application in topical pharmaceutical and cosmetic products [19,20,21,22].
In contrast, chitosan is a naturally derived cationic polysaccharide obtained through the deacetylation of chitin and has attracted considerable interest as a multifunctional biomaterial. Aside from serving as a biodegradable and biocompatible polymeric carrier, chitosan possesses intrinsic antimicrobial, antibiofilm, antioxidant, bioadhesive, and wound-healing properties that may synergistically enhance the biological activity of incorporated pharmabiotic ingredients [23,24]. Electrostatic interactions between positively charged amino groups of chitosan and negatively charged microbial cell surfaces increase membrane permeability, destabilize biofilm architecture, and may facilitate improved antimicrobial efficacy. Moreover, its film-forming ability, high water-holding capacity, and favorable rheological characteristics contribute to prolonged skin retention and enhanced topical performance [23,24,25,26,27,28,29,30].
These complementary characteristics suggest that both Carbopol® and chitosan are attractive polymeric platforms for the development of microbiome-inspired pharmabiotic gel-serums, although their comparative influence on formulation performance has not been comprehensively elucidated. Recent studies have further highlighted the growing interest in postbiotic-based approaches for topical and dermocosmetic applications. Theodorou et al. (2024), in a review of clinical studies involving probiotic- and postbiotic-containing cosmeceuticals, emphasized the increasing use of microbial-derived preparations in skin-care products while also highlighting the need for adequate characterization and standardization [30]. Rusic et al. (2024) evaluated a cosmetic serum containing postbiotics, further demonstrating the growing interest in incorporating microbial-derived bioactives into topical formulations [31]. More recently, Machado et al. (2025), in a scoping review of cutaneous biotic formulations, reported considerable heterogeneity in microbial preparations, formulation strategies, and evaluated outcomes, which limits direct comparison among available studies [32].
Collectively, these recent studies demonstrate the growing potential of postbiotic-based topical formulations but also reveal important limitations in the current literature, particularly the lack of standardized microbial preparations, heterogeneous formulation approaches, and limited systematic evaluation of how carrier systems influence the performance of incorporated microbial-derived ingredients. Despite these advances, comparatively little attention has been given to the influence of the polymeric carrier on the physicochemical and biological performance of postbiotic and paraprobiotic preparations. In particular, direct comparisons of postbiotic and paraprobiotic fractions derived from the same microbial source and incorporated into different polymeric gel matrices remain limited. Addressing this gap may provide further insight into how both the pharmabiotic fraction and the carrier matrix contribute to overall formulation performance.
To address this gap, the present study aimed to develop multifunctional pharmabiotic gel-serums containing Lacticaseibacillus casei-derived postbiotic and paraprobiotic fractions using Carbopol® and chitosan as alternative polymeric delivery systems. The developed formulations were characterized in terms of macroscopic appearance, pH, spreadability, viscosity, and rheological properties. Their antimicrobial activity was subsequently evaluated against Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa, Candida albicans, and Candida krusei. In addition, minimum inhibitory concentrations, effects on preformed S. aureus and P. aeruginosa biofilm biomass, and DPPH radical-scavenging activity were investigated. By evaluating both the pharmabiotic fraction (postbiotic and paraprobiotic) and the polymeric carrier (Carbopol® and chitosan), this study provides an integrated framework for assessing their influence on the physicochemical, rheological, and biological performance of the developed gel-serums.
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Materials
Lacticaseibacillus casei-derived postbiotic and paraprobiotic fractions used in this study were prepared through controlled fermentation and inactivation procedures performed within the scope of the present project. Medium molecular weight (MMW) chitosan (Sigma-Aldrich, St. Louis, MO, USA) and Carbopol® 940 (Zag Kimya, İstanbul, Türkiye) were employed as natural and synthetic gelling polymers, respectively. Citric acid monohydrate (Merck, Darmstadt, Germany) was used for chitosan solubilization, whereas sorbitol (Sigma-Aldrich, USA), polyethylene glycol 400 (PEG 400; Merck, Germany), and urea (Merck, Germany) were incorporated as humectants and formulation auxiliaries. Polysorbate 20 (Sigma-Aldrich, USA) served as the surfactant, while sodium salicylate (Sigma-Aldrich, USA) and ammonium benzoate (Merck, Germany) were included as preservative components. Triethanolamine (Merck, Germany) was used to neutralize Carbopol® formulations. Ultrapure water obtained from a laboratory purification system was used throughout the study. All chemicals and reagents were of analytical grade and were used without further purification.
Soyer, P.; Öztürk, A.A. Development of Pharmabiotic Gel-Serums with Lacticaseibacillus casei Postbiotics and Paraprobiotics Using Chitosan and Carbopol. Gels 2026, 12, 838. https://doi.org/10.3390/gels12090838
