Therapeutic and Industrial Applications of Curdlan With Overview on Its Recent Patents

Curdlan is an exopolysaccharide, which is composed of glucose linked with β-(1,3)-glycosidic bond and is produced by bacteria, such as Alcaligenes spp., Agrobacterium spp., Paenibacillus spp., Rhizobium spp., Saccharomyces cerevisiae, Candida spp., and fungal sources like Aureobasidium pullulan, Poria cocos, etc. Curdlan has been utilized in the food and pharmaceutical industries for its prebiotic, viscosifying, and water-holding properties for decades. Recently, the usefulness of curdlan has been further explored by the pharmaceutical industry for its potential therapeutic applications. Curdlan has exhibited immunoregulatory and antitumor activity in preclinical settings. It was observed that curdlan can prevent the proliferation of malarial merozoites in vivo; therefore, it may be considered as a promising therapy for the treatment of end-stage malaria. In addition, curdlan has demonstrated potent antiviral effects against human immunodeficiency virus (HIV) and Aedes aegypti virus. It has been suggested that the virucidal properties of curdlans should be extended further for other deadly viruses, such as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and the current severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2/COVID-19). The prebiotic property of curdlan would confer beneficial effects on the host by promoting the growth of healthy microbiota in the gut and consequently help to reduce gastrointestinal disorders. Therefore, curdlan can be employed in the manufacture of prebiotics for the management of various gastrointestinal dysbiosis problems. Studies on the mechanism of action of curdlan-induced suppression in microbial and tumor cells at the cellular and molecular levels would not only enhance our understanding regarding the therapeutic effectiveness of curdlan but also help in the discovery of new drugs and dietary supplements. The primary focus of this review is to highlight the therapeutic interventions of curdlan as an anticancer, anti-malaria, antiviral, and antibacterial agent in humans. In addition, our review provides the latest information about the chemistry and biosynthesis of curdlan and its applications for making novel dairy products, functional foods, and nutraceuticals and also details about the recent patents of curdlan and its derivatives.


INTRODUCTION
Exopolysaccharides are natural polymers of high-molecularweight residual sugar polymers secreted by a wide variety of non-pathogenic microorganisms, including bacteria, yeast, and fungi. The extracellular natural polymeric substances belong to the carbohydrates category and provide many important health benefits, such as promotion of digestibility, biocompatibility, and endurance of long-term stability over a wide range of pH and environmental temperatures (1). Recently, researchers have found promising applications of natural polymeric substances, for example, curdlan, in the pharmaceuticals, food packaging, making biofilms, and in the dairy industry where they improve the viscosity and texture of the food products (2). Overall, curdlan-type exopolysaccharides have many emerging potential applications in making nutraceuticals, functional foods, cosmetics, drugs, and dairy products (3).

Physical Properties of Curdlan
Curdlan is a water-insoluble exopolysaccharide secreted by a wide variety of non-pathogenic bacteria, yeast, and fungi as shown in Table 2. It is comprised of a homopolymer of Dglucose-linked with β-(1,3)-glucan (12). The unique thermal characteristics of curdlan are utilized in biofilm making to improve water barrier capacity and thermal stability in singleand multilayer food packaging (13). Apart from its high thermal stability, curdlan also possesses stability at lower freezing temperatures, and hence is used in frozen foods packaging. According to recent reports, curdlan biofilms with 5% caprolactam have been shown to possess better mechanical strength and other physical properties as compared to other films (14). Curdlan can form two types of gels based on their thermal properties, viz., low-set gel and high-set gel (15). The low-set curdlan gel is produced when the aqueous solution of curdlan is heated up to 60 • C and then cooled to 40 • C in a thermally reversible process. On the other hand, the high-set curdlan gel is generated when the aqueous solution of curdlan is heated >80 • C in a thermally irreversible process (16). In chemical terms, there is a cross-linking between the curdlan micelles in the low-set gel, which are occupied by single-helix molecules through hydrogen bonds, whereas the curdlan micelles are cross-linked by a triplestranded helix through hydrophobic interactions in the high-set gel (17). Curdlan gels are used for the encapsulation of a number of drugs like indomethacin, salbutamol sulfate, and prednisolone to provide sustained release of these drugs (18). They are also used for grafting copolymers in the pharmaceutical industry (19).
Curdlan is one of the emerging exopolysaccharide, which was discovered and studied by (14,20). It has the ability to produce polysaccharide-based membrane film, which has proven effective in possessing gas barrier properties (20,21). Due to its nontoxic attributes, curdlan was approved as a food additive by the US Food and Drug Administration (22). Prebiotics are nondigestible oligosaccharides that promote the growth of beneficial bacteria in the gut. Curdlan being an oligosaccharide has a prebiotic potential and therefore can be used to promote the growth of useful intestinal microbiota (23).
Curdlan has shown a wide range of pharmacotherapy functions, such as anti-HIV, immunomodulation, and antitumor activity (24). A recent study revealed that curdlan can inhibit the invasion of malarial merozoites and therefore can be considered as a possible auxiliary candidate for treating intense malaria (25). Additionally, curdlan has exhibited potential antiviral activity against human immunodeficiency virus (HIV) and Dengue fever virus (26). In view of its virucidal activities, curdlan may be considered a potential candidate against other deadly viruses, such as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and the current severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2/COVID- 19).
There are many published reports regarding the therapeutic properties and excipient properties of curdlan (26), but the underlying mechanism of curdlan at the cellular and molecular level remains unknown. This review is intended to highlight the applications of curdlan in drugs and cosmetics, nutraceuticals, and functional foods and in potential industrial usages. In addition, our review provides the latest information regarding the chemistry and microbial sources used for the biosynthesis of curdlan, along with the recent patents applied by the food and drug manufacturers for the utility of curdlan.

CHEMISTRY OF CURDLAN Chemical Structures/Formulas of Curdlan Products
Chemical structures and formulas of curdlan compounds are shown in Table 1. The analytical studies have confirmed that curdlan is a D-glucose homopolymer linked in the manner of β-(1,3)-glycosidic bond (30). Curdlan is unbranched and has an average degree of polymerization of around 450. In an alkaline solution of 0.3 Mol, the molecular weight of curdlan is in the range of 5.3 × 10 4 to 2.0 × 10 6 Da. The bacterial and fungal species produced curdlan, the average weight lies between 2.06 × 10 4 and 5.0 × 10 6 Da (31).
The general molecular formula of curdlan is (C 6 H 10 O 5 ) n . It consists of approximately 135 residues of glucosyl, and the infrared spectrum shows a peak of β-arrangement gel at 890 cm −1 , while α-arrangement gel shows a peak at 840 cm −1 (32)(33)(34).

Molecular Conformers of Curdlan
The molecular structures of curdlan in the aqueous systems have been determined by several investigators. For instance, Zhang et al. (37) identified three soluble curdlan conformers, including single-helix, triple-helix, and random coil. The single-stranded curdlan is composed of 2-25 glucose units that exhibit better water solubility with improved permeability. Different biological activities are associated with short disordered curdlan chains (1, 3, 1, 2)-β-D-glucan (branched glucan) Streptococcus sp.
(38). Some curdlans have a helical (ordered) conformation at low NaOH concentrations (37). Ikeda and Shishido demonstrated using atomic force microscopy that as alkali concentration increases from 0.19 to 0.24 N, curdlan conformation changes from helix conformation to random coil structure (39). Cheung et al. reported that in 0.3 mol NaOH, the ultrasonic degradation was found to be much faster than in 0.1 mol NaOH, suggesting that curdlan is more sensitive to the degradation in the randomcoil conformation than in the helix conformation (40). Saitô et al. confirmed that curdlan is fully soluble in an alkaline solution having a concentration >0.2 N NaOH and exists as a random coil, but the polymer adopts an "ordered state" after neutralization, which consists of a combination of single and triple helices (41). Zhang and Nishinari reported that at low concentrations, curdlan exhibits a relatively extended flexible chain structure in dimethyl sulfoxide, whereas chain entanglement occurs at a higher concentration (42).

Microbial Source
Curdlan can be produced as exopolysaccharides by different types of microbes, for example, bacteria, yeast, and fungi. Some major microbes are involved in the biosynthesis of curdlan, and the yields achieved are shown in Table 2.
Alcaligenes faecalis is a Gram-negative bacterium, which can be isolated from the soil and can yield maximum curdlan. The NaOH solution was substituted by feeding ammonia water at an initial stage to control pH at 7.0, and then the NaOH solution was added to control pH at 5.6 for the curdlan production after ND, not described.
ammonia was consumed. As a result, 72 g/L of curdlan was produced at controlled nitrogen condition (4). Agrobacterium radiobacter is also a Gram-negative soil bacterium, which yields 41.24 g/L of curdlan after 96 h of fermentation (43).
Recently, the biological activity as an immune stimulator of peripheral blood mononuclear cells to increase the interferonγ secretion was shown by curdlan developed by the mutant strain of Agrobacterium sp. ATCC 31750 (ATCC, Manassas, VI, USA), having a molecular weight of 300,000 Da and consisting exclusively of the 1,3-glucose residue linkage (44). Paenibacillus polymyxa is a Gram-positive bacterium, which at 50 • C and at pH 7 yields 6.89 g/L of curdlan (6). It is used as a drug delivery carrier for the sustained release of drugs and as a support matrix for the immobilization of enzymes (45). Rhizobium radiobacter is another Gram-negative bacterium, which can yield 31 g/L of curdlan after 72 h of fermentation (46). A recent study confirmed that curdlan obtained from R. radiobacter is highly thermostable with excellent water-holding capacity and can be used as an antioxidant and prebiotic agent. However, further studies are needed to check the biological potential of curdlan produced from R. radiobacter (47).

Biosynthesis of Curdlan
The biosynthesis of extracellular polysaccharides can be divided into three steps. First, substrate uptake for the intracellular formation of polysaccharides and extrusion from the cells. Second, through active transport and group translocation involving substrate phosphorylation, a carbohydrate substratum reaches the cell. Finally, the substrate is guided either along catabolic pathways or those that lead to the synthesis of polysaccharides.
In the biosynthesis of curdlan, uridine diphosphate (UDP)glucose constitutes the primary precursor that is synthesized via the conversion of glucose-1-phosphate to UDP-glucose by the enzyme UDP-glucose pyrophosphorylase. Later, the polymer structure occurs by the combination with the transition of monosaccharides to the carrier lipid from UDP-glucose. Subsequently, the polymer is extruded from the cells after further chain elongation (Figure 1) (48).
A study conducted by Mohsin et al. showed that curdlan can be synthesized from the orange peels using A. faecalis. The cocktail mixture included about 78% water, 10% soluble sugars, 35% cellulose, and 15% pectin from fresh orange peels. However, because of high moisture content, the cost for the preservation of curdlan is high, and hence drying process is an effective approach to store peels. By this technique, the dried orange peels yielded about 23.24 g/L of curdlan. The fermentation substrate showed significant effects on the development of curdlan and presented a novel engineered approach for the production of curdlan in an environment-friendly manner (49).

Food and Dairy Products
Curdlan gel is a colorless, odorless, and non-toxic substance that has water retention ability. All these versatile properties have led to its use as a food additive in the dairy industry (50,51). Since curdlan is not easily hydrolyzed by the digestive enzymes, it can be added in prebiotics and to reduce calories and to promote healthy gut microbiota, and thus prevent obesity (52). It is also used as an additive in the preparation of frozen foods, pasta, and canned meat (53).
The hydrothermal hydrolysis of curdlan creates 1,3-glucooligosaccharides (GOS), which have proven beneficial in the production of functional foods and biopharmaceuticals (54). On account of the bacteriostatic, biocompatible, biodegradable, and hydrophilic characteristics, the quaternary ammonium salt solution of curdlan (Qcurd) is widely used along with its derivatives to encapsulate several bioactive food components (55). Several curdlan-induced benefits in food and dairy products are summarized in Table 3.

Therapeutic Products
Biologically active β-(1,3)-glucan oligosaccharides were developed by the combination of heat treatment and hydrolysis of curdlan (62). In vitro batch fermentation using human fecal microorganisms showed that the bioactive oligosaccharides significantly increased the population of Lactobacillus in the gut and hence can be considered as a potential prebiotic (63).
In vivo studies suggested that glucan oligosaccharide enhanced immunoregulatory activity in cyclophosphamide (CTX)-induced immunosuppressed mice by stimulating the release of nitric oxide (NO), cytokines, and increased immunoglobulins release and splenic lymphocyte proliferation (64).
Curdlan itself and GOS activate macrophages via complement receptor 3 (CR3) and toll-like receptor 2 (TLR2) by mitogenactivated protein kinase (MAPK) pathway and nuclear factor NF-κB pathway. The activated macrophages were found to be non-toxic or compatible with bone marrow-derived macrophages isolated from healthy male C57BL/6J mice. The GOS promoted M1 phenotype polarization of bone marrowderived macrophages by upregulation of CD11C and CD86 expressions (65). In view of these findings, GOS may be considered safe to enhance immune responses when added to the development of functional foods (66).
Curdlan sulfate (CRDS) is a sulfated glycoconjugate that is biochemically similar to rosette inhibiting drugs like heparin and has been tried for treating severe malaria in 18 African children. CRDS inhibited erythrocyte rosette formation in vitro at 10-100µg/ml in Plasmodium falciparum laboratory strains and revealed anti-malarial activity in clinical isolates (67). Palm sulfopropyl (SP) and palm carboxymethyl/sulfopropyl (CM/SP) curdlan derivatives were able to potentiate the action of actinomycin D or doxorubicin in B16 tumor cells by inhibiting HEp-2 tumor cell growth and by upregulating doxorubicin and actinomycin D cytostatic activities (68). Further clinical trials are needed to establish the long-term safety and efficacy of CRDS in humans.
Barbosa-Lorenzi et al. found that curdlan acts as an agonist for the fungi receptor dectin-1 and induces selective mast cell degranulation without provoking synthesis of leukotriene and cytokine. Hence, curdlan may be employed against fungal infections in human skin (69). Curdlan pellicle produced by Gluconacetobacter xylinus is one of the best bio-based materials because of its super network structure and unique physiochemical properties and could be used for a wide range of clinical and tissue engineering applications (70).

Role of Curdlan as an Adjuvant
In order to enhance the immune response, adjuvants are added to vaccines to produce more antibodies and long-lasting immunity, thus minimizing the required antigen dose. In several studies, β-D-glucan has proven useful as an adjuvant carrier as it is a natural non-digestible polysaccharide that can be easily detected when bound to receptors, such as dectin-1. There is emerging interest in exploring the ability of curdlan to gain endogenous information in the desired organs or cells (71). Protective immunity to Staphylococcus aureus was observed by vaccination of mice with fungal β-glucan particles (GP) loaded with S. aureus antigens. The findings suggest that the glucan particles vaccine system has the potential as a novel approach for developing vaccines against S. aureus (72). Chemo-and radio-therapies cause several side effects in patients with cancer. Neutropenia is one of the most significant side effects of chemotherapy, which disrupts the process of blood formation in the bone marrow and can cause lymphocytopenia, thrombocytopenia, and granulocytopenia and consequently leading to immune system suppression (73). Curdlan represents a novel and promising therapeutic approach in treating cervical cancer and can be used as an adjuvant to reduce side effects of cancer chemotherapy treatment and help in recovering from compromised hematopoiesis in the therapy injured bone marrow (74).
In clinical trials, β-(1,3)-glucan has been used as adjuvant therapy with significant beneficial effects on the survival and quality of life of patients with cancer through the activation of the immune system (75). In comparison with the aluminum adjuvant, curdlan increased the immunogenicity of the hepatitis B vaccine and promoted an immune response in mice by increasing the specific antibody response to hepatitis B surface antigen (HBsAg) (76). Sulfated yeast β-glucan showed an excellent adjuvant effect on the influenza vaccine (H5N1) in a mouse model. The anti-influenza vaccine increased the influx of macrophages and dendritic cells, increased antigen-specific T cell levels, and induced splenocyte proliferation in mice (77). In another study done on mice, curdlan alone substantially boosted the immune response against influenza through stimulation of lymphocyte proliferation and enhanced cytokine production (78). Well-designed clinical studies are warranted to verify the results obtained in animal models.

Curdlan as an Antioxidant and Anti-inflammation Agent
It has been hypothesized that β-(1,3)-glucan products are capable of increasing the lifespan, slowing the emergence of age-related biomarkers and exerting the antioxidant impact of Nothobranchius guentheri and hence may be potentially useful for promoting health and well-being and extension of lifespan in humans (79). Recent studies have shown that beta-glucans can decrease hyperglycemia, hyperlipidemia, and high blood pressure. By minimizing oxidative stress-induced mitochondrial damage, curdlan/β-(1,3)-glucan may contribute to new approaches to manage diabetes mellitus associated complications, neurodegenerative disorders, and cardiovascular diseases (80). It has also been suggested that curdlan obtained from S. cerevisiae protects the human plasma components from strong biological oxidants and inflammatory mediators due to its free radical scavenging capability (81). A study conducted on isolated rat hepatocytes showed that curdlan obtained from the fungal source can capture free radicals and may be a possible candidate for prophylaxis and treatment against depleted uranium toxic effects (82). Furthermore, the systemic administration and local application of beta-glucan were effective against burn-induced oxidative tissue damage in rats (83).

Protection Against Hyperglycemia in Patients With Diabetes Infected With COVID-19
Curdlan obtained from yeast Aureobasidium pullulans, also known as biological response modifier glucan (BRMG), has the ability to bring the blood sugar levels under control and has the potential in enhancing and regulating immune parameters. Fasting plasma glucose (FPG) level is one of the indicators of COVID-19 infection, which causes high mortality among patients with diabetes and is responsible for causing serious pathological complications in vital organs. Consequently, BRMG may help to control FPG levels and may be used as a preventive measure against the COVID-19 virus (84). COVID-19 has also been linked to an increased risk of mortality in patients with comorbidities including diabetes. Diabetics have a higher risk of disease severity in COVID-19 because they have a higher incidence of inflammatory processes due to persistent glucose recognition by C-type lectin receptors (84). Reduced PI3K/Akt activity is a crucial factor in the development of diabetes. Dectin-1 (belongs to the C-type lectin receptors family), complement receptor-3, lactosylceramide, and scavenger and toll-like receptors are among the receptors that BRMGs activate. The immune system of patients with diabetes is typically impaired. The immune system can be boosted by BRMG (85). Diabetes is linked to glycation-induced chronic inflammation of beta cells (β-cells) in the pancreas and elevated hemoglobin A1C (HbA1C) and glycated hemoglobin levels with clinical relevance as a marker of diabetes diagnosis and glycemic regulation. The immune-boosting activity of BRMG is linked to its potential anti-glycation effect (normalization of HbA1c) in patients with diabetes (86). In addition, the development of a hypercoagulable prothrombotic state is favored by an imbalance between coagulation and fibrinolysis, with increased levels of clotting factors and relative inhibition of the fibrinolytic system, endothelial dysfunction, and enhanced platelet aggregation and activation, all of which are complications of diabetes, explains the high risk of patients with diabetes to COVID-19 in terms of intensity/severity (85). Yeast BRMGs have been found to regulate the coagulation activation by controlling the cytokine activity and reversing diabetes-induced oxidative stress, which causes clotting parameters to be disrupted due to increased platelet aggregation and thrombin levels (87). Under the prevailing circumstances, it is of utmost importance to reduce COVID-19-induced mortality and serious complications such as coagulopathy, especially among the vulnerable populations (85). In order to fight against COVID-19-like pathogens, BRMG may be used to boost the immune system in humans. BRMG may also prevent blood clotting from occurring due to radiation exposure or diabetesinduced oxidative stress that enhances platelet aggregation and increases thrombin levels. It was reported that BRMG increases the level of IL8, while decreases the levels of several interleukins (IL-1β, IL-2, IL-4, IL-6, and IL-12), tumor necrosis factor (TNF)-alpha, interferon-gamma (IFN-γ), and active soluble fasligand (sFasL) levels, thereby maintaining the effective optimal defense against hyper-inflammation-free viral infections (87). Hyperinflammation results by preventing the onset of apoptosis and by increasing the development of sFAS and by suppressing the cytokine storms through a cascade of molecular events involved in activating Treg cells and reducing IL6, and at the same time preventing the chemoattraction of monocytes and macrophages, T cells, NK cells, and dendritic cells by reducing CXCL10 and CCL2 cells (88). Overall, BRMG enhances the immune function in the upper respiratory tract to protect against microbial pathogens (89). The immune boosting activity is one of the most important attributes of β-glucans. Keeping in view these observations, further clinical trials are required to validate the antiviral and immune system prophylaxis of BRMG.
The virucidal properties of BRMG should be explored further for other deadly viruses, including the lethal viruses like SARS, MERS, and the virulent strains of SARS-CoV-2/COVID-19. Although SARS-CoV-2 is primarily a respiratory virus, the viral host receptor angiotensin-converting enzyme 2 (ACE2) is found in the cytoplasm of gastrointestinal epithelial cells, and the viral nucleocapsid protein is found in the cytoplasm of rectal, duodenal, and gastric epithelial cells, suggesting that the intestine may play a role in COVID-19 pathogenesis and may be a possible route of infection. β-glucans with immune effects in the intestine could thus be a beneficial supplementation method for the COVID-19 treatment (90). A β-glucan extract from the edible shiitake mushroom Lentinus edodes was recently confirmed to have a differential in vitro immunomodulatory and pulmonary cytoprotective effects, suggesting that it may be used to treat COVID-19. According to the findings, β-glucans delivered as a customized cocktail may be a good match for a potential nutraceutical-based COVID-19 intervention (91). In a randomized, placebo-controlled, and double-blind trial, Mah et al. found that the severity of upper respiratory tract infection (URTI) was lower among marathon runners (n = 76) who consumed dairy beverages containing 250 mg soluble baker's yeast beta-glucan (Wellmune R Austin, TX, USA) as opposed to athletes given placebo (n = 133). The severity was evaluated by the Jackson index, and results showed that the yeast beta-glucan significantly reduced the severity of nasal discharge and URTI in marathon runners (92). Therefore, the above evidence suggests the application of curdlan in the treatment of hyperglycemia in patients infected with COVID-19.
Wound Healing Properties of Curdlan β-Glucans increase wound repair by increasing the infiltration of macrophages, which promote tissue granulation, collagen deposition, and skin re-epithelialization. Curdlan-based wound dressings have a great stability and resistance to wound protease, prevent bacterial infections, and are effective in wound healing. According to a clinical study conducted by Majtan and Jesenak, the application of 3% cream containing curdlan isolated from S. cerevisiae caused 55.23% reduction in ulcers after 90 days of treatment (93). In humans, fungal β-D-glucans reduce blood cholesterol and improve absorption of glucose in cells and thereby help in wound healing (94).

Immunomodulation Functions of Curdlan
Besides the above-mentioned usages, curdlan appears to have some other immunomodulation applications. Recently, Song et al. reported improvement in the in vivo humoral and cellmediated immune responses in aged mice after the antigen challenge thereby suggesting the antiaging efficacy of curdlan (95). Curdlan derivatives also have anti-allergic activities, which have been verified in animal models and clinical trials in humans  (96). Burg et al. observed that the daily oral administration of fungal β-glucan (400 µg/day) significantly reduced the influx of eosinophils into the lungs of ovalbumin challenged mice compared with control counterparts, thereby suggesting the protective action of curdlan against the development of asthma (97). The overactive type II helper T-lymphocytes response is thought to occur in individuals who experience allergic asthma attacks. It has been observed that β-(1,3)-glucan attenuates Th2 cytokines response by modulating the activity of macrophages, which produce pro-inflammatory mediators, such as prostaglandin E2, tumor growth factor, and IL-10 (98). Some investigators suggest that curdlan may be helpful in the prevention of chronic fatigue syndrome because it increases the proliferation of splenic lymphocytes and enhances the production of cytokines together with the stimulation of humoral and cellular immunity (99).

RECENT PATENTS OF CURDLAN
Besides other functional and biological properties, curdlan has been identified as an antiviral and antibacterial substance. As an immunomodulatory agent, curdlan can bind as a phagocytic receptor ligand and can act as dectin-1 agonist (69). In addition, curdlan can act as C-type lectin receptor and hence can achieve targeted immunotherapy (100). Apart from this, curdlan has unique gelling properties and thus can be applied to increase elasticity and the overall gel strength (13). Curdlan can also be used to form edible preservative films (14,15). Recent patents of curdlan are mostly concentrated toward its immunomodulatory functions and improvement of physical properties of foods and drugs in the packaging processes. A comprehensive list of recent patents on curdlan, its applications, inventors, and patent holders are summarized in Table 4.

CONCLUSIONS AND FUTURE DIRECTIONS
A wide variety of exopolysaccharides or curdlans are produced by using bacteria, fungi, and yeast. The unique physicochemical properties of soluble and insoluble curdlans have been found highly valuable materials in different industries, such as dairy, food manufacturing, cosmetics, and pharmaceuticals. As shown in Table 4, curdlan hydrocolloids play very important roles in making novel functional foods, pharmaceuticals, and dairy products like prebiotics. Some curdlans are dectin-1 receptor agonists and hence can be employed as adjuvant carriers for vaccines, and their immunomodulatory activity can be beneficial in cancer chemotherapy and prevention of autoimmune disorders. A number of curdlans have shown antiaging, antidiabetic, antioxidant, anti-inflammation, wound healing, and cardio-and neuro-protection capabilities. Some curdlans have revealed the potential for treating malaria, pathogenic bacteria, and viral diseases (HIV and SARS-CoV-2/COVID-19). However, further well-designed studies are needed to verify the preliminary results obtained in a limited number of clinical observations. It is also important to evaluate the immunomodulatory activities of different curdlans in suitable animal models and randomized and well-controlled clinical studies in humans. More research is also warranted to understand the mechanism of action of curdlans at the cellular and molecular levels to develop novel products. In addition, there is a need to study the formulation characteristic of curdlans under different environmental conditions for creating stable formulations with long-lasting shelf lives.
The current patents on curdlan are mostly concentrated toward biopolymers, curdlan gels, polysaccharide-based membrane film, immunomodulatory functions, and improvement of physical properties of foods and drugs in the packaging processes. Microencapsulation and nanotechnology studies are needed for the development and delivery of antiviral, anticancer, and parenteral therapies of curdlan. Good manufacturing procedures and quality control methods must be used in making curdlan preparations.

AUTHOR CONTRIBUTIONS
GK, HB, and HT visualized the presented idea, contributed to manuscript writing, and supervised the project. VC, AV, and SB-P contributed in doing literature searches and prepared manuscript draft. GK and HB revised and approved the manuscript. All authors contributed to the article and approved the submitted version.