The Use of Essential Oils and Their Isolated Compounds for the Treatment of Oral Candidiasis: A Literature Review

In this literature review, we present the main scientific findings on the antifungal activity of essential oils (EOs) applicable for a new drug formulation to treat oral candidiasis. Seven literature databases were systematically searched for eligible in vitro and clinical trials. Selected articles were screened for biological activity, botanical species, phytochemical composition, study design, and methodological quality. A total of 26 articles were included in the review, of which 21 were in vitro studies and 5 clinical trials. The most promising EOs were obtained from Allium tubeorosum, Cinnamomum cassia, Cinnamomum zeylanicum, and Coriandrum sativum L. Among the phytochemicals, citral and thymol were the most active. Clinical trials indicated that the EOs from Pelargonium graveolens and Zataria multiflora are potentially effective to treat oral candidiasis. Further nonclinical and clinical studies with these EO are warranted to determine their potential use and safety for the treatment of oral candidiasis.


Introduction
Oral candidiasis is an infection caused by Candida spp. which manifests clinically as erythematous, ulcerated, sensitive white lesions, with soft consistency and easy removal, commonly affecting the palate, oral mucosa, tongue, or oropharynx [1]. Due to the opportunistic pathogenicity of yeasts, oral candidiasis is more prevalent in immunocompromised individuals [2].
Yeast colonization of complete dentures occurs mainly due to the strong adhesion of yeast cells to acrylic resin of base materials [1]. Direct adhesion of yeasts onto dental surfaces is a critical pathogenic factor for the onset of dental stomatitis. Yeast cells can co-aggregate with various bacterial species from the oral plaque and integrated into a robust biofilm pellicle on the surface of dentures. Oral biofilms can be considered microbial reservoirs and significantly affect the oral and systemic health of denture users [1,3].
Polyene and azole drugs, such as nystatin and miconazole, respectively, have been commonly prescribed for the treatment of oral candidiasis [2]. However, recent years have seen failures in antifungal therapy due to increasing microbial resistance rates and high drug toxicity, which have altogether contributed to rise the prevalence of morbidity and mortality indicators related to fungal infections [4]. is scenario has encouraged the search for novel substances capable of controlling and treating yeast infections while having low toxicity to the host. Some naturally occurring products are considered an important source of new molecules with biological properties, displaying antifungal efficacy comparable or stronger than that of drugs currently available for clinical use. Essential oils (EOs) are a class of natural products with pharmacological properties, which include antimicrobial, antiseptic, anti-inflammatory, and antioxidant activities [5][6][7][8][9]. ese compounds are described as a mixture of volatile constituents produced as secondary metabolites by aromatic plants. With the chemical characteristic of lipophilicity, EOs have the ability to interact with fungal cell membranes and lipid structures. Among their mechanisms of action, EO can disrupt the activity of enzymes involved in ergosterol synthesis or complex with membrane ergosterol, thereby creating pores in the membrane and disrupting permeability [10][11][12]. In addition, EO can affect cell wall biosynthesis, interfere with protein synthesis or cell division, and stimulate the production of reactive oxygen species, causing growth inhibition or cell death [13,14]. In this literature review, we present the main scientific findings on the antifungal activity of EO and their isolated phytochemicals on Candida spp. commonly responsible for oral infections. In vitro and clinical (controlled clinical trials in humans) studies were selected and are further discussed in this review.

Study Question.
is literature review was conducted to address the specific question: "Is there scientific evidence to support the use of EO and/or their isolated constituents for the treatment of oral candidiasis or to warrant further nonclinical and clinical research?"

Search Strategy and Study Selection.
e PRISMA guidelines (Transparent Reporting of Systematic Reviews and Meta-Analyses) [15] were followed. Seven databases were systematically searched for studies of experimental oral candidiasis and randomized controlled clinical trials published up to 1 March 2020 (Table 1).

Eligibility Criteria.
A systematic screening of the articles was performed by two independent examiners according to the following inclusion criteria: (1) Biological activity: clinical effects of an EO-containing formulation on denture stomatitis or oral candidiasis in in vitro or clinical trials.  Table 2.
Randomized controlled trials of herbal interventions were analyzed based on the CONSORT guidelines [17]. e Jadad scale [18] was used to check study validity and methodological quality (randomization, blinding, and loss of follow-up). Based on these requirements, clinical studies were assigned scores ranging from 0 to 5, in which a score <3 was indicative of poor quality.

Search Strategy.
Using a previously defined strategy, bibliographic searches were carried out using specific keyword combinations. A total of 395 articles were retrieved, of which 26 were considered eligible and included in the final review ( Figure 1). Twenty-one studies with in vitro design and five clinical trials were included and are further discussed.

In Vitro Antifungal Activity.
e antifungal activity of thirty-one EO and four phytochemicals against Candida spp. strains (clinical isolates and reference strains) was analyzed. As shown in Tables 3 and 4, the most promising EOs were obtained from Allium tuberosum, Cinnamomum cassia, Cinnamomum zeylanicum, and Coriandrum sativum. Citral and ymol were the most active isolated constituents, with MIC values lower than 100 μg/mL, indicating very strong antifungal activity (Table 5)[.

Effects of Intervention.
According to the pre-established criteria, five clinical studies were included in this review: Pelargonium graveolens, Zataria multiflora, and Melaleuca alternifolia in three formulations. e main methodological characteristics and outcomes of selected studies are shown in Table 6 and Figure 2. An experimental gel containing Pelargonium graveolens EO healed completely (34%) or partially (56%) patients with prosthetic stomatitis as compared to those who received only the gel with a placebo. In addition, the gel was effective in reducing the fungal load as well as in decreasing erythema in patients with prosthetic stomatitis as compared to those treated with the placebo. Another experimental gel containing Zataria multiflora EO was also effective in reducing the fungal load in participants' saliva and denture samples as well as n reducing local inflammation.

Discussion
e main antimicrobial mechanisms of EO and their constituents are associated with their ability to increase cell membrane permeability due to lipophilicity of their molecules, resulting in extravasation of ions and cellular contents and cell lysis [39][40][41]. In this review, the selected data suggest that some EO and phytochemicals are promising for the treatment of oral candidiasis and warrant further nonclinical, clinical, and toxicological investigation for pharmaceutical purposes [42][43][44].
Next, a brief summary of the most active EO and isolated compounds will be presented based on in vitro and clinical studies. Information on ethnopharmacological knowledge, biological properties, and chemical composition is further discussed.

Essential Oils and Phytochemicals with Promising Antifungal Activity against Candida spp.
e Allium genus, which belongs to the Amaryllidaceae family, contains approximately 700 species of plants, such as Allium cepa    (onion), Allium sativum (garlic), Allium schoenoprasum (chives), and Allium tuberosum (garlic chives). All are important due to their commercial character and nutritional value [45]. Allium tuberosum is a perennial plant that grows in many countries in Asia and whose aerial parts are edible green vegetables common to the Chinese. A. tuberosum has an odor similar to the smell of garlic and other Allium plants due to the presence of sulfur-containing compounds [46].

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Several pharmacological activities are attributed to this species, including antidiabetic and hepatoprotective [47], antiparasitic [48], antibacterial [49], and antifungal activities against fungi of the Aspergillus genus [50]. is species has been reported to have strong antifungal activity against Candida parapsilosis isolates and inhibitory effects on biofilm formation [19].
Cinnamomum cassia, popularly known as China cinnamon, is an herb belonging to the Lauraceae family, occurring in several countries such as India, China, Uganda, Vietnam, Bangladesh, and Pakistan. It is intensely aromatic, with a sweet taste and bitter touch. Its peels have been used in different ways, either as a flavoring in various Asian cuisines or in traditional medicine for the treatment of diabetes mellitus and peptic ulcer [51]. e major compound of C. cassia is cinnamaldehyde (75-90%). Other phytoconstituents, present in trace amounts, include eugenol, benzoic acid, cinnamic acid, salicylic acid, cinnamyl alcohol, and their corresponding esters and aldehydes [52].
C. cassia has been shown to have anti-inflammatory, antioxidant, anticancer, antipyretic, antiangiogenic, larvicidal, and antifungal properties [53]. C. cassia was reportedly active against four Candida spp. strains, namely, C. albicans and C. tropicalis, C. glabrata, and C. krusei, as well as against Aspergillus, Fusarium, and three dermatophyte isolates (Microsporum gypseum, Trichophyton rubrum, and T. mentagraphytes). C. Cassia EO was effective in reducing the number of pseudohyphae in C. albicans cultures, which is considered an important virulence factor [54]. Mouse models and in vitro assays have also proved the antiproliferative activity of C. cassia EO against oral candidiasis (C. albicans infection). Cinnamaldehyde was reported as the main compound responsible for the antifungal effects observed in C. cassia EO [55].
Cinnamomum zeylanicum, popularly known as cinnamon, is a very common spice that has been used by different cultures around the world for several centuries. It is obtained from the bark and leaves of trees of the genus Cinnamomum, a perennial tropical plant that has two main varieties, namely, Cinnamomum zeylanicum and Cinnamomum cassia. In addition to its culinary uses, in native Ayurvedic medicine, cinnamon is used as an alternative to treat respiratory, digestive, and gynecological diseases [56]. Four of the main components of the EO obtained from C. zeylanicum bark are trans-cinnamaldehyde, cinnamaldehyde, eugenol, and linalool, which represent 82.5% of the total EO composition [57]. In vitro and in vivo studies in animals and humans have shown important biological activities attributed to C. zeylanicum EO, such as anti-inflammatory, antimicrobial, reduction of cardiovascular diseases, and increase of cognitive function [58]. Some studies reported that C. zeylanicum EO has antifungal     Evidence-Based Complementary and Alternative Medicine 7    Evidence-Based Complementary and Alternative Medicine 9 activity against Candida spp. most likely by disrupting yeast cell wall [14,24,25], which suggests that this EO may be a promising candidate for the treatment of oral candidiasis. Coriandrum sativum L. is a small plant belonging to the Apiaceae family, popularly known as coriander. Coriander leaves and seeds are widely used in folk medicine as a cholesterol-lowering agent, digestive stimulant, and antihypertensive [11], in addition to its use as a spice in food preparation. e main components present in C. sativum EO are linalool (55.09%), α-pinene (7.49%), 2,6-octadien-1ol, 3,7-dimethyl-acetate, geraniol (4.83%), 3-cyclohexene-1methanol, α, α, 4-trimethyl-(4.72%), hexadecanoic acid (2.65%), acid tetradecanoic (2.49%), 2-α-pinene (2.39%), citronellyl acetate (1.77%), and undecanal (1.29%) [59]. Pharmaceutical formulations containing C. sativum also revealed antibacterial [60], antioxidant [61], hepatoprotective, and anticonvulsant properties. C. sativum EO also showed strong antifungal effects against Candida spp. strains [16].

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Citral (3,7-dimethyl-2-6-octadienal) is a racemic mixture composed of geranial (trans-citral, citral A) and neral (cis-citral, citral B) isomers, which are acyclic and monounsaturated aldehydes naturally occurring in many citric fruits, as well as in other herbs or spices [62]. Citral has become a raw material of great importance due to its characteristic lemon aroma and has been used as a flavoring ingredient in the food, perfumery and cosmetic industries [63]. Citral showed fungicidal activity against Candida spp. strains isolated from denture wearers after 2 hours of exposure and caused major morphological changes [34]. Leite et al. [64] demonstrated a strong antifungal activity of citral against C. albicans strains via mechanisms other than cell wall biosynthesis or ergosterol complexation. us, citral can be considered a promising candidate for the development of novel antifungal leads.
ymol is a monoterpene found in essential oils extracted from plants belonging to the Lamiaceae family such as the genera ymus, Ocimum, Origanum, Satureja, ymbra, and Monarda [65][66][67]. is molecule is a phytoconstituent with several biological activities described, including anti-inflammatory and antinociceptive [68], local anesthetic [69], and antifungal and antibacterial [70] activities. ymol has been reported to have strong antifungal activity against strains of the Candida genus, acting on the fungal cell membrane and producing a synergistic effect when used with nystatin to inhibit the growth of these strains [36].

Clinical Studies of Essential Oils for the Treatment of Oral
Candidiasis. While numerous studies are carried out to determine the antifungal activity of EO in vitro, only a few formulations reach the clinical stage and even less become a commercial product. As seen in this review, few clinical trials have been carried out to test experimental formulations containing EO and/or isolated constituents against oral candidiasis. Currently, the most common formulations for the treatment of oral candidiasis are for external use, such as oral solutions, gels, and creams, which are normally safe [71].
Sabzghabaee et al. [37] evaluated the clinical efficacy of a gel containing Pelargonium graveolens EO for the treatment of prosthetic stomatitis. is study presented a low risk of bias for aspects related to randomization and blinding and showed high methodological quality according to Jadad's scale [18]. Another clinical study, conducted by Amanlou et al. [38], showed that Zataria multiflora EO is also effective to treat prosthetic stomatitis. Denture wearers applied the gel containing 0.1% of Z. multiflora EO four times a day for two weeks. e presence of erythema on the palate surface of participants was significantly reduced as well as CFU counts of yeast strains. Although limitations related to randomization were observed in the study by Amanlou et al. [38], it showed a low risk of bias, which suggests that Z. multiflora EO may be a favorable therapeutic alternative for the treatment of prosthetic stomatitis.
Despite the favorable outcomes of EO on oral candidiasis and prosthetic stomatitis reported by the authors of the studies selected in this review, only the studies with P. graveolens (popular names: fragrant-leaf geranium-Port., rose geranium-Engl., and geranium-Span.) and Z. multiflora (popular name: thyme of shiraz-Engl.) met high  Evidence-Based Complementary and Alternative Medicine 11 Note: statistically significant reduction (+) or not (−) in the CFU count and in the signs and symptoms of oral candidiasis in relation to the positive control or placebo. † Good result. methodological quality standards. Further research should consider the chemical standardization of these EO and the adoption of appropriate methodological strategies for further clinical testing. is literature review shows that the most promising EOs were obtained from Allium tubeorosum, Cinnamomum cassia, Cinnamomum zeylanicum, and Coriandrum sativum L. Among the phytochemicals, the citral and the thymol were the most active. e clinical trials selected in this review provided evidence that the EO from Pelargonium graveolens and Zataria multiflora are potentially effective to treat oral candidiasis. Further nonclinical and clinical studies with these EO are warranted to determine their potential use and safety for the treatment of oral candidiasis.  Evidence-Based Complementary and Alternative Medicine 13