2.1. Candida Infection Affecting Salivation

In the early stages of oral candidiasis, Candida attaches to the oral mucosa and begins to multiply [64,70]. Several proteins, such as secretory IgA, lactoferrin, histatins, and defensins, downregulate adhesion and multiplication of Candida [10,71,72,73]. Among the proteins, histatins and defensins are particularly effective as antifungal factors, which are produced in epithelial cells and salivary glands [74]. Histatins 1, 3, and 5 are present within saliva, accounting for about 80% of total salivary histatins [75]. Human β-defensin-1 was isolated from both the major and minor salivary glands, especially from ductal cells and not acinar cells [76]. Therefore, when salivation or the antifungal agent levels of saliva are reduced, oral microbial hyperproliferation is permitted and oral candidiasis can more easily manifest [73].

Salivary flow showed a significantly negative correlation between stimulated salivary flow rates and Candida colony-forming units (CFUs) in the patients with xerostomia [77]. This trend also appeared in patients with reduced salivation after radiation therapy [78]. Antifungal therapy for candidiasis patients can expect to relieve pain, redness, and oral mucosa atrophy. Notably, antifungal therapy often increases the amount of saliva by removing Candida. A clinical study investigated the effects of Candida elimination on stimulating whole salivary flow rate [79]. The patients with successful elimination of Candida showed significantly increased stimulated whole salivary flow rate, whereas patients with unsuccessful elimination of Candida did not show increased stimulated whole salivary flow rate. Sympathetic stimuli, like acute pain and stress from Candida infection, can reduce salivary flow rate. In other words, parasympathetic stimuli result in increased saliva flow rate; on the other hand, sympathetic stimuli result in more viscous saliva secretions [24,80]. On the basis of this evidence, researchers have suggested that the increased stimulated whole salivary flow rate after treatment was the result of reduced sympathetic stimulation by oral pain reduction [79]. The study states that a decrease in sympathetic stimulation could lead to changed watery salivary secretion. However, 13.5% of the patients with successful elimination of Candida did not show increased stimulated whole salivary flow rate [79]. The unrestored salivary flow rate may be a result of salivary gland destruction from the Candida infection, and the salivary glands could not restore function even after successful Candida treatment [20,62].

2.2. Candida Infection and Host Immune Response

Oral Candida infection on salivary glands causes host immune responses by activation of T lymphocytes. The T cells mediate inflammation by stimulating the production of inflammatory cytokines, such as TNF-α, IL-1ß, and IL-6. These T cells also stimulate the production of inflammatory chemokines and recruit neutrophils and macrophages. The rapid and localized induction of these cytokines form the first line of defense that limits the transmission of invading Candida. However, recurrent or chronic infections can provide an elevated inflammatory environment, leading to organ dysfunction [81]. TNF-α and IL-1ß play well-known roles in the pathogenesis of chronic inflammatory diseases. These cytokines may affect salivary gland damage [82,83,84]. The role of these cytokines in the etiology has been determined experimentally in Sjögren’s syndrome with dry mouth [85]. TNF-α suppresses the transcription of Aquaporin-5 and destroys human salivary gland acinar cells. Aquaporin-5 is critical for saliva production and a specific channel protein found in the acinar cells that allows for rapid transcellular migration of water in response to an hydrostatic/osmotic pressure gradient [86,87]. There is a cycle of destruction where Candida causes immune mediated salivary gland destruction, following reduced salivary flow and consequent Candida infection.

3.1. Tentative Diagnosis Using Clinical Features and Characteristics

The diagnosis of oral candidiasis can usually be made through a complete medical history and physical examination [88]. Most commonly, candidiasis demonstrates as acute pseudomembranous candidiasis, acute atrophic candidiasis, chronic atrophic candidiasis, chronic hyperplastic candidiasis, angular cheilitis, or median rhomboid glossitis (Figure 1) [89]. (a) Pseudomembranous candidiasis accounts for approximately 35% of oral candidiasis cases. In these cases, the pseudomembrane can be easily removed, revealing underlying mucosa, with minimal bleeding. The pseudomembranous white matter consists of debris, fibrin, and exfoliated epithelium invaded by Candida and its hyphae. Acute pseudomembranous candidiasis can be chronic, either intermittently or constantly affecting the patient. The condition may occur in infants, immune-compromised patients (leukemia and AIDS), or patients taking medication such as antibiotics, immunosuppressants, or topical corticosteroids [18,90,91]. (b) Acute atrophic candidiasis, also known as erythema candidiasis, is usually associated with a burning sensation in oral mucosa. It presents as a raw-looking red lesion and occurs prior to the formation of the pseudomembrane or appears after the removal of the pseudomembrane. Acute atrophic candidiasis usually occurs on the dorsal surface of tongue and is characterized by absent papilla due to the use of topical antibiotics or systemic long-term corticosteroids or antibiotics [18,90,91]. (c) Chronic atrophic candidiasis, referred to AS “denture stomatitis”, is usually associated with wearing dentures and inhibited salivary flow. It appears as erythematous inflammation and edema in denture occluded areas. These lesions are caused by dentures rubbing against the oral mucosa, creating a moist and warm environment that is ideal for the growth of Candida. Chronic atrophic candidiasis can be symptomatic, causing soreness and burning, or asymptomatic and only found on routine examination [92,93]. (d) Chronic hyperplastic candidiasis is a rare type of oral candidiasis and appears as a rough or nodular lesion, which complicates the diagnosis by differentiating from oral cancer. It typically appears as white patches on the commissures of the oral mucosa. The main cause of chronic hyperplastic candidiasis is C. albicans, but other systemic co-factors, such as vitamin deficiency and generalized immune suppression, may contribute. Clinically, the lesions are asymptomatic and regress after proper antifungal treatment and correction of underlying nutritional deficiencies or other co-factors. If the lesion is not treated, it can develop into dysplasia or carcinoma [94]. (e) Candida-associated angular cheilitis is inflammatory fissures that emanate from the commissure of the mouth. Angular cheilitis is frequently found in the clinic, including cases involving a combination of Candida and bacterial organisms. Signs and symptoms may include bleeding, blisters, cracks, crusts, itchiness, pain, redness, and swelling. Predisposing factors can be loss of vertical height in the denture wearer, habitual lip licking, mouth breathing, or nutritional deficiencies, particularly with vitamin B12 or iron [95]. (f) Median rhomboid glossitis is a term used to describe the area of a smooth, red, flat, or raised nodule in the middle of the dorsal surface of the tongue. The affected area of the tongue usually does not have a normal coating of filiform papilla covering the entire upper surface of the tongue. High levels of Candida can be discovered from these lesions, which are associated with the frequent use of steroid inhalers or cigarettes [18,90,91].

Figure 1. Clinical manifestations of oral candidiasis: (a) acute pseudomembranous candidiasis, (b) acute atrophic candidiasis, (c) chronic atrophic candidiasis, (d) chronic hyperplastic candidiasis, (e) angular cheilitis, and (f) median rhomboid glossitis. Clinical photographs were taken under patients’ informed-consent agreement, with approved Institutional Review Board, PNUDH-2017-026, from the Pusan National University Dental Hospital.

3.2. Definite Diagnosis Using Cytology and Culture

Diagnosis can be confirmed by smear, oral rinse sample, whole saliva sample, culture, or oral biopsy [88]. Specimens for cytology can be obtained by scraping the lesion with a tongue blade. PAS staining of specimens reveals the existence of Candida hyphae and budding yeast. Moreover, 10% potassium hydroxide (KOH), gram, and methylene blue staining can be used instead of PAS. The sensitivity of smear is 51.6%, which is less than that of sample (oral rinse or whole saliva sample) culture. Candida species at a low concentration of 200 to 500 cells per milliliter of saliva could be detected using cell culture method rather than cytology method. Of the asymptomatic healthy population carry Candida in the oral cavity. Therefore, it is necessary to identify a threshold amount of Candida species (>270 CFU/mL), to distinguish oral candidiasis from oral carriage [96]. A definitive diagnosis of candidiasis requires the confirmation of tissue invasion by Candida, using biopsy with PAS staining. Biopsies are always required in hyperplastic candidiasis in order to discard the existence of epithelial dysplasia [97].

4.1. Prevention

Clinicians should notice that patients with immunocompromised disease, such as AIDS and diabetes, or individuals who have the risk factors of usage of medication (antibiotics, steroids, or immunosuppressants), impaired salivary gland function, dentures, poor oral hygiene, or a high-carbohydrate diet can develop candidiasis easily. Therefore, periodical oral examinations, oral hygiene instruction, and periodic prophylaxis could prevent oral candidiasis. Oral hygiene includes cleaning the tongue with a tongue cleaner, cleaning teeth and dentures with a toothbrush [98], and rinsing oral mucosa with chlorhexidine. In addition, dentures should be removed at night and meticulously washed and soaked in a disinfectant solution, such as chlorhexidine and sodium hypochlorite (1%) [15,99]. To reduce the destruction of salivary glands due to repetitive candidiasis, periodical oral examinations with prophylaxis and proper oral hygiene instruction should be recommended and practiced.

4.2. Treatments of Candida Infection

The treatment of oral candidiasis is based on four basic principles [98,100]: Assess the Candida infection type, diagnose the infection early and accurately, correct the predisposing factor or underlying disease, and administer antifungal agents appropriately. In order to select the proper medications, studies consider factors including local or systematic approach, type of Candida, clinical findings [90], and medication efficacy and toxicity [101]. Commonly used antifungal medications are included in Table 1.

Table was adapted with permission of CEDRO, Centro Espanol de Derechos Reprograficos, from “Current treatment of oral candidiasis: A literature review”, 6, 5, 2014 [98]); permission conveyed through Copyright Clearance Center, Inc. (Danvers, MA, USA).

Based on the histopathological information via microscopic examination and fungal culture, clinicians should choose the most appropriate antifungal medication. Polyene was the first broad spectrum antifungal agent discovered in the 1940s and 1950s [102]. Polyenes, such as nystatin and amphotericin B, bind to and weaken ergosterols in fungal cell membranes that can initiate the leakage of K+ and Na+ ions, thus contributing to fungal cell death. Polyenes are considered fungicidal and have broad activity against most fungal organisms. Amphotericin B is an antifungal drug used for serious fungal infections and nystatin is used to treat Candida infections of the skin, vagina, mouth, and esophagus [102,103]. Although resistance to polyene medication is rare, some fungal species exhibit intrinsic resistance to polyenes [104,105]. Nystatin is only effective topically, and amphotericin B, which is effective orally and intravenously, is well-known for its severe and potentially lethal side effects such as high fever, kidney damage, and multiple-organ damage. The search for antifungal agents with an acceptable toxicity profile first led to the discovery of azole. Therefore the first azoles were discovered in 1944, but were not approved for use in humans until the early 1960s [102]. Azoles inhibit 14-α-sterol demethylase, a cytochrome P-450 enzyme involved in ergosterol synthesis [106], resulting in the accumulation of toxic sterol intermediaries and loss of membrane integrity. Most azoles are fungistatic and have a broad spectrum against filamentous fungi and yeasts [107,108]. The search for azole antifungal agents with an acceptable toxicity profile led to the discovery of the first ketoconazole. Later, the triazoles fluconazole and itraconazole were developed with an improved safety profile and comparatively broader range of antifungal activity. Analogs have been developed to overcome limitations, such as a suboptimal spectra of activity, need to develop resistance, unfavorable pharmacokinetics, drug–drug interactions, and toxicity [109]. Candida species resistance to the azole medications (e.g., itraconazole, clotrimazole, and fluconazole), including Candida glabrata, Candida tropicalis, or Candida parapsilosis are susceptible to polyene medication. Polyene medications are not well absorbed from the gastrointestinal tract but are effective for topical application [89]. Topical antifungal therapy is recommended as the primary treatment option for mild cases of Candida infection. If the lesion is refractory to topical treatment or recurs frequently, systemic antifungal therapy is suggested. However, systemic antifungal therapy must be considered as the primary treatment for patients with immunocompromised conditions due to the risk of candidemia [103].

The removal of Candida biofilm is necessary, in combination with appropriate medication. Successful treatment of candidiasis depends upon biofilm control, using daily oral hygiene and professional prophylaxis. The Candida biofilm is a thick extracellular polymeric substances layer with a dense network of yeasts, pseudohyphae, and hyphae [110]. The biofilm allows Candida to easily attach between cells and other surfaces, such as dentures. The biofilm provides barriers between Candida and the surrounding environment, thus protecting Candida from antifungal medications [90]. Therefore, the removal of Candida biofilm from the dentures, as well as from all sides of the oral cavity, contributes to lowering the failure rate of antifungal treatment; it is essential for the effective treatment of Candida infection.

4.3. Treatments of Salivary Gland Dysfunction