Background
Oral candidiasis is sub-classified into three major variants: pseudomembranous, erythematous, and hypertrophic [
1,
2]. In addition to these variants, angular cheilitis, median rhomboid glossitis, and denture stomatitis have been recognized as
Candida-associated lesions; moreover,
Candida super-infection in oral lichen planus is frequently encountered. The various forms of oral candidiasis are caused by interactions between the host defenses and fungal virulence factors.
Atrophic glossitis is caused by complete or partial lingual papillary atrophy. It exhibits a smooth, glossy appearance with a red or pink background and is primarily a manifestation of an underlying condition. Nutritional deficiencies (iron, folic acid, vitamin B
12, riboflavin, and niacin) are the most common causes [
3]. Other etiological factors of atrophic glossitis include hyposalivation and candidiasis [
2,
4‐
6]. An increase in
Candida colony counts, a low salivary flow rate, and advanced age were identified as being risk factors that were closely associated with the development of atrophic glossitis [
7,
8]. However, the
Candida species that are the most relevant to the development of atrophic glossitis remain unclear.
Candida albicans is the primary causative organism of oral candidiasis. Recent studies have suggested that non-
albicans Candida (NAC) species, such as
C. glabrata,
C. parapsilosis,
C. tropicalis, and
C. krusei are also pathogenic in humans [
2,
8‐
13]. One of these studies showed that
C. albicans was frequently isolated from the tongue, while
C. glabrata was most frequently isolated from the angle of the mouth, suggesting that certain
Candida species are associated with oral candidiasis at particular sites [
8]. In a study on the relationship between
Candida species and local and systemic predisposing factors, only
C. albicans infections were significantly associated with the use of inhaled steroids and antibiotics and super-infection with oral lichen planus, whereas the presence of removable dentures was significantly associated with the detection of NAC or a mixed flora consisting of
C. albicans and NAC [
9]. Similarly, the incidence of colonization with more than one
Candida species was found to be higher in cases of denture stomatitis than in cases involving other forms of oral candidiasis [
10].
C. albicans and
C. glabrata were the most prevalent species detected in cases of mixed infections [
10,
14]. However, no association has been confirmed between a specific
Candida species and any particular sign or symptom of oral candidiasis.
The purpose of this study was to clarify the species of Candida that are important for the development of atrophic glossitis in xerostomia patients.
Discussion
The purpose of this study was to clarify the species of Candida that are most strongly associated with the development of Candida-associated atrophic glossitis because recent studies have suggested the importance of NAC pathogenesis in oral candidiasis. Our logistic regression analysis showed an association between increased amounts of C. albicans colonies and atrophic glossitis. However, no significant relationship between NAC and atrophic glossitis was observed.
With regard to the methods that are used to obtain samples from the oral cavity, although an oral rinse method is helpful for quantification, this method does not evaluate localized sites of infection [
19]. In the present study, swab testing was used to evaluate local colonization on the dorsum of the tongue. Swab testing is simple to perform and is thought to be useful for quantitative estimation since the results obtained from rinsing and swab methods have been found to be correlated [
20,
21]. A chromogenic media-based commercial system, CHROMagar™ Candida, was used for quantitative estimation; this selective and differential medium is used for the isolation and identification of
Candida species [
19]. The particular advantage of this system is its ability to detect mixed yeast infections in clinical samples [
22]. Studies have suggested that
C. albicans,
C. tropicalis,
C. glabrata, and
C. krusei can be identified accurately using CHROMagar™ Candida [
23‐
26]. Although it has been reported that the dark green appearance of
C. dubliniensis can also be reliably distinguished from that of
C. albicans [
27], a more detailed method, such as molecular fingerprinting, is needed to discriminate between these two species since
C. albicans and
C. dubliniensis share many phenotypic characteristics [
28]. In the present study, no molecular methods were applied; thus,
C. dubliniensis colonies could not be distinguished from
C. albicans on CHROMagar™ Candida plates. However,
C. dubliniensis is an opportunistic oral pathogen that is typically isolated from patients infected with human immunodeficiency virus (HIV) [
29]. Since the present study did not include any such patients, there is little concern about the potential for confusion.
C. albicans, but not NAC, proved to be associated with atrophic glossitis in the present study. This result was consistent with the observations reported by Terai et al. [
5,
30], who found that
C. albicans is predominantly detected as a single infection. The detection of NAC or a mixed flora consisting of
C. albicans and NAC is therefore limited in patients with atrophic glossitis. Although
C. glabrata was detected in 50 of the 135 (37.0%) patients with
Candida, the second-most common isolated species after
C. albicans (Table
2), no significant association was observed between
C. glabrata and atrophic glossitis (Table
6). Since the clinical manifestation of erythematous candidiasis is related to the proteinase production capacity of
C. albicans [
31], a low level of protease secretion might explain why
C. glabrata did not contribute to atrophic glossitis.
The virulence factors of
Candida include adherence, evasion of the host defenses, and the invasion and destruction of host tissue [
1]. Hyphae penetrate perpendicularly, traversing the surface epithelium up to the spinous cell layer [
32]. The tip of the penetrating hyphae is known to be rich in proteinases, lipases, and several other enzymes.
C. glabrata is unable to grow in filamentous forms and cannot invade the epithelial layer [
33]. In contrast to other
Candida species, the amounts of secreted aspartyl proteinases (SAPs) and phospholipases (PLs) produced by
C. glabrata are extremely low [
11,
34]. In
C. albicans and
C. glabrata mixed infections,
C. albicans promotes the penetration of
C. glabrata into the oral epithelium as
C. glabrata cannot do so alone [
33]. Once
C. albicans penetrates the epithelial layer, the final step in the infection process is damage, which is characterized by the loss of the superficial epithelium [
35].
C. albicans induces both apoptosis and necrosis in the oral epithelial cells [
36]. In a study using reconstituted human oral epithelia, the hyphal elements of
C. albicans invaded and led to marked disorganization of the epithelium [
33]. In contrast,
C. glabrata induced less epithelial damage [
33]. Different levels of epithelial loss, namely, the severity of atrophic glossitis, may be related to the levels of disorganization of the epithelium. The removal of the keratin layer by oral dynamics, i.e., the movement of the tongue and other muscles, might be enhanced in certain situations, especially in patients with hyposalivation, due to the reduced lubricant function of the saliva. Erythematous candidiasis is the most prevalent variant in patients with hyposalivation [
37]. Furthermore, in the present study, a logistic regression analysis revealed that a low SSFR was associated with atrophic glossitis (Table
6).
High vascularity is a characteristic finding of erythematous candidiasis in addition to epithelial atrophy or a lack of a keratinized epithelial surface layer [
32]. One hypothesis regarding the mechanism underlying these findings is that erythematous candidiasis is accompanied by the activation of a partially reactive defense mechanism and that it may represent a clinical expression in response to candidal antigens [
38,
39]. Because patients with systemic immunosuppressive diseases were excluded from the present study, the oral mucosa in the patients was presumed to be immunocompetent. Atrophic glossitis may be induced even in patients with normal immunity; as such, factors other than immunosuppression, such as hyposalivation, are necessary for the establishment of atrophic glossitis. With its thin epithelial layers, the oral mucous membrane might be sensitized to react to
Candida and other virulence factors, thereby causing inflammation.
A chi-square test revealed that denture wearing was associated with the presence of atrophic glossitis (Table
4). The influence of salivation and denture wearing on
Candida colony counts was evaluated using the Kruskal-Wallis H test. The number of
C. albicans colonies (Fig.
2) was found to be significantly higher in the hyposalivation group than in the normosalivation group. This result is in accordance with the findings of a previous study, which demonstrated an inverse association between the salivary flow rate and
C. albicans counts in saliva [
40‐
42]. However, no significant difference was observed in the amounts of
C. albicans colonies between the denture wearers and denture non-wearers.
C. glabrata exhibits superior cell surface hydrophobicity and a greater tendency to adhere to the surface of denture acrylic resin than other
Candida species [
43]. In addition, NAC shows a greater ability to form biofilms on the surface of dental acrylic resin in comparison to
C. albicans, while the combination of
C. albicans and
C. glabrata shows the highest biofilm-forming ability [
44]. The morphological differences and growth patterns exhibited by
C. albicans and
C. glabrata may allow these species to occupy the same location within the oral cavity with limited competition for space, thereby allowing their co-existence [
33]. Thus, the presence of
Candida on the denture surface is a prominent etiological factor for denture stomatitis [
45]. However, there seems to be no correlation between the colonization of
Candida on the mucosal surface of the denture base and that on the dorsum of the tongue. Although denture wearers with hyposalivation are considered at substantial risk for oral candidiasis, increased
C. albicans colony counts on the dorsum of the tongue is essential for
Candida-related atrophic glossitis.