Human saliva is a multifunctional secretion. This complex fluid plays important roles that influence oral health through its physical and chemical properties. Alterations in its quantity or quality directly affect the oral cavity. Quantitative changes in saliva have been described as a potential factor in the increased incidence of Candida infection in the oral cavity [29]. A decreased salivary flow rate reduces its flushing action to control of the amount of oral microorganisms. Impaired clearance of oral microbes results in the adhesion and colonization of oral microorganisms, including Candida, to the oral mucosa causing an increased risk of oral Candida infection. Consequently, increased colonization by oral Candida occurs with a high frequency in patients with xerostomia [30, 31]. Although our results demonstrated no significant difference in the salivary flow rate between the healthy subjects and OLP patients, a negative correlation between the salivary flow rate and the amount of salivary Candida in the healthy volunteers and OLP patients before topical 0.1% FAO treatment was observed in the present study.
Defending against the ingress of microorganisms into the body is one of the most important functions of human saliva. This secretion comprises a number of local immune proteins, such as lactoferrin, an antimicrobial component of saliva. The mechanism and biological role of lactoferrin have undergone numerous investigations in the past decade [11,12,13,14]. It has been demonstrated that the quantity of salivary lactoferrin protein is correlated with decreased saliva [28]. Our results indicated that neither salivary flow rate nor salivary lactoferrin levels were significantly decreased in the OLP patients in our study.
Topical steroids, such as 0.1% FAO, are widely recommended as the initial drug of choice for treating OLP. However, the most common adverse effect when treating OLP with a topical steroid is the increased adhesion and growth of Candida in the oral cavity [6]. Currently, the biological role of salivary lactoferrin in inhibiting Candida growth is unresolved. Decreased salivary lactoferrin may cause oral Candida overgrowth [15]. The relationship between decreased lactoferrin secretion and using a topical steroid was also demonstrated in an in vitro study using nasal and bronchial mucosa explants [32]. However, we found no correlation between using topical 0.1% FAO and a quantitative change in salivary lactoferrin in this study.
As previously mentioned, lactoferrin participates in the host defense system. Because lactoferrin is an iron-binding protein, its function requires the presence of iron in the saliva. A few studies have investigated the amount of iron in OLP patients’ saliva; however, the results have been inconsistent [33, 34]. Although a correlation between topical steroid use and iron level in oral fluid has not been determined, several studies have also demonstrated that lactoferrin can directly interact with the fungal cell wall in an iron-independent manner and this may play an important role in its antifungal activity against Candida [19,20,21,22,23]. Our study found no significant decrease in salivary lactoferrin levels nor increase in Candida colonization in the OLP patients who were undergoing topical 0.1% FAO therapy compared with healthy individuals. However, in cases of OLP treated with topical steroids that had an increased incidence of oral candidiasis, it is also possible that the infection may be due to the impairment of other factors.
Candida microbes living as normal flora in the oral cavity can be detected in approximately one-half of the world’s population and Candida albicans is the most frequently identified species in the oral cavity [6, 16]. Although none of the subjects had signs or symptoms of oral candidiasis in the present study, we observed the presence of oral Candida in over half of the subjects, and C. albicans was the most common species found in the oral cavity in both healthy participants and OLP patients, which is consistent with a previous study [6]. Interestingly, our study demonstrated a marked difference in the Candida species isolated from the saliva of healthy subjects and OLP patients. This finding should be evaluated in a future study.
The major strength of this study is the use of a prospective design. This allowed us to compare the levels of Candida colonization and salivary lactoferrin in OLP patients before and after a finite period of topical 0.1% FAO treatment in the same subjects. Moreover, we included only OLP patients who had never received any topical steroid therapy before the start of this study to minimize any effect from previous treatment. In addition, the OLP patients and healthy controls had no history of systemic conditions that are known to affect oral Candida colonization and lactoferrin levels. Although these strict inclusion and exclusion criteria limited the pool of the eligible sample population, these were an important strength of the present study.
Interestingly, although our results revealed a decreased lactoferrin concentration and lactoferrin flow rate in the OLP patients undergoing topical 0.1% FAO treatment compared with the other groups, there were no significant differences in salivary flow rate, lactoferrin concentration or salivary lactoferrin flow rate between the groups. However, these decreases did not result in a significant difference in the Candida count between these groups. Therefore, the limitations of the present study should be considered. Although the sample size in this study had sufficient statistical power according to the sample size calculation, the small sample size in each group may be a key limitation. Furthermore, the limited duration of the present study is also a concern. Thus, we suggest that these findings should be further investigated in future studies with a larger sample size and longer duration. The progress in further research will be helpful in our understanding the mechanisms and the biological role of salivary lactoferrin in inhibiting the occurrence of oral candidiasis in OLP patients undergoing topical steroid therapy. In the future, an advanced understanding of the pathogenesis of oral candidiasis may aid in developing new prevention methods that decrease the risk of oral candidiasis in patients undergoing topical steroid treatment.