The colonization of C. albicans depends on several factors: the acquisition or entry of cells into the oral cavity, the attachment and growth of these cells, the penetration of tissues, and the removal of cells from the oral cavity. In this study, the NOD/SCID.e2f1
−/− mice with decrease of saliva showed negative colonization of C. albicans as compared with NOD/SCID wild type mice. On the tooth surface coated with a salivary pellicle microbial adherence interactions may involve adsorbed saliva molecules. The saliva pellicle increases the adherence of C. albicans cells to HA beads . Other investigators previously have shown the presence of serum and salivary pellicles can help C. albicans colonization on acylic strips and denture lining materials [32, 33]. Therefore, we speculate the NOD/SCID mice produce saliva containing receptors to bind to C. albicans and create a saliva-exposing environmental mucosal surface in the oral cavity.
In contrast, innate defenses include the epithelial barrier and anti-candidal compounds in saliva such as lysozyme , histatins , lactoferrin , and calprotectin [37, 38]. Innate host defenses are critical in the maintenance of oral health. Saliva includes lysozome, lactoferrin, and histatins that are thought to be the three major non-immunological antimicrobial proteins to modulate Candida populations in the oral cavity . However, the decreased saliva negatively modulated C. albicans populations in this study. In our previous study, the saliva from NOD/SCID.e2f1
+/+ and e2f1
−/− mice was characterized in protein concentration/ml, amylase activities, and protein concentration per min/ml . There were no significant differences in protein concentration and amylase activity between the mice. The protein concentration per min in 1 ml saliva was significantly lower in NOD/SCID.e2f1
−/− mice as compared to NOD/SCID.e2f1
+/+ mice. Here we did not measure the activities of lysozyme, lactoferrin, or histatins in saliva from NOD/SCID.e2f1
+/+ mice and NOD/SCID.e2f1
−/− mice, but the activities of the innate immunity and the receptors to C. albicans attachment may not be sufficiently provided in oral cavity from NOD/SCID.e2f1
−/− as compared with NOD/SCID.e2f1
+/+ mice. Mouse salivary proteins are poorly existed in NOD/SCID.e2f1
−/− mice. Taken together, mouse saliva works positively for the initial colonization of C. albicans rather than protecting by innate immunity, an opposite function to S. mutans colonization .
The colonization decreased in a time-dependent manner without the effects of saliva volume difference in NOD/SCID.e2f1
+/+ and NOD/SCID.e2f1
−/− mice. The colonization may be decreased by washing effects with an appropriate volume of saliva. Moreover, cell-mediated immunity plays an important role in the resistance to mucosal candidiasis [40, 41]. The mucosal surface provides a protective barrier against bacterial and fungal infections in the oral cavity. The β-defensins are small cationic amphipathic peptides that exhibit a broad spectrum of activity against gram-positive and gram-negative bacteria and fungal species . NOD/SCID.e2f1
+/+ and NOD/SCID.e2f1
−/− mice do not have mature T and B cells but have a protective barrier with cell-mediated immunity. Therefore, cell-mediated immunity may not be associate with the difference of C. albicans colonization between NOD/SCID.e2f1
+/+ and NOD/SCID.e2f1
−/− mice. The positive function for C. albicans in the oral cavity indicates a useful animal model for initial colonization of C. albicans in NOD/SCID wild type mice as compared to NOD/SCID.e2f1
Previous report suggested that the indigenous bacterial flora could suppress the extent of colonization of C. albicans by interfering with its ability to attach to mucosal surface in comparison with germ-free rats and conventional rat . This was explained by competing for epithelial receptor sites required for Candida attachment or by enzymatically altering the surfaces of the yeast cells. Our mouse model system using oral inoculation of C. albicans is different from previous report using germ free and conventional rat and indicates better model to explore effects of salivary components and indigenous microorganisms on the colonization under the natural background condition than previous model system. Sucrose works as a substrate for production of glucan by oral streptococci. The drinking of sucrose water before inoculation provides a source of glucan for the restoration of indigenous microorganisms. CHX was uniformly effective against strains of common borne microorganisms. The treatment with CHX disinfected the indigenous microorganism before inoculation in both no-sucrose and sucrose drinking water [44, 45]. However, the indigenous microorganisms contaminated the sample of C. albicans in the mice fed 1% sucrose water. The colonization of C. albicans was not affected by restoration of indigenous microorganisms. Therefore, the initial colonization of C. albicans is not affected by later colonization of indigenous microorganisms in the oral cavity. It is suggested that the colonized cells of C. albicans are not removed by the enzymatical or physical effect by indigenous microorganisms.
Yeast-form cells adhere more effectively than hypal cells to endothelial cells under conditions of flow . Although cells locked in the filamentous state also display reduced virulence [47–49], the ability to form hyphae is important for C. albicans to cause disease after dissemination: cells locked in the yeast form remain avirulent until they are permitted to form hyphae, after which, mice succumb to the infection . C. albicans hyphae are impaired in their ability to adhere to the human oral cavity by the bacterium Streptococcus gordonii . To colonize and infect the oral environment, yeast cells must first adhere to host cells and tissues or prosthetic materials within the oral cavity or co-aggregate with the oral microbiota [52–54]. In this study, the hyphae form of C. albicans showed similar results as the yeast form but the restoration of the indigenous microorganisms was weaker with the hyphal form than that with the yeast form. This may indicate restriction effects by the hyphal form on the restoration.
We provide here an original new animal model system, which may be a useful model to explore oral initial colonization of C. albicans. The salivary protein flow may play important roles in maintaining the commensal behavior of C. albicans and becoming an opportunistic pathogen under the immune deficiency condition such as SCID. These mice are required for these investigations to determine several factors that contribute to the susceptibility for candidal infections.