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Review
. 2021 Feb;100(2):133-140.
doi: 10.1177/0022034520956975. Epub 2020 Sep 13.

Critically Appraising the Significance of the Oral Mycobiome

P I Diaz  1   2 , A Dongari-Bagtzoglou  3
Affiliations
Review

Critically Appraising the Significance of the Oral Mycobiome

P I Diaz et al. J Dent Res. 2021 Feb.

Abstract

Recent efforts to understand the oral microbiome have focused on its fungal component. Since fungi occupy a low proportion of the oral microbiome biomass, mycobiome studies rely on sequencing of internal transcribed spacer (ITS) amplicons. ITS-based studies usually detect hundreds of fungi in oral samples. Here, we review the oral mycobiome, critically appraising the significance of such large fungal diversity. When harsh lysis methods are used to extract DNA, 2 oral mycobiome community types (mycotypes) are evident, each dominated by only 1 genus, either Candida or Malassezia. The rest of the diversity in ITS surveys represents low-abundance fungi possibly acquired from the environment and ingested food. So far, Candida is the only genus demonstrated to reach a significant biomass in the oral cavity and clearly shown to be associated with a distinct oral ecology. Candida thrives in the presence of lower oral pH and is enriched in caries, with mechanistic studies in animal models suggesting it participates in the disease process by synergistically interacting with acidogenic bacteria. Candida serves as the main etiological agent of oral mucosal candidiasis, in which a Candida-bacteriome partnership plays a key role. The function of other potential oral colonizers, such as lipid-dependent Malassezia, is still unclear, with further studies needed to establish whether Malassezia are metabolically active oral commensals. Low-abundance oral mycobiome members acquired from the environment may be viable in the oral cavity, and although they may not play a significant role in microbiome communities, they could serve as opportunistic pathogens in immunocompromised hosts. We suggest that further work is needed to ascertain the significance of oral mycobiome members beyond Candida. ITS-based surveys should be complemented with other methods to determine the in situ biomass and metabolic state of fungi thought to play a role in the oral environment.

Keywords: Candida; Malassezia; caries; colonization; internal transcribed spacer; opportunistic infections.

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Conflict of interest statement

The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article.

Figures

Figure.
Figure.
Main components of the oral mycobiome and their significance. Two community types (mycotypes) have been detected in saliva (Hong et al. 2020) and are represented in the figure by the red and green circles. Either mycotype is numerically dominated by Candida or Malassezia. The less diverse Candida mycotype is associated with a bacteriome of lower diversity enriched for aciduric species. The more diverse Malassezia mycotype is enriched for other fungi present in low proportions. The Malassezia mycotype is associated with a more diverse bacteriome enriched for inflammophilic species. Factors hypothesized to drive selection of mycotypes are depicted in the top portion of the figure. Smoking, intake of corticosteroids, a diet rich in carbohydrates, and wearing a removable prosthesis favor a Candida mycotype, while a nonimpaired immune system, a diet low in carbohydrates and high in lipids, and good oral hygiene favor a Malassezia mycotype. While evidence exists for some of these associations, further studies are needed to understand some of these correlations (e.g., the relationship of Malassezia and dietary lipids). The bottom of the figure shows conditions in which mycotypes may be able to serve as predictors. Carrying the Candida mycotype may increase risk of oral candidiasis (Diaz et al. 2019) and may predispose to increased caries (Abusleme et al. 2018; Xiao et al. 2018). The size of the circles depicting mycotypes represents hypothesized differences in the total fungal load in each of these communities.
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