Differences in tongue microbiota of older adults with a high fungal load according to tongue coating status

ABSTRACT The tongue microbiota of older adults requires careful attention because it can be aspirated into the lower respiratory tract due to impaired swallowing function. This study focused on differences in the tongue microbiota of older adults with a high fungal load, with and without tongue coating. We initially quantified the total fungal load of tongue microbiota in 145 individuals receiving commuting rehabilitation services (mean age, 87.6 ± 6.5 years) using quantitative polymerase chain reaction targeting the fungal ribosomal internal transcribed spacer (ITS) region. The fungal and bacterial compositions of 58 individuals with high fungal loads were determined based on the fungal ITS region and bacterial 16S rRNA gene amplicon sequencing, respectively. No significant differences were observed in the fungal load and composition of the tongue microbiota between individuals with no/slight and substantial tongue coating (Winkel Tongue Coating Index ≤3 and >3, respectively), and Candida albicans was the predominant fungal species of the tongue microbiota regardless of the tongue coating status. In contrast, individuals with no/slight tongue coating exhibited significantly lower bacterial loads and higher relative abundances of acidogenic taxa, including Streptococcus mutans and Lactobacillus gasseri , compared with those with substantial tongue coating. These bacterial communities were also distinct from those of individuals with no/slight tongue coating and a low fungal load. These findings indicate that the reduction in tongue coating accompanied by fungal overgrowth in older adults reflects alterations in the bacterial community, rather than differences in fungal composition. IMPORTANCE Tongue microbiota status requires particular attention, especially among older adults, as microbiota can be aspirated into the lower respiratory tract due to impaired swallowing function, leading to pulmonary infections. Bacterial overgrowth on the tongue dorsum manifests as an accumulation of tongue coating, which serves as an indicator of tongue hygiene. However, an increase in fungal components in older adults is occasionally accompanied by a loss of tongue coating unrelated to good tongue hygiene, thereby complicating visual assessments. This study demonstrated that loss of tongue coating along with fungal overgrowth reflects the coincidence of alterations in the bacterial community and an increased prevalence of decayed teeth, indicating a higher risk of dental caries. The study findings highlight the importance of assessing both dental health status and tongue microbiota in older adults with slight tongue coating.