Altered Salivary Microbiome and Increased Immune Markers in Mouth-Breathing Children: Implications for Oral Health

Objective: To investigate the impact of mouth breathing (MB) on the salivary microbiome in children by comparing the differences in biochemical indicators, immune proteins and microbial community structure between mouth-breathing children and nose-breathing children, as well as among mouth-breathing children with different malocclusion types. Methods: Saliva samples were collected from 30 mouth-breathing children (MB group) and 10 nose-breathing children (control group), aged 8 to 12, between August 2023 and August 2024. The MB group was further subdivided into Angle Class I, II and III malocclusion subgroups, with 10 cases each. Ion concentrations were measured using an automatic biochemical analyser, pH was determined using a pH metre, immune protein levels were assayed by enzyme-linked immunosorbent assay and the structure and diversity of the salivary microbiota were analysed using 16S rRNA high-throughput sequencing. Results: Compared with the control group, the MB group showed no significant difference in salivary pH (P > .05), but a significantly lower chloride ion (Cl⁻) concentration (P < .05). Conversely, the concentrations of secretory immunoglobulin A (SIgA) and peroxiredoxin-5 (PRDX5) were significantly higher in the MB group (P < .05). No statistically significant differences in these indicators were observed among the different malocclusion subgroups. Spearman correlation analysis revealed a positive correlation between PRDX5 and SIgA concentrations (rs = 0.808, P < .001) and negative correlations between both PRDX5 and SIgA concentrations and Cl⁻ concentration (rs = -0.588 and -.600, respectively; P < .001) in the MB group. Microbiota analysis indicated that the species richness (Chao1 index) of the salivary microbiota was significantly higher in the MB group. At the genus level, the relative abundances of Dialister, Streptobacillus, Anaeroglobus and Scardovia were significantly increased in the MB group (P < .05). Conclusion: MB alters the physicochemical properties of children's saliva, triggering an enhanced local immune response and a state of oxidative stress. This leads to dysbiosis of the salivary microbial community, characterised by a higher abundance of pathogenic bacteria associated with dental caries and periodontal disease and shows a correlation with pathogens linked to upper respiratory tract inflammation. These findings suggest that MB may impact oral and potentially systemic health by disrupting the oral microenvironment.