The sociobiome–oral microbiome pathway in dental caries among Indigenous Australians

ABSTRACT Indigenous Australians experience disproportionately high rates of dental caries, yet the biological pathways linking socioeconomic disadvantage to oral health remain unclear. This study examined how individual– and neighbourhood-level socioeconomic status (SES) shape the oral microbiome and mediate dental caries risk in Indigenous adults. A cross-sectional study of 100 Indigenous Australians (≥18 years) collected demographic, SES, and oral health behaviour data, followed by dental examinations for dental caries assessment, followed by collection of saliva and plaque samples. The microbiome was profiled using 16S rRNA sequencing, with analyses of microbial diversity, composition, differential abundance, and mediation of SES–caries associations. Saliva exhibited greater observed and Shannon diversity than plaque (both p < 0.01), with significant compositional differences (adonis p < 0.001). In saliva, alpha diversity was reduced with age, secondary education, low income, ownership of a healthcare card, and caries presence (all p < 0.05). SES explained greater variation in saliva than plaque composition, with associations for income (R²=3.8%, p<0.01), education (R²=2.0%, p<0.01), and caries (R²=2.2%, p<0.01). Differentially abundant taxa in low-income and caries groups included Rikenellaceae RC9 gut group , F0058 , Filifactor , and Treponema . Mediation analyses showed 75.6% of the income effect on caries was mediated by microbiome shifts (ME=0.28, SE=0.32), compared with 21% for education (ME=0.03, SE=0.02). Socioeconomic disadvantage has a significant impact on the oral microbiome, influencing caries risk through income-related microbial dysbiosis. Saliva emerges as a sensitive biomarker of SES gradients. Addressing oral health inequities requires both structural policies targeting income inequality and microbiome-informed interventions. IMPORTANCE This research provides novel biological insights into how socioeconomic disadvantage contributes to the higher burden of dental caries among Indigenous Australians. Although social determinants of health are well recognised, the pathways connecting these determinants to oral disease remain unclear. By demonstrating that low income and education affect oral microbiome diversity and composition, and that a significant part of income-related caries risk is mediated through microbiome changes, this study highlights an important mechanism behind oral health inequalities. Identifying saliva as particularly responsive to socioeconomic differences makes it a useful, non-invasive biomarker for tracking risk in vulnerable populations. These findings emphasise the need for two approaches: structural interventions to reduce social and income gaps, and microbiome-based strategies to address microbial imbalance and disease risk. Together, they strengthen the evidence for more effective, culturally sensitive efforts to promote oral health equity.