A two-hit ecological framework linking social context to caries-associated microbiome shifts in children

Background Dental caries arises from an ecological imbalance within a complex community. How chronic social context relates to ecological heterogeneity and dysbiosis-associated microbial shifts in school-age children remains unclear.Objective To investigate the associations of left-behind status and caries burden with the salivary microbiome and to explore a two-hit ecological framework linking social context to caries-associated microbial shifts.Design In this cross-sectional study, 127 rural children were classified using a 2 × 2 framework based on left-behind status and caries burden. Saliva samples underwent shotgun metagenomic sequencing. Ecological analyses and covariate-adjusted multivariable models were performed.Results Alpha diversity did not differ across groups. Global community centroids were similar, whereas within-group dispersion was higher in left-behind children, suggesting greater ecological heterogeneity. After covariate adjustment, no genus-level associations remained significant, whereas several KEGG level 3 pathways related to translation and carbohydrate utilization were positively associated with dmft. Stratified analyses showed concordant caries-related enrichment of Streptococcus, Veillonella, and carbohydrate-utilization pathways across social strata. Ecological subtyping identified Neisseria- and Veillonella-anchored community types.Conclusion The findings are consistent with a two-hit ecological framework in which social context is associated with greater ecological heterogeneity and cariogenic pressure is associated with reproducible functional shifts. Given the cross-sectional design, this framework should be considered hypothesis-generating.