Longitudinal shifts in oral microbiome composition and metabolic pathways associated with preterm birth

Oral dysbiosis in pregnant women with oral diseases has been associated with adverse pregnancy outcomes. However, the inter-individual variability in oral microbiome composition of pregnant women without any oral disease, and its role in preterm birth, has not been studied yet. Here, we have collected saliva from 20 term birth (TB) and 20 preterm birth (PTB) delivering women without any self-reported oral disease at three trimesters ( n = 120). Microbial DNA was subjected to 16S rRNA gene sequencing for taxonomic classification, and microbial pathways were investigated by PICRUSt2. In a subset of samples, shotgun metagenomic sequencing was done to identify microbial species, their gene families, and their pathways. TB and PTB women were distributed into three distinct oral community types (OCTs). Haemophilus parainfluenzae and Rothia mucilaginosa were associated with TB and PTB, respectively. The chorismate biosynthesis pathway, essential for folic acid biosynthesis, was significantly enriched in TB, whereas the enterobactin biosynthesis pathway that produces iron chelators (siderophores) was significantly enriched in PTB. The heterolactic fermentation pathway that reduces oral pH was enriched in PTB. Our data suggest that oral microbiome changes might have an impact on birth outcomes in women even without any history of self-reported oral disease during the pregnancy period. IMPORTANCE The importance of this study lies in demonstrating that compositional and functional shifts in the oral microbiome are associated with pregnancy outcomes. Using a longitudinal design across three trimesters in an Indian cohort, we show that pregnant women segregate into distinct oral community types with consistent associations to term birth (TB) and preterm birth (PTB). Importantly, the TB-associated microbiome was enriched in taxa and pathways linked to vitamin and amino acid biosynthesis, including chorismate and threonine metabolism, which are critical for fetal growth. In contrast, PTB was associated with pathways related to iron scavenging and acidification of the oral environment, suggesting a metabolically stressed and dysbiotic state. These findings highlight the oral microbiome as a previously underappreciated, modifiable factor in pregnancy outcomes and underscore its potential relevance for early risk stratification and preventive strategies against PTB.