Shotgun metagenomic mapping of saliva reveals insights into diversity and function of the oral microbiome in pregnancy.

The oral microbiome is a complex and dynamic microecosystem that fluctuates continually throughout the lifespan of a woman. Nevertheless, the function of the oral microbiome in reproductive health is not yet fully understood. Monitoring oral health and providing necessary dental care before and during pregnancy could help maintain a balanced oral microecology and support healthier microbial transfer to newborns. Here, we aimed to compare the salivary microbiome of pregnant and non-pregnant women using shotgun metagenomics to describe their taxonomic and functional composition and assess whether the resulting data is better explained by the reproductive stage. We conducted a comparative cross-sectional study involving pregnant women (n = 71; gestational age 37-42 weeks) and non-pregnant women (n = 143 with regular menstrual cycles; 3 saliva samples per participant across different menstrual phases). Shallow shotgun metagenomic sequencing was used to characterize both taxonomic and functional profiles of the oral microbiome. Socransky's color complex analysis was performed to assess group differences in key microbial complexes. Quantitative PCR was used to validate the abundance of selected oral bacteria. Participant data, including demographic, behavioral, clinical, and oral health variables (such as dentist visits), were collected and incorporated as covariates to adjust for potential confounding effects. Additionally, a sensitivity analysis was performed by excluding participants with identified behavioral or clinical risk factors. Ten phyla including Actinomycetota, Bacteroidota, Chloroflexota Bacillota, Fusobacteriota, Pseudomonadota, Spirochaetota, Synergistota Candidatus Saccharimonadota and Mycoplasmatota, 102 genera, and 410 species were identified. Pregnant women had lower saliva microbiome diversity, driven by reduced richness but unchanged evenness. The microbial composition varied between the groups, even after adjusting for confounding factors. Differential abundance analysis, adjusted for potential confounders, identified 25 species that significantly differed between groups (q < 0.05), with 13 taxa more than three-fold higher in pregnant women. Notably, red complex species were more abundant in pregnant women (p < 0.05). Functional pathway analysis identified 40 modules that differed by pregnancy status. These results further suggest a connection between pregnancy and changes to the oral microbiome in women. As many of these changes are in a pro-inflammatory direction, further research is warranted to assess its potential impact on pregnant women and their newborns.