In this study, we developed a method for predicting PTB based on random forest classifier using oral microbiome compositions. Recently, several sporadic reports have suggested a bidirectional relationship between oral microbiome and pregnancy [11]. However, prenatal oral microbiome is not well understood yet. Some research has shown that oral microbial dysbiosis combined with gingival inflammation can lead to adverse pregnancy outcomes, including low birth weight, PTB, preeclampsia, and miscarriages [19]. Nevertheless, these results have been inconsistent due to methodologies employed in studies that only target known pathogens.
Fusobacterium nucleatum is the most prevalent oral microbiome studied. Fusobacterium nucleatum is a Gram-negative, anaerobic, filamentous oral microbiome. It is considered one of the most abundant species in the oral microbiome. It can also be isolated from vaginal microbiome [20, 21]. Intra-amniotic Fusobacterium nucleatum infection leading to PTB has been reported in human and animal studies [22]. Other studies have shown that other oral pathogens including Porphyromonas gingivalis and intrauterine Bergeyella spp. can be isolated from the placenta of women who deliver prematurely [23, 24]. In the present study, although Bergeyella spp. was enriched in the FTB group, it was one of the DAT between the PTB and FTB group. It was also one of the 20 most important taxa consisting of Random forest classifiers. Furthermore, Campylobacter gracilis was one of the FTB-enriched DAT that can aid colonization by periodontal pathogens including Porpyhromonas gingivalis in subgingival microbiome [25]. Lactobacillus gasseri was also one of the FTB-enriched DAT. It is known that Lactobacillus gasseri in vaginal microbiome can decrease early PTB risk [21, 26, 27].
We found that decisive species differentiating between two groups were mainly abundant in the FTB group, with DAT consisting of 26 FTB-enriched DAT and six PTB-enriched DAT. We hypothesize that deficiency of species having a protective impact might have triggered the pathophysiology of PTB. Two different mechanisms have been proposed to explain the relationship between unhealthy microbiota composition and adverse pregnancy outcomes. The first mechanism proposed that periodontal bacteria originating in the gingival biofilm could translocate from the unhealthy oral cavity and cross the placenta, reach the intra-amniotic fluid and fetal circulation and directly affect the fetoplacental unit, resulting in bacteremia [28]. The second mechanisms proposed that systemic dissemination of endotoxins and/or inflammatory mediators derived from periodontal plaque and secreted by the subgingival inflammatory site could be carried to the fetoplacental unit [29–30]. Although certain microbiota has the same species, their subgroups can have both positive and negative influences on pregnancy outcomes. Following this line of thought, we believe that composition or dysbiosis of the oral microbiome is more important than the presence of specific microbiota.
It is worth nothing that microbial changes occurring during pregnancy might be nature consequences of a healthy pregnancy. Three reasons can explain the susceptibility to oral diseases such as periodontitis during pregnancy. These diseases are common in pregnant women due to hormonally driven hyper-reactivity of the gingiva to bacteria in the subgingival biofilm. Other factors that increase the risk of poor oral health during pregnancy include changes in dietary habits (frequent snacking or increased consumption of carbohydrate-rich foods), stomach acids from nausea and vomiting that contribute to the breakdown of tooth enamel, and a decreased likelihood of seeking dental care during pregnancy. We plan to implement pathway analyses to investigate direct link between the microbiome and PTB.
Due to limited power resulting from a small number of participants, our study verified that oral microbiota might provide potential biomarkers for predicting pregnancy complications using machine learning methods including random forest classification. Additionally, the fact that the entire microbiome was not analyzed was a limitation of this study because our analysis only used relative values measured by 16S rRNA sequencing, not 16S metagenome sequencing.
Despite these limitations, this prospective study demonstrated the potential of a PTB prediction model using oral microbiome in mouthwash. Further multi-center and larger-scale studies are needed to confirm out results before applying techniques developed in this study in the clinical field.