Microbial infection of the intrauterine environment is a major cause of preterm birth. The current paradigm indicates that intrauterine infections predominantly originate from the vaginal tract, with the organisms ascending into the sterile uterus. With the improvements in technology, an increasing number of bacterial species have been identified in intrauterine infections that do not belong to the vaginal microflora. We have demonstrated previously that intrauterine infections can originate from the oral cavity following hematogenous transmission. In this study, we begin to systemically examine what proportion of the oral microbiome can translocate to the placenta. Pooled saliva and pooled subgingival plaque samples were injected into pregnant mice through tail veins to mimic bacteremia, which occurs frequently during periodontal infections. The microbial species colonizing the murine placenta were detected using 16S rRNA gene-based PCR and clone analysis. A diverse group of bacterial species were identified, many of which have been associated with adverse pregnancy outcomes in humans although their sources of infection were not determined. Interestingly, the majority of these species were oral commensal organisms. This may be due to a dose effect but may also indicate a unique role of commensal species in intrauterine infection. In addition, a number of species were selectively "enriched" during the translocation, with a higher prevalence in the placenta than in the pooled saliva or subgingival plaque samples. These observations indicate that the placental translocation was species specific. This study provides the first insight into the diversity of oral bacteria associated with intrauterine infection.Preterm birth (PTB) is a significant public health concern, affecting 12.7% of births in the United States. This rate reflects a 36% increase in the past 25 years. Among these, the very preterm births, i.e., those occurring before 32 weeks of gestation, are the most costly and medically relevant group, accounting for approximately 2% of all deliveries. Intrauterine infection has been recognized as a major cause of PTB. The infection rate is inversely related to the gestational age and is highly prevalent among the very preterm deliveries (24). Intrauterine infection has also been implicated in other adverse pregnancy outcomes, such as late miscarriage and stillbirth (25). A wide variety of bacterial species have been identified in the infections, with Ureaplasma urealyticum, Mycoplasma hominis, Bacteroides spp., Gardnerella vaginalis, and Fusobacterium nucleatum (24, 31) as the most prevalent. It has been suggested that the majority of these bacterial species are of relatively low virulence. However, once inside the uterus, they stimulate the synthesis and release of proinflammatory cytokines, neutrophil infiltration and activation, and prostaglandin and metalloprotease synthesis and release, leading to cervical ripening, membrane weakening and rupture, uterine contractions, and PTB (24).The current paradigm ind...