Functional heterogeneity and health impacts of oral bacterial ectopic colonization

The oral cavity is the second most colonized anatomical site after the intestinal tract, and due to its unique and diverse microbial community, it constitutes a distinct biome. The oral microbiota is an integral part of the human microbiome ecosystem, maintaining the balance of the oral microecology. However, when this balance of the oral microecology is disrupted, oral bacteria not only cause oral diseases but also spread to other parts of the body through various pathways. These oral bacteria colonizing outside the oral cavity may exhibit different functional roles from those in the oral cavity, thereby contributing to or exacerbating pathological processes in distant organs, such as experimental colitis, hepatic steatosis, and opportunistic bacteremia. By analyzing compositional changes in the oral and gut microbiota, the risk of certain diseases can be predicted at an early stage, and timely interventions can be implemented. This review summarizes the colonization and disease-causing effects of several major oral bacteria (Streptococcus, Veillonella, Prevotella, Lactobacillus, Helicobacter pylori, Fusobacterium nucleatum, Porphyromonas gingivalis, Tannerella forsythia) in the oral cavity and other sites. To provide conceptual rigor, we explicitly distinguish among passive ectopic detection, stable ectopic colonization, and functionally active pathogenicity. Furthermore, we highlight how ectopic oral bacteria undergo context-dependent metabolic adaptations in new niches, rather than simply replicating their oral profiles.