The bidirectional effects and mechanisms of the oral and gut microbiomes: a narrative review

Among the microbial ecosystems of the human body, the gut and oral microbiota constitute the two largest communities, collectively harboring thousands of bacteria, fungi, and viruses. Under physiological conditions, these microbiotas maintain internal homeostasis and stability, thereby protecting the host against pathogenic colonization. However, when pathogens such as Porphyromonas gingivalis translocate from the oral cavity to the gut, disruption of gut microbial homeostasis may occur, increasing the risk of disease development. Potential mechanisms underlying this association include the establishment of new symbiotic relationships, the disruption of the intestinal barrier, the activation or suppression of inflammatory cells—particularly the balance between T helper 17 (Th17) cells and regulatory T cells (Tregs)—and the induction of systemic inflammation. Conversely, gut microbiota dysbiosis, as observed in patients with inflammatory bowel disease, irritable bowel syndrome (IBS), or colorectal cancer, is also associated with alterations in the composition and diversity of the oral microbiota. Factors such as immune cell migration, malnutrition, and taste disturbances may contribute to oral microbial imbalance. In this review, we summarize the bidirectional influences on the composition and diversity of the oral and gut microbiomes and propose potential mechanisms underlying their interactions. A deeper understanding of these processes will enhance our knowledge of microbiota–host interactions and systemic health, and may shed light on the prevention and treatment of systemic diseases related to oral and gut microbiota dysbiosis.