Does the mouth seed the small bowel? Mechanisms and hypothesis-generating framework via the oral–gut axis

Background As the starting segment of the digestive tract, the oral cavity harbors a complex microbial community that closely interacts with the distal gut microbiota. Recent evidence suggests that oral hygiene status and oral dysbiosis may correlate with gut microbial homeostasis and play a potential role in the initiation or exacerbation of small intestinal bacterial overgrowth (SIBO).Objective This review aims to integrate existing evidence, outline a hypothesis-generating framework, identify critical knowledge gaps, and provide new perspectives for future interdisciplinary mechanistic research and clinical translation in SIBO and related gastrointestinal disorders.Design This narrative review synthesizes and evaluates preliminary clinical correlations, metagenomic surveys, and animal models that support the oral microbiota as a potentially modifiable factor associated with SIBO, and discusses the proposed underlying molecular pathways.Results Existing evidence supports the oral microbiota as a potentially modifiable factor associated with SIBO. However, direct high-level evidence testing oral hygiene interventions against definitive SIBO outcomes is currently lacking.Conclusions The oral-gut axis may play a critical role in SIBO pathogenesis. Future research requires high-quality clinical trials to validate the impact of oral hygiene interventions on SIBO outcomes, bridging the current critical knowledge gaps.