The Oral–Gut Microbiome Axis: Microbial Transit, Ectopic Colonization, Systemic Associations, Salivaomics, and Clinical Control Points

Abstract

The oral cavity is a continuous microbial source for the gastrointestinal tract. Adults swallow approximately 0.5–1.5 L of saliva each day, carrying a large and variable bacterial load. Gastric acidity removes most ingested organisms, but survival increases with food buffering, acid tolerance, mucin or biofilm protection, and proton-pump inhibitor use. Oral taxa are therefore repeatedly detected in stool, intestinal mucosa, and diseased tissues. The strongest mechanistic evidence concerns Fusobacterium nucleatum in colorectal cancer and Porphyromonas gingivalis in inflammatory and metabolic pathways, although detection alone does not prove stable colonization or causality. This narrative review summarizes microbial transit, key taxa, salivaomics, systemic associations, and practical oral control points. It also identifies clinical control points, including probiotics and polyols such as xylitol, as selective microbiome-modulating adjuncts at varying stages of evidence. The evidence suggests a strong clinical message: treat oral dysbiosis for established dental reasons, while paired oral and gut studies determine when downstream systemic benefits are achieved.