Abstract
The oral microbiome, the complex microbial ecosystem of the mouth, is increasingly proposed as a “sixth vital sign,” offering a non-invasive window into systemic health, analogous to traditional metrics like blood pressure and heart rate. This concept is based on the oral cavity’s role as both an interface with the environment and a gateway to internal systems. This article reviews the rationale for conceptualizing the oral microbiome as a “sixth vital sign,” summarizes the extensive evidence connecting oral microbial dysbiosis (imbalance) to a range of systemic diseases, and critically evaluates the strengths and limitations of this framework. The rationale for this concept rests on the oral cavity’s accessibility for sampling, the microbiome’s modifiability, and its profound interconnectedness with systemic health. Evidence demonstrates strong associations between oral dysbiosis—particularly stemming from conditions like periodontitis—and numerous systemic disorders. Key mechanistic pathways include the translocation of oral pathogens (e.g., Porphyromonas gingivalis, Fusobacterium nucleatum) into the bloodstream, the generation of chronic low-grade systemic inflammation, and disruptions via the oral-gut-brain axis. This review details significant links to cardiovascular disease, metabolic disorders (diabetes, obesity), neurological conditions (Alzheimer’s disease), autoimmune diseases (rheumatoid arthritis), and various cancers. Despite compelling associations, the “sixth vital sign” concept remains largely metaphorical at present. Its translation into clinical practice is hindered by significant challenges, chiefly the need to establish causality beyond association, the lack of standardized sampling and analytical methods, and the absence of defined “normative” microbial benchmarks. The concept’s immediate value is in reinforcing the integration of oral health as an essential component of systemic health monitoring and management.
Keywords: Dysbiosis, Host-microbe Interactions, Oral Microbiome, Systemic Disease, Vital Sign.