The oral microbiome begins developing within a few minutes after birth and some recent research is investigating the role of oral bacterial communities, not only as a potential target for antibiotics in oral diseases (like caries) that can be attributed to the presence of a single organism, but also as a useful indicator of systemic disease.
The review begins with an introduction to the focal infection theory that was promulgated during the 19th and early 20th centuries, which states that “foci” of sepsis are responsible for the initiation and progression of a variety of inflammatory diseases, ranging from arthritis and peptic ulcers to cardiovascular and neurodegenerative diseases. Therefore, oral infections, especially periodontitis-considered recently as an autoimmune disease that affects collagen-may be a triggering factor for systemic diseases.
The authors cover studies that have examined the contributions of oral microorganisms to systemic disorders including pulmonary diseases, cardiovascular diseases, adverse pregnancy outcomes, rheumatoid arthritis, diabetes and Alzheimer’s disease. Although the body of evidence from both animal and human studies supports the infectious aetiology of many of these systemic diseases, in many cases only correlations were identified and underlying mechanisms are not well understood.
Possible pathways linking oral infections with non-oral diseases include metastatic spread of infection from the oral cavity as a result of transient bacteremia, metastatic injury from the effects of circulating oral microbial toxins (e.g. lipopolysaccharide from periodontal Gram-negative bacteria), and metastatic inflammation caused by immunological injury from oral microorganisms, among others. Periodontitis and some systemic diseases also have shared risk factors, and periodontitis has been suggested to affect the host’s susceptibility to systemic disease through subgingival biofilms acting as reservoirs of inflammatory mediators derived from Gram-negative bacteria.
On the whole, the multifactorial aetiology of systemic chronic diseases and adverse outcomes that have been associated to periodontal diseases makes it difficult to establish a causal role for oral pathobionts in systemic infection. Although it is therefore too early to recommend therapy based on the links between oral bacteria and systemic diseases, future well-designed and large-scale prospective studies are needed in humans in order to explore how oral microbiome dysbiosis may impact either initiation or exacerbation of chronic systemic disorders. Currently what can be said is that good oral health is relevant not only for preventing oral infections but also for maintaining good overall health.
Andreu Prados Andreu Prados holds a Bachelor of Science Degree in Pharmacy & Human Nutrition and Dietetics. Science writer specialised in gut microbiota and probiotics, working also as lecturer and consultant in nutrition and healthcare. Follow Andreu on Twitter @andreuprados