16. Systemic Diseases and the Oral Microbiota

The oral microbiota comprises species that span the continuum from commensal to pathogenic. The pathogens are best known for their ability to cause infections that are normally limited to the oral cavity, including dental caries and periodontitis. However, when these organisms or their components enter the circulation or connective tissues or encounter the mucosal immune system, they may be linked to the occurrence of systemic disease. A strong causal association has been suggested for oral bacteria in the systemic diseases infective endocarditis and disseminated intravascular coagulation (DIC). Both are life-threatening diseases. Oral microorganisms have also been associated with pathogenic mechanisms in nephritis, rheumatoid arthritis, and Behet's disease. In some epidemiological studies, oral infections, such as periodontal diseases, have been associated with the occurrence of diverse diseases such as atherosclerosis and preterm low-birth-weight infants. Principles of entry from the oral cavity into the systemic compartment, systemic dissemination of microbes, and subsequent host responses that have been learned from molecular, animal, and human studies of infective endocarditis may be applicable to other systemic infections. This chapter explores the underlying biological plausibility that oral microbes may contribute more generally to systemic disease.