Bacteremias of oral origin and potential implications for systemic diseases

The human oral cavity harbors a wide array of different bacterial species (>100 bacterial genera and >700 species). Most of these bacterial species are thought to be of a commensal nature, engaging in complex biofilm formation and facilitating the colonization of more noxious and pathogenic bacteria. Oral bacteria are the main infectious cause of two of the most endemic and historic diseases known to man: dental caries and periodontal disease. A number of unique characteristics of the mouth such as higher temperature, permanent humidity, a variety of substrate surfaces for attachment (teeth, tongue, cheeks and other oral soft-tissue surfaces) and decreased oxygen tension facilitate the formation of a biofilm that continuously challenges the host mucosal immune system. When undisturbed, mature oral microbial communities are formed. These are largely anaerobic Gram-negative bacteria under the gum-line and facultative gram-positive bacteria above the gum-line. Oral microbial communities ultimately overwhelm the local immune response and facilitate local infection. Epithelial cells, endothelial cells, take up the microorganisms and their antigens and innate immune cells, leading to immune dysregulation. With no obvious clinical signs or symptoms experienced by the infected individual, a long-standing chronic oral inflammatory response develops and ultimately progresses to a systemic inflammation stage. During disease development, translocation of oral bacteria to the bloodstream, i.e., bacteremia, ensues, modifying immune homeostasis in the process. This results in continued systemic infection and inflammation with potential systemic consequences. This chapter will focus on reviewing the current evidence of bacteremias of oral origin and how they affect different systems. Human evidence as well as experimental evidence obtained from in vitro and in vivo studies will be discussed to explain the potential implications to other systemic diseases.