## 6 The Oral Microbiota in Health and IBD  
While most studies have focused on the microbiota of the large intestine, the oral cavity also contains diverse microbiota. To date, over 700 oral bacterial species have been identified in the human oral microbiome database (HOMD) [94]. Saliva, shedding of soft surfaces, buccal mucosa, palatal mucosa, tongue dorsum, and supragingival plaque contribute to the oral microbiome [95; 96]. Likewise, hard and mucosal surfaces within the oral cavity maintain environments suitable for commensal bacteria to flourish and protect the integrity of the gingival and epithelial cells [96]. Similar to other regions of the digestive tract, the oral cavity is subject to constant environmental exposures that may impact pH, temperature, oxygen, and available nutrients due to continuous exposure to environmental insults such as food, beverage, and potential pathogens. Throughout all of this, microbial communities within resident biofilms remain relatively stable and diverse in healthy individuals [97].  
Periodontal disease and dental carries contain distinct microbial profiles compared to healthy oral cavity patients [51; 98]. Community shifts can occur as a result of syntrophic metabolism between microbes, microbial byproducts, and competition for nutrients [50]. For example, low molecular weight fermentable carbohydrates, namely glucose, shift microbial communities that have been linked to oral diseases such as periodontal disease as well as non-oral diseases, such as cardiovascular disease [99; 100; 101; 102]. A recent study evaluating over 200 children also showed oral microbiota may be a determining factor in predicting weight gain trajectories [103]. The Ecological Plaque Hypothesis (EPH) and Keystone-Pathogen Hypothesis (KPH) propose mechanisms for the switch from a healthy to diseased oral state. These hypotheses agree that cariogenic, acidogenic, and aciduric bacteria primarily contribute to cavity production; fermentation from these bacteria produce acidic metabolites that promote oral diseases [104]. Indeed, research supporting the KPH suggests that _Porphytomonas gingivalis_ (_P. gingivalis_), a bacterium which originates in the oral cavity where it typically resides as a symbiont, is highly expressed in oral diseases where it has the ability to over-activate the immune system, leading to periodontal disease, poor gum health, and dental carries [98; 105; 106]. Further studies also suggest that pathogenic periodontal bacteria contribute to the development of metabolic, autoimmune, and inflammatory diseases, such as in IBD [107] by entering the bloodstream through lesions in the oral cavity [50]. _P. gingivalis_ is an anaerobic, sacacharolytic, Gram negative rod and an oral-originating pathobiont that ferments protein as opposed to carbohydrates. _P. gingivalis_ utilizes virulence factor cysteine proteases called gingipin to break down proteins, such as hemin acquired from hemoglobin. However, when homeostasis is lost, _P. gingivalis_ causes periodontal disease and can cause further disease progression in distant tissues [108; 109]. Together, these studies suggest that the oral cavity microbiota plays an important role in regulating human health and may also serve as a mechanism for the diagnosis and/or prognosis of patient health outcomes, as in IBD.