The mouth is colonized by a rich and diverse microbial community made up of more than 700 different species of bacteria, certain species of fungi, and even protozoa.
The oral cavity contains both beneficial and pathogenic microbes. Pathogenic microorganisms often reside on the pellicle, a layer of proteins and other large molecules that lines the surface of the enamel and has a thickness of around 10μm. The pathogens coat the enamel and form a biofilm layer referred to as plaque.
Initially, plaque affects the teeth, causing the formation of cavities or tooth decay. In particular, a combination of two pathogens, the bacteria Streptococcus mutans and the fungus Candida albicans, causes this dental caries, which is a common and chronic condition that can affect any age group.
Also highly prevalent among humans are the inflammatory diseases caused by Porphyromonas gingivalis such as periodontitis (inflamed gingiva). Researchers have also shown that periodontitis exacerbates arthritis, is a precursor to arthritis, and is likely to be both a causative and contributory factor in this autoimmune condition. P. gingivalis is also a contributor in other inflammatory conditions including cardiovascular disease. The bacteria avoid detection by the immune system, allowing it to create ongoing low-grade vascular inflammation, which promotes the progression of atherosclerosis.
Oral bacteria have also been shown to play a role in the development of cancer. Again, P. ginigivalis and another microbe found in the oral cavity called Fusobacterium nucleatum, cause chronic inflammation that plays a key role in the development and progression of cancer, particularly oral squamous cell carcinoma.
Studies have shown that oral squamous cell carcinoma surfaces have higher numbers of these two bacteria, compared with other mucosal surfaces. Bacteria in the oral cavity may play a role in cancer at any stage of the disease, from precancerous lesions to metastasis.
Microbes in the mouth can also be used to find out about other diseases such as human immunodeficiency virus (HIV). In a 2014 study published in PLOS Pathogens, Ghannoum and colleagues characterized the oral bacteriome and mycobiome (fungal biota) in patients infected with HIV and people not infected.
They found that while the bacteriome was not significantly different between the two groups, the mycobiome was and that Candida was the predominant fungus in both groups. The researchers found that increased Candida colonization was associated with decreased levels of the fungus Pichia, suggesting an antagonistic interaction between the two fungi.
Growth assays were then performed, which demonstrated that Pichia exhibit anti-Candida activity and inhibition of Candida by Pichia was further confirmed in a murine model of oral candidiasis. The finding also agreed with previous research suggesting that Pichia can out-compete Candida for nutrients.