Oral microbiology

Oral microbiology is the study of the microorganisms (microbiota) of the oral cavity and their interactions between oral microorganisms or with the host. The environment present in the human mouth is suited to the growth of characteristic microorganisms found there. It provides a source of water and nutrients, as well as a moderate temperature. Resident microbes of the mouth adhere to the teeth and gums to resist mechanical flushing from the mouth to stomach where acid-sensitive microbes are destroyed by hydrochloric acid. Recent approaches in the microbiome studies suggest that different niches in the oral cavity such as tooth surfaces, tongue dorsal, gingival crevices, buccal mucosa and saliva provide different conditions (different pH, oxygen levels, nutrient availability) and microenvironments that favour different types of microorganisms.

Anaerobic bacteria in the oral cavity include: Actinomyces, Arachnia (Propionibacterium propionicus), Bacteroides, Bifidobacterium, Eubacterium, Fusobacterium, Lactobacillus, Leptotrichia, Peptococcus, Peptostreptococcus, Propionibacterium, Selenomonas, Treponema, and Veillonella. The most commonly found protists are Entamoeba gingivalis and Trichomonas tenax. Genera of fungi that are frequently found in the mouth include Candida, Cladosporium, Aspergillus, Fusarium, Glomus, Alternaria, Penicillium, and Cryptococcus, among others. Many of these microorganisms are now classified within the broader of oral microbiome, where their roles in health and disease are being re‑evaluated using techniques such as deep sequencing. In some individuals, overrepresentation or underrepresentation of certain species in these microbial groups has been associated with changes in oral microbiome (known as dysbiosis) that are more consistent with disease-related phenotypes. Recent studies also suggest that many microorganisms are specialised to particular niches, such as subgingival crevice or dorsum of the tongue, and are not evenly distributed throughout the oral cavity. Bacteria accumulate on both the hard and soft oral tissues in biofilms. Bacterial adhesion is particularly important for oral bacteria.

Oral bacteria have evolved mechanisms to sense their environment and evade or modify the host. Bacteria occupy the ecological niche provided by both the tooth surface and mucosal epithelium. Factors of note that have been found to affect the microbial colonization of the oral cavity include the pH, oxygen concentration and its availability at specific oral surfaces, mechanical forces acting upon oral surfaces, salivary and fluid flow through the oral cavity, and age. It has been observed that the oral microbiota differs between men and women in conditions of oral health, but especially during periodontitis. However, a highly efficient innate host defense system constantly monitors the bacterial colonization and prevents bacterial invasion of local tissues. A dynamic equilibrium exists between dental plaque bacteria and the innate host defense system. Oral microorganisms play a role in the two major dental diseases: dental caries and periodontal disease. The concept of the oral microbiota suggests that oral health or disease results from the collective activities and interaction of these microorganisms, not from individual microorganisms acting alone.