Chapter 31 Normal oral flora, the oral ecosystem and plaque biofilms
Oral flora comprises a diverse array of organisms and includes eubacteria, archaea, fungi, mycoplasmas, protozoa and possibly a viral flora that may persist from time to time. These organisms usually live in harmony in a range of habitats including the teeth, gingival sulcus, tongue, cheek, hard and soft palate and tonsils. Collectively the oral flora have been termed oral microbiota, and more recently, the oral microbiome. Bacteria are by far the predominant group of organisms, and there are probably some 500 to 700 common oral species or phylotypes of which only 50 to 60% are cultivable. The remaining unculturable flora are currently being identified using molecular techniques. This, together with the fact that the oral cavity has a wide range of sites (habitats) with different environmental conditions, makes the study of oral microbiology complex and difficult. Interestingly, despite the enormous diversity and complexity of the oral flora, many organisms commonly isolated from neighbouring ecosystems such as the gut and skin are not found in the mouth, indicating the unique and selective ecology of the oral cavity with regard to microbial colonization.
The main bacterial genera found in the oral cavity are well characterized using mostly traditional culture-based techniques. Oral bacteria can be classified primarily as Gram-positive and Gram-negative organisms, and secondarily as either anaerobic or facultatively anaerobic according to their oxygen requirements. Some oral microbes are more closely associated with disease than others, and a proportion of these appear to be uncultivable. The following is a synopsis of the major bacterial genera isolated from the oral cavity. Students should refer to the appropriate chapters in Part 3 for detailed information on these organisms.
Due to continuing advances in molecular technology, especially those based on 16S ribosomal RNA (rRNA) sequences, microbial taxonomy is always in a state of flux. This poses a challenge to both the student and the scientist alike. Despite these changes, some prefer using the traditional nomenclature, while others use the new terminology, leading to further confusion. Hence in the following text, both the old and the recent taxonomic changes of oral bacteria are highlighted.
Gram-positive cocci in chains, non-motile, usually possessing surface fibrils, occasionally capsulate; facultative anaerobes; variable haemolysis but α-haemolysis most common; selective medium: mitis salivarius agar (MSA).
These comprise a large proportion of the plaque biofilms. The classification of this group of organisms is fraught with difficulties, but the advent of new tests such as lipid analysis and molecular approaches have eased the problem to some extent. Most of the oral anaerobes were previously classified under the genus Bacteroides. However, advances in taxonomic methods have shown that they belong to two major genera, now termed Porphyromonas and Prevotella, which differ in their ability to metabolize sugar. Some of these organisms produce characteristic brown-black pigments on blood agar and are referred to collectively as ‘black-pigmented anaerobes’ (see Fig. 17.1).