Role of the Oral Environment
Introduction
In certain aspects, the oral cavity may be regarded as a single microbial ecosystem. A major regulatory factor is the flow rate of saliva, which decreases to almost 0.0 mL/min during sleep, is approximately 0.4 mL/min at rest, and increases to 2.0 mL/min after stimulation.
Although saliva is not a good medium for supporting the growth of many bacteria, 1.0 mL of whole saliva may contain more than 200 million microorganisms, representing more than 300 different species. Most originate from local environments in the oral cavity, but a minority belong to the so-called normal microflora of saliva and obtain nutrients from salivary proteins.
All the surfaces of the oral cavity are colonized by microorganisms. The facultatively anaerobic streptococci constitute an essential part of the microflora that constantly colonize the mucous membranes and the teeth. Microorganisms are regularly swallowed with saliva and the amount within the oral cavity fluctuates, simply because the microbial deposits building up on mucous membranes and, in particular, on tooth surfaces grow and multiply, thus providing a reservoir for the oral environment. Fluctuations also occur during sleeping and waking hours, and also as a result of such activities as eating and drinking and oral hygiene procedures.
Because the composition of the microflora in mixed saliva is mainly a result of the microorganisms that colonize oral surfaces, the salivary microflora to some extent reflects the gross composition of the microbial deposits on the various oral surfaces.
The oral cavity presents two types of surface for colonization by bacteria, the soft tissues and the hard tooth surfaces, modified to some extent by a coating of saliva, or, in the case of the hard surfaces, by a pellicle formed by adsorption of salivary components. A distinct and important difference between the two types of surface is that the soft tissue surfaces are lost when the cells are shed; thus, readherence of bacteria is essential for survival. In contrast to the hard surfaces which will support heavy deposits of bacteria in dental plaque, the soft tissue surfaces do not support formation of complex layers of bacteria (biofilms).
It is also likely that microbes attached to desquamated epithelial cells spread, via saliva, to different tooth surfaces and typically colonize sheltered regions: interproximal spaces, gingival margins, and occlusal fissures (Saxton, 1975).
