Role of Specific Cariogenic Microflora
Introduction
Microorganisms implicated in the etiology of dental caries must be acidogenic as well as aciduric. To initiate carious lesions in enamel, the microorganisms must also be able to colonize the tooth surface and survive in competition with less harmful species, forming biofilms¾the so-called dental plaque. As early as 1960, Fitzgerald and Keyes showed that certain microorganisms isolated from human dental plaque, when inoculated in germ-free rodents on a high-sucrose diet, resulted in the spread of rampant caries. Therefore, dental caries should be regarded as an infectious, transmissible disease.
There is abundant support for the so-called specific plaque hypothesis, introduced by Loesche (1982, 1986), which proposes that some specific species of the plaque flora be regarded as major pathogens in the etiology of dental caries. Included in the major pathogens are those bacteria associated with caries in humans and also able to induce carious lesions in experimental animals. The most important are the mutans streptococci: there are seven species, of which two, S mutans and S sobrinus, are closely associated with caries in humans. The remainder are found in animals or, if present in humans, are not highly cariogenic. The relationship of S sobrinus to caries in humans is not as well understood as that between S mutans and caries, and only recent studies identify the species separately.
The second genus closely associated with caries is Lactobacillus, commonly isolated from carious dentin (Edwardsson, 1974), thought to be its main habitat in the mouth.
Compared to the extensive research into identification of the mutans streptococci, much less attention has been paid to speciating lactobacilli isolated from carious lesions.
Also associated with the etiology of dental caries, but considered to be less cariogenic than S mutans, S sobrinus, and Lactobacillus, are Actinomyces odontologica, Actinomyces naeslundii, and some other species of MS. To clarify the role of S mutans in the etiology of dental caries, many cross-sectional and longitudinal human studies have been conducted over the past two to three decades, particularly by the Krasse and Bratthall research groups in Sweden.
