【正文】 bacteria, with a few filaments During the course of the 2nd day the bacterial deposits are colonized by multiple filamentous anisms with a perpendicular orientation to the surface Distinct morphological changes may be recorded on the surface of the microbiota when paring the bacterial deposits after 24 (Fig. 512) and 48 h(). Whereas the 24hold bacterial deposit prises a mass of coccoid bacteria from which a few filaments extend, the 48hold microbiota is almost entirely dominated by filamentous anisms28 Initial colonization of root cementum occurs in principle as outline for enamel surface, but process more rapidly Because of the irregular surface to topography of root surfaces, colonization does not take place according to a particular pattern 48hold bacterial deposits on root cementum and enamel surfaces from the same individual. Note that the microbial deposits are thicker and more densely packed on root cementum mature dental plaque During the early days, plaque growth occurs mainly as a result of cell division, but continuous adsorption of single microanisms from saliva also contributes to the expansion of the bacterial deposit The corn cob are posed of a central filament coated with spherical anisms, and appear to have a direct interspecies relationship machinated by surface fibrils As the microbiota grows older, characteristic structural changes are noted deep to the surface. The most striking change is the formation of an inner layer of densely packed grampositive pleomorphic bacteria next to the tooth surface Ultrastructure of 2 weekold dental plaque from 3 individuals with different colonization patterns. Note that , in addition to differences in thickness, the outer parts of the deposits vary in position and structure. Relative proportions of selected anisms in developing supragingival plaque on the labial surface of incisors. Plaque samples were obtained 1,3,5,7, and 9 days after thorough prophylaxis. Chemical position of plaque plaque contain about 80% water 20% solids protein 40% to 50% carbohydrates account for 13% to 18% lipids account for 10% to 14% other Direct smears Count of 2 1011 microanisms/g In centrifuge 3 1011 anisms/g 2/3 of plaque must consist of bacteria that means 70% the area is posed of microanisms and 30% of intercellular material (matrix) Carbohydrates of plaque ? Glucose is the main carbohydrate found in hydrolyzed extracts of plaque ? arabinose ? ribose ? galactose ? fucose Much of the carbohydrate exists in the form of extracellular polymers ? glucans (homopolymers of glucose) ? fructans (homopolymers of fructose ) ? Heteropolysaccharides Plaque microanisms forming extracellular polysaccharides Glucans Fructans Heteropolysaccharides Streptococcus sanguis Actinomyces viscosus Actinomyces viscosus Streptococcus mutans Streptococcus mutans Lactobacillus buclneri Streptococcus salivarius Streptococcus salivarius lactobacillus cellobiosus Streptococcus mitior Lactobacillus casei Lactobacillus casei Lactobacillus acidophilus Neisseria sp. Cariogenic bacteria ? Rats raised under bacteriafree conditions did not develop caries ? In 1960 Keyes performed a series of experiments that established dental caries as infectious disease Diagram of caries formation due to passage of bacteria from rat dams to their pups, elimination of the disease by treatment of animals with antibiotics, and the occurrence of caries following various types of inoculations. The infection was reintroduced by contact with infected animals, inoculation with isolated strains of “cariesinducing”streptococci, and transfer of bacterial plaque or feces. DR. ROBERT KOCH The agent must be present in every case of the disease。 The agent must be isolated from the host and grown in a lab dish。 The disease must be reproduced when a pure culture of the agent is inoculated into a healthy susceptible host。 The same agent must be recovered again from the experimentally infected host Koch’s postulates Specific bacteria and caries Lactobacilli these bacteria were thought to play an important role in caries etiology when it was first found that early carious plaque contained elevated levels of lactobacilli pared with plaque from noncarious surfaces Lactobacill Caries free group: 100/ml Caries active group: 100000/ml In a group of cariesfree children the mean number of lactobacilli per 1ml of saliva was in the hundreds, while in caries active children the mean number per1ml was in the range of 100,000. The early observation on changes in lactobacillus levels in the oral cavity led many dental scientists to consider these bacteria as the specific microbial etiological factor in human caries For a number of reasons, the lactobacilli failed to qualify as an exclusive etiological agent in human caries formation: The affinity of lactobacilli for tooth surface is low, % High levels of lactobacilli tend to exist after caries has developed, caries can frequently be initiated in the absence of detectable lactobacilli. The lactobacilli are secondary invaders, they may contribute to the progression of decay due to their acidogenic and aciduric properties Conclusion Streptococci In vitro studies with the oral streptococci have demonstrated many features that support their role as cariogenic agents ? Relatively rapid generation ? To produce large quantities of acid ? Aciduric ? To utilize a wide range of fermentable carbohydrates ? To produce extracellula