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Lec. 1 Microbiological Aspect of Dental Caries

الكلية كلية طب الاسنان     القسم التقويم والاطفال وطب الاسنان الوقائي     المرحلة 5
أستاذ المادة حسن فليح فرحان السلطاني       6/7/2011 11:44:04 AM

Lec.1                                            Preventive dentistry                                     د.حسن الوطيفي       

Microbiological Aspect of Dental Caries
Introduction: 
Cariology:
the science that study the dental caries. It study the relation between dental plague (MO.) and dental caries. Current concept of the etiology of caries have been derived from both laboratory animal models, from clinical studies and epidemiological surveys in humans, most of these studies suggested that dental caries is a disease of bacterial origin these results was, obtained from many evidence which is summarized below.
Evidences of Bacterial Role in Caries Etiology:
There are many opinions as to how and which microorganisms produce caries lesions, it is uniformly agreed that caries cannot occur without microorganisms.
The evidences, implicating microorganisms in the etiology of caries are summarized below:
1- Germ-free animals do not develop caries.
2- Antibiotics fed to animals are effective in reducing the incidence and severity of caries.
3- Totally unerupted and unexposed teeth do not develop caries, yet, when exposed to oral environment and micro flora can become carious.
4- Oral bacteria can dematerialize enamel and dentin in vitro and produce caries like lesions.
5- Microorganisms have been histologically demonstrating invading carious enamel and dentin. They can be isolated and cultured from carious lesions. 

Orland et al (1955) established principles that had been debated for more than a century, namely, that dental caries is bacterial infection. These studies demonstrated that germ-fee rats on a highly cariogenic diet containing sucrose did not develop caries. When the gnotobiotic rats on the same diet were infected with combination of an enterococcus and a proteolytic bacillus or an enterococcus and a pleomophic bacterium, caries developed.


Localization of the Oral Flora Related to Caries:
Different organisms were display some selectivity as to which tooth surface they attack and suggest that there are at least four types of processes involved:

1- Pit and Fissure Caries:
This is the most common caries lesion found in modern humans. Many organisms can colonize in fissures, which provide a mechanical retention for the bacteria. Gnotobiotic rats monoinfected with either S. mutans, S. salivarius, S sanguis, streptococcal species, lacto-acidophilus, A. viscous, A naeslundii or Actinomyces israelii develop fissure lesions, A wide variety of microbes may be able to initiate pit and fissure caries.
2- Smooth Surface Caries:
A limited number of organisms have proved able to colonize smooth surfaces in large enough numbers to cause decay in test animals. S. mutans is very significant in this respect.
3- Root Caries:
In rodents, gram-positive filamentous rods, including Actinomyces species have been associated with this type of lesions. S. sanguis, besides causing enamel caries, may at time also cause root caries. In humans the mutants group of streptococci has been associated with surfaces with root caries when compared to intact root surfaces.
4- Deep Dentinal Caries:                                                                                                         
Because the environment in deep dentinal lesions is different from that at other locations it is not unexpected that the flora have is also different. The predominant organisms are Lactobacillus, which counts for approximately one third of all bacteria. Frequently isolated grrm-positive anaerobic rods and filaments are arachnia, Bifidobacterium, Eubacteruim  and propioni bacterium. Actinomyces, Rothia and Bacillus also occurs in the forefront of deep dentinal lesion. The incidence of gram Positive facultative cocci is low.

Virulence of Cariogenic Microorganisms:
virulence: Is generally defined as the ability of an organisms to overcome the defense mechanisms of the host and to cause a damage to host tissues. 
A long number laboratory investigations have been aimed of discovery the factors which determine virulence, or carcinogenicity of oral microorganisrns, since S.mutans has been the most thoroughly studied of these organisms most of the discussion to follow is based on findings with this organism. However reasonable limits the basic concepts involved are applicable to other cariogenic organisms as well.

Intrinsic Microbial  Factors in Virulence
The following characteristics have been found to be determinants of cariogenicity in S. mutans:

1-acid producing ability:
Ultimately caries is initiated by acid dernineralizadtion of the surface enamel. Therefore acid production is an essential prerequisite for a cariogenic organism. In support of this concept it has been found that mutants of cariogenic parent strains of S. mutans which are impaired in their ability to produce lactic acid from sugars are also less cariogenic when tested in gnotobiotic rats.

2- Aciduric potential:
The continued production of acid by microorganisms can eventually result in their death due to dropping PH below live sustaining levels. As will be shown later mutants are more tolerant of acids than most streptococci and it has the ability to survive in these acidic condition and become cariogenic.


3- formation and utilization of storage polysaccharides:
Most cariogenic organisms have the ability, when excess amounts of sugar are available to convert a portion to intercellular storage polysaccharides (ISP), Later when exogenous carbohydrates are exhausted these organisms metabolize these reserve for energy requiring reaction with the production of lactic acids. This has the effect of prolonging the concentration of acid in plague and there by prolong the cariogenic challenge to the tooth. Laboratory studies have shown that mutant strains of S. mutans which lack this capability are less cariogenic in animals than their parent strains which produce abundant amount of (ISP). Although are some naturally occurring strains of S. mutans which are still highly cariogenic in animals despite the inability to form large amount of ISP.


4- Formation of Insoluble Extra cellular Glucans from Sucrose:
The formation of insoluble extra cellular adherent polysaccharides from sucrose is a common characteristic of S. mutans and cariognic strains of S. sanguis, S. mitiar and S. Salivarius.
1. These polysaccharides play an important role in the adherence and build up of these organisms on the tooth and they form a protective matrix for other organisms as well. 
2. They may also serve, to concentrate for the plaque organisms required growth substances present in suboptimal concentrations in the saliva.
3.  Finally the may act as a barrier to the diffusion of acids from the plague and the ingress of salivary buffers, thus prolonging the concentration of acids in proximity to the tooth surfaces.  
The importance of these polysaccharides in plaque formation and carcinogenicity of S. mutans has been demonstrated by the use of mutants species of S. mutans which are deficient in their production of (ECP), Such mutants are less able to colonize the smooth surface of the teeth of animals and hence produce less smooth surface caries However their ability to cause caries in pits and fissures is generally due to the retentive and protected nature of the latter sites.
5- Formation of protease enzyme: this enzyme destroyed the secretary IgA and impaired the oral specific immune system.



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