MICRO BIOLOGY OF PERIODONTAL DISEASE- A REVIEW

: Periodontal diseases are inflammatory and destructive diseases of the dentogingival complex associated with specific periodontal pathogens inhabiting periodontal pockets. Periodontal diseases lead to damage of the periodontal tissues supporting the teeth (bone and connective tissue) and affect the quality of life of the affected individuals: poor alimentation, tooth loss, social and financial problems. Although it is generally considered that the disease has multifactorial etiology, data show that some specific Gram-negative microorganisms in the subgingival plaque biofilm play a major role in the initiation and progression of periodontitis. Porphyromonas gingivalis, Treponema denticola and Tannerella forsythia form a consortium in the subgingival biofilm and are regarded as the principal periodontopathogenic bacteria. Other microorganisms that have been implicated as predominant species in the disease process are: migros, Eikenella corrodens. In periodontitis, the initiation of the disease is the colonization of the tissues by these pathogenic species. The next step is bacterial invasion or invasion by pathogenic products into the periodontal tissues, interactions of bacteria or their substances with host cells, and this directly/indirectly causes degradation of the periodontium, resulting in tissue destruction.


Introduction:
The reserch on the microbiology of periodontal health and diseases has been the focus of intense investigation for several decades. Microbial biofilm in the gingival sulcus around teeth is the most important cause of the periodontal diseases. It is a structured community of bacterial cells enclosed in a self produced polyemeric matrix. [1] Although more than 700 different species are present in subgingival microbiota [2][3][4] only a few of these species are actually involved in the initiation and progression of the periodontal disease process. Moreover, research on the etiopathogenesis of periodontal diseases has suggested the role of environmental behavioral [6][7], and [8] risk factors in periodontal disease progression but most, if not all forms of periodontitis, should be viewed primarily as infectious diseases. Many technological advances have occurred in molecular techniques in the last few decades which have provided us the capability of performing high-throughput analysis of a large number of samples, circumventing some of the limitation of culturing techniques.

Historical Aspect
Dental plaque the period from 1880 to 1930 is known as 'Golden age of microbiology'[9]. Wet mounts or stained smear. Scientists identified four different groups of potential etiologic agents (amoebae,siprochetes,fusiforms and streptococci) for various periodontal diseses using the techniques available at that time (wet mounts or stained smear microscopy). Researchers suggested the specific plaque hypothesis based on these findings. However with the advancements in bacterial identification techniques, many other bacterial species were identified in dental plaque derived from periodontitis patients. Studies conducted between 1930 and 1970 failed to identified any specific microorganism as the etiologic agent of periodontal diseases which leads to the proposal of non-specific plaque hypothesis, according to which gross accumulation of dental plaque would be necessary and sufficient to cause periodontitis.
Later on, with the advancement in the field of microbiology, immunology and molecular biology, numerous studies concluded a pututative pathogenic role of many bacteria, mainly gram negative species in the etiopathogenesis of the diseases.
Virulence Factors produced by these micro organisms have been identified and their role in the periodontal destruction is well established.

BACTERIAL TAXONOMY
Systemic classification and categorization of organisms into ordered groups is called taxonomy. A working knowledge of bacterial taxonomy is essential for understanding various bacterial species involved in the periodontal disease progression. Although higher organisms are classified according in their evolutionary pathways (i.e. phylogenetically), bacteria cannot be similarly categorized because of insufficiency in their morphological features.
 Periodontal diseases are caused by a variety of micro-organisms that reside at or below the gingival margin in the form of plaque biofilm. Because there is a complex microbiota involved in the formation of plaque biofilm, first we need to distinguish the pathogenic bacterial species from the host compatible species.
 Secondly, we need to devise treatment methods that can decrease the pathogens while maintaining hostcompatible species.  To establish an association of an organism with disease pathogenesis, three rules for experimental proof for the pathogenicity of an organism were presented in 1883 by the German bacteriologist, Robert Koch, and a fourth rule was appended by smith (1905)[10]. These rules of proof are often referred to as Koch's Postulates.
These are, 1. The suspected casual organism must be constantly associated with the disease. 2. The suspected casual organism must be isolated from an infected host and grown in pure culture. 3. When a healthy susceptible host is included with the pathogen from pure culture, symptoms of the original disease must develop. 1. The organism must be found in relatively high numbers in proximity to the periodontal lesion. 2. The organism must be absent, or present in much smaller numbers in periodontally healthy subjects or in subjects with other froms of periodontal disease. 3. The organism must have high level of serum, salivary and gingival crevicular fluid antibody developed against it is periodontally diseased subjects. 4. The organism must be found to produce virulence factors in vitro, which can be correlated with clinical histopathology. 5. The organism must mimic similar pathogenic properties in an appropriate animal model. 6. Clinical improvement following treatment must eliminate the putative pathogen from the periodontal lesion.

SUBGINGIVAL MICROBIAL COMPLEXES
In a landmark study, Socransky et al. (1998) examined over 13000 subgingival plaque samples from 185 adult subjects and used DNA hybridization methodology and community coordination techniques to demonstrate the presence of specific microbial groups within the dental plaque.

Conclusion
The results of current genome study projects of several periodontopathogens will provide detailed information about the etiology of periodontal diseases, and will likely show new possibilities for the treatment and prevention of periodontal diseases.In the near future, it is expected that the correlation between biofilm maturation and activation of specific genes of the inner microorganisms will be clarified at the molecular level.