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Properties of Dental Plaque Biofilm


The dental plaque is a complex, metabolically interconnected, highly organized microbial ecosystem. Dental plaque has an open structure due to the presence of channels and voids. It provides Protection from host defenses, desiccation etc. by production of extracellular polymers to form a functional matrix.1 Dental plaque is a microbial biofilm. Biofilms are defined as “matrix–enclosed bacterial population’s adherent to each other and/or to surface or interfaces”.2Biofilms are surface-associated communities of microorganisms embedded in an extracellular polymeric substance, which upon contact with the host may affect tissue haemostasis and result in disease.3

Periodontitis and caries are infectious diseases of the oral cavity in which oral biofilms play a causative role. The presence of micro-organisms in the oral cavity and their virulence decide the occurrence of a particular disease.4 The three main hypothesis that explain the disease occurrence in oral cavity

  • Specific plaque hypothesis(Loesche 1976): In contrast to the above, this suggests the importance of individual bacterial species within dental plaque as causative of disease5
  • Non-specific plaque hypothesis (Theilade E 1986): The bacterial dental plaque that accumulates around teeth is a relatively homogenous mass that causes periodontal disease when it accumulates to the point of over whelming the hosts defense mechanism.6
  • Ecologic plaque hypothesis (PD Marsh 1989): Based on the theory that unique local environment influences the composition of oral micro flora and any disturbance in this balance may lead to increase in pathogenic micro flora over harmless normal oral micro flora.7

Chronic periodontitis is the most common form of periodontitis causing bone loss and attachment loss. This disease has a slow progression and is more prevalent in adults.8,9 Calculus and bacterial plaque are among the etiologic factors; thus, treatment is mainly comprised of removal of supra- and sub-gingival calculus to reduce bacterial content. However, despite this treatment, progressive attachment loss continues in some patients indicating that mechanical treatment is not successful in reducing some periodontal pathogens. Therefore, antibiotic therapy is recommended to reduce the number of these resistant pathogens.8,10 Factors that may contribute to the higher drug resistance in microbial biofilms include

  1. Altered metabolism
  2. Extracellular polymeric substance
  3. Proteomic regulation
  4. Genomic regulation
  5. Persister cells
  6. Stress response

Periodontal disease is one of the most common microbial infections in adults. It is an inflammatory disease of bacterial origin that affects the tooth-supporting tissues. There are two major types of periodontal disease: gingivitis and periodontitis. Gingivitis involves a limited inflammation of the unattached gingiva, and is a relatively common and reversible condition. In contrast, periodontitis is characterized by general inflammation of the periodontal tissues, which leads to the apical migration of the junctional epithelium along the root surface and progressive destruction of the periodontal ligament and the alveolar bone (11). Periodontitis progresses in cyclical phases of exacerbation, remission and latency, a phenomenon that is closely linked to the effectiveness of the host immune response.

Experts now distinguish among generalized and localized chronic periodontitis, generalized and localized aggressive periodontitis (AP), periodontitis associated with systemic diseases, periodontitis associated with endodontic lesions and necrotizing ulcerative periodontitis (12). Of these, chronic periodontitis is the most frequently encountered in the adult population. In addition, certain conditions may be predisposing or aggravating factors for periodontitis, including accumulation of subgingival plaque, smoking and conditions associated with some immune disorder (e.g., diabetes mellitus, AIDS) (13). More than 500 microbial species have been identified in subgingival plaque, which can thus be considered to represent a complex ecological niche (14). Under the influence of local and systemic factors, some of these bacterial species in the subgingival dental biofilm constitute the primary etiologic agents of periodontal disease. Among these species, the most important are Aggregatibacter actinomycetemcomitans (A.a.), Porphyromonas gingivalis (P.g.), Tannerella forsythia (T.f.), Treponema denticola (T.d.), Fusobacterium nucleatum (F.n.), Prevotella intermedia (P.i.), Campylobacter rectus (C.r.), and Eikenella corrodens (E.c.) (15,16). Although A. actinomycetemcomitans is associated with localized aggressive periodontitis, P. gingivalis is considered the major etiologic agent of chronic periodontitis (15,17). Although the presence of periodontal pathogens is essential for the onset of periodontitis, these organisms are not sufficient for the disease to progress. In fact, the host immune response modulates progression of the disease toward destruction or healing (18). However, overproduction of certain mediators, such as interleukin-1β, tumor necrosis factor alpha and prostaglandins, lead to the chronic, persistent inflammation which is in the origin of tissue destruction (19,20). In fact, these mediators can activate one or more tissue degradation factors, notably matrix metalloproteinases, plasminogen and polymorphonuclear serine proteases, which cause bone resorption (21,22).

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Mechanical debridement of the dental biofilm and elimination of local irritating factors are the basis of initial periodontal therapies. Longitudinal studies have demonstrated the effectiveness of this approach, which is based on scaling and root planing, reinforcement of the patient oral hygiene practices and regular follow-up to eliminate new deposits (23,24). The effectiveness of this treatment is reflected by the disappearance of clinical symptoms, reduction or elimination of periodontal pathogens and regeneration of beneficial bacterial flora. Not all patients or all sites respond uniformly and favorably to conventional mechanical therapy. Given the infectious nature of periodontal disease and the limited results that can be achieved with conventional mechanical therapies, the use of antibiotics is warranted for certain forms of periodontitis.


The academic argument over the importance of a specific or non-specific bacterial etiology for periodontal diseases may never be totally resolved. However, there is little doubt that certain specific organisms are closely associated with some forms of periodontal disease (6). Unlike the majority of general infections, all the suspected periodontal pathogens are indigenous to the oral flora (25,26). Consequently, the long-term and total elimination of these organisms with antibiotics will be very difficult to achieve as immediate repopulation with the indigenous bacteria will occur when the therapy is completed (27). Nevertheless, in certain forms of periodontitis the loss of connective tissue attachment is rapid. Extremely virulent, gram – negative organisms populate the deep pockets, and bacteria can actually invade the connective tissue (28,29). Under these circumstances, antibiotics provide a useful adjunct to root planing, which by itself may not remove all subgingival deposits and certainly would not affect any invading organisms that had already penetrated the soft tissue.

Ideal properties of antibiotic

  • Unique target
  • Narrow spectrum-kills only selective pathogen, not normal microbiota
  • High therapeutic index-ratio of toxic level to therapeutic level
  • No/fewer reactions
  • Various routes of administration- IV, IM, oral
  • Good pharmacokinetics, pharmacodynamics properties
  • E.g. good absorption, good distribution to the site of infection
  • No resistance/ slower emergence of resistance

Common antibiotics for periodontitis

Antibiotic Dosage
Metronidazole 500 mg/t.i.d/8 days
Clindamycin 300 mg//t.i.d./8 days
Doxycycline/Minocycline 100-200 mg/q.d./21 days
Ciprofloxacin 500 mg/b.i.d/8 days
Azithromycin 500 mg/q.d./4-7 days
Metronidazole+ amoxicillin 250 mg/t.i.d./8 days (each drug)
Metronidazole+ ciprofloxacin 500/b.i.d./8 days (each drug)

Antibiotics are classified based on their mechanism of action, as follows:31

Agents that inhibit synthesis of bacterial cell walls (e.g. penicillins and cephalosporins);

Agents that interfere with the cell membrane of the microorganism, affecting permeability ( e.g. some antifungal agents);

Agents that inhibit protein synthesis by affecting the function of 30S or 50S ribosomal subunits (e.g. tetracyclines, macrolides and clindamycin);

Agents that block important metabolic steps of the microorganisms (e.g. sulfonamides and trimethoprim);

Agents that interfere with nucleic acid synthesis (e.g. metronidazole and quinolones).


Therapeutic success of an antimicrobial depends on the activity of the antimicrobial agent against the infecting organisms. Periodontitis is a mixed microbial infection making the choice of antibiotic regimen difficult. Certain antibiotics target specific parts of the subgingival biofilm. For example, metronidazole targets the gram-negative strict anaerobes from the red and orange Socransky complexes 40,41 such as Fusobacterium nucleatum, Tanerella forsythia, Porphyromonas gingivalis and Treponema denticola, while members of the genera Actinomyces, Streptococcus and Capnocytophaga are minimally affected by metronidazole. Metronidazole also has a limited effect on the species Aggregatibacter actinomycetemcomitans, which is a facultative anaerobe rather than a strict anaerobe. Amoxicillin has a broader spectrum lowering counts of gram negative anaerobes as well as decreasing the counts and proportions of Actinomyces species during and after antibiotic therapy.40,42 Micro-organisms can be intrinsically resistant to antimicrobials or can develop acquired resistance by emergence of resistant strains of bacteria that would otherwise be considered to be sensitive to the antimicrobial. The literature reports a wide range of antibiotics used in conjunction with non-surgical and surgical mechanical debridement for the treatment of both chronic and aggressive periodontitis. The most commonly used antibiotics include tetracyclines, penicillins (amoxicillin), metronidazole, macrolides (spiramycin, erythromycin, azithromycin), clindamycin and ciprofloxacin. The most common combination antibiotic regimen reported is metronidazole and amoxicillin combined.

What is the ideal duration, dosage and timing of the antibiotic?

The dosage and duration of the antibiotic prescribed also varies widely among studies and there is no consensus on the ideal regimen. In principle it is important to prescribe an antibiotic in sufficient dose for adequate duration. Another important clinical question is when to start the antibiotics in relation to the mechanical phase of treatment. Indirect evidence suggests that antibiotic intake should start on the day of debridement completion and debridement should be completed within a short period of time (< 1 week).

How critical is patient compliance when using adjunctive antibiotics?

The issue of patient compliance has been infrequently addressed in publications evaluating the effects of systemic antibiotics. Some studies have shown that as little as 20 per cent of patients comply with antibiotic regimens prescribed.40,44 One advantage of the antibiotic azithromycin may be that due to its pharmacologic properties and long half life, only one tablet (500 mg) per day during three consecutive days is required as opposed to one tablet three times a day for seven days with other antibiotic regimens.40,45 Compliance in terms of oral hygiene and maintenance care should also be addressed. It should be recognized that in studies where beneficial results following adjunctive antibiotics were reported, patients had received maintenance care and had good plaque control. If a patient was non-compliant with oral hygiene measures and maintenance protocols, then a favourable treatment outcome following adjunctive antibiotics was unlikely. Prescription of antibiotics is no substitute for adequate debridement, good oral hygiene and regular maintenance care.

What are the common side effects following systemic antibiotics?

Within the literature there is a general lack of reporting on the presence or absence of adverse events following the adjunctive use of systemic antibiotics. Most adverse effects, which have been reported, are minor and related to gastrointestinal problems such as diarrhoea and nausea. However, serious adverse events such as allergic and anaphlyactic reaction and pseudomembranous colitis, may occur and patients should be informed of the potential for adverse events both minor and major when prescribing systemic antibiotics. Anaphylactic responses to penicillin occur approximately once every 10 000 courses administered, with 10 per cent of these being fatal.40,46 The use of antibiotics should be carefully considered choosing agents that maximize antimicrobial activity and minimize potential drug interactions and adverse reactions. A thorough medical history should be taken prior to antibiotic prescription. An increase in microbial resistance following the use of systemic antibiotics has been evaluated in few studies. Feres et al.47 identified antibiotic-resistant species in subgingival plaque and saliva samples from chronic periodontitis patients treated by scaling and root planing followed by orally administered amoxicillin or metronidazole. There was an increase in the percentage of resistant subgingival species following antibiotic administration. However, levels returned to baseline after a relatively short period of time (90 days).

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In Spain, where systemic antibiotics are readily available over the counter without prescription and widely used in the general population, it has been shown that there was an increase in the microbial resistance patterns of oral bacteria to commonly prescribed antibiotics compared to the Netherlands where antibiotics use is more restricted.48 This underlines the importance of development of microbial resistance to antibiotics and the importance of responsible use to prevent the global spread of resistant strains of bacteria.


Antibiotics are amongst the most widely prescribed pharmaceutical agents in modern medicine. Although only a small number of these drugs have been used in the treatment of periodontal diseases, it is essential that the main contraindications for their use and their possible unwanted effects are known to the periodontist. Generally, the contraindications for use are related to the impaired metabolism and excretion of the drugs. Consequently, disease or impaired function of the hepatic or renal tracts should warrant caution in prescribing systemic antibiotics. When penicillins are prescribed it is vitally important to determine whether or not there is a history of hypersensitivity to the drug. The unwanted effects of penicillin are often mild and characterized by rashes, urticaria, joint pains, and dermatitis, although severe anaphylactic reactions have been reported and can be fatal.

David herrera 43 concluded in his systematic review that If systemic antimicrobials are indicated as part of periodontal therapy, they should be adjunctive to mechanical debridement. Lack of data prevents us from making any conclusion regarding the preferred type of adjunctive debridement (non-surgical versus surgical). Furthermore, there is not enough evidence to support the use of adjunctive systemic antimicrobials with periodontal surgery. There is no direct evidence to recommend a specific protocol for the use of adjunctive systemic antimicrobials with non-surgical mechanical debridement. However, indirect evidence suggests that antibiotic intake should start on the day of debridement completion; debridement should be completed within a short time (preferably <1 week) and with an adequate quality, because these may help to improve the results

Antimicrobial resistance in biofilms

Antimicrobial resistance can be classified into 3 groups: intrinsic, mutational and acquired resistance.31-33 Intrinsic resistance refers to an inherent resistance to an antibiotic that is a naturally occurring feature of the microorganism. Mutational resistance occurs due to a spontaneous chromosomal mutation that produces a genetically-altered bacterial population that is resistant to the drug. Mutations resulting from the change of a single nucleotide base can result in resistance, as has been well documented for aminoglycosides and for rifampin.31,32 Finally, acquired resistance refers to the horizontal acquisition from another microorganism of a genetic element that encodes antibiotic resistance. This process can occur by transduction, transformation or conjugation. Transduction is a process by which exogenous DNA is transferred from one bacterium to another by the intervention of a bacteriophage, while transformation is the process by which bacteria acquire segments of DNA that are free in the environment. In conjugation the passage of genetic material occurs by direct cell-to-cell contact, through a sex pilus or bridge. This is the most common mechanism of transferring antibiotic resistance genes. 31-33 In general, bacteria use 3 main strategies to become resistant to different antibiotics: (a) preventing the drug from reaching its target 34,35 (b) altering the target 36,37 and (c) inactivating the antibiotic 38,39

Various mechanisms to account for the increased resistances to antimicrobials in biofilms have been postulated. Several of these mechanisms seem to occur in conjunction with the final stages of biofilm maturation .49,50 Reduced penetration into the biofilm may result in antibiotic inactivation because of secretion of certain enzymes, such as β-lactamases, or binding of the agent by the exopolysaccharide matrix. The exopolysaccharide could inhibit antimicrobial penetration by either binding the antimicrobial 49, 51,52 or serving as a protective coating that prevents or delays diffusion through the biofilm .53,54 The complex heterogeneity within biofilms is evidenced by studies analyzing different microenvironments throughout the biofilm that differ in metabolic activity55, pH , and oxygen distribution56. The ‘biofilm phenotype’ is a collective term used to describe a biologically programmed response to growth on a surface that involves specific physiologies and patterns of protein and gene expression that are quite different from those of planktonic cells 57,58 and have been linked to aspects of antimicrobial resistance (57,59,60). Increased resistance to antimicrobials is likely a combination of all of these mechanisms and may involve many, if not all, of these factors working together in unison as the biofilm matures.


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