Introduction

Throughout life, all the interface surfaces of the body are exposed to colonization by a wide range of microorganisms. In general, the establishing microbial population size is restricted by the microenvironmental conditions and the organisms live in harmony with the host. In the oral cavity, however, teeth provide a permanent, moist, non-shedding surface on which extensive bacterial deposits can develop. The accumulation and metabolic activity of these deposits, i.e., bacterial plaque, is considered the primary cause of dental caries, gingivitis, periodontitis and stomatitis. Understanding the nature of plaque, how it forms and matures assists in the treatment and control of these diseases. Knowing that dental plaque is a biofilm adds to understanding its nature. Over the past few years, biofilm has been extensively studied due to its importance in both general environmental management and in medicine, both of which have major implications for human and animal health. Elucidation of biofilm dynamics is pertinent to both veterinary and human dentistry, improving our understanding of the nature of plaque.^1,2

It has been estimated that more than 99% of all the planet's bacteria live as adherent biofilm under conditions very different from those provided in laboratory environments. This is not surprising as living in a biofilm is highly advantageous to the organisms present. They are protected from extreme conditions by the external "slime" matrix they secrete and they can cooperate to make maximal use of available resources. This protection and cooperation confers increased resistance to the effects of antimicrobial agents, hence the poor response to antibiotics of infections involving biofilms. It is therefore encouraging that researchers are now cultivating dental plaque organism in biofilms in constant depth film fermenters as this method should more accurately predict clinical response of plaque to antimicrobial drugs.³

Formation of Biofilms

The first bacteria to attach to a smooth surface attach either via their glycocalyx mucopolysaccharides or by electrostatic forces. Once bacteria stick, they also begin producing homoserine lactone, which acts as a communication signal stimulating other free-roving bacteria to produce sigma factors that activate genes that stimulate them to join the community.⁴ Next, new arrivals of varied genus and species of bacteria, many of which produce fibrillar polysaccharide exopolymers, form a thick slime layer. New sigma factors also cause the expression of genes needed for communal living, and the suppression of other genes, which were needed during their prior planktonic existence. They are now phenotypically distinct from their planctonic counterparts.⁵

The biofilm bacteria act very much like tissue cells of multicellular organisms, in which the wider needs of the community take precedence over those of the individual. Its neighbours as nutrients use toxic wastes produced by one species. Non-useful wastes are transported away by circulating fluids. The cells' biochemical resources are pooled, allowing the community to use varied enzyme systems to break down potential food supplies that are otherwise unavailable to the individual organisms. Communication, cell specialization, and a basic circulatory system are all present in biofilms. Biofilms even have their own predators, internal and external parasites in the form of bacteriophagic amoeba and nematodes. Many bacteria can only be grown in culture when others are included to provide required nutrition's. This characteristic is similar to an organ that cannot survive outside its supporting organism. Biofilm communities are units of existence, activity, ecology, proliferation, survival and evolution⁶.

Within a biofilm, the surrounding slime matrix effectively protects its resident bacteria from antimicrobial drugs. The exact nature of this protection remains unclear. The matrix is difficult to dissolve, and resident bacteria can develop resistance to antimicrobial drugs by producing more polysaccharides to form a thicker layer. The matrix itself may mechanically protect bacteria against desiccation, bacteriophages, amoeboid predators, protozoa, and immune system clearance. The mechanism of hindrance of antimicrobial drugs remains unclear, but may involve a neutralizing ability rather than or in addition to inhibition of diffusion. The bacteria themselves, although genetically identical to their planctonic counterparts, make use of a very different biochemistry due to their newly expressed set of genes. As many as 30-40% of bacterial cell wall proteins differ, sometimes eliminating antimicrobial target sites. Their lower metabolic activity and slower absorption of environmental factors increase the chances they can acquire enhanced resistance to chemical antimicrobial compounds. Biocide rinses, including powerful chlorine-based disinfectants and bleach solutions, are largely ineffective and could even select for resistant organisms. A disinfectant may need to be 1000 to 500,000 times more concentrated to kill the bacteria in biofilms than in a monoculture, and bacteria in biofilms can be up to 1500 times more resistant to antimicrobial drugs than the same bacteria in a single colony. However, simple mechanically disruption of the biofilm, as occurs if their substrate is wiped with a brush or other abrasive material, can easily disperse the organisms and make them susceptible.⁷

Significance of Biofilm Behaviour in Plaque Control

Dental pellicle, a thin clear layer of glycoprotein deposited from saliva and gingival crevicular fluid, adheres to mineralised tooth surfaces within minutes after cleaning. This facilitates the attachment by the pioneering species of bacteria that start the biofilm (plaque) formation. These first bacteria are typically gram-positive organisms, often Actinomyces spp. and streptococci. As the biofilms thickens and matures, the community attracts new residents and provides an environment that is conducive to the growth of anaerobes and gram-negative organisms. As with other biofilms, mechanical disruption remains the best method of removing plaque. Although once or twice daily tooth brushing of pets' teeth by their owners may be ideal, this may sometimes be difficult or impractical. A method that enhances the abrasive dental self-cleaning that occurs when an animal chews fibrous foods can be very helpful. Various dietary factors can influence the accumulation of plaque and the calcified residue of dead plaque, i.e., calculus. Food particle size, shape density, moisture level, fibre content and source can all have an influence. Abrasive diets are more successful in some individuals and certain teeth than others due to variability in occlusion, tooth crowding, and eating habits.⁸

The realization that plaque is a biofilm helps us understand its formation, development, removal, and control. Since plaque bacteria are protected physically and metabolically from disinfectants and antibiotics, mechanical removal represents the best method of control.

Microbial Destructive Processes of Periodontium

Periodontal disease is initiated and sustained by factors produced by the subgingival microbiota. Microbial biofilm accumulation on the surface of teeth adjacent to the gingival tissues brings the oral sulcular and junctional epithelial cells into contact with the waste products, enzymes, antigens, toxins and surface components of colonizing bacteria. Some of these substances can directly injure host cells and tissues. Other microbial constituents may activate inflammatory or cellular and humoral immune systems, which secondarily damage the periodontium. It is the latter pathway, which accounts for most periodontal injury.^5,6

Plaque microorganisms may damage cellular and structural components of the periodontium via release of their proteolytic and noxious waste products. As well as the formation of noxious substances by the microbiota of the gingival pocket, microbial invasion of soft tissues should be considered. Invasion of the dentogingival epithelium by spirochetes has been conclusively documented in acute necrotizing ulcerative gingivitis.³

Microorganisms produce a large variety of soluble enzymes in order to digest extracellularly host proteins and other molecules and thereby obtain nutrients for growth. They also release numerous metabolic products, such as ammonia, indole, hydrogen sulfide and butyric acid. Among the enzymes released by bacteria are proteases capable of digesting collagen, elastin, fibronectin, fibrin and various other components of the intercellular matrix of epithelial and connective tissues.

The effect of many structural, enzymatic and waste products is to stimulate, probably noxiously, host cell cytokine production. The cytokines thus produced are predominantly pro-inflammatory and possess multiple effects, which serve to enhance the inflammatory response. They also enhance matrix metalloproteinase activity as well as recruiting leucocytes to the area.^4,5

Microbes are capable of producing a variety of substances, which either directly or indirectly harm the host. The main detrimental effect, however, may be the host's own immune response to the foreign microbial antigens.¹

References

1.  Lang NP, Mombelli A, Attstrom R. Dental plaque and calculus. In: Clinical periodontology and implant dentistry. Lindhe J, Karring T, Lang NP eds. 3rd ed, Munksgaard, 2002, pp. 102-34.

2.  Jensen I, Logan E et al. Reduction in accumulation of plaque, stain, and calculus in dogs by dietary means. J Vet Dent 1995; 12: 161-3.

3.  Williams JF, Molinari JA, Andrews N. Microbial contamination of dental unit waterlines: Origins and characteristics. Compend Cont Educ Dent 1996;17:538.

4.  Shearer BJ. Biofilm and the dental office. J Am Dent Assoc 1996;127: 181-9.

5.  Costerton JW, Lewandowski Z, Caldwell DE et al. Microbial biofilms. Ann Rev Microbiol 1995;49: 711-45.

6.  Gorrel C, Rawlings JM. The role of tooth brushing and diet in the maintenance of periodontal health in dogs. J Vet Dent 1996;13: 139-43.

7.  Greenfield JI, Sampath L et al. Decreased bacterial adherence and biofilm formation on chlorhexidine and silver sulfadiazine- impregnated central venous catheters implanted in swine. Crit Care Med 1995;23: 894-900.

8.  Rateitschak KH. Color atlas of dental medicine. Periodonology.New York, Thieme, 1989.