PERIODONTAL MICROBIOLOGY

TOP ABSTRACT MICROBIOLOGY OVERVIEW PERIODONTAL MICROBIOLOGY BIOFILM DENTAL PLAQUE BIOFILM DENTAL PLAQUE BIOFILM MANAGEMENT CONCLUSIONS REFERENCES

 
From the mid-1960s through the 1970s, the nature of dental plaque became a significant focus for dental scientists and the dental research community. Emphasis was placed on factors contributing to the diversity of microbial ecosystems, including pH, oxidation-reduction potential and nutritional requirements. In 1976, Loesche³ recognized the importance of the plaque ecosystem and proposed both a nonspecific and a specific plaque hypothesis for oral disease progression. The non-specific plaque hypothesis maintained that periodontal disease resulted from an "elaboration of noxious products by the entire plaque flora." Large accumulations of plaque would produce large amounts of noxious products, which would essentially overwhelm the host defense and cause periodontal disease. Thus, all the microorganisms within plaque were viewed as contributing to the development of periodontal disease, and identifying a single microorganism was not important. Oral hygiene measures that seek to remove as much of the total plaque mass as possible became paramount for the maintenance of oral health.

In contrast, the "specific plaque hypothesis" stated that only certain organisms within the plaque complex were pathogenic, and pathogenicity depended on the overgrowth or selection of more virulent microorganisms. This hypothesis postulated that specific pathogens result in periodontal disease because these organisms are associated with cellular challenges that result from the host’s inflammatory and immune responses.

After recognizing that early plaque colonizers are predominantly gram-positive and later organisms are predominantly gram-negative, Socransky and colleagues^4,5 defined the organisms within the subgingival microbiota, placing them in five "complexes." This concept emphasized that microorganisms create their own habitat, interact with each other and are implicated in disease severity^4,5 (Figure 1). The organisms in the plaque reflected the environmental conditions. The most virulent combinations were strict anaerobes, and the less virulent microorganisms thrived in a relatively low-oxygen (micro-aerophilic) environment.

View larger version (29K): [in this window] [in a new window]   Figure 1. Relationship of species within a microbial complex (domain) and between the microbial complex of the subgingival microbiota. Adapted with permission of Blackwell Scientific from Socransky and colleagues.5

	BIOFILM

TOP ABSTRACT MICROBIOLOGY OVERVIEW PERIODONTAL MICROBIOLOGY BIOFILM DENTAL PLAQUE BIOFILM DENTAL PLAQUE BIOFILM MANAGEMENT CONCLUSIONS REFERENCES

 
In recent years, dental plaque has been evaluated and discussed as a biofilm. In 2002, Donlan and Costerton⁷ offered the most salient description of a biofilm. They stated that a biofilm is "a microbially derived sessile community characterized by cells that are irreversibly attached to a substratum or interface or to each other, are embedded in a matrix of extracellular polymeric substances that they have produced, and exhibit an altered phenotype with respect to growth rate and gene transcription."^7(p.168) In fact, a biofilm is an accumulation of microbial cells within a matrix, optimizing the use of the available nutritional resources.

The preferred method of growth of any microbial species is in the attached or sessile phenotype. Nevertheless, every microorganism identified as a human pathogen has the potential to exist in either a planktonic phenotype or a biofilm.⁸ However, microorganisms have a propensity to exist as an attached multispecies biofilm. Biofilms are found throughout the body and in the environment and can be found lining dental unit waterlines, catheters and prosthetic heart valves.

Evidence is accumulating that the aggregated organisms in biofilms are not merely passive neighbors, but rather are involved in a wide range of physical, metabolic and molecular interactions.

A biofilm is organized to maximize energy, spatial arrangements and movement of nutrients and byproducts. Physical composition, degree of organization and multi-species organization characterize the four stages of biofilm growth (Figure 2). Whether the organism is planktonic or exists as part of a biofilm, there are four similar phases in the lifecycle. Stage I is the quiescent or least metabolically active state. Conversion or transformation from Stage I to Stage II requires significant genetic up-regulation. Stage III involves maturity of the biomass, and total organism concentration can approach 10¹¹ or 10¹² colony-forming units per milliliter. At this phase, new antigens may be expressed, genetic exchange enhanced and membrane transport maximized. Stage IV (apoptosis or death) signals detachment, eroding or sloughing from the biofilm.

View larger version (58K): [in this window] [in a new window]   Figure 2. Four stages of dental plaque biofilm growth: Stage I attachment (lag [not inert, but metabolically reduced]), Stage II growth (log [exponential growth]), Stage III maturity (stationary) and Stage IV dispersal (death).

Evidence suggests that gene expression may be altered within a biofilm, which may be in response to the specific surface on which the bacterium has settled.⁹ Boles and colleagues¹⁰ proposed that the environmental heterogeneity that develops in biofilms can accelerate diversity in bacterial populations as a form of "biological insurance" in which cells are better prepared to cope with adverse conditions. Communication among bacteria within the biofilm usually is carried out by bacterial products that are able to diffuse away from one cell and enter another.⁹ Gram-positive bacteria generally communicate via small diffusible peptides, while many gram-negative bacteria secrete acyl homoserine lactones to communicate.

	DENTAL PLAQUE BIOFILM

TOP ABSTRACT MICROBIOLOGY OVERVIEW PERIODONTAL MICROBIOLOGY BIOFILM DENTAL PLAQUE BIOFILM DENTAL PLAQUE BIOFILM MANAGEMENT CONCLUSIONS REFERENCES

 
Dental plaque incorporates all of the features of biofilm architecture and microbial community interaction, but it is different in that it has more than 700 contributing oral microbial species in the oral cavity and a distinct method of conditioning the tooth surface.¹¹ Only 20 to 25 percent of the oral environment is tooth surface,¹² and mucosal surfaces are important contributors to periodontal microbial biofilms.

For tooth surfaces, pellicle formation is the preconditioning stage that defines the reversible-irreversible attachment of the colonizing bacteria. Attachment is defined as a slime layer forming around the colonizing pioneer bacteria, which consist mainly of gram-positive cocci and rods that divide and form microcolonies. If this early supragingival plaque is unregulated owing to the absence of effective oral hygiene, the bacterial composition can mature into a more complex flora in a three-stage scenario. The first stage is predominantly gram-positive cocci and is represented by the streptococcal species, the second stage is cross-linking via fusobacterium species, and the third stage is predominantly gram-negative organisms. Mature oral biofilms are robust and resilient, acting as reservoirs of antibiotic resistance and virulence in deep periodontal pockets. Their uncontrolled growth eventually may lead to periodontal disease.

A defining characteristic of the multispecies dental plaque biofilm, as well as other microbial biofilms, is communication either from cell to cell or from microcommunity to macrocommunity. This dynamic communication, called "quorum sensing," and regulation provide a mechanism for bacteria to monitor each other’s presence and to modulate gene expression in response to changes in population density.¹³

Determining the pathogenicity of dental plaque biofilm. Dental biofilm pathogenicity in the oral cavity is magnified by two biofilm characteristics: increased antibiotic resistance and the inability of the community to be phagocytized by host inflammatory cells.

Dental biofilm pathogenicity in the oral cavity is magnified by two biofilm characteristics: increased antibiotic resistance and the inability of the community to be phagocytized by host inflammatory cells.

Three mechanisms can account for increased antibiotic resistance. One is the failure of the antibiotic agent to penetrate the extracellular matrix into the full depth of the biofilm.¹² The second mechanism suggests that at least some of the cells in a biofilm experience nutrient limitation and, therefore, exist in a slow-growing or starved state. Slow-growing or nongrowing cells are not highly susceptible to antimicrobial agents.¹⁴ Finally, individual antibiotic resistance increases as a result of genetic changes such as mutations or gene transfer and can result in a loss of susceptibility among the multispecies microcommunities.

	DENTAL PLAQUE BIOFILM MANAGEMENT

TOP ABSTRACT MICROBIOLOGY OVERVIEW PERIODONTAL MICROBIOLOGY BIOFILM DENTAL PLAQUE BIOFILM DENTAL PLAQUE BIOFILM MANAGEMENT CONCLUSIONS REFERENCES

 
The treatment of infectious diseases has been driven by clinicians’ recognition of Koch’s postulates. Dentistry’s understanding of the predominant phenotypes associated with the dental biofilm has necessitated a shift in the treatment paradigm. The shift in the treatment paradigm incorporates the ecological plaque hypothesis, which states that disease prevention should not only focus on the inhibition of putative pathogens, but also on interference with environmental factors that drive selection and enrichment for these bacteria as reported by Marsh.¹ Prevention via maintenance of a normal health-associated ecosystem is key.

The key characteristics of biofilm that could be targets for pathogen management include its behavior as an adhesive mass with viscoelastic properties, its activity as a coordinated multispecies community in which cells communicate via small molecules, and its inflammatory disease potential.

In the pathogen management process, first "focused" or "targeted" energy is delivered to the biofilm via regular meticulous toothbrushing and flossing or via professional sonication and scaling and root planing to overcome viscoelasticity and reduce the pathogenic burden. Second, antimicrobial therapies, including using mouthrinses, can interfere with the shift from Stage I biofilm to Stage II biofilm by application at key intervals to impede the attachment and maturation of the biofilm. Third, use of inflammatory modulators such as low-dose doxycycline (not acting as an antibiotic) may need to be considered to address local tissue inflammation. The underlying risk factors for the periodontal disease also should be identified and addressed.

	CONCLUSIONS

TOP ABSTRACT MICROBIOLOGY OVERVIEW PERIODONTAL MICROBIOLOGY BIOFILM DENTAL PLAQUE BIOFILM DENTAL PLAQUE BIOFILM MANAGEMENT CONCLUSIONS REFERENCES

 
Dental plaque biofilm cannot be eliminated. However, the pathogenic nature of the dental plaque biofilm can be reduced by reducing the bioburden (total microbial load and different pathogenic isolates within that dental plaque biofilm) and maintaining a normal flora with appropriate oral hygiene methods that include daily brushing, flossing and rinsing with antimicrobial mouthrinses. This can result in the prevention or management of the associated sequelae, including the development of periodontal diseases and possibly the impact of periodontal diseases on specific systemic disorders.¹⁵