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Dental Plaque Biofilms

By Jill S. Nield-Gehrig, RDH, MA*

Bacteria are the primary etiologic agents in periodontal disease. More than 500 bacterialstrains may be found in dental plaque.1 These bacteria have evolved to survive in theenvironment of the tooth surface, gingival epithelium, and oral cavity. Recent technicaladvances have led to the recognition that dental plaque is a biofilm. Changes in thinkingabout the structure of dental plaque have improved our understanding of why peri-odontitis is so difficult to treat and will affect the strategies used to prevent and controlperiodontitis in the future.

Bacterial LifestylesBacteria may be free-floating or attached to a surface. Recent advances in researchtechnology have allowed researchers to study bacteria in their natural environment.

These studies have revealed that most bacteria live in complex communities calledbiofilms. A biofilm is a well-organized community of bacteria that adheres to surfacesand is embedded in an extracellular slime layer. Once a bacterium attaches to asurface, it activates a whole different set of genes that gives the bacterium differentcharacteristics from those that it had as a free-floating organism. It has been estimatedthat more than 99% of all bacteria on the earth live as attached bacteria.2 Biofilms can

be found on medical and dentalimplants living in intravenousand urinary catheters, contactlenses, and prosthetic devices,such as heart valves, biliarystents, pacemakers, andartificial joints (Figure 1).Bacteria can also be life-threatening; Legionnaire'sdisease, which killed 29 peoplein 1976, was the result of abacterial biofilm in the hotel'sair conditioning system.

The Structure of Biofilm

CommunitiesA biofilm community comprisesbacterial microcolonies, anextracellular slime layer, fluidchannels, and a primitivecommunication system. Thebasic properties of the biofilm

structure are summarized in the Table. As the bacteria attach to a surface and to each

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other, they cluster together to formsessile, mushroom-shapedmicrocolonies that are attached to thesurface at a narrow base (Figure 2).Each microcolony is a tiny,

independent community containingthousands of compatible bacteria.Different microcolonies may containdifferent combinations of bacterialspecies. Bacteria in the center of amicrocolony may live in a strictanaerobic environment, while otherbacteriaat the edges of the fluid channels maylive in an aerobic environment (Figure3). Thus, the biofilm structure providesa range of customized living envi-ronments (with differing pHs, nutrientavailability, and oxygen

concentrations) within which bacteria withdifferent physiological needs can survive.

The extracellular slime layer is a protectivebarrier that surrounds the mushroom-shaped bacterial microcolonies. The slimelayer protects the bacterial microcoloniesfrom antibiotics, antimicrobials, and hostdefense mechanisms. A series of fluidchannels penetrates the extracellular slimelayer (Figure 4). These fluid channelsprovide nutrients and oxygen for thebacterial micro colonies and facilitate

movement of bacterial metabolites, wasteproducts, and enzymes within the biofilmstructure. Each bacterial microcolony useschemical signals to create a primitivecommunication system used tocommunicate with other bacterialmicrocolonies (Figure 5).

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Formation of Dental Plaque BiofilmsThe moment a baby passes through thebirth canal and takes its first breath,microbes begin to reside in its mouth.

Later, as the teeth erupt, additionalbacteria establish colonies on the toothsurfaces. Dental bacterial plaque is abiofilm that adheres tenaciously to toothsurfaces, restorations, and prostheticappliances. Understanding the formation,composition, and characteristics of theplaque biofilm assists in its control(Figure 6). The pattern of plaque biofilmdevelopment can be divided into threephases:

1. Attachment of bacteria to a solidsurface;2. Formation of microcolonies on thesurface; and3. Formation of the mature, subgingivalplaque biofilms (Figure 7).

The initial attachment of bacteria beginswith pellicle formation. The pellicle is a

thin coating of salivary proteins thatattach to the tooth surface within minutesafter a professional cleaning. The pellicleacts like double-sided adhesive tape,adhering to the tooth surface on one sideand on the other side, providing a stickysurface that facilitates bacterialattachment to the tooth surface. Followingpellicle formation, bacteria begin to attachto the outer surface of the pellicle.

Bacteria connect to the pellicle and eachother with hundreds of hairlike structurescalled fimbriae. Once they stick, thebacteria begin producing substances thatstimulate other freefloating bacteria to

join the community. Within the first 2days in which no further cleaning isundertaken, the tooth's surface is

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colonized predominantly by gram-positive facultative cocci, which areprimarily streptococci species. It appearsthat the act of attaching to a solidsurface stimulates the bacteria to

excrete an extracellular slime layer thathelps to anchor them to the surface andprovides protection for the attachedbacteria.

Microcolony formation begins once thesurface of the tooth has been coveredwith attached bacteria. The biofilmgrows primarily through cell division ofthe adherent bacteria, rather thanthrough the attachment of new bacteria.Next, the proliferating bacteria begin togrow away from the tooth. Plaquedoubling times are rapid in earlydevelopment and slower in more maturebiofilms. Bacterial blooms are periodswhen specific species or groups ofspecies grow at rapidly acceleratedrates. A second wave of bacterialcolonizers adheres to bacteria that arealready attached to the pellicle.

Coaggregation is the ability of newbacterial colonizers to adhere to thepreviously attached cells. The bacteriacluster together to form sessile,mushroom-shaped micro colonies thatare attached to the tooth surface at anarrow base. The result ofcoaggregation is the formation of acomplex array of different bacterialinked to one another.

Following a few days of undisturbedplaque formation, the gingival margin becomes inflamed and swollen. Theseinflammatory changes result in the creation of a deepened gingival sulcus. The biofilmextends into this subgingival region and flourishes in this protected environment,resulting in the formation of a mature subgingival plaque biofilm. Gingival inflammationdoes not appear until the biofilm changes from one composed largely of gram-positivebacteria to one containing gram-negative anaerobes. A subgingival bacterial

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microcolony, predominantly composed of gram-negative anaerobic bacteria, becomesestablished in the gingival sulcus between 3 and 12 weeks after the beginning ofsupragingival plaque formation. Most bacterial species currently suspected of beingperiodontal pathogens are anaerobic, gram-negative bacteria.

Control and Removal of DentalPlaque BiofilmsThe recent recognition thatsubgingival plaque is a biofilm helpsconsiderably in understanding itspersistence and resistance to thehost's defense system. The formationof biofilms by subgingival bacteriaprovides the bacteria with anadvantage that permits long-termsurvival within the sulcus or pocketenvironment.

Bacterial microcolonies are protectedby one another and by theextracellular slime layer and areunusually resistant to antibiotics(administered systemically),antimicrobials (administered locally),and the body's defense system.

Antibiotic doses that kill free-floating

bacteria, for example, need to beincreased as much as 1,500 times tokill biofilm bacteria (and at these high

doses, the antibiotic would kill the patient before the biofilm bacteria).2,3 It is likely thatseveral mechanisms are responsible for biofilm resistance to antibiotics andantimicrobial agents. The slime layer may prevent the drugs from penetrating fully intothe depth of the biofilm. Bacteria can develop resistance to antimicrobial drugs byproducing a thicker protective slime layer. The slime layer may protect the bacteriaagainst leukocytes (defensive cells of the body's immune system). Antibiotic orantimicrobial therapy usually will not kill the biofilm; the biofilm can be destroyed, how-ever, by simply wiping them off (disrupting their attachment to a surface).

Due to the structure of biofilms, physical removal of bacterial plaque biofilms is themost effective means of control. Subgingival plaque within pockets cannot be reachedby brushes, floss, or oral rinses. Therefore, frequent periodontal debridement ofsubgingival root surfaces by a dental hygienist or dentist is an essential component inthe treatment of periodontitis.

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ConclusionMore than 500 bacterial strains havebeen identified in dental biofilm. Expertsagree that most forms of periodontaldisease are caused by specific

pathogens, particularly gram-negativebacteria. The recognition that dentalplaque is a biofilm helps to explain whyperiodontal diseases have been sodifficult to prevent and to treat.Periodontal pathogens within a biofilmenvironment behave very differentlyfrom free-floating bacteria. The

protective extracellular slime matrix makes bacteria extremely resistant to antibiotics,antimicrobial agents, and host defense mechanisms. Mechanical removal is the mosteffective treatment currently available for the control of dental plaque biofilms.

References 1. Kroes I, Lepp PW, Reiman DA Bacterial diversity within the human subgingivalcrevice. Proc Natl Acad Sci USA1999;96(25):14547-14552.2. Coghlan A Slime City. New Scientist1996;2045:32-36.3. Elder MJ, Stapelton F, Evans E, DartJK. Biofilm-related infections in ophthalmologyEye1995;9(Pt. 1):102-109.4. Nield-Gehrig JS and Willmann DE. Foundations of Periodontics for the DentalHygienist. Philadelphia: Lippincott Williams & Wilkins 2003:67-73.

*Textbook author, international speaker, and dental hygiene consultant, Asheville,North Carolina. She can be reached at jng_consulting@bellsouth.net.

Portions of this presentation have been adapted with permission from Nield-Gehrig JSand Willmann DE. Foundations of Periodontics for the Dental Hygienist; @ 2003Lippincott Williams & Wilkins (http://www.lww.com/).