Biofilms are complex, surface-adherent, spatially organized polymicrobial communities containing bacteria surrounded by a polysaccharide matrix.
Oral biofilms that form on the surfaces of teeth and biomaterials in the oral cavity are also known as den- tal plaque. When the human diet is rich in ferment- able carbohydrates, the most prevalent organisms shown to be present in dental plaque are adherent acidogenic and aciduric bacteria such as streptococci and lactobacilli, which are primarily responsible for dental caries. Other consequences of long-term oral biofilm accumulation can also include periodontal diseases and periimplantitis (inflammation of the soft and hard tissues surrounding an implant), depend- ing on the location of attachment of the biofilm.
Biofilm formation on hard surfaces in the oral cavity is a sequential process. A conditioning film from saliva (known as pellicle) containing adsorbed macromolecules such as phosphoproteins and gly- coproteins is deposited on tooth structure and bio- materials within minutes after a thorough cleaning.
This stage is followed by the attachment of plank- tonic (free-floating) bacteria to the pellicle. Division of the attached initial colonizing bacterial species produces microcolonies, and subsequent attachment of later colonizing species results in the formation of matrix-embedded multispecies biofilms. These bio- films can mature over time if they are not detached by mechanical removal or intrinsic factors.
Biofilm formation occurs via complicated physi- cochemical and cellular interactions between the substrate, pellicle, and bacteria. These interactions occur at several levels and can include physical prox- imity, metabolic exchange, signal molecule–medi- ated communication, exchange of genetic material, production of inhibitory factors, and coaggregation (“specific cell-to-cell recognition between geneti- cally distinct cell types,” as defined by Kolenbrander et al., 2006).
The pellicle contains a variety of receptor mol- ecules that are recognized primarily by streptococci (Fig. 2.14). This is evident in healthy individuals, who typically have biofilms containing a thin layer of adherent gram-positive cocci. The ability to bind to nonshedding surfaces such as enamel gives strep- tococci a tremendous advantage and is consistent with the observation that streptococci constitute 60% to 90% of the initial bacterial flora on enamel in situ. Furthermore, the streptococci are less sensitive to exposure to air than most oral bacteria because they are facultatively anaerobic and can participate in modifying the biofilm environment to a more reduced state, a condition often considered to favor an ecological shift toward gram-negative anaerobes.
Interactions among human oral bacteria are pivotal to the development of oral biofilms (see Fig. 2.14). In the first 4 hours of biofilm formation, gram-positive cocci appear to predominate, particularly mitis group streptococci. After 8 hours of growth, the majority of the bacterial population continues to be largely coc- coid, but rod-shaped organisms are also observed.
By 24 to 48 hours, thick deposits of cells with vari- ous morphologies can be detected, including coccoid, coccobacillary, rod-shaped, and filamentous bacteria.
Within 4 days of biofilm growth, an increase in the numbers of gram-negative anaerobes is observed, and particularly of Fusobacterium nucleatum. The lat- ter organism has the unique ability to coaggregate with a wide variety of bacteria and is believed to play a pivotal role in the maturation of biofilm because it forms coaggregation bridges with both early and late colonizers. As the biofilm matures, a shift is observed
toward a composition of largely gram-negative mor- photypes, including rods, filamentous organisms, vibrios, and spirochetes. These shifts in the microbial composition of biofilm are important because they correlate with the development of gingivitis (inflam- mation of gingival tissues).
Even though biofilms accumulate on restorative, orthodontic, endodontic, and implant biomaterials, the remainder of this section focuses on biofilms that accumulate on the surfaces of restorative and
implant materials only. The precise mechanisms of bacterial adhesion and biofilm formation on the sur- faces of dental materials have not yet been identified despite decades of research effort but are accepted to be complex processes that depend on a large num- ber of factors. In vitro studies have shown that the adhesion of salivary proteins and bacteria at small distances (5–100 nm) from the surfaces of biomateri- als is influenced by a combination of Lifshitz-van der Waals forces, electrostatic interactions, and acid-base Adhesin receptor
Fusobacterium nucleatum
Early colonizers S. flueggei
colonizersLate
atypicaV.
Statherin Sialylatedmucins Proline-richprotein Salivaryagglutinin Bacterial cellfragment Sialylatedmucins Salivaryagglutinin Alpha-amylase Proline-richprotein
C. sputigena
C. ochracea
C. gingiv
alis
H. parainfluenzae
A. naeslundii A. israeli
i
P. acnes
P. loescheii
P. denticola P. gingiv
alis T. denticola
E. corrodens Eubacter
ium spp. P. intermedia
A. actinom
ycetemcomitans
S. oralis
S. mitis S.
gordonii
Acquired pellicle Tooth surface
gordoniiS.
S. oralis S. sanguis
FIG. 2.14 Spatiotemporal model of oral bacterial colonization, showing recognition of salivary pellicle receptors by early colonizing bacteria and coaggregations between early colonizers, fusobacteria, and late colonizers of the tooth surface. Starting at the bottom, primary colonizers bind via adhesins (round-tipped black line symbols) to complementary salivary receptors (blue-green vertical round-topped columns) in the acquired pellicle coating the tooth surface. Secondary colonizers bind to previously bound bacteria. Sequential binding results in the appearance of nascent surfaces that bridge with the next coaggregating partner cell. The bacterial strains shown are Actinobacillus actinomycetemcomitans, Actinomyces israelii, Actinomyces naeslundii, Capnocytophaga gingivalis, Capnocytophaga ochracea, Capnocytophaga sputigena, Eikenella cor- rodens, Eubacterium spp., Fusobacterium nucleatum, Haemophilus parainfluenzae, Porphyromonas gingivalis, Prevotella denticola, Prevotella intermedia, Prevotella loescheii, Propionibacterium acnes, Selenomonas flueggei, Streptococcus gordonii, Streptococcus mitis, Streptococcus oralis, Streptococcus sanguis, Treponema spp., and Veillonella atypica. (From Kolenbrander PE, Andersen RN, Blehert DS, et al. Communication among oral bacteria. Microbiol Mol Biol Rev. 2002;66(3):486–505.)
bonding. Other properties such as substrate hydro- phobicity, surface free energy, surface charge, and surface roughness have commonly been investigated in vitro for correlation with the number of adhering bacteria. Many of the aforementioned surface prop- erties are described in later chapters.
The role of surface roughness in biofilm formation has been widely investigated. Smooth surfaces have been shown to attract less biofilm in vivo than rough surfaces. It has also been observed that hydrophobic surfaces that are located supragingivally attract less biofilm in vivo than more hydrophilic surfaces over a 9-day period. An increase in the mean surface rough- ness parameter (Ra) above a threshold value of 0.2 μm or an increase in surface free energy were found to result in more biofilm accumulation on dental materi- als. When both of those surface properties interact with each other, surface roughness was observed to have a greater effect on biofilm accumulation. The creation of a rough restoration surface caused by abrasion, ero- sion, air polishing or ultrasonic instrumentation, or a lack of polishing after the fabrication of a restoration has also been associated with biofilm formation.
Bacterial adhesion in vivo is considerably reduced by the formation of a pellicle, regardless of the com- position of the underlying substrate. Pellicle forma- tion has also been shown to have a masking effect on specific surface characteristics of biomaterials to a certain extent. Surfaces having a low surface energy were observed to retain the smallest amount of adherent biofilm because of the lower binding forces between bacteria and substrata even after several days of exposure in the human oral cav- ity. Reciprocally, the higher surface energy of many restorative materials compared with that of the tooth surface could result in a greater tendency for the sur- face and margins of the restoration to accumulate debris, saliva, and bacteria. This may in part account for the relatively high incidence of secondary (recur- rent) carious lesions seen in enamel at the margins of resin composite and amalgam restorations.
Investigations of oral biofilms on restorative materials can generally be divided into in vivo, in situ, and in vitro studies, with the latter comprising monospecies or multispecies investigations. Biofilms that are formed on restorative materials can vary in thickness and viability. In vivo and in situ studies of biofilm formation on dental materials have produced inconsistent results, and a trend for accumulation on materials has not been determined so far.
Levels of cariogenic organisms (capable of pro- ducing or promoting caries) such as Streptococcus mutans have been shown to be higher in biofilms adjacent to posterior resin restorations than in bio- films adjacent to amalgam or glass ionomer res- torations. The formation of oral biofilms has been associated with an increase in the surface roughness
of resin composites, degradation of the material due to acid production by cariogenic organisms, hydroly- sis of the resin matrix, and a decrease in microhard- ness of the restoration’s surface. Esterases of salivary and bacterial origin have also been implicated as sources of degradation. In addition, it has been theo- rized that planktonic bacteria can enter the adhesive interface between the restorative material and the tooth, leading to secondary caries and pulp pathol- ogy. By contrast, trace amounts of unpolymerized resin, resin monomers, and the products of resin bio- degradation, such as 2,2-bis[4(2,3-hydroxypropoxy) phenyl]propane (BisHPPP), triethylene glycol mono- methacrylate (TEGMA), triethylene glycol (TEG), and methacrylic acid (MA), have been shown to modulate the growth of oral bacteria in the vicinity of resin restorations. All of these factors create a cycle of bacteria-surface interaction that further increases surface roughness and encourages bacterial attach- ment to the surface, thereby placing the adjacent enamel at greater risk for secondary caries.
Bacterial adhesion to casting alloys and dental amalgams has received limited attention in recent times as dental amalgam is being discontinued in response to global concerns about mercury (Hg) in the environment. Biofilms on gold-based casting alloys are reported to be of low viability, possibly because of the bacteriostatic effect of gold. Biofilms on amalgam are also reported to have low viability, which could be attributed to the presence of the Hg(II) form of mercury in dental amalgam. Interestingly, amalgam restorations have been shown to promote the levels of Hg-resistant bacteria in vitro and in vivo. Resistance to antibiotics, and specifically tetracycline, was observed to be concurrent with Hg resistance in oral bacteria.
However, it is worth noting that Hg-resistant bacteria were also found in children without amalgam fillings or previous exposure to amalgam.
Information regarding the morphology of bio- films on ceramic restorations is limited, although it is generally accepted that ceramic crowns accumu- late less biofilm than adjacent tooth structure. The recent demonstration of increased surface roughness of zirconia surfaces in vitro after the use of hand and ultrasonic scaling instruments could be theorized to produce greater biofilm accumulation on zirco- nia restorations subsequent to dental prophylaxis procedures.
Biofilms that adhere to denture base resins pre- dominantly contain Candida species of yeast. However, initial adhesion of bacteria such as streptococci to the denture base may have to occur before Candida species can form biofilms. This is attributed to the observa- tion of bacteria on dentures within hours and Candida species after days, and to the ability of Candida spe- cies to bind to the cell wall receptors in streptococci.
Biofilms on dentures have commonly been associated
with denture stomatitis (chronic inflammation of the oral mucosa) in elderly and immunocompromised patients. Removal of biofilms from dentures typically requires mechanical and/or chemical means and is a significant clinical problem because of biofilm adher- ence to the denture base resins.
The accumulation of biofilms on glass ionomer and resin-modified glass ionomer biomaterials is a factor that has been associated with an increase in the surface roughness of those biomaterials.
Fluoride-releasing materials, and glass ionomers and compomers in particular, can neutralize acids pro- duced by bacteria in biofilms. Fluoride can provide cariostatic benefits and may affect bacterial metabo- lism under simulated cariogenic conditions in vitro.
Although the large volume of saliva normally present in the oral cavity is hypothesized to result in fluoride concentrations that are too low for oral cavity–wide antibacterial protection, the amount of fluoride released could theoretically be sufficient to minimize demineralization in the tooth structure adjacent to glass ionomer and resin-modified glass ionomer res- torations. In addition, glass ionomer materials can be recharged by daily exposure to fluoride-containing dentifrices, thereby compensating for the signifi- cant decrease in fluoride release that occurs over time. Interestingly, more studies are needed because clinical studies have not clearly demonstrated that fluoride-releasing restorative materials significantly reduce the incidence of secondary caries compared with nonfluoride-releasing biomaterials.
The accumulation of biofilms on titanium and titanium alloys that are used in dental implants has received much attention because biofilms play a significant role in determining the success of an implant. The sequence of microbial colonization and biofilm formation on dental implants has been shown to be similar to that on teeth, but differs in early colonization patterns. Several in vivo stud- ies have confirmed that a reduction in mean Ra of implant materials below the threshold value of 0.2 μm has no major effect on adhesion, colonization, or microbial composition. Compared with polished titanium surfaces, titanium implant surfaces that were modified with titanium nitride (TiN) showed significantly less bacterial adhesion and biofilm for- mation in vivo, thereby potentially minimizing bio- film accumulation and subsequent periimplantitis.
Other contributing factors such as the hydrophobic- ity, surface chemistry, and surface free energy of the implant material have been found to play vital roles in bacterial adhesion to dental implant materials. In addition, the surface characteristics of the bacteria, the design of the implant and the abutment, and the microgap between the implant and abutment have also been shown to influence microbial colonization on dental implants.
The most common reason for the replacement of dental restorations is secondary caries at the gingi- val tooth-restoration margin. It is estimated that 50%
to 80% of resin restorations are replaced annually in the United States alone. The cost of replacing resto- rations is estimated to be in the billions of dollars worldwide, and the number and cost of replacing restorations is increasing annually. Although bacte- riological studies of secondary caries indicate that its etiology is similar to that of primary caries, the mechanisms by which secondary caries occur are a focus of ongoing investigations.
The removal of tenaciously adherent oral bio- films from hard surfaces is crucial to caries control and is most effectively accomplished by mechanical brushing with toothpaste, especially in interproxi- mal regions and posterior teeth along with the use of adjunctive chemical agents. Although tooth brushing has been associated with increased surface roughness of restorations over time due to the process of wear, which could permit additional bacterial attachment on the surface, mechanical removal has been shown to be more effective than chemical intervention. This is because bacteria in biofilms are typically well pro- tected from the host immune response, antibiotics, and antibacterials when embedded within a complex biofilm matrix. Furthermore, most antimicrobial agents have commonly been tested against plank- tonic bacteria, which are killed by much lower con- centrations of antimicrobials than biofilm bacteria.
Chemical control of biofilms has also been limited by concerns regarding the development of resistant microorganisms resulting from the prolonged use of antimicrobials, and acceptance of the hypothesis that the microflora should not be eliminated but should instead be prevented from shifting from a favorable ecology to an ecology favoring oral disease.
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