Characterization of salivary alpha-amylase binding to Streptococcus sanguis.

The purpose of this study was to identify the major salivary components which interact with oral bacteria and to determine the mechanism(s) responsible for their binding to the bacterial surface. Strains of Streptococcus sanguis, Streptococcus mitis, Streptococcus mutans, and Actinomyces viscosus were incubated for 2 h in freshly collected human submandibular-sublingual saliva (HSMSL) or parotid saliva (HPS), and bound salivary components were eluted with 2% sodium dodecyl sulfate. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western transfer, alpha-amylase (EC 3.2.1.1) was the prominent salivary component eluted from S. sanguis. Studies with 125I-labeled HSMSL or 125I-labeled HPS also demonstrated a component with an electrophoretic mobility identical to that of alpha-amylase which bound to S. sanguis. Purified alpha-amylase from human parotid saliva was radiolabeled and found to bind to strains of S. sanguis genotypes 1 and 3 and S. mitis genotype 2, but not to strains of other species of oral bacteria. Binding of [125I]alpha-amylase to streptococci was saturable, calcium independent, and inhibitable by excess unlabeled alpha-amylases from a variety of sources, but not by secretory immunoglobulin A and the proline-rich glycoprotein from HPS. Reduced and alkylated alpha-amylase lost enzymatic and bacterial binding activities. Binding was inhibited by incubation with maltotriose, maltooligosaccharides, limit dextrins, and starch.     

Adherence of mutans streptococci to other oral bacteria.

Adherence of mutans streptococci to strains of Actinomyces viscosus, Streptococcus sanguis, and Streptococcus mitis immobilized on a nitrocellulose membrane was measured. Strains of Streptococcus mutans, S. sobrinus, and S. rattus bound in a lactose-independent manner to a variety of the actinomyces and streptococci. Most of these reactions could proceed in the presence of whole saliva although adherence of S. rattus BHT to the streptococci was inhibited by salivary molecules. In contrast, adherence of S. mutans 10449 and KPSK2 to A. viscosus, S. sanguis, and S. mitis was enhanced by salivary molecules. S. mutans KPSK2, S. sobrinus OMZ 176, and S. rattus FA-1 binding to A. viscosus NC3 and S. sanguis G9B exhibited saturation kinetics. Adherence to A. viscosus NC3 was of a higher avidity than adherence to S. sanguis G9B. Attachment of S. mutans KPSK2 to S. sanguis G9B and of S. mutans OMZ 176 to A. viscosus NC3 and S. sanguis G9B was inhibited by heat treatment of the mutans streptococci. Attachment of S. mutans KPSK2 to A. viscosus NC3 and of S. rattus FA-1 to A. viscosus NC3 and S. sanguis G9B was unaffected by heat. These observations suggest that the mutans streptococci can adhere to a variety of early plaque bacteria by several distinct mechanisms. Such interactions may be important in the colonization of tooth surfaces by the mutans streptococci.     

Adherence of Streptococcus sanguis to saliva-coated hydroxyapatite: evidence for two binding sites.

The characteristics of bacterial adherence to saliva-coated hydroxyapatite were examined for a salivary aggregating strain of Streptococcus sanguis, strain 12, and for its nonaggregating variant, strain 12na. Both strains were found to adhere in similar numbers to saliva-coated hydroxyapatite that had been preincubated at 4 degrees C overnight. Preincubation of saliva-coated hydroxyapatite overnight at 37 degrees C reduced subsequent adherence of S. sanguis 12 by approximately 10%, whereas adherence of S. sanguis 12na was reduced by over 80%. Preincubation at 37 degrees C in the presence of neuraminidase reduced adherence of S. sanguis 12 by over 90% and caused some additional reduction in adherence of S. sanguis 12na. The data were analyzed with Langmuir isotherms, Scatchard plots, and Hill plots. Some evidence of cooperativity was seen. A peak in the Scatchard plot for S. sanguis 12 binding to saliva-coated hydroxyapatite preincubated at 4 degrees C disappeared after preincubation at 37 degrees C, suggesting the loss of a salivary receptor. Many more organisms were found to bind when adherence was measured by assays counting the number of organisms remaining in suspension after the beads had settled. These weakly binding organisms, which were removed by washing, demonstrated adherence characteristics similar to those of the firmly bound organisms. Both strains were strongly hydrophobic. It is proposed that the binding of S. sanguis 12 and 12na involves two types of receptor on the salivary pellicle. One type of receptor is stable at 37 degrees C, but sensitive to neuraminidase; the second type is inactivated by prolonged incubation at 37 degrees C. S. sanguis 12 may bind to both types of receptor, whereas S. sanguis 12na binds only to the second type. The neuraminidase-sensitive receptor might be involved in saliva-mediated aggregation.     

Characteristic differences between saliva-dependent aggregation and adhesion of streptococci.

Comparison of saliva-mediated aggregation of Streptococcus sanguis, Streptococcus mitis, and Streptococcus mutans and adhesion of these organisms to saliva-coated hydroxyapatite showed that there was no relationship between these two activities. Adsorption of salivary aggregating activity to bacteria appears to have little effect on the ability of the residual saliva to support adherence; conversely, adsorption of salivary adherence factors to hydroxyapatite does not affect aggregation. Although heating saliva significantly reduces bacterial aggregation, it has little or no effect on adherence. A comparison of aggregation and adhesion with serial dilutions of saliva demonstrated that adhesion could still be detected at 100 to 500-fold-lower concentrations of salivary protein that bacterial aggregation. These findings support the concept that aggregation and adherence involve two distinct mechanisms of microbial clearance in the oral cavity.     

Adherence of oral streptococci to salivary glycoproteins.

We used an overlay method to study the ability of human salivary glycoproteins to serve as receptors for several strains of streptococci that colonize the oral cavity. Parotid and submandibular-sublingual salivas were collected as ductal secretions, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and transferred to nitrocellulose membranes. The resulting blots were overlaid with [35S]methionine-labeled bacteria, and salivary components to which the bacteria bound were detected by autoradiography. Potential glycoprotein receptors were identified for 8 of the 16 strains tested. In three cases (Streptococcus sanguis 72-40 and 804 and Streptococcus sobrinus OMZ176), highly specific interactions with a single salivary component were detected. Removal of sialic acid residues from the low-molecular-weight salivary mucin prevented adherence of one of these strains (S. sanguis 72-40), suggesting that this saccharide either mediates binding or is a critical component of the receptor site. In the remaining five strains (Streptococcus gordonii G9B and 10558, S. sanguis 10556, and Streptococcus oralis 10557 and 72-41), interactions with multiple salivary components, including the low-molecular-weight salivary mucin, highly glycosylated proline-rich glycoproteins, and alpha-amylase, were detected. These results suggest that some oral streptococci can bind specifically to certain of the salivary glycoproteins. The interactions identified may play an important role in governing bacterial adherence and clearance within the oral cavity.     

Cell-to-cell interaction of Streptococcus sanguis and Propionibacterium acnes on saliva-coated hydroxyapatite.

Cell-to-cell interaction (coaggregation) between Propionibacterium acnes PK93 and Streptococcus sanguis DL1 was measured on saliva-coated hydroxyapatite beads (SHA) at bacterial concentrations between 1.3 X 10(6) and 6.7 X 10(8) cells per ml. Four hundredfold more DL1 than PK93 cells adhered to the saliva-coated beads, and the adherence of S. sanguis was proportional to cell input. SHA precoated with 3 X 10(8) DL1 cells bound 75 to 80% of available PK93 cells at all input amounts tested, up to an input of 8 X 10(7) cells. Adherence of PK93 to DL1-coated SHA approached saturation at an input of approximately 10(9) PK93 cells, when 1.5 X 10(8) bound. The coaggregation on SHA occurred either in buffer or saliva and was inhibited by N-acetylgalactosamine and by lactose; the attachment of DL1 to SHA was not inhibited by these sugars. S. sanguis 34 and heat-treated DL1 cells, neither of which form coaggregates with PK93, attached to SHA, but such cells did not bind PK93 cells. The findings of this study indicate that bacteria unable to attach to saliva-coated hydroxyapatite can indeed adhere to such a surface by strong lectin-mediated cell-to-cell interactions with bacteria already attached to the surface.     

Effect of bacterial aggregation on the adherence of oral streptococci to hydroxyapatite.

Several in vitro assay systems to measure the adherence of human dental plaque bacteria to solid surfaces such as teeth, glass, and hydroxyapatite have been published. In many studies a variety of macromolecular solutes have been used to study the adherence process. Often these solutes are able to aggregate the test bacterial and thus may alter the outcome of adherence experiments. In this study, the effects of the aggregation of Streptococcus sanguis on adherence to spheroidal hydroxyapatite is described. Adherence of preformed aggregates and of bacteria which were aggregating during the adherence reaction was examined. Bacteria were aggregated with whole saliva, concanavalin A, and wheat germ lectin. Further effects of the coaggregation of S. mitis and Actinomyces viscosus to saliva-coated spheroidal hydroxyapatite are presented. These studies suggest that formation of large aggregates resulted in a decrease in the numbers of organisms which adhered. In contrast, the formation of small aggregates actually increased the numbers of bacteria that adhered. All increases in adherent bacteria occurred at low concentrations of aggregating substance in which visible bacterial aggregation was not evident. The data indicate that adequate dose-response experiments must be performed to ensure that solutes used as probes to study adherence mechanisms do not affect the adherence simply as a result of aggregation of the test microorganisms.     

Hydrophobicity and adherence of oral streptococci after repeated subculture in vitro.

Fresh isolates of Streptococcus mutans, Streptococcus sanguis, and Streptococcus salivarius from human dental plaque were all highly hydrophobic. After repeated subculture in vitro on blood agar, strains of S. mutans serotype c showed decreased hydrophobicity, whereas serotype d/g strains did not. Parallel to the decreased hydrophobicity in the serotype c strains, an impaired ability to adhere to hydroxyapatite was observed. A similar but less pronounced decrease in hydrophobicity in one S. sanguis strain resulted in a marked decrease in adherence to hydroxyapatite.     

Further characteristics of beta2-microglobulin binding to oral streptococci.

A total of 85 strains of oral bacteria representing Streptococcus mutans, S. sanguis, S. Mitior, S. salivarious, S. milleri, S. infrequens, S. durans, S. lactis, S. faecalis, S. faecium, S. equinus, Streptococcus species group E, Actinomyces, and one group A Streptococcus were tested for binding of aggregated human beta 2-microglobulin. Positive affinity between bacteria and aggregated human beta 2-microglobulin was detected in 36% of the strains. No apparent correlation with bacterial species, serotype, or group was noted. No positive strains were detected among seven group I:A S. sanguis strains (P < 0.01). Binding constants for one S. mutans strain indicated heterogeneous binding structures on the bacterial surface. The number of binding sites for aggregates of human beta 2-microglobulin involving multipoint attachment varied from 70 to 1,700 per bacterial cell. With whole saliva as buffer, a general increase in affinity was seen. Variations in salt concentrations of the buffers revealed different salt-dependent species-associated uptake patterns. Oral bacteria tended to have an uptake maximum at a salt concentration similar to that seen in saliva. Binding structures for aggregated beta 2-microglobulin on oral streptococci were sensitive to pepsin, heat, and formaldehyde treatment. Bacterial binding structures for aggregated beta 2-microglobulin might represent one of several factors of importance for bacterial attachment in the oral cavity. Experimental conditions reflecting the salivary milieu increased the degree of interaction, emphasizing the importance of physiological test systems for such studies.     

Influence of preformed antibody on experimental Streptococcus sanguis endocarditis.

The influence of preformed, anti-whole organism antibody on the development of Streptococcus sanguis endocarditis was examined in both in vivo and in vitro systems. Antibody prevented, rather than potentiated, endocarditis in rabbits. The infectious dose in 30 control animals was 10(6.5) +/- 0.33 (mean +/- standard deviation); this increased to 10(7.71 +/- 0.05 in 36 immunized animals (P less than 0.01). No differences in bacterial clearance mechanisms were apparent between groups. Antibody also prevented the adherence of S. sanguis to the constituents of nonbacterial thrombotic endocarditis (fibrin and platelets) in vitro. When preincubated in high-titer antisera, adherence of S. sanguis was reduced compared with controls (adherence ratio mean +/- standard error of the mean, X 10(4): 174 +/- 5 versus 427 +/- 10, P less than 0.001). Preadsorption of immune sera with intact S. sanguis restored adherence to normal values, whereas preadsorption with dextran was partially effective. These studies demonstrate that preformed antibody had a protective role in vivo and suggest that a possible mechanism is blockade of adherence, a crucial early step in the pathogenesis of endocarditis.     

Coagglutination reactions between Candida albicans and oral bacteria

An agglutination assay for detecting intermicrobial adherence between the cells of Candida albicans and various oral bacteria is described. Strains of Streptococcus sanguis, S. salivarius, S. mutans, S. mitis, Fusobacterium nucleatum and Actinomyces viscosus all coagglutinated with C. albicans. No interaction could be demonstrated between the cells of Bacteroides melaninogenicus and those of C. albicans. Preliminary investigations of these interactions suggest that binding of F. nucleatum and A. viscosus to C. albicans is mediated by bacterial proteins, possibly lectins. Other mechanisms must account for the binding of oral streptococci to C. albicans. The possible implications of these findings in relation to oral mucosal colonisation and oral candidal clearance are discussed.     

Analysis of adherence of Streptococcus defectivus and endocarditis-associated streptococci to extracellular matrix.

Pathogenesis of nutritionally variant streptococcal (NVS) endocarditis initiates with bacterial attachment to and colonization of the damaged heart valve surface. Underlying extracellular matrix (ECM) exposed to the environment during damage to cardiac endothelium provides additional receptors that could be involved in bacterial adherence. The ability of NVS and endocarditis-associated streptococci to bind ECM was investigated by using an enzyme-linked immunosorbent assay system that incorporated ECM secreted by baby hamster kidney and human umbilical vein endothelial cells in culture. Streptococcus defectivus, the major species isolated from NVS endocarditis cases, bound ECM of fibroblasts and endothelial cells, indicating that the ECM molecule involved in the binding was a common constituent of diverse matrices. The specific binding of S. defectivus to ECM was demonstrated by saturation binding and specific antibody inhibition studies. Of the 15 S. defectivus strains analyzed, 13 bound ECM, whereas Streptococcus adjacens and NVS serotype III strains were unable to bind the matrix. This selective binding suggested that S. defectivus binds to heart valves through a mechanism different from those of other NVS in subacute bacterial endocarditis. A survey of non-NVS streptococcal endocarditis isolates demonstrated that S. mutans, S. mitis, S. sanguis, and S. faecalis also bound ECM, whereas other viridans species were unable to bind the matrix.     

Salivary-agglutinin-mediated adherence of Streptococcus mutans to early plaque bacteria.

Interspecies binding is important in the colonization of the oral cavity by bacteria. Streptococcus mutans can adhere to other plaque bacteria, such as Streptococcus sanguis and Actinomyces viscosus, and this adherence is enhanced by saliva. The salivary and bacterial molecules that mediate this interaction were investigated. Salivary agglutinin, a mucinlike glycoprotein known to mediate the aggregation of many oral streptococci in vitro, was found to mediate the adherence of S. mutans to S. sanguis or A. viscosus. Adherence of S. mutans to saliva- or agglutinin-coated S. sanguis and A. viscosus was inhibited by antibodies to the bacterial agglutinin receptor. Expression of the S. sanguis receptor (SSP-5) gene in Enterococcus faecalis increased adhesion of this organism to saliva- or agglutinin-coated S. sanguis and A. viscosus. This interaction could be inhibited by antibodies to the agglutinin receptor. The results suggest that salivary agglutinin can promote adherence of S. mutans to S. sanguis and A. viscosus through interactions with the agglutinin receptor on S. mutans.     

Molecular aspects of immunoglobulin A1 degradation by oral streptococci

Using a panel of 143 strains classified according to a novel taxonomic system for oral viridans-type streptococci, we reexamined the ability of oral streptococci to attack human immunoglobulin A1 (IgA1) molecules with IgA1 protease or glycosidases. IgA1 protease production was an exclusive property of all strains belonging to Streptococcus sanguis and Streptococcus oralis (previously S. mitior) and of some strains of Streptococcus mitis biovar 1. These are all dominant initiators of dental plaque formation. Degradation of the carbohydrate moiety of IgA1 molecules accompanied IgA1 protease activity in S. oralis and protease-producing strains of S. mitis biovar 1. Neuraminidase and beta-galactosidase were identified as extracellular enzymes in organisms of these taxa. By examination with enzyme-neutralizing antisera, four distinct IgA1 proteases were detected in S. sanguis biovars 1 to 3, S. sanguis biovar 4, S. oralis, and strains of S. mitis, respectively. The cleavage of IgA1 molecules by streptococcal IgA proteases was found to be influenced by their state of glycosylation. Treatment of IgA1 with bacterial (including streptococcal) neuraminidase increased susceptibility to protease, suggesting a cooperative activity of streptococcal IgA1 protease and neuraminidase. In contrast, a decrease in susceptibility was observed after extensive deglycosylation of the hinge region with endo-alpha-N acetylgalactosaminidase. The effector functions of IgA antibodies depend on the carbohydrate-containing Fc portion. Hence, the observation that oral streptococci may cleave not only the alpha 1 chains but also the carbohydrate moiety of IgA1 molecules suggests that the ability to evade secretory immune mechanisms may contribute to the successful establishment of these bacteria in the oral cavity.     

In Vitro Attachment of Streptococci to the Tooth Surface

The ability of Streptococcus strains to adhere to the tooth surface in vitro was investigated. Polished enamel slabs, with and without acquired pellicles, were incubated with buffer suspensions of oral streptococci, and attached bacteria were counted under a microscope using incident light. Low numbers of bacteria adhered to uncoated enamel; the presence of an acquired pellicle significantly enhanced the attachment of all strains tested. The adherence of Streptococcus sanguis was significantly greater than that of Streptococcus salivarius, and both of these strains adhered in greater numbers than did Streptococcus mutans. When bacteria were suspended in whole saliva, the adherence of S. salivarius and S. mutans was inhibited, whereas the adherence of S. sanguis was enhanced in some experiments and inhibited in others. The adherence of S. sanguis and S. salivarius was consistently inhibited by parotid fluid; this inhibitory effect persisted after thorough washing and resonication of the bacterial cells. Incubation in oral fluids was associated with the attachment of bacterial clumps to the pellicle, and parallel investigation revealed agglutination of S. sanguis and S. salivarius by whole saliva and, in particular, parotid fluid. The results are discussed in terms of surface microecology, and are related to the development of dental plaque.     

Adherence of oral streptococci: evidence for nonspecific adsorption to saliva-coated hydroxylapatite surfaces

It is proposed that binding of oral streptococci to saliva-coated hydroxylapatite (SHA) surfaces is a multifactorial process involving both specific and nonspecific receptors. In this context, specific binding is described as a high-affinity, saturable interaction between the cell and binding surface. Conversely, nonspecific binding is considered to be a nonsaturable, generalized, low-affinity reaction. Experimental differentiation of specific binding from nonspecific binding was achieved with a competition assay which utilized a large excess of nonradiolabeled bacteria to compete with the 3H-labeled cells for attachment to receptors on 1.5 mg of SHA crystals. Competition assays of Streptococcus sanguis and Streptococcus mitis adhesion clearly demonstrated that the total binding isotherm was composed of a saturable specific binding reaction and a minor nonspecific binding component. This was further substantiated by analysis of nonlinear Scatchard plots of the total binding data. The competition data for Streptococcus mutans binding indicated that ca. 50% of the S. mutans binding appeared to be specific, although saturation of the SHA surfaces with bacterial cells could not be demonstrated. Experiments measuring desorption of radiolabeled cells from SHA crystals into buffer showed that ca. 50% of the bound S. mutans cells were removed after 4 h, whereas less than 5% of the S. sanguis cells were eluted from the SHA surfaces. The kinetics of attachment were studied by using an extract of Persea americana as a noncompetitive inhibitor of adherence. The total cell binding data for these experiments suggested a very rapid binding reaction followed by a slower rate of attachment. It was concluded from these three different experimental approaches that adherence of selected oral streptococci to SHA surfaces involves specific, high-affinity and nonspecific, low-affinity binding reactions. The concept is developed that in vitro streptococcal attachment to SHA can be described as a two-reaction process in which the low-affinity interaction of the cell with the SHA surface precedes the establishment of the stronger, specific bonds needed for the maintenance of streptococci in the oral cavity.     

Adherence of Streptococcus sanguis clinical isolates to smooth surfaces and interactions of the isolates with Streptococcus mutans glucosyltransferase.

Streptococcus sanguis isolated from human dental plaque were grown in Todd-Hewitt broth. Cells were collected by centrifugation and lyophilized after extensive washing with water. The cell-associated glucosyltransferase (GTase) activities of S. sanguis strains were assayed with [14C]sucrose. Strain differences in GTase activity were significant within the same serotype or biotype or both. The ability of S. sanguis cells to adhere to smooth glass surfaces was generally weak, irrespective of significant cell-associated GTase activity synthesizing water-insoluble, gel-like glucans. Resting cells of most S. sanguis strains bound extracellular GTase from Streptococcus mutans strain B13 (serotype d), resulting in the strong adherence of the S. sanguis cells to smooth glass surfaces in the presence of sucrose. Conversely, S. mutans B13 cells also could bind extracellular GTase from some strains of S. sanguis examined. The sucrose-dependent adherence of S. mutans cells was not altered, although S. sanguis strains from which the extracellular GTases were obtained did not produce significant adherence in the presence of sucrose. In view of these findings, it was suggested that S. mutans GTase could affect the adherence of S. sanguis to smooth tooth surfaces in the oral cavity.     

Restricted distribution of Streptococcus milleri carbohydrate type antigens amongst other viridans streptococci.

The distribution of oral Streptococcus milleri carbohydrate type antigens in other viridans streptococcus species was examined. Rantz-Randall extracts of cells of the test strains grown in broth containing glucose were allowed to react with typing or grouping antisera for S. milleri serotypes a-k, or Lancefield groups A-G and K. Of 93 strains comprising more than 12 streptococcal species that included S. mutans and S. sanguis complexes, only 15 S. salivarius strains and one S. mitis strain were immunologically related to S. milleri serotype f. Unlike S. milleri strains, S. salivarius type f strains belonged to Lancefield group K, whereas the S. mitis strain was closely related to S. milleri serotype f but did not react with any of the Lancefield grouping antisera tested. Results suggest that oral S. milleri strains can be distinguished serologically from other oral viridans streptococci and that the typing antisera used in our researches might differentiate S. milleri isolates from the mouth from those associated with systemic purulent infections.     

Adherence of oral streptococci to keratinized and nonkeratinized human oral epithelial cells.

The ability of Streptococcus mutans, Streptococcus sanguis, Streptococcus mitis, and Streptococcus salivarius to adhere to keratinized versus nonkeratinized human oral epithelial cells was compared. S. mitis and S. salivarius exhibited significantly greater adherence to keratinized cells than to nonkeratinized cells. S. mutans and S. sanguis adhered equally well to either epithelial cell type. It is concluded that keratinization of epithelial cells may be a significant factor in the adherence of certain oral streptococci to the oral mucosa.     

Coaggregation of oral Bacteroides species with other bacteria: central role in coaggregation bridges and competitions.

Seventy-three freshly isolated oral strains representing 10 Bacteroides spp. were tested for their ability to coaggregate with other oral gram-negative and gram-positive bacteria. None coaggregated with any of the gram-negative strains tested, which included Capnocytophaga gingivalis, C. ochracea, C. sputigena, and Actinobacillus actinomycetemcomitans. Strains of Bacteroides buccae, B. melaninogenicus, B. oralis, and B. gingivalis failed to coaggregate with any of the gram-positive strains tested. However, six Bacteroides spp. coaggregated with one or more species of gram-positive bacteria. Most isolates of B. buccalis, B. denticola, B. intermedius, B. loescheii, B. oris, and B. veroralis coaggregated with strains of Actinomyces israelii, A. viscosus, A. naeslundii, A. odontolyticus, Rothia dentocariosa, or Streptococcus sanguis. The strongest coaggregations involved B. denticola, B. loescheii, or B. oris; 22 of 25 strains coaggregated with A. israelii. Only B. loescheii interacted with certain strains of S. sanguis; these coaggregations were lactose inhibitable and were like coaggregations between A. viscosus and the same strains of S. sanguis. In fact, B. loescheii and A. viscosus were competitors for binding to S. sanguis. Many bacteroides also acted as coaggregation bridges by mediating coaggregations between two noncoaggregating cell types (e.g., S. sanguis and A. israelii). Evidence for binding-site competition and coaggregation bridging involving noncoaggregating cell types from three different genera provides support for the hypothesis that these intergeneric cell-to-cell interactions have an active role in bacterial colonization of the oral cavity.