Comparison of the initial streptococcal microflora on dental enamel in caries-active and in caries-inactive individuals

Mutans streptococci and other caries-associated acidogenic bacteria in five-year-old children in South Africa

Correlations between mutans streptococci and caries are inconsistent; some studies show that early caries may occur with other bacterial species possessing the ability to survive, interact and compete with mutans streptococci. This study looked at the relationship between mutans streptococci, Lactobacillus, Actinomyces and Veillonella in saliva and plaque collected from four tooth sites in two ethnic groups, with and without caries. All samples were collected and cultured onto selective media. Colonies were enumerated and identified using biochemical tests. Pearson correlations showed low yet statistically significant correlations between plaque mutans streptococci counts and the number of decayed surfaces (ds) and decayed, missing and filled surfaces (dmfs) of primary teeth in blacks and in coloreds (historical race classification). Salivary mutans streptococci counts correlated with ds and dmfs in coloreds. No relationship was found between ds, dmfs and Lactobacillus, Actinomyces or Veillonella numbers. Significant associations were evident between plaque mutans streptococci and Veillonella and Lactobacillus in black children with caries and between plaque mutans streptococci and Actinomyces and Lactobacillus and Veillonella in colored children. Hence, oral microbes are in constant flux but have interrelationships in children with caries.

Identification of oral mitis group streptococci by arbitrarily primed polymerase chain reaction

"Mitis group" streptococci are commensal but may play some role in dental caries, septicemia or endocarditis. Rapid genotypic identification would aid studies of dental plaque ecology, or diagnostic use. AP-PCR with 58 unpaired arbitrary primers was used to characterize 7 Streptococcus gordonii, 11 Streptococcus sanguis, 2 Streptococcus crista, 5 Streptococcus parasanguis, 18 Streptococcus oralis, and 36 Streptococcus mitis (22 biovar 1 and 14 biovar 2). S. parasanguis 16S rRNA variable region primer RR2 produced species-specific bands with all S. gordonii and S. sanguis. Human V beta 1 T-cell receptor primer 434 yielded concordant genotypic identification of all phenotypically defined S. crista and S. parasanguis, 83% of S. oralis, and 74% of S. mitis biovar 1. Amplicon patterns for S. mitis biovar 2 were heterogeneous. Findings suggest that primers RR2 and 434 in succession will allow rapid identification of genotypic groups corresponding closely to mitis group species established by phenotype.

Inhibition of Prevotella and Capnocytophaga immunoglobulin A1 proteases by human serum

Oral Prevotella and Capnocytophaga species, regularly isolated from periodontal pockets and associated with extraoral infections, secret specific immunoglobulin A1 (IgA1) proteases cleaving human IgA1 in the hinge region into intact Fab and Fc fragments. To investigate whether these enzymes are subject to inhibition in vivo in humans, we tested 34 sera from periodontally diseased and healthy individuals in an enzyme-linked immunosorbent assay for the presence and titers of inhibition of seven Prevotella and Capnocytophaga proteases. All or nearly all of the sera inhibited the IgA1 protease activity of Prevotella buccae, Prevotella oris, and Prevotella loescheii. A minor proportion of the sera inhibited Prevotella buccalis, Prevotella denticola, and Prevotella melaninogenica IgA1 proteases, while no sera inhibited Capnocytophaga ochracea IgA1 protease. All inhibition titers were low, ranging from 5 to 55, with titer being defined as the reciprocal of the dilution of serum causing 50% inhibition of one defined unit of protease activity. No correlation between periodontal disease status and the presence, absence, or titer of inhibition was observed. The nature of the low titers of inhibition in all sera of the IgA1 proteases of P. buccae, P. oris, and P. loescheii was further examined. In size exclusion chromatography, inhibitory activity corresponded to the peak volume of IgA. Additional inhibition of the P. oris IgA1 protease was found in fractions containing both IgA and IgG. Purification of the IgG fractions of five sera by passage of the sera on a protein G column resulted in recovery of inhibitory IgG antibodies against all three IgA1 proteases, with the highest titer being for the P. oris enzyme. These finding indicate that inhibitory activity is associated with enzyme-neutralizing antibodies.

Interruption of the Streptococcus gordonii M5 sspA/sspB intergenic region by an insertion sequence related to IS1167 of Streptococcus pneumoniae

Streptococcus gordonii M5 and DL1 each express two related adhesin polypeptides, SspA and SspB, which are members of the antigen I/II family of streptococcal surface proteins. The sspA and sspB genes are tandemly arranged in both strains, with sspA residing upstream of sspB. The genes are separated by approximately 400 nucleotides in S. gordonii DL1 and 1300 nucleotides in S. gordonii M5. The nucleotide sequence of the sspA/sspB intergenic region of strain M5 is reported and the difference in length compared to S. gordonii DL1 shown to arise from the presence of an insertion sequence, designated ISSg1, consisting of 1197 bp. The nucleotide sequence of ISSg1 is highly homologous to IS1167 to Streptococcus pneumoniae and is related to a lesser extent to other members of the IS1096 family of bacterial insertion sequences. It contains a single ORF of 1026 bp, encoding a putative transposase polypeptide of 342 amino acids. The deduced transposase sequence exhibits 93% identity with the transposase polypeptides encoded by IS1167. However, the S. gordonii protein lacks a 90 residue central domain that is present in the IS1167 transposase and in the transposase polypeptides encoded by the related IS elements. In addition, the organization of the inverted repeats flanking the transposase gene in S. gordonii differs from IS1167. Extension products generated from a sspB-specific primer indicated that transcription initiates within the intergenic region in both S-gordonii strains, suggesting that sspA and sspB are independently transcribed. Transcription appears to initiate 42 bases upstream of sspB in S. gordonii DL1. In contrast, sspB transcription in M5 initiates at least 125 bases upstream of sspB, in close proximity to the terminal inverted repeat of ISsg1. These results indicate that the sspB promoter of S. gordonii M5 and DL1 are not conserved and suggest that ISSg1 sequences may play a role in directing the expression of sspB in S. gordonii M5.

Saliva mediated adherence, aggregation and prevalence in dental plaque of Streptococcus mutans, Streptococcus sanguis and Actinomyces spp, in young and elderly humans

Salivary components in the pellicle mediate bacterial adherence to the tooth. Such components may also aggregate bacteria in saliva and prevent them becoming established in dental plaque. In the present study, the adherence and aggregation of Streptococcus mutans strain Ingbritt, S. sanguis strain 10556 and Actinomyces viscosus-strain 19246 mediated by parotid and whole saliva from groups of young and elderly people were examined. Significant differences were found between test strains, salivary secretions and age groups. S. sanguis 10556 and A. viscosus 19246 generally adhered more strongly than S. mutans Ingbritt, which adhered better to pellicles from parotid saliva than from whole saliva Strain 19246 bound in higher numbers to parotid saliva pellicles from elderly compared to young individuals. Strain 10556 adhered better to whole saliva than parotid saliva pellicles, and the difference was significant among the young individuals, indicating reduced adherence ability in elderly whole saliva. The streptococci were aggregated by parotid and whole saliva, and S. sanguis aggregation was less with whole saliva from the elderly than from the young participants. Besides a correlation between whole saliva aggregation of S. mutans and proportions of bacteria in plaque, no correlations were found for the individual binding properties of saliva and prevalence of bacteria in vivo. However, the level of saliva-mediated adherence in vitro was in the following order: S. mutans. Actinomyces S sanguis, which corresponded to their isolation frequency in plaque. These findings emphasize the importance of initial adherence to salivary receptors in bacterial colonization on teeth. Further studies are needed to reveal if individual patterns in the in vitro binding characteristics of saliva lead to variation of colonization in vivo.

Mechanisms of adhesion by oral bacteria

Adherence to a surface is a key element for colonization of the human oral cavity by the more than 500 bacterial taxa recorded from oral samples. Three surfaces are available: teeth, epithelial mucosa, and the nascent surface created as each new bacterial cell binds to existing dental plaque. Oral bacteria exhibit specificity for their respective colonization sites. Such specificity is directed by adhesin-receptor cognate pairs on genetically distinct cells. Colonization is successful when adherent cells grow and metabolically participate in the oral bacterial community. The potential roles of adherence-relevant molecules are discussed in the context of the dynamic nature of the oral econiche.

Regional variation in root dentinal tubule infection by Streptococcus gordonii

The purpose of this study was to investigate the pattern of bacterial invasion of dentinal tubules at different regions in human roots. Specimens were obtained from single-rooted teeth that had their root canals prepared in a standard manner. Roots were then sectioned longitudinally through the canals and the resulting specimens chemically treated to remove the smear layers. Specimens were immersed in a suspension of Streptococcus gordonii for 3 weeks and then prepared for histological analysis. Sections from the cervical, midroot, and apical areas were examined. The pattern of bacterial infection of the cervical and midroot areas was similar, characterized as a heavy infection with bacteria penetrating as deep as 200 microns. Invasion of the apical dentin was significantly different, with a mild infection and maximum penetration of 60 microns.

Identification and characterization of a protease from Streptococcus oralis C104

Streptococcus oralis is among the earliest colonizers of the tooth surface during plaque formation. As such, its enzymatic activities may influence ecologic succession on the tooth surface. In the current study, we use zymograms and preparative polyacrylamide gel electrophoresis to identify and purify a protease from S. oralis (sanguis) C104. Proteases from S. oralis C104 were detected in cell pellets at 133, 146 and 176 kDa as clear proteolytic bands on gelatin-substrate zymograms. Preparation of the major (146 kDa) protease were obtained by continuous-elution electrophoresis. The protease was active over the pH range of 7 to 9 with optimum activity between pH 8 and 9. Protease activity was inhibited by several serine protease inhibitors including phenylmethylsulfonyl fluoride, di-isopropyl-phosphofluoridate and aprotinin. The protease showed highest hydrolytic activity against azoalbumin and Bz-Pro-Phe-Arg-NA. Immunofluorescence studies with a polyclonal antiserum to the 146-kDa protease suggest it is present on the cell surface of S. oralis C104. Zymograms of cell pellets from other S. oralis strains as well as S. sanguis and Streptococcus mitis suggest that functionally similar proteases are elaborated by many early colonizers of the tooth surface.

Biological significance of IgA1 proteases in bacterial colonization and pathogenesis: critical evaluation of experimental evidence

IgA1 protease activity, which allows bacteria to cleave human IgA1 in the hinge region, represents a striking example of convergent evolution of a specific property in bacteria. Although it has been known since 1979 that IgA1 protease is produced by the three leading causes of bacterial meningitis in addition to important urogenital pathogens and some members of the oropharyngeal flora, the exact role of this enzyme in bacterial pathogenesis is still incompletely understood owing to lack of a satisfactory animal model. Cleavage of IgA1 by these post-proline endopeptidases efficiently separates the monomeric antigen-binding fragments from the secondary effector functions of the IgA1 antibody molecule. Several in vivo and in vitro observations indicate that the enzymes are important for the ability of bacteria to colonize mucosal membranes in the presence of S-IgA antibodies. Furthermore, the extensive cleavage of IgA sometimes observed in vivo, suggests that IgA1 protease activity results in a local functional IgA deficiency that may facilitate colonization of other microorganisms and the penetration of potential allergens. It has been hypothesized that IgA1 protease activity of Haemophilus influenzae, Neisseria meningitidis, and Streptococcus pneumoniae, under special immunological circumstances, allows these bacteria to take advantage of specific IgA1 antibodies in a strategy to evade other immune factors of the human body. The decisive factor is the balance between IgA antibodies against surface antigens of the respective bacteria and their IgA1 protease. Recent studies have shown that serine-type IgA1 proteases of H. influenzae, meningococci, and gonococci belong to a family of proteins used by a diverse group of Gram-negative bacteria for colonization and invasion.

Co-adhesion of oral microbial pairs under flow in the presence of saliva and lactose

Co-aggregation (interactions between two suspended micro-organisms) between oral microbial pairs has been studied extensively and is believed to be an important factor in dental plaque formation. However, co-adhesion (interactions between suspended and already-adhering micro-organisms) may well be equally important. The aim of this paper was to determine the influence of saliva and lactose on the co-adhesion of streptococci (S. oralis 34 and S. sanguis PK1889) to actinomyces (A. naeslundii T14V-J1 or 5951) adhering on glass under flow from buffer and saliva in the absence and presence of lactose. The kinetics of co-adhesion as well as co-adhesion in a stationary end-point of co-aggregating and non-co-aggregating pairs was studied in a parallel plate flow chamber by analysis of the spatial arrangement of co-adhering micro-organisms as a function of time. For co-aggregating pairs, initial deposition rates of streptococci in the immediate vicinity of adhering actinomyces (local initial deposition rates) were up to 5 to 10 times higher than the non-local initial deposition rates in buffer and in saliva, respectively. In a stationary end-point of co-adhesion, 5 to 6 times more streptococci co-adhered with the adhering actinomyces than averaged over the entire substratum surface. A non-co-aggregating pair showed only minor preferential (co-)adhesion near the adhering actinomyces. Co-adhesion in buffer was fully lost when lactose was added. However, addition of lactose to saliva did not inhibit co-adhesion, but co-adhesion became more reversible. Detachment of micro-organisms from the substratum due to the passage of an air-liquid interface, as occurs in the oral cavity during eating, drinking, and speaking, was minimal when deposition was carried out from buffer to bare glass. Major detachment of streptococci adhering to the substratum occurred when adhesion was mediated through a salivary conditioning film on the glass, while detachment of adhering actinomyces and streptococci co-adhering with them remained low. It is suggested that, in the development of dental plaque, adhering actinomyces may act as strongholds for other micro-organisms, like streptococci, to adhere.

Multiple phase variation in haemolytic, adhesive and antigenic properties of Streptococcus gordonii

Streptococcus gordonii gave rise to beta-haemolytic variants (Bhp+ for beta-haemolysin production) at frequencies of 10(-4)-10(-3) on agar medium containing washed horse erythrocytes. Bhp+ variants reverted to the wild-type alpha-haemolytic phenotype (Bhp-) at the same frequencies. There was a significant probability (> or = 0.1) that phase variation in Bhp and phase variation in the previously described Spp (sucrose promoted phenotype) would occur concomitantly, but there was no correlation between these phenotypes. There was evidence also of independent phase variation in adhesion to saliva-coated hydroxyapatite (Asp for adhesion to salivary pellicles), in lactose-sensitive coaggregation (Cls for coaggregation, lactose-sensitive) and in the concentrations of particular cell surface antigens (Cap for cell antigen profile) in strains that had undergone phase changes in Spp and/or Bhp. Phase variation in all these phenotypes were transitions between high and low levels of activity and each appeared to occur as an independent event. Significant associations (P << 0.0001 by contingency table analysis) between particular phenotypes such as Bhp and Asp and between Asp, Cls and Cap phenotypes, however, were apparent. The results suggest that S. gordonii cells become predisposed to phase variation and that the resulting independent phenotypic changes may give rise to phenotypically diverse streptococcal populations able to accommodate rapid and transient environmental changes in the mouth.

In vivo cleavage of immunoglobulin A1 by immunoglobulin A1 proteases from Prevotella and Capnocytophaga species

Immunoglobulin A1 (IgA1) proteases secreted by oral Prevotella and Capnocytophaga species specifically cleave IgA1 at the same peptide bond in the hinge region, leaving intact monomeric Fab and Fc fragments. Assuming that Prevotella- and Capnocytophaga-induced Fab fragments of IgA1 expose a specific immunogenic neoepitope at the cleavage site, we established an enzyme-linked immunosorbent assay to measure human serum antibodies to this neoepitope as indirect evidence of in vivo activity of Prevotella and Capnocytophaga IgA1 proteases. The assay used a monoclonal antibody with specificity for the neoepitope, and the ability to block binding of the monoclonal antibody to the neoepitope was investigated. Absorption of sera with Prevotella melaninogenica-induced Fab fragments of IgA1 resulted in removal of antibodies blocking binding of the monoclonal antibody, whereas absorption with Fab fragments induced by bacterial IgA1 proteases of other cleavage specificities did not remove blocking antibodies. Consequently, we assume that the antibodies detected had been induced by a neoepitope an the Fab fragment of IgA1 exposed exclusively after cleavage with IgA1 proteases from Prevotella and Capnocytophaga, indicating in vivo activity of these IgA1 proteases. Evidence, though indirect, of in vivo activity of Prevotella and Capnocytophaga IgA1 proteases was present in 42 of 92 sera examined and in a significantly higher proportion of sera from adults with periodontal disease compared with control individuals. No correlation with disease was observed for the juvenile periodontitis groups.

Dental plaque as a biofilm

Dental plaque is the diverse microbial community found on the tooth surface embedded in a matrix of polymers of bacterial and salivary origin. Once a tooth surface is cleaned, a conditioning film of proteins and glycoproteins is adsorbed rapidly to the tooth surface. Plaque formation involves the interaction between early bacterial colonisers and this film (the acquired enamel pellicle). To facilitate colonisation of the tooth surface, some receptors on salivary molecules are only exposed to bacteria once the molecule is adsorbed to a surface. Subsequently, secondary colonisers adhere to the already attached early colonisers (co-aggregation) through specific molecular interactions. These can involve protein-protein or carbohydrate-protein (lectin) interactions, and this process contributes to determining the pattern of bacterial succession. As the biofilm develops, gradients in biologically significant factors develop, and these permit the co-existence of species that would be incompatible with each other in a homogenous environment. Dental plaque develops naturally, but it is also associated with two of the most prevalent diseases affecting industrialised societies (caries and periodontal diseases). Future strategies to control dental plaque will be targeted to interfering with the formation, structure and pattern of development of this biofilm.

Isolation and characterization of coaggregation-defective (Cog-) mutants of Streptococcus gordonii DL1 (Challis)

Streptococcus gordonii DL1 (Challis) bears coaggregation-mediating surface adhesins which recognize galactoside-containing surface polysaccharides on Streptococcus oralis 34, Streptococcus oralis C104, and Streptococcus SM PK509. Fifty-nine spontaneously-occurring coaggregation-defective (Cog-) mutants of S. gordonii DL1 unable to coaggregate with partner streptococci were isolated. Six representative Cog- mutants were characterized by their coaggregation properties with four Actinomyces naeslundii strains (T14V, PK947, PK606, PK984), Veillonella atypica PK1910, and Propionibacterium acnes PK93. The six representative Cog- mutants showed altered coaggregation with their streptococcal partners, A. naeslundii PK947, and P. acnes PK93. Based on the coaggregation phenotypes of these mutants, a model for the lactose-inhibitable coaggregation between S. gordonii DL1 and its partner bacteria is proposed. The potential use of these mutants in studies of oral biofilms is discussed.

Salivary amylase promotes adhesion of oral streptococci to hydroxyapatite

Recent studies have demonstrated that several species of oral streptococci, such as Streptococcus gordonii, bind soluble salivary alpha-amylase. The goal of the present study was to determine if amylase immobilized onto a surface such as hydroxyapatite can serve as an adhesion receptor for S. gordonii. Initially, human parotid saliva was fractionated on Bio-Gel P60, and fractions were screened for their ability to promote adhesion of S. gordonii to hydroxyapatite. Fractions containing alpha-amylase and proline-rich proteins promoted the adhesion of [3H]-labeled S. gordonii to hydroxyapatite. Similar findings were obtained with purified amylase and acidic proline-rich protein 1 (PRP1). Incubation of S. gordonii G9B in the presence of starch and maltotriose increased the binding of this strain to amylase-coated hydroxyapatite, while the adhesion of S. sanguis 10556 to amylase-coated hydroxyapatite was not affected by these saccharides. These results suggest that amylase may serve as a hydroxyapatite pellicle receptor for amylase-binding streptococci. Furthermore, starch and starch metabolites may enhance the adhesion of amylase-binding streptococci to amylase in dental pellicles to augment the formation of dental plaque.

Development of a flow method for susceptibility testing of oral biofilms in vitro

Bacteria in biofilms are known to be more resistant than bacteria in batch cultures to antimicrobial agents. The purpose of the present study was to develop a flow method for formation of oral biofilms permitting susceptibility testing of plaque bacteria. A brain heart infusion (BHI) Streptococcus sanguis 804 culture was pumped through a modified Robbins Device (MRD) with 25 exchangeable silicone disks at 40 ml/h. After 24-48 h disks were removed and biofilm cells dispersed by vortex mixing and low-output ultrasonication. Colony forming units (cfu)/cm2 were determined after aerobic incubation on blood agar plates. Optimal biofilm formation was found after growth for 48 h at 37 degrees C in BHI + 1% sucrose, using saliva-coated silicone disks in inverted MRDs, yielding on average 4.4 x 10(5) cfu/cm2. Similar results were obtained for S. sanguis ATCC 10556 and five clinical isolates. Testing the susceptibility of S. sanguis to chlorhexidine gluconate showed increased resistance of biofilms compared to batch culture. Thus an appropriate biofilm model for susceptibility testing of oral microorganisms has been established.

Clonal diversity of the Streptococcus mitis biovar 1 population in the human oral cavity and pharynx

A total of 250 isolates of oral streptococci were recovered from swabs of oropharyngeal surfaces of 3 members of one family. All isolates were examined by biochemical and serological means, and 106 isolates were identified as Streptococcus mitis biovar 1. These were typed by restriction endonuclease analysis using the enzymes EcoRI and HaeIII and further characterized by their whole-cell polypeptide profile patterns in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In addition, rabbit antisera raised against 8 reference strains of oral streptococci were used to characterize representative isolates both by their carbohydrate and protein antigens by Ouchterlony and Western blot analyses. Very limited biochemical diversity was observed among the 106 S. mitis biovar 1 isolates. In contrast, 24 different genotypes defined by restriction endonuclease analysis were detected, and each individual carried 6-13 types. Limited sharing of genotypes was observed between the 3 members of the same family and between the pharyngeal and buccal mucosa of single individuals. The antigenic analyses showed remarkable antigenic diversity between the 24 genotypes. The results provide a basis for studying the population dynamics of an oral commensal species and its interaction with the salivary immune system.

Site-related streptococcal attachment to buccocervical tooth surfaces. A correlative micromorphologic and microbiologic study

Scanning electron (SEM) microscopy of epoxy replicas made from dental impressions has shown that in buccal gingival recession the root surfaces are devoid of cementum, leaving the dentin exposed. In this study replication techniques were applied to correlate the micromorphology of the buccocervical region with early streptococcal attachment. The subjects were 27 healthy young adults. The buccocervical surfaces of all the premolars were meticulously cleaned. The subjects fasted for 2 h before impression-taking. Replicas were made from impressions in hydrophilic A-silicone, and streptococcal attachment was visualized by light microscopy of mitis-salivarius agar replicas incubated anaerobically for 48 h. The surface micromorphology was documented by SEM of corresponding epoxy replicas. Colonization only 2 h after cleaning was very sparse. Sites with healthy or inflamed gingivae had markedly different colonization patterns in the sulcular region. In 4 subjects with a total of 12 sites where gingival recession, undetected clinically, was disclosed by SEM, representative colonies were retrieved and identified microbiologically to species level. Two healthy sites per subject were also sampled. Streptococcus mutans and S. sobrinus were identified from eight sites with exposed root dentin. S. oralis predominated on the enamel surfaces. The method offers a valuable complement to in situ and in vitro microbiologic studies of exposed dentin and a novel technique for sampling clinical isolates of streptococci.

Emergence in human dental plaque and host distribution of amylase-binding streptococci

Salivary amylase is known to bind specifically to several species of oral streptococci. To assess the importance of this interaction in bacterial colonization of the oral cavity, we determined the proportion and identity of amylase-binding bacteria (ABB) in dental plaque of humans and various salivary amylase-secreting and non-secreting mammalian species. The numbers of ABB in undisturbed plaque collected over time from tooth surfaces of six human volunteers or from 14 other mammalian species were determined by means of a replicating assay. The mean proportion of ABB cultured aerobically from human teeth at 2 h was 10.5% (SD 10), at 8 h 7.9% (8), at 24 h 13% (11), and at 48 h 12% (9). The mean proportion of anaerobically cultured ABB found at 2 h was 3% (SD 4), at 8 h 5% (5), at 24 h 12% (9), and at 48 h 16% (12). Amylase-binding bacteria cultured from these samples resembled Streptococcus mitis, Streptococcus gordonii, Streptococcus salivarius, Streptococcus crista, or unidentified streptococci. In addition, only animals exhibiting salivary amylase activity in their saliva harbored ABB (ranging from 2 to 31% of the total flora), with the exception of the pig, where no ABB were found to colonize, despite considerable amylase activity in saliva. Only strains resembling S. mitis and S. salivarius and unspeciated strains were isolated from these mammals. These results suggest that amylase-binding streptococci are the predominant ABB in human plaque, and their numbers generally increase as plaque develops. Since ABB colonized only the oral cavities of hosts demonstrating salivary amylase activity, the ability to bind amylase may play an important role in oral colonization by these bacteria.

Saliva-bacterium interactions in oral microbial ecology

Saliva is thought to have a significant impact on the colonization of microorganisms in the oral cavity. Salivary components may participate in this process by one of four general mechanisms: binding to microorganisms to facilitate their clearance from the oral cavity, serving as receptors in oral pellicles for microbial adhesion to host surfaces, inhibiting microbial growth or mediating microbial killing, and serving as microbial nutritional substrates. This article reviews information pertinent to the molecular interaction of salivary components with bacteria (primarily the oral streptococci and Actinomyces) and explores the implications of these interactions for oral bacterial colonization and dental plaque formation. Knowledge of the molecular mechanisms controlling bacterial colonization of the oral cavity may suggest methods to prevent not only dental plaque formation but also serious medical infections that may follow microbial colonization of the oral cavity.

Mechanisms of dental plaque formation

Much effort has been placed on elucidating the diverse mechanisms of microbial adhesion to tooth surfaces. Both specific and non-specific types of adhesion have been envisaged. Pioneer colonizers represent a selected part of the oral microflora, and it has been assumed that specific adhesin-receptor interactions between the microbial surface and the pellicle account for this specificity. Whereas microbial adhesion to tooth surfaces is a general prerequisite for initiation of plaque formation, microbial multiplication is probably the dominant feature in the build-up of dental plaque. Local environmental factors which influence the establishment and composition of the ultimate plaque community are therefore of greater importance than initial adhesion per se. The highly individual and site-related characteristics of the plaque flora illustrate the selective power of the environment. Environmental conditions are not uniform. Thus, each site represents its own conditions are not uniform. Thus, each site represents its own distinct ecosystem, and the microbial composition at the site depends on the outcome of a variety of host-microbial and microbial-microbial interactions. The relative in vivo significance of these interactions is difficult to assess.

Use of restriction fragment polymorphism analysis of rRNA genes to assign species to unknown clinical isolates of oral viridans streptococci

This study evaluated restriction fragment length polymorphisms of rRNA genes (ribotyping) for genotypic identification of 53 oral isolates classified as "Streptococcus sanguis" by colony morphology. Isolates were from 8-h buccal plaque on lower first permanent molars of 20 subjects. DNA was digested with AatII and hybridized with digoxygenin-labeled cDNA of Escherichia coli 16S and 23S rRNA. Strains were ribotyped again with AlwNI or PvuII on the basis of the presence or absence of a 2,290-bp AatII band. Band patterns were compared with reference ribotypes for Streptococcus gordonii, Streptococcus sanguis, Streptococcus crista, Streptococcus oralis, Streptococcus mitis, and Streptococcus parasanguis strains. Forty-eight isolates could be assigned to a species (22 S. sanguis, 14 S. oralis, 12 S. gordonii). Multiple species were seen in 14 subjects; multiple strains of the same species occurred in 11 subjects. Our findings suggest that ribotyping can be used for genotypic identification of S. sanguis, S. oralis, and S. gordonii isolates.

Species identification of oral viridans streptococci by restriction fragment polymorphism analysis of rRNA genes

Oral streptococci formerly classified as Streptococcus sanguis have been divided into six genetic groups. Methods to identify those species by genotype are needed. This study compared restriction fragment polymorphisms of rRNA genes (ribotypes) for seven S. gordonii, three S. sanguis, four S. oralis, three S. mitis, one S. crista, and seven S. parasanguis strains classified in previous DNA hybridization studies, as well as one clinical isolate. DNA was digested with HindIII, PvuII, HindIII and PvuII combined, EcoRI, BamHI, AatII, AlwNI, and DraII. DNA fragments were hybridized with a digoxigenin-labeled cDNA probe obtained by reverse transcription of Escherichia coli 16S and 23S rRNA. S. oralis, S. mitis, and S. parasanguis all showed an isolated 2,290-bp band in AatII ribotypes that was absent from S. gordonii, S. sanguis, and S. crista. The last three groups showed species-specific bands with AatII and also with PvuII. S. oralis could be distinguished from S. mitis and S. parasanguis in AlwNI and DraII ribotypes. S. mitis and S. parasanguis could not be distinguished, since they shared multiple bands in PvuII, AlwNI, and EcoRI patterns. The clinical isolate in the panel was very similar to S. sanguis by all enzymes used. Our findings suggest that ribotyping may be useful for genotypic identification of oral viridans streptococci. Initial digests of clinical isolates might be made with AatII, followed by PvuII or AlwNI. Isolates then could be identified by comparing ribotype patterns with those of reference strains. This approach could facilitate clinical studies of these newly defined species.

Similar proportions of immunoglobulin A1 (IgA1) protease-producing streptococci in initial dental plaque of selectively IgA-deficient and normal individuals

By comparing the initial colonization of cleaned teeth in immunoglobulin A (IgA)-deficient, IgM-compensating individuals with that in normal individuals, no significant difference in the proportion of IgA1 protease-producing streptococci was found. Thus, as one of several bacterial means of immune evasion, the ability to cleave secretory IgA1 does not appear essential to the successful adherence of oral streptococci.

Inactivation of the gene encoding surface protein SspA in Streptococcus gordonii DL1 affects cell interactions with human salivary agglutinin and oral actinomyces

Cell surface protein SSP-5 in the oral bacterium Streptococcus gordonii M5 binds human salivary agglutinin in a Ca(2+)-dependent reaction (D.R. Demuth, E.E. Golub, and D. Malamud, J. Biol. Chem. 265:7120-7126, 1990). The region of the gene encoding an N-terminal segment of a related polypeptide (SspA) in S. gordonii DL1 (Challis) was isolated following polymerase chain reaction amplification of genomic DNA. The sspA gene in S. gordonii DL1 was insertionally inactivated by homologous recombination of the erythromycin resistance (Emr) determinant ermAM onto the streptococcal chromosome. The SspA polypeptide (apparent molecular mass, 210 kDa) was detected on Western blots (immunoblots) of spheroplast extracts and extracellular culture medium proteins from wild-type strain DL1 but was absent from Emr mutants. One SspA- mutant (designated OB220) was not altered in rate or extent of aggregation by whole saliva or parotid saliva but showed reduced aggregation in the presence of purified salivary agglutinin. Mutant bacteria were unaffected in their ability to adhere to hydroxylapatite beads coated with whole or parotid saliva and were unaltered in cell surface hydrophobicity. However, the SspA- strain OB220 was deficient in binding salivary agglutinin and in binding to six strains of Actinomyces naeslundii. Therefore, expression of SspA polypeptide in S. gordonii is associated with both agglutinin-dependent and agglutinin-independent aggregation and adherence reactions of streptococcal cells.

Saliva-binding protein (SsaB) from Streptococcus sanguis 12 is a lipoprotein

Two lipoprotein consensus sequences (Leu-X-X-Cys) are found in the presumptive signal peptide region (positions 12 to 15 and 17 to 20) of saliva-binding protein (SsaB) from Streptococcus sanguis 12. Three analogs of SsaB containing Cys-->Gly mutations were constructed by site-directed mutagenesis of pSA2, the recombinant plasmid expressing SsaB. [3H]palmitate was incorporated into SsaB only when the native Cys-20 residue was present. These data show that SsaB is a lipoprotein and that Cys-20 is the critical site for acylation.

Identification of a gene, rgg, which regulates expression of glucosyltransferase and influences the Spp phenotype of Streptococcus gordonii Challis

Streptococcus gordonii Challis was previously shown to give rise to phase variants expressing high (Spp+) or low (Spp-) levels of extracellular glucosyltransferase (GTF) activity. Here, shotgun cloning of an S. gordonii Spp+ chromosomal digest resulted in a chimeric plasmid (pAM5010) able to complement the Spp- phenotype. In addition, introduction of pAM5010 into an Spp+ strain resulted in a 10-fold increase in GTF expression. Deletion analysis of pAM5010 identified a 1.2-kb DNA segment which exhibited the same functional properties as pAM5010. Nucleotide sequence analysis of this region revealed a gene approximately 1 kb in size. The gene was designated rgg. Disruption of the chromosomal rgg gene open reading frame in an Spp+ strain resulted in strain DS512, which displayed an Spp(-)-like phenotype and had 3% of wild-type GTF activity. A plasmid containing the rgg gene was able to complement the DS512 phenotype and significantly increase GTF expression above wild-type levels. Sequence analysis and other data showed that the S. gordonii GTF determinant, designated gtfG, is located 66 bp downstream of the rgg gene. The sequence also revealed interesting inverted repeats which may play a role in the regulation of gtfG. We conclude that rgg positively regulates the expression of GTF and influences expression of the Spp phenotype.

Use of a replica-plate assay for the rapid assessment of salivary protein-bacteria interactions

A replica-plate assay was used to screen for the interaction of salivary molecules with dental plaque bacteria. Bacterial colonies cultured from supragingival plaque on sheep-blood (SB) agar were replica-plated onto nitrocellulose membranes overlaying SB or mitis-salivarius agar. Membranes with attached colonies were removed and incubated with 125I-amylase or 125I-proline-rich glycoprotein (PRG). Positive interactions were detected by autoradiography. Only strains of Streptococcus gordonii and Actinomyces viscosus bound amylase, and strains of A. viscosus bound PRG. The results suggest that amylase and PRG bind to selected species of aerobic dental plaque bacteria.

Ecological implications of glucosyltransferase phase variation in Streptococcus gordonii

When sucrose is provided as a substrate for glucosyltransferase (GTF), Spp+ cells of the oral bacteria Streptococcus gordonii grow embedded in an insoluble glucan mass associated with surfaces. Spp- phase variants with lower GTF activity, which either arise from or are grown with Spp+ cells, segregate preferentially as unattached cells in the culture supernatants. Conversely, Spp+ revertants preferentially accumulate on surfaces. GTF phase variation, therefore, may facilitate the dispersion of S. gordonii cells throughout the oral cavity.

Sucrose-promoted accumulation of growing glucosyltransferase variants of Streptococcus gordonii on hydroxyapatite surfaces

Streptococcus gordonii exhibits a phase variation involving expression of high (Spp+) or low (Spp-) glucosyltransferase activity. The related bacterial accumulation on hydroxyapatite (HA) and saliva-coated HA surfaces was examined and found to be significant. Spp+ cells growing anaerobically in a defined medium utilize about 30% of the glucose available from sucrose to make insoluble glucans. These glucans formed cohesive masses on HA beads, which contained 80 to 90% of the total bacteria. The bacterial polymer mass had a volume of about 40 microns3 and contained more than 5 x 10(10) viable cells per cm3. In the absence of sucrose, the beads were saturated by 1 x 10(8) to 2 x 10(8) Spp+ cells. Spp- bacteria, which make 30-fold less glucan than do Spp+ bacteria, did not accumulate on surfaces in numbers significantly above the saturation level of 1 x 10(8) to 2 x 10(8) cells in the presence or absence of sucrose. Insoluble glucan synthesized by Spp+ cells from sucrose also enabled these bacteria to accumulate on saliva-coated HA seven times more effectively than the Spp- cells and 10 times more effectively than the Spp+ cells grown in medium without sucrose.

Delineation of a segment of adsorbed salivary acidic proline-rich proteins which promotes adhesion of Streptococcus gordonii to apatitic surfaces

Cells of several strains of Streptococcus gordonii attached in much higher numbers to experimental pellicles formed from samples of submandibular or parotid saliva on hydroxyapatite (HA) beads than to buffer controls. The nature of the salivary components responsible were investigated by preparing experimental pellicles from chromatographic fractions of submandibular saliva obtained from Trisacryl GF 2000M columns. Adhesion of S. gordonii Blackburn was promoted by two groups of fractions. The adhesion-promoting activity in the first group of fractions was associated with the family of acidic proline-rich proteins (PRPs), while that of the second group is as yet unidentified. Experimental pellicles prepared by treating HA with 2 micrograms of pure 150-amino-acid-residue PRPs (PRP-1, PRP-2, and PIF-s) promoted adhesion of S. gordonii Blackburn cells to an extent comparable to that obtained with unfractionated saliva. However, pellicles prepared from a 106-residue PRP (PRP-3) were significantly less effective, and those prepared from the amino-terminal tryptic peptide (residues 1 to 30) of the PRP and the salivary phosphoprotein statherin were completely ineffective in promoting adhesion. Although adhesion of several strains of S. gordonii was promoted by adsorbed PRP-1, the adhesion of several strains of Streptococcus sanguis or Streptococcus oralis was either not affected or only weakly enhanced by this protein. S. gordonii cells bound avidly to PRPs adsorbed onto HA beads, but the streptococci did not appear to bind PRPs in solution, since concentrations of PRP as high as 200 micrograms/ml did not inhibit binding of bacterial cells to pellicles prepared from pure PRP. S. gordonii cells also attached well to PRP or a synthetic decapeptide representing residues 142 to 150 of the PRP when the peptide was linked to agarose beads. Studies with a series of synthetic decapeptides indicated that the minimal segment of PRP which promoted high levels of S. gordonii adhesion was the carboxy-terminal dipeptide Pro-Gln (residues 149 and 150).

Ecology of viridans streptococci in the oral cavity and pharynx

Recently published taxonomic studies of viridans streptococci have resulted in several changes in the nomenclature and definition of oral streptococcal species. With this background, the ecology of streptococci in the oropharyngeal cavities was reinvestigated. The results based on the examination of 1426 streptococcal isolates confirmed and extended earlier findings. Apart from mature supragingival plaque, which contained a mixture of all orally encountered streptococci, each site showed a characteristic streptococcal flora. Initial dental plaque formation is primarily associated with Streptococcus sanguis, Streptococcus mitis biovar 1 and Streptococcus oralis. Our investigation showed that S. sanguis and S. mitis biovar 1 were the most prominent streptococci, also on buccal mucosa. In contrast, S. oralis was almost exclusively found in initial dental plaque. Streptococcus gordonii, formerly part of S. sanguis, was found in small numbers on the oropharyngeal mucosa and in mature supragingival plaque. The dorsum of the tongue was dominated by S. mitis biovar 2 and Streptococcus salivarius, the latter of which was predominant also on the pharyngeal mucosa. Streptococcus anginosus was by far the most predominant streptococcus in subgingival plaque. Immunoglobulin A1 (IgA1) protease-producing streptococci were primarily isolated from initial dental plaque and from the buccal mucosa. This lends further support to the concept of IgA1 proteases being important for the ability of streptococci to evade the local immune defence during their initial colonization of certain oral surfaces.

Ability to bind salivary alpha-amylase discriminates certain viridans group streptococcal species

A collection of 144 viridans group streptococcal strains recently characterized as part of a taxonomic study was examined for the ability to bind salivary alpha-amylase. This property was found in most strains of Streptococcus gordonii and Streptococcus mitis and in occasional strains of Streptococcus anginosus and Streptococcus salivarius. In contrast, all strains of Streptococcus sanguis, Streptococcus oralis, Streptococcus vestibularis, and Streptococcus mutans lacked alpha-amylase-binding capacity. A rapid and easy assay described in this paper may be an important supplementary test for identification of oral streptococci.