Isolation of a coaggregation-inhibiting cell wall polysaccharide from Streptococcus sanguis H1

Interbacterial Adhesion Networks within Early Oral Biofilms of Single Human Hosts

Specific interbacterial adhesion, termed coaggregation, is well established for three early colonizers of the plaque biofilm: streptococci, actinomyces, and veillonellae. However, little is known about interactions of other early colonizers and about the extent of interactions within the bacterial community from a single host. To address these gaps, subject-specific culture collections from two individuals were established using an intraoral biofilm retrieval device. Molecular taxonomy (Human Oral Microbe Identification Microarray [HOMIM]) analysis of biofilm samples confirmed the integrity and completeness of the collections. HOMIM analysis verified the isolation of Streptococcus gordonii and S. anginosus from only one subject, as well as isolation of a previously uncultivated streptococcal phylotype from the other subject. Strains representative of clonal diversity within each collection were further characterized. Greater than 70% of these streptococcal strains from each subject coaggregated with at least one other coisolate. One-third of the strains carry a known coaggregation mediator: receptor polysaccharide (RPS). Almost all nonstreptococcal isolates coaggregated with other coisolates. Importantly, certain Rothia strains demonstrated more coaggregations with their coisolated bacteria than did any Streptococcus or Actinomyces strain, and certain Haemophilus isolates participated in twice as many. Confocal microscopy of undisturbed biofilms showed that Rothia and Haemophilus each occur in small multispecies microcolonies. However, in confluent high-biomass regions, Rothia occurred in islands whereas Haemophilus was distributed throughout. Together, the data demonstrate that coaggregation networks within an individual's oral microflora are extensive and that Rothia and Haemophilus can be important initiators of cell-cell interactions in the early biofilm.IMPORTANCE Extensive involvement of specific interbacterial adhesion in dental plaque biofilm formation has been postulated based on in vitro coaggregation between oral bacteria from culture collections that are not subject specific. In the present study, subject-specific culture collections were obtained from early plaque biofilm of two volunteers, and coaggregations within each culture collection were assayed. Coaggregations, several of which involved a coaggregation-mediating cell surface molecule known from well-studied streptococci, were widespread. Unexpectedly, the little-studied organisms Haemophilus and Rothia participated in the greatest numbers of interactions with community members; these two organisms showed different distributions within the undisturbed biofilm. The data show that coaggregation networks encompass most organisms within the biofilm community of each individual, and they indicate prominent participation of organisms such as Haemophilus and Rothia in early plaque biofilm formation.

Probing of microbial biofilm communities for coadhesion partners

Investigations of interbacterial adhesion in dental plaque development are currently limited by the lack of a convenient assay to screen the multitude of species present in oral biofilms. To overcome this limitation, we developed a solid-phase fluorescence-based screening method to detect and identify coadhesive partner organisms in mixed-species biofilms. The applicability of this method was demonstrated using coaggregating strains of type 2 fimbrial adhesin-bearing actinomyces and receptor polysaccharide (RPS)-bearing streptococci. Specific adhesin/receptor-mediated coadhesion was detected by overlaying bacterial strains immobilized to a nitrocellulose membrane with a suspended, fluorescein-labeled bacterial partner strain. Coadhesion was comparable regardless of which cell type was labeled and which was immobilized. Formaldehyde treatment of bacteria, either in suspension or immobilized on nitrocellulose, abolished actinomyces type 2 fimbrial adhesin but not streptococcal RPS function, thereby providing a simple method for assigning complementary adhesins and glycan receptors to members of a coadhering pair. The method's broader applicability was shown by overlaying colony lifts of dental plaque biofilm cultures with fluorescein-labeled strains of type 2 fimbriated Actinomyces naeslundii or RPS-bearing Streptococcus oralis. Prominent coadhesion partners included not only streptococci and actinomyces, as expected, but also other bacteria not identified in previous coaggregation studies, such as adhesin- or receptor-bearing strains of Neisseria pharyngitis, Rothia dentocariosa, and Kingella oralis. The ability to comprehensively screen complex microbial communities for coadhesion partners of specific microorganisms opens a new approach in studies of dental plaque and other mixed-species biofilms.

Capnocytophaga spp. involvement in bone infections: a review

Capnocytophaga are commensal gliding bacteria that are isolated from human and animal oral flora and are responsible for infections both in immunocompromised and immunocompetent hosts. Accumulation of microbial plaque, loss of collagen attachment, and alveolar bone resorption around the tooth can lead to local Capnocytophaga spp. bone infections. These capnophilic bacteria, from oral sources or following domestic animal bites, are also causative agents of bacteraemia and systemic infections as well as osteomyelitis, septic arthritis, and infections on implants and devices. The present literature review describes the main aetiologies of bone infections due to Capnocytophaga spp., the cellular mechanisms involved, methods used for diagnosis, antimicrobial susceptibility, and effective treatments.

Adherence of Bilophila wadsworthia to cultured human embryonic intestinal cells

Adherence of Bilophila wadsworthia to the cultured human embryonic intestinal cell line, Intestine 407 (Int 407), varied among the strains tested from strongly adherent (76-100% cells positive for one or more adherent bacteria) to non- or weakly adherent (0-25% positive cells). Although negative staining revealed that infrequent cells of an adherent strain, WAL 9077, the adherent type-strain, WAL 7959, and a non-adherent strain, WAL 8448, expressed loosely associated fimbrial structures, a role for these structures in adhesion could not be confirmed with either scanning or thin-section electron micrography. Ruthenium red staining of thin-section preparations and subsequent electron microscopy failed to reveal an extensive extracellular polysaccharide layer. SDS-PAGE analysis of crude outer membrane fractions of WAL 9077 and WAL 8448 demonstrated clear differences in their major and minor outer membrane protein components. Thus, we postulate that the adherence of B. wadsworthia to Int 407 cells is mediated by an outer membrane or cell wall component.

Identification of the Pseudomonas aeruginosa 1244 pilin glycosylation site

Previous work (P. Castric, F. J. Cassels, and R. W. Carlson, J. Biol. Chem. 276:26479-26485, 2001) has shown the Pseudomonas aeruginosa 1244 pilin glycan to be covalently bound to a serine residue. N-terminal sequencing of pilin fragments produced from endopeptidase treatment and identified by reaction with a glycan-specific monoclonal antibody indicated that the glycan was present between residue 75 and the pilin carboxy terminus. Further sequencing of these peptides revealed that serine residues 75, 81, 84, 105, 106, and 108 were not modified. Conversion of serine 148, but not serine 118, to alanine by site-directed mutagenesis, resulted in loss of the ability to carry out pilin glycosylation when tested in an in vivo system. These results showed the pilin glycan to be attached to residue 148, the carboxy-terminal amino acid. The carboxy-proximal portion of the pilin disulfide loop, which is adjacent to the pilin glycan, was found to be a major linear B-cell epitope, as determined by peptide epitope mapping analysis. Immunization of mice with pure pili produced antibodies that recognized the pilin glycan. These sera also reacted with P. aeruginosa 1244 lipopolysaccharide as measured by Western blotting and enzyme-linked immunosorbent assay.

Structural characterization of the Pseudomonas aeruginosa 1244 pilin glycan

An antigenic similarity between lipopolysaccharide (LPS) and glycosylated pilin of Pseudomonas aeruginosa 1244 was noted. We purified a glycan-containing molecule from proteolytically digested pili and showed it to be composed of three sugars and serine. This glycan competed with pure pili and LPS for reaction with an LPS-specific monoclonal antibody, which also inhibited twitching motility by P. aeruginosa bearing glycosylated pili. One-dimensional NMR analysis of the glycan indicated the sugars to be 5N beta OHC(4)7NfmPse, Xyl, and FucNAc. The complete proton assignments of these sugars as well as the serine residue were determined by COSY and TOCSY. Electrospray ionization mass spectrometry (MS) determined the mass of this molecule to be 771.5. The ROESY NMR spectrum, tandem MS/MS analysis, and methylation analysis provided information on linkage and the sequence of oligosaccharide components. These data indicated that the molecule had the following structure: alpha-5N beta OHC(4)7NFmPse-(2-->4)beta-Xyl-(1-->3)-beta-FucNAc-(1-->3)-beta-Ser.

Microbial biofilms: from ecology to molecular genetics

Biofilms are complex communities of microorganisms attached to surfaces or associated with interfaces. Despite the focus of modern microbiology research on pure culture, planktonic (free-swimming) bacteria, it is now widely recognized that most bacteria found in natural, clinical, and industrial settings persist in association with surfaces. Furthermore, these microbial communities are often composed of multiple species that interact with each other and their environment. The determination of biofilm architecture, particularly the spatial arrangement of microcolonies (clusters of cells) relative to one another, has profound implications for the function of these complex communities. Numerous new experimental approaches and methodologies have been developed in order to explore metabolic interactions, phylogenetic groupings, and competition among members of the biofilm. To complement this broad view of biofilm ecology, individual organisms have been studied using molecular genetics in order to identify the genes required for biofilm development and to dissect the regulatory pathways that control the plankton-to-biofilm transition. These molecular genetic studies have led to the emergence of the concept of biofilm formation as a novel system for the study of bacterial development. The recent explosion in the field of biofilm research has led to exciting progress in the development of new technologies for studying these communities, advanced our understanding of the ecological significance of surface-attached bacteria, and provided new insights into the molecular genetic basis of biofilm development.

Adhesive interactions between medically important yeasts and bacteria

Yeasts are being increasingly identified as important organisms in human infections. Adhesive interactions between yeasts and bacteria may contribute to yeast retention at body sites. Methods for studying adhesive interactions between bacterial strains are well known, and range from simple macroscopic methods to flow chamber systems with complex image analysis capabilities. The adhesive interactions between bacteria and yeasts have been studied employing several of the methods originally developed for studying adhesive interactions between bacteria. However, in many of the methods employed the larger size of the yeasts as compared with bacteria results in strong sedimentation of the yeasts, often invalidating the method adapted. In addition, most methods are semi-quantitative and do not properly control mass transport. Consequently, adhesive interaction mechanisms between yeasts and bacteria identified hitherto, including lectin binding and protein-protein interactions, must be regarded with caution. Extensive physico-chemical characteristics of yeast cell surfaces are not available and a physico-chemical mechanism has not yet been put forth. A new method for quantifying adhesive interactions between yeasts and bacteria is proposed, based on the use of a parallel plate flow chamber, in which the influence of adhering bacteria upon the kinetics of yeast adhesion and aggregation of the adhering yeasts is quantitatively evaluated, under carefully controlled mass transport.

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.

Adhesin receptors of human oral bacteria and modeling of putative adhesin-binding domains

Adherence by bacteria to a surface is critical to their survival in the human oral cavity. Many types of molecules are present in the saliva and serous exudates that form the acquired pellicle, a coating on the tooth surface, and serve as receptor molecules for adherent bacteria. The primary colonizing bacteria utilize adhesins to adhere to specific pellicle receptor molecules, then may adhere to other primary colonizers via adhesins, or may present receptor molecules to be utilized by secondary colonizing species. The most common primary colonizing bacteria are streptococci, and six streptococcal cell wall polysaccharide receptor molecules have been structurally characterized. A comparison of the putative adhesin disaccharide-binding regions of the six polysaccharides suggests three groups. A representative of each group was modeled in molecular dynamics simulations. In each case it was found that a loop formed between the galactofuranose beta (Galf beta) and an oxygen of the nearest phosphate group on the reducing side of the Galf beta, that this loop was stabilized by hydrogen bonds, and that within each loop resides the putative disaccharide-binding domain.

Adherence of Candida albicans to a cell surface polysaccharide receptor on Streptococcus gordonii

Candida albicans ATCC 10261 and CA2 bound to cells of the oral bacteria Streptococcus gordonii, Streptococcus oralis, and Streptococcus sanguis when these bacteria were immobilized onto microtiter plate wells, but they did not bind to cells of Streptococcus mutans or Streptococcus salivarius. Cell wall polysaccharide was extracted with alkali from S. gordonii NCTC 7869, the streptococcal species to which C. albicans bound with highest affinity, and was effective in blocking the coaggregation of C. albicans and S. gordonii cells in the fluid phase. When fixed to microtiter plate wells, the S. gordonii polysaccharide was bound by all strains of C. albicans tested. The polysaccharide contained Rha, Glc, GalNAc, GlcNAc, and Gal and was related compositionally to previously characterized cell wall polysaccharides from strains of S. oralis and S. sanguis. The adherence of yeast cells to the immobilized polysaccharide was not inhibitable by a number of saccharides. Antiserum raised to the S. gordonii NCTC 7869 polysaccharide blocked adherence of C. albicans ATCC 10261 to the polysaccharide. The results identify a complex cell wall polysaccharide of S. gordonii as the coaggregation receptor for C. albicans. Adherent interactions of yeast cells with streptococci and other bacteria may be important for colonization of both hard and soft oral surfaces by C. albicans.

Isolation and structural characterization of adhesin polysaccharide receptors

The procedure for the purification of the adhesin polysaccharide receptor and its hexasaccharide repeating unit from whole S. oralis ATCC 55229 by chemical, enzymatic, and chromatographic techniques has been described. Chemical, chromatographic, and mass spectrometric procedures allow preliminary structural characterization of the hexasaccharide repeating unit and polysaccharide. The structural characterizations of the hexasaccharide and polysaccharide are completed using several 1D and 2D NMR techniques. Identification of the anomeric 1H and 13C signals of the glycosyl residues permits, by virtue of their chemical shifts and coupling constants (3JHH and 1JCH), the determination of the configurations of the glycosidic linkages. The HMBC connectivities permit the establishment of the hexasaccharide sequence as Rhap alpha(1-->2)Rhap alpha(1-->3)Galp alpha(1-->3)Galp beta(1-->4)Glcp beta(1-->3)Gal. The 1H NMR chemical shifts of the polysaccharide, as determined by the combination of COSY and TOCSY experiments, and the observed interglycosidic NOESY cross-peaks reveal the structure of the polysaccharide to be [formula: see text] where the position of the glycerol (Gro) phosphate moiety has been determined by [1H, 31P] NMR spectroscopy.

Carbohydrate composition analysis of bacterial polysaccharides: optimized acid hydrolysis conditions for HPAEC-PAD analysis

The capsular polysaccharide from Haemophilus influenzae type b (polyribosyl ribitol-phosphate; PRP) and the capsular polysaccharides from Streptococcus pneumoniae types 6B, 14, 18C, and 23F (Pn6B, Pn14, Pn18C, and Pn23F) were subjected to acid hydrolysis using hydrofluoric (HF) and/or trifluoroacetic acid (TFA) and high-pH anion-exchange chromatography with pulsed amperometric detection in an effort to identify optimum hydrolysis conditions for composition analysis of their carbohydrate components. With the exception of PRP, composition analyses of polysaccharides containing a phosphate moiety in the repeating unit structure (Pn6B, Pn18C, and Pn23F) are significantly improved by subjecting the sample to HF hydrolysis (65 degrees C, 1 h) followed by TFA hydrolysis (98 degrees C, 16 h). This results in essentially quantitative hydrolysis of the phosphodiester bond to the carbohydrate components, which otherwise remained predominantly phosphorylated and poorly accounted for in the analysis. Optimum analysis of PRP was achieved following a 2-h hydrolysis with TFA at 80 degrees C, whereas Pn14 showed optimum results after a 16-h hydrolysis with TFA at 98 degrees C. These analyses also provide information about the relative susceptibility to acid hydrolysis of the various glycosidic and phosphodiester bonds in these polysaccharides, with evidence to suggest that the acid lability of a given bond can be dramatically different from one polysaccharide to another.

Binding of mutagens by fractions of the cell wall skeleton of lactic acid bacteria on mutagens

The binding effect of cells and cell fractions, cell wall skeleton, cytoplasm, whole cells, and cell wall skeleton treated by lysozyme and alpha-amylase at 37 degrees C for 5 h, on Trp-P-1 (3-amino-1,4-dimethyl-[5H]pyrido [4,3-b]indole) and Trp-P-2 (3-amino-1-methyl-[5H]-pyrido[4,3-b]indole) were investigated. The cell and cell wall skeleton of Streptococcus cremoris Z-25 had greater binding activity, but cytoplasm and extract of cell wall skeleton did not bind Trp-P-1 and Trp-P-2. When the cells or cell wall skeleton were treated with lysozyme and alpha-amylase, unbound Trp-P-1 and Trp-P-2 concentrations were greater than that of the untreated control. It is possible that cell walls may be involved in the binding of mutagenic pyrolyzates to lactic acid bacteria. The cell wall skeleton of S. cremoris Z-25, Lactobacillus acidophilus IFO 13951, and Bifidobacterium bifidum IFO 14252 showed binding of Trp-P-1, 2-amino-6-methyldipyrido(1,2-a:3',2'- d)imidazole, 2-amino-5-phenylpyridine, 2-amino-3-methylimidazo(4,5-f)quinoline, 2-amino-3,4-dimethylimidazo(4,5-f) quinoline, and 2-amino-3,8-dimethylimidazo(4,5-f)quinoxaline. The cell wall skeleton of S. cremoris group and Streptococcus lactis also showed the binding activity with A N-nitrosodimethylamine. The binding of Trp-P-1 to cell walls was very high, and the binding of mutagenic pyrolyzates was variable with different bacterial species. The peptidoglycan complex and polysaccharides liberated from cell wall skeleton of S. cremoris Z-25 showed strong binding of Trp-P-2. Peptidoglycans has a binding effect of about 19.86 micrograms/mg; polysaccharides had a binding effect of 14 micrograms/mg.

Platelet-interactive products of Streptococcus sanguis protoplasts

To isolate a more native, platelet-interactive macromolecule (class II antigen) of Streptococcus sanguis, cultured protoplasts were used as a source. Protoplasts were optimally prepared from fresh washed cells by digestion with 80 U of mutanolysin per ml for 75 min at 37 degrees C while osmotically stabilized in 26% (wt/vol) raffinose. Osmotically stabilized forms were surrounded by a 9-nm bilaminar membrane, as shown by transmission electron microscopy. Protoplasts were cultured in chemically defined synthetic medium and osmotically stabilized by ammonium chloride. Spent culture media were harvested daily for 7 days. Each day, soluble proteins were isolated from media, preincubated with platelet-rich plasma, and tested for inhibition of platelet aggregation induced by S. sanguis cells. Products released from S. sanguis protoplasts and reactive with an anti-class II antigen immunoaffinity matrix were able to inhibit S. sanguis-induced platelet aggregation. As resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, anti-class II-reactive protoplast products included silver-stained bands of 67, 79, 115, 216, and 248 kDa. The 115-kDa protein fraction was isolated by gel filtration and ion-exchange chromatography. This form of the class II antigen contained N-formylmethionine at its amino terminus. Rhamnose constituted 18.2% of the total residual dry weight and nearly half of its carbohydrate content. Diester phosphorus constituted 1% of this fraction. After trypsinization of the protoplast products from either preparation, a 65-kDa protein fragment was recovered. This protoplast protein fragment and the S. sanguis cell-derived 65-kDa class II antigen, previously implicated in the induction of platelet aggregation, were shown to be functionally and immunologically identical.

Insertional inactivation of the gene encoding a 76-kilodalton cell surface polypeptide in Streptococcus gordonii Challis has a pleiotropic effect on cell surface composition and properties

A library of Streptococcus gordonii DL1-Challis DNA was constructed in lambda gt11. Phage plaques were screened for production of antigens that reacted with antiserum to S. gordonii cell surface proteins. A recombinant phage denoted lambda gt11-cp2 was isolated that carried 1.85 kb of S. gordonii DNA and that expressed an antigen with a molecular mass of 29 kDa in Escherichia coli. Antibodies that reacted with the expression product were affinity purified and were shown to react with a single polypeptide antigen with a molecular mass of 76 kDa in S. gordonii DL1-Challis. A segment (0.85 kb) of the cloned DNA within the transcription unit was ligated into a nonreplicative plasmid carrying an erythromycin resistance determinant and transformed into S. gordonii DL1-Challis. The plasmid integrated onto the chromosome, and expression of the 76-kDa polypeptide antigen was abolished. The gene inactivation had no obvious effect on bacterial growth or on a number of phenotypic properties, including hydrophobicity and adherence. However, it abolished serum-induced cell aggregation, mutant cells had reduced aggregation titers in saliva and in colostrum immunoglobulin A, and it also reduced coaggregation with some Actinomyces species. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles of cell envelope proteins from wild-type and mutant strains showed that as well as lacking the surface-exposed 76-kDa polypeptide, mutant cell envelopes were deficient in several other polypeptides, including those that bound to immunoglobulin A. Expression of the gene encoding the 76-kDa polypeptide in S. gordonii appeared to be critical for functional conformation of the cell surface.

Coaggregation of Streptococcus sanguis and other streptococci with Candida albicans

Thirteen strains of viridans group streptococci and two strains of other streptococci were tested for coaggregation with Candida albicans. Streptococcus sanguis strains generally exhibited low levels of adherence to 28 degrees C-grown exponential-phase yeast cells, but starvation of yeast cells for glucose at 37 degrees C (or at 28 degrees C) increased their coaggregating activity with these streptococci by at least tenfold. This was a property common to four C. albicans strains tested, two of which were able to form mycelia (6406 and MEN) and two of which were not (MM2002 and CA2). The expression of the coaggregation adhesin during yeast cell starvation was inhibited by addition of trichodermin or amphotericin B. The strains of S. sanguis, Streptococcus gordonii, and Streptococcus oralis tested for coaggregating activity encompassed a diverse range of physiological and morphological types, yet all exhibited saturable coaggregation with starved C. albicans cells. There was no correlation of cell surface hydrophobicity, of either yeast or streptococcal cells, with their abilities to coaggregate. Strains of Streptococcus anginosus also coaggregated with starved yeast cells; Streptococcus salivarius and Streptococcus pyogenes coaggregated to a lesser degree with C. albicans, and the coaggregation with S. pyogenes was not promoted by yeast cell starvation; Streptococcus mutans and Enterococcus faecalis did not coaggregate with yeast. The coaggregation reactions of S. sanguis and S. gordonii with C. albicans were inhibited by EDTA and by heat or protease treatment of the yeast cells and were not reversible by the addition of lactose or other simple sugars. These observations extend the range of intergeneric coaggregations that are known to occur between oral microbes and suggest that coaggregations of C. albicans with viridans group streptococci may be important for colonization of oral surfaces by the yeast.

Purification and characterization of a Bacteroides loeschei adhesin that interacts with procaryotic and eucaryotic cells

The adhesin of Bacteroides loeschei PK1295 that mediates coaggregation with Streptococcus sanguis 34 and hemagglutination of erythrocytes was purified to electrophoretic homogeneity. The lectinlike protein has an estimated native Mr of 450,000 and consists of six subunits of identical molecular weight (Mr 75,000). The purified adhesin appears to be a basic protein with a pI between 7.4 and 8.0. Amino acid and N-terminal sequence analyses were carried out with the purified protein. These indicated that the protein contains a large number of Asx and Glx residues as well as basic amino acid residues. The binding site of the pure adhesin retained its native configuration during purification. When preincubated with streptococcal partner cells at pH 4.6, the adhesin prevented B. loeschei cells from coaggregating with the streptococci. An adhesin preparation adjusted to a pH of 6.8 rapidly agglutinated both streptococci and neuraminidase-treated erythrocytes. Galactosides inhibited the agglutination reactions.

Inhibition of coaggregation between Fusobacterium nucleatum and Porphyromonas (Bacteroides) gingivalis by lactose and related sugars

The coaggregation of Fusobacterium nucleatum PK1594 and Porphyromonas (Bacteroides) gingivalis PK1924 was inhibited equally well by lactose, N-acetyl-D-galactosamine, and D-galactose, which caused 50% inhibition of coaggregation at 2 mM sugar concentration. Other sugars such as D-galactosamine, D-fucose (6-deoxy-D-galactose), and alpha-methyl- and beta-methyl-D-galactosides also inhibited coaggregation. Sugar specificity was apparent, since neither L-fucose, L-rhamnose, N-acetyl-D-glucosamine, nor N-acetylneuraminic acid was an inhibitor. Protease treatment of the fusobacterium completely abolished coaggregation, whereas it had no effect on the coaggregating activity of the porphyromonad. Although numerous lactose-inhibitable coaggregating pairs are known to occur among gram-positive bacteria, this report and the accompanying survey (P. E. Kolenbrander, R. N. Andersen, and L. V. H. Moore, Infect. Immun. 57:3194-3203, 1989) are the first studies demonstrating the extensive nature of this type of interaction between gram-negative human oral bacteria. The significance of galactoside-inhibitable coaggregations between these two potential periodontal pathogens is discussed.