Actinobacillus actinomycetemcomitans strains Y4 and N27 adhere to hydroxyapatite by distinctive mechanisms

Aggregatibacter actinomycetemcomitans, a potent immunoregulator of the periodontal host defense system and alveolar bone homeostasis

Aggregatibacter actinomycetemcomitans is a perio-pathogenic bacteria that has long been associated with localized aggressive periodontitis. The mechanisms of its pathogenicity have been studied in humans and preclinical experimental models. Although different serotypes of A. actinomycetemcomitans have differential virulence factor expression, A. actinomycetemcomitans cytolethal distending toxin (CDT), leukotoxin, and lipopolysaccharide (LPS) have been most extensively studied in the context of modulating the host immune response. Following colonization and attachment in the oral cavity, A. actinomycetemcomitans employs CDT, leukotoxin, and LPS to evade host innate defense mechanisms and drive a pathophysiologic inflammatory response. This supra-physiologic immune response state perturbs normal periodontal tissue remodeling/turnover and ultimately has catabolic effects on periodontal tissue homeostasis. In this review, we have divided the host response into two systems: non-hematopoietic and hematopoietic. Non-hematopoietic barriers include epithelium and fibroblasts that initiate the innate immune host response. The hematopoietic system contains lymphoid and myeloid-derived cell lineages that are responsible for expanding the immune response and driving the pathophysiologic inflammatory state in the local periodontal microenvironment. Effector systems and signaling transduction pathways activated and utilized in response to A. actinomycetemcomitans will be discussed to further delineate immune cell mechanisms during A. actinomycetemcomitans infection. Finally, we will discuss the osteo-immunomodulatory effects induced by A. actinomycetemcomitans and dissect the catabolic disruption of balanced osteoclast-osteoblast-mediated bone remodeling, which subsequently leads to net alveolar bone loss.

Aggregatibacter actinomycetemcomitans - a tooth killer?

Strong evidence is available on Aggregatibacter actinomycetemcomitans (A.a) on its role as the causative agent of localised juvenile periodontitis (LJP), a disease characterised by rapid destruction of the tooth-supporting tissues. This organism possesses a large number of virulence factors with a wide range of activities which enable it to colonise the oral cavity, invade periodontal tissues, evade host defences, initiate connective tissue destruction and interfere with tissue repair. Adhesion to epithelial and tooth surfaces is dependent on the presence of surface proteins and structures such as microvesicles and fimbriae. Invasion has been demonstrated in vivo and in vitro. The organism has a number of means of evading host defences which include: (i) production of leukotoxin; (ii) producing immunosuppressive factors; (iv) secreting proteases capable of cleaving IgG; and (v) producing Fc-binding.

Learned and unlearned concepts in periodontal diagnostics: a 50-year perspective

In the past 50 years, conceptual changes in the field of periodontal diagnostics have paralleled those associated with a better scientific understanding of the full spectrum of processes that affect periodontal health and disease. Fifty years ago, concepts regarding the diagnosis of periodontal diseases followed the classical pathology paradigm. It was believed that the two basic forms of destructive periodontal disease were chronic inflammatory periodontitis and 'periodontosis'- a degenerative condition. In the subsequent 25 years it was shown that periodontosis was an infection. By 1987, major new concepts regarding the diagnosis and pathogenesis of periodontitis included: (i) all cases of untreated gingivitis do not inevitably progress to periodontitis; (ii) progression of untreated periodontitis is often episodic; (iii) some sites with untreated periodontitis do not progress; (iv) a rather small population of specific bacteria ('periodontal pathogens') appear to be the main etiologic agents of chronic inflammatory periodontitis; and (v) tissue damage in periodontitis is primarily caused by inflammatory and immunologic host responses to infecting agents. The concepts that were in place by 1987 are still largely intact in 2012. However, in the decades to come, it is likely that new information on the human microbiome will change our current concepts concerning the prevention, diagnosis and treatment of periodontal diseases.

Influence of different biomaterials on the viability of Aggregatibacter actinomycetemcomitans

OBJECTIVES

The aim of the present in vitro study was to evaluate the effects of different biomaterials used for regenerative periodontal surgery on the growth of the periodontopathogen Aggregatibacter actinomycetemcomitans.

METHODS

Three commercially available biomaterials of synthetic origin (hydroxyapatite/beta-tricalcium phosphate, nanostructured hydroxyapatite paste, oily calcium hydroxide suspension), a bovine-derived xenograft as well as an enamel matrix derivative (EMD) were added in different concentrations to calibrated suspensions of A. actinomycetemcomitans ATCC 43718/33384 (serotype b/c). Equal aliquots (0.1 ml) for the viability assay were taken after 5 min, 1h, 3h, 8h and 24h, plated on blood agar and incubated in an anaerobic environment for 48 h at 37°C. Viable cell counts were expressed as colony forming units (cfu)/0.1 ml.

RESULTS

The results demonstrated that none of the investigated biomaterials could inhibit the growth of A. actinomycetemcomitans serotype b. A marked growth reduction of A. actinomycetemcomitans serotype c was observed in the presence of oily calcium hydroxide suspension and nanostructured hydroxyapatite. In contrast, no significant growth inhibition could be observed in the presence of hydroxyapatite/beta-tricalcium phosphate, enamel matrix derivative and bovine-derived xenograft.

CONCLUSIONS

The results of the present study suggest that none of the investigated biomaterials possesses antimicrobial properties against A. actinomycetemcomitans serotype b. Therefore, the use of these biomaterials for regenerative procedures should be weighted critically in the presence of A. actinomycetemcomitans serotype b.

Molecular pathogenicity of the oral opportunistic pathogen Actinobacillus actinomycetemcomitans

Periodontitis is mankind's most common chronic inflammatory disease. One severe form of periodontitis is localized aggressive periodontitis (LAP), a condition to which individuals of African origin demonstrate an increased susceptibility. The main causative organism of this disease is Actinobacillus actinomycetemcomitans. A member of the Pasteurellaceae, A. actinomycetemcomitans produces a number of interesting putative virulence factors including (a) an RTX leukotoxin that targets only neutrophils and monocytes and whose action is influenced by a novel type IV secretion system involved in bacterial adhesion; (b) the newly discovered toxin, cytolethal distending toxin (CDT); and (c) a secreted chaperonin 60 with potent leukocyte-activating and bone resorbing activities. This organism also produces a plethora of proteins able to inhibit eukaryotic cell cycle progression and proteins and peptides that can induce distinct forms of proinflammatory cytokine networks. A range of other proteins interacting with the host is currently being uncovered. In addition to these secreted factors, A. actinomycetemcomitans is invasive with an unusual mechanism for entering, and traveling within, eukaryotic cells. This review focuses on recent advances in our understanding of the molecular and cellular pathogenicity of this fascinating oral bacterium.

Biofilm growth and detachment of Actinobacillus actinomycetemcomitans

The gram-negative, oral bacterium Actinobacillus actinomycetemcomitans has been implicated as the causative agent of several forms of periodontal disease in humans. When cultured in broth, fresh clinical isolates of A. actinomycetemcomitans form tenacious biofilms on surfaces such as glass, plastic, and saliva-coated hydroxyapatite, a property that probably plays an important role in the ability of this bacterium to colonize the oral cavity and cause disease. We examined the morphology of A. actinomycetemcomitans biofilm colonies grown on glass slides and in polystyrene petri dishes by using light microscopy and scanning and transmission electron microscopy. We found that A. actinomycetemcomitans developed asymmetric, lobed biofilm colonies that displayed complex architectural features, including a layer of densely packed cells on the outside of the colony and nonaggregated cells and large, transparent cavities on the inside of the colony. Mature biofilm colonies released single cells or small clusters of cells into the medium. These released cells adhered to the surface of the culture vessel and formed new colonies, enabling the biofilm to spread. We isolated three transposon insertion mutants which produced biofilm colonies that lacked internal, nonaggregated cells and were unable to release cells into the medium. All three transposon insertions mapped to genes required for the synthesis of the O polysaccharide (O-PS) component of lipopolysaccharide. Plasmids carrying the complementary wild-type genes restored the ability of mutant strains to synthesize O-PS and release cells into the medium. Our findings suggest that A. actinomycetemcomitans biofilm growth and detachment are discrete processes and that biofilm cell detachment evidently involves the formation of nonaggregated cells inside the biofilm colony that are destined for release from the colony.

Nonspecific adherence and fibril biogenesis by Actinobacillus actinomycetemcomitans: TadA protein is an ATPase

Cells of Actinobacillus actinomycetemcomitans, a gram-negative pathogen responsible for an aggressive form of juvenile periodontitis, form tenaciously adherent biofilms on solid surfaces. The bacteria produce long fibrils of bundled pili, which are required for adherence. Mutations in flp-1, which encodes the major subunit of the pili, or any of seven downstream tad genes (tadABCDEFG) cause defects in fibril production, autoaggregation, and tenacious adherence. We proposed that the tad genes specify part of a novel secretion system for the assembly and transport of Flp pili. The predicted amino acid sequence of TadA (426 amino acids, 47,140 Da) contains motifs for nucleotide binding and hydrolysis common among secretion NTP hydrolase (NTPase) proteins. In addition, the tadA gene is the first representative of a distinct subfamily of potential type IV secretion NTPase genes. Here we report studies on the function of TadA. The tadA gene was altered to express a modified version of TadA that has the 11-residue epitope (T7-TAG) fused to its C terminus. The TadA-T7 protein was indistinguishable from the wild type in its ability to complement the fibril and adherence defects of A. actinomycetemcomitans tadA mutants. Although TadA is not predicted to have a transmembrane domain, the protein was localized to the inner membrane and cytoplasmic fractions of A. actinomycetemcomitans cells, indicating a possible peripheral association with the inner membrane. TadA-T7 was purified and found to hydrolyze ATP in vitro. The ATPase activity is stimulated by Triton X-100, with maximal stimulation at the critical micellar concentration. TadA-T7 forms multimers that are stable during sodium dodecyl sulfate-polyacrylamide gel electrophoresis in nonreducing conditions, and electron microscopy revealed that TadA-T7 can form structures closely resembling the hexameric rings of other type IV secretion NTPases. Site-directed mutagenesis was used to substitute Ala and Gln residues for the conserved Lys residue of the Walker A box for nucleotide binding. Both mutants were found to be defective in their ability to complement tadA mutants. We suggest that the ATPase activity of TadA is required to energize the assembly or secretion of Flp pili for tight adherence of A. actinomycetemcomitans.

Nonspecific adherence by Actinobacillus actinomycetemcomitans requires genes widespread in bacteria and archaea

The gram-negative coccobacillus, Actinobacillus actinomycetemcomitans, is the putative agent for localized juvenile periodontitis, a particularly destructive form of periodontal disease in adolescents. This bacterium has also been isolated from a variety of other infections, notably endocarditis. Fresh clinical isolates of A. actinomycetemcomitans form tenacious biofilms, a property likely to be critical for colonization of teeth and other surfaces. Here we report the identification of a locus of seven genes required for nonspecific adherence of A. actinomycetemcomitans to surfaces. The recently developed transposon IS903phikan was used to isolate mutants of the rough clinical isolate CU1000 that are defective in tight adherence to surfaces (Tad(-)). Unlike wild-type cells, Tad(-) mutant cells adhere poorly to surfaces, fail to form large autoaggregates, and lack long, bundled fibrils. Nucleotide sequencing and genetic complementation analysis revealed a 6.7-kb region of the genome with seven adjacent genes (tadABCDEFG) required for tight adherence. The predicted TadA polypeptide is similar to VirB11, an ATPase involved in macromolecular transport. The predicted amino acid sequences of the other Tad polypeptides indicate membrane localization but no obvious functions. We suggest that the tad genes are involved in secretion of factors required for tight adherence of A. actinomycetemcomitans. Remarkably, complete and highly conserved tad gene clusters are present in the genomes of the bubonic plague bacillus Yersinia pestis and the human and animal pathogen Pasteurella multocida. Partial tad loci also occur in strikingly diverse Bacteria and Archaea. Our results show that the tad genes are required for tight adherence of A. actinomycetemcomitans to surfaces and are therefore likely to be essential for colonization and pathogenesis. The occurrence of similar genes in a wide array of microorganisms indicates that they have important functions. We propose that tad-like genes have a significant role in microbial colonization.

Tenacious adhesion of Actinobacillus actinomycetemcomitans strain CU1000 to salivary-coated hydroxyapatite

Adherence of Actinobacillus actinomycetemcomitans to hard-tissue surfaces was evaluated by comparing a phenotypically stable, well-maintained clinical isolate (strain CU1000) to Streptococcus gordonii G9B, an extensively studied oral-colonizing bacterium. Standard innocula of radiolabelled bacteria were added to saliva-coated hydroxyapatite (SHA) and the ratio of bound to unbound cells counted. Several other clinical isolates as well as laboratory strain Y4 were studied. In other experiments, cell detachment from SHA was compared in static and shaking vessels to calculate controlled desorption of cells over time. A sonic-displacement assay was used to measure avidity of binding to HA and SHA. To better define the attachment properties of CU1000, bacteria were treated with a variety of agents including detergents, salts and enzymes before or after incubation with SHA. Results indicated that CU1000 bound better than G9B (a minimum of 10-fold greater; p < or = 0.05) and did not desorb from SHA, while G9B desorbed to equilibrium in 4 h. Furthermore, Langmuir isotherm calculations indicated that, unlike G9B, CU1000 did not follow second-order adsorption kinetics and thus did not achieve saturation. In addition, of the agents tested only periodate reduced attachment and resulted in detachment of CU1000 from surfaces. These experiments suggest that clinical isolates of A. actinomycetemcomitans possess unique binding properties that promote adsorption to and impede desorption from SHA. The characteristics described for the actinobacillus in this study have been previously underestimated, appear to be mediated by glycoconjugates, and may resemble attachment described for several biofilm-forming, non-oral pathogens.

Identification and molecular analysis of rough-colony-specific outer membrane proteins of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans, a gram-negative bacterium isolated from the human mouth, has been implicated in the pathogenesis of early-onset periodontitis. Primary isolates cultured from subgingival plaque exhibit an adherent, rough colony phenotype which spontaneously converts to a nonadherent, smooth phenotype upon in vitro subculture. The rough colony variant produces abundant fimbriae and autoaggregates, while the smooth colony variant is planktonic and produces scant fimbriae. To begin to understand the significance of colony variation in biofilm formation by A. actinomycetemcomitans, outer membrane protein profiles of four isogenic rough and smooth colony variants were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two proteins with relative molecular masses of 43 and 20 kDa were expressed by the rough colony variants exclusively. Expression of these proteins was not found to be dependent on growth phase, oxygen tension, or type of complex medium. N-terminal amino acid sequences of these proteins obtained by Edman degradation were compared with sequences from the University of Oklahoma A. actinomycetemcomitans genome database. Two contiguous open reading frames (ORFs) encoding proteins having sequence homology with these proteins were identified. The 43-kDa protein (RcpA [rough colony protein A]) was similar to precursor protein D of the general secretion pathway of gram-negative bacilli, while the 20-kDa protein (RcpB [rough colony protein B]) appeared to be unique. The genes encoding these proteins have been cloned from A. actinomycetemcomitans 283 and sequenced. A BLASTX (gapped BLAST) search of the surrounding ORFs revealed homology with other fimbria-related proteins. These data suggest that the genes encoding the 43-kDa (rcpA) and 20-kDa (rcpB) proteins may be functionally related to each other and to genes that may encode fimbria-associated proteins.

Interaction of the salivary low-molecular-weight mucin (MG2) with Actinobacillus actinomycetemcomitans

Periodontitis is associated with the presence of certain Gram-negative bacteria in the oral cavity, among these Actinobacillus actinomycetemcomitans. In order to determine which types of salivary components interact with A. actinomycetemcomitans two strains (HG 1175 and FDC Y4) were incubated with whole saliva and individual glandular secretions, viz. parotid, submandibular, and sublingual saliva. Immunochemical analysis by immunoblotting of bacteria-bound salivary proteins showed that IgA, the low-molecular mucin MG2, parotid agglutinin, and a 300 kDa sublingual and submandibular glycoprotein, were bound to the bacterial strains tested. In addition, adherence of A. actinomycetemcomitans to salivary proteins in a solid-phase was studied. After electrophoresis and transfer of salivary proteins to nitrocellulose membranes A. actinomycetemcomitans adhered only to MG2. In this assay periodate treatment, mild acid hydrolysis or neuraminidase digestion of the saliva glycoproteins abolished binding of two clinical isolates (HG 1175 and NY 664), suggesting that sialic acid residues on MG2 are involved in the binding. In contrast, adherence of the smooth laboratory strain Y4 was not affected by removal of sialic acid residues or even periodate treatment of MG2.

Comparative virulence of periodontopathogens in a mouse abscess model

OBJECTIVE(S)

This report compares the virulence of selected strains of P. gingivalis, A. actinomycetemcomitans, C. rectus, F. nucleatum and T. denticola in a murine model as a measure of pathogenic potential of these oral microorganisms. The characteristics of the tissue destruction associated with these monoinfections were then related to a potential model for bacterial synergism in progressing periodontitis.

DESIGN AND METHODS

All bacterial strains were grown to mid-logarithmic to early stationary growth phase, harvested and used at various doses to challenge BALB/c normal and BALB/c dexamethasone (DEX) treated mice to mimic a neutrophil dysfunction. The characteristics of tissue destruction, and overt tissue destructive capacity of these species were examined as a function of challenge dose and time.

OUTCOME MEASURES

The mice were examined for an interval of approximately 15 days post-challenge and the presence/absence of lesions, localized or generalized nature of the lesion (including size in mm2), and lethality of the infection were assessed.

RESULTS

Comparison of the virulence of the various P. gingivalis strains related to lethality and lesion size associated with destruction of the connective tissue, indicated a virulence capacity of P. gingivalis strains 53977>W50 = T22>3079>33277>381. C. rectus elicited localized necrotic lesions which were limited to the epithelial layers of the skin. The size of the lesions also indicated a graded difference in virulence, such that C. rectus strains 234>576>>33238. A. actinomycetemcomitans caused the formation of classic localized abscesses with a PMN infiltrate and inflammatory exudates. Although each of the A. actinomycetemcomitans strains exhibited a similar virulence pattern in this murine model, A. actinomycetemcomitans serotype b representative strains were potentially more pathogenic with a virulence capacity of 3113D-N = 3975A>JP2 > or = Y4>29523>33384. Both C. rectus and A. actinomycetemcomitans strains showed clear evidence that recent clinical isolates were more virulent than laboratory strains. Challenge with F. nucleatum resulted in tissue destructive responses which were different from those observed with the other strains used in this study. A rapid onset of dose-dependent lesion development, related to the formation of either closed abscesses or open lesions, was observed with F. nucleatum. Tissue involvement was also greater at lower F. nucleatum doses when compared to the other bacteria. F. nucleatum challenge of DEX-treated mice resulted in a shift to open lesions. T. denticola appeared to be more tissue invasive than the other species examined in this study. Challenge of mice with T. denticola resulted in involvement of multiple tissues, including epithelial and connective tissues, as well as appearing to invade muscle layers and deeper tissues. In addition to invading deeper tissues, the resulting lesions took considerably longer to resolve. In the DEX-treated mice (neutrophil depleted), P. gingivalis, C. rectus, and A. actinomycetemcomitans were significantly more virulent. In contrast, while DEX treatment altered the characteristics of lesions caused by F. nucleatum, the extent of lesions produced by F. nucleatum and T. denticola was not substantially enhanced.

CONCLUSIONS

The results obtained from this study suggest that different microorganisms have the ability to provide individual pathologies which may act in an additive/synergistic fashion contributing to the tissue destruction noted in periodontitis.

Adhesion of Actinobacillus actinomycetemcomitans to a human oral cell line

Two quantitative, rapid assays were developed to study the adhesion of Actinobacillus actinomycetemcomitans, an oral bacterium associated with periodontal disease, to human epithelial cells. The human oral carcinoma cell line KB was grown in microtiter plates, and adherent bacteria were detected by an enzyme-linked immunosorbent assay with purified anti-A. actinomycetemcomitans serum and horseradish peroxidase-conjugated secondary antibody or [3H]thymidine-labeled bacteria. Adhesion was found to be time dependent and increased linearly with increasing numbers of bacteria added. Variation in the level of adhesion was noted among strains of A. actinomycetemcomitans. Adhesion was not significantly altered by changes in pH (from pH 5 to 9) but was sensitive to sodium chloride concentrations greater than 0.15 M. Pooled human saliva was inhibitory for adhesion when bacteria were pretreated with saliva before being added to the cells. Pretreatment of the KB cells with saliva did not inhibit adhesion. Protease treatment of A. actinomycetemcomitans reduced adhesion of the bacteria to KB cells. The data are consistent with the hypothesis that a protein(s) is required for bacterial adhesion and that host components may play a role in modulating adhesion to epithelial cells.

Characteristics of adherence of Actinobacillus actinomycetemcomitans to epithelial cells

Actinobacillus actinomycetemcomitans smooth variants [SUNY 75(S), SUNY 465, 652] were investigated for their ability to adhere to KB epithelial cells. Both the type of medium (broth versus agar) and anaerobicity influenced adherence levels and cell surface characteristics. Optimal adherence was observed with all three strains after growth of the bacterial cells in broth under anaerobic conditions, a condition which was associated with extracellular microvesicles. Adherence of SUNY 75(S) also was correlated with extracellular amorphous material, whereas adherence of SUNY 465 was also associated with fimbriation which accompanied a smooth to rough phenotype shift. The relationship between adherence and extracellular vesicles, extracellular amorphous material, and fimbriation suggests that all of these components may function in A. actinomycetemcomitans adherence to epithelial cells. The phenotype shift observed in SUNY 465 cells is further evidence that A. actinomycetemcomitans SUNY 465 is predisposed to variant shifts which are associated with changes in adherence and invasion properties.

Polymorphonuclear leukocyte chemiluminescence induced by Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus in serum and saliva

The ability of different strains of Actinobacillus actinomycetemcomitans (A.a.) and Haemophilus aphrophilus (H.a.) to trigger activation of an oxidative burst in human polymorphonuclear leukocytes (PMNL) was examined by measuring the luminol-amplified light emission--chemiluminescence (CL)--from these cells. Bacterial cells were incubated with PMNL from one healthy subject, in the presence of either active serum, heat-inactivated serum, saliva, or saliva and active serum. In the presence of active serum, all five H.a. strains and two out of five A.a. strains triggered a CL response. The CL induced in the presence of heat-inactivated serum was considerably less than that achieved with fresh serum. In the presence of only saliva, all strains induced considerably weaker CL responses than those induced in the presence of saliva with active serum. In the presence of serum, intracellular reactions appeared to be the main source of CL, while addition of saliva and active serum increased the extracellular CL. The results indicate that strain-dependent differences exist among A.a. strains in their ability to trigger the oxygen-dependent bactericidal mechanisms of human PNML. In contrast, the CL patterns of H.a. strains were equivalent. Various factors in the environment, such as activated complement and salivary compounds, affect the interaction of these species with neutrophils.

Physicochemical and structural investigation of the surfaces of some anaerobic subgingival bacteria

The surfaces of nine clinical isolates of Porphyromonas gingivalis, Prevotella intermedia, Actinobacillus actinomycetemcomitans, and Peptostreptococcus micros and that of laboratory strain P. gingivalis W83 were studied by using contact angle measurements, X-ray photoelectron spectroscopy, infrared spectroscopy, microelectrophoresis of whole cells, and transmission electron microscopy of whole and sectioned cells. P. intermedia strains were hydrophilic, as judged from their small water contact angles, and had highly negative zeta potentials, consistent with the presence of a prominent ruthenium red (RR)-staining layer and fibrillar appendages which are probably partly carbohydrate. The two clinical isolates of P. gingivalis were also hydrophilic and highly negatively charged despite the presence of prominent fibrils, which usually yield less negative zeta potentials. This finding suggests that the RR-staining layer dominates the suspension characteristics of P. gingivalis and P. intermedia strains. P. gingivalis W83 had no demonstrable fibrils and a morphologically distinct RR-staining layer, and it was more hydrophobic than the two clinical isolates of P. gingivalis. P. micros isolates were hydrophobic and much less negatively charged than the other species. The A. actinomycetemcomitans strains displayed long, prominent fibrils and a very thin RR-staining layer, which resulted in high hydrophobicity but distinctly different zeta potentials for the two. Physicochemical data on microbial cell surfaces usually have clear and predictable relationships with each other. For the strains in this study that did not follow these relationships, their aberrant behavior could be explained as due to a masking effect caused by specific surface architecture. We conclude that this combined analysis provides a detailed image of subgingival bacterial surface architecture.

Cell surface hydrophobicity and electrokinetic potential of Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus

Laboratory strains and fresh isolates of Actinobacillus actinomycetemcomitans and Haemophilus aphrophilus were examined for cell surface hydrophobicity and for electrokinetic properties under different experimental conditions. Fresh isolates of A. actinomycetemcomitans and H. aphrophilus were hydrophobic. Laboratory strains of A. actinomycetemcomitans were 20-30% less hydrophobic than fresh isolates. No difference was observed between laboratory and fresh isolates of H. aphrophilus. The pH of the suspending buffer, growth phase or incubation atmosphere did not significantly affect the hydrophobicity of the 2 species, whereas agar cultures of H. aphrophilus were less hydrophobic than broth cultures. All A. actinomycetemcomitans strains treated with sterile filtered saliva showed a concentration-dependent decrease in hydrophobicity of at most 30%. H. aphrophilus strains were not affected by the same treatment. Laboratory strains of H. aphrophilus were more negatively charged than A. actinomycetemcomitans, whereas fresh isolates of the 2 species exhibited similar surface charge. In the presence of saliva the mean cell surface charge of laboratory strains decreased by 56% for A. actinomycetemcomitans and by 73% for H. aphrophilus. The results indicate that the 2 species differ in expression of cell structures accounting for hydrophobicity and surface charge and that environmental factors might differently influence the physical properties of the two species analyzed.