Heterogeneity of antibodies reactive with the dominant antigen of Actinobacillus actinomycetemcomitans

Inflammatory bone loss in experimental periodontitis induced by Aggregatibacter actinomycetemcomitans in interleukin-1 receptor antagonist knockout mice

The interleukin-1 receptor antagonist (IL-1Ra) binds to IL-1 receptors and inhibits IL-1 activity. However, it is not clear whether IL-1Ra plays a protective role in periodontal disease. This study was undertaken to compare experimental periodontitis induced by Aggregatibacter actinomycetemcomitans in IL-1Ra knockout (KO) mice and wild-type (WT) mice. Computed tomography (CT) analysis and hematoxylin-and-eosin (H&E) and tartrate-resistant acid phosphatase (TRAP) staining were performed. In addition, osteoblasts were isolated; the mRNA expression of relevant genes was assessed by real-time quantitative PCR (qPCR); and calcification was detected by Alizarin Red staining. Infected IL-1Ra KO mice exhibited elevated (P, <0.05) levels of antibody against A. actinomycetemcomitans, bone loss in furcation areas, and alveolar fenestrations. Moreover, protein for tumor necrosis factor alpha (TNF-α) and IL-6, mRNA for macrophage colony-stimulating factor (M-CSF), and receptor activator of NF-κB ligand (RANKL) in IL-1Ra KO mouse osteoblasts stimulated with A. actinomycetemcomitans were increased (P, <0.05) compared to in WT mice. Alkaline phosphatase (ALP), bone sialoprotein (BSP), osteocalcin (OCN)/bone gla protein (BGP), and runt-related gene 2 (Runx2) mRNA levels were decreased (P, <0.05). IL-1α mRNA expression was increased, and calcification was not observed, in IL-1 Ra KO mouse osteoblasts. In brief, IL-1Ra deficiency promoted the expression of inflammatory cytokines beyond IL-1 and altered the expression of genes involved in bone resorption in A. actinomycetemcomitans-infected osteoblasts. Alterations consistent with rapid bone loss in infected IL-Ra KO mice were also observed for genes expressed in bone formation and calcification. In short, these data suggest that IL-1Ra may serve as a potential therapeutic drug for periodontal disease.

Sensitivity of the periodontal pathogen Aggregatibacter actinomycetemcomitans at mildly acidic pH

BACKGROUND

Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans), a capnophilic facultative anaerobe, is associated with localized aggressive periodontitis and endocarditis. When grown in broth, the cells begin to die rapidly after overnight growth. The cells also often lose viability on plates within a few days. The aim of this study is to identify the cause of the rapid loss of cell viability.

METHODS

Cell viabilities, as measured by colony forming units, were determined for cells obtained from isolated colonies and from the dense part of a streak on plates. The effect of pH on cell viability was determined by growing cells in broth at various initial glucose concentrations and with or without added bicarbonate.

RESULTS

A. actinomycetemcomitans cells were highly sensitive to even a mildly acidic pH of ≈6. Because the bacteria grew at a glucose concentration that is commonly used in many laboratories, there was a dramatic decrease in cell viability as the pH went <6, which happened long before the culture reached saturation. This was easily avoided by using a lower initial glucose concentration, and under these conditions, the addition of bicarbonate to the growth medium was not necessary. Cells resuspended in buffer without nutrients lost viability much faster at pH 6 than at a higher pH. On plates, the cell viability was much higher in isolated colonies than in the dense area of the streak.

CONCLUSIONS

A. actinomycetemcomitans cells rapidly lost viability at even a mildly acidic pH. The problem was easily rectified by growing cells at a low glucose concentration.

Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis subgingival presence, species-specific serum immunoglobulin G antibody levels, and periodontitis disease recurrence

BACKGROUND AND OBJECTIVE

The biological and clinical effects of antibody against periodontal pathogenic bacteria are incompletely understood. This study evaluated the inter-relationships among periodontal levels of cultivable Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis, species-specific serum immunoglobulin G (IgG) antibody levels, and periodontitis disease activity.

MATERIAL AND METHODS

Forty-three adults who had previously been treated for periodontitis and who also harbored cultivable A. actinomycetemcomitans or P. gingivalis were evaluated semiannually for clinical disease recurrence over a 36-month period. Each patient provided subgingival microbial samples, for the recovery of A. actinomycetemcomitans and P. gingivalis, from the two deepest pockets in each dentition sextant. A. actinomycetemcomitans and P. gingivalis serum IgG antibody levels were assessed using enzyme-linked immunosorbent assay (ELISA), together with whole-cell sonicate extracts from A. actinomycetemcomitans serotypes a-c and P. gingivalis ATCC 33277. Data were analyzed using the Mantel-Haenszel chi-square and Fisher exact two-tailed tests.

RESULTS

Eighteen (60.0%) of 30 A. actinomycetemcomitans-positive subjects, and 10 (76.9%) of 13 P. gingivalis-positive subjects, exhibited recurrent periodontal breakdown within 36 months of periodontal therapy. Nineteen (67.9%) of the 28 patients with active periodontitis had A. actinomycetemcomitans or P. gingivalis serum antibody levels below designated threshold values. In comparison, 10 (66.7%) of 15 culture-positive clinically stable subjects showed A. actinomycetemcomitans or P. gingivalis serum antibody levels above threshold values. The difference between specific antibody levels in periodontitis-active and periodontitis-stable patients was statistically significant (p = 0.032).

CONCLUSIONS

Serum levels of IgG antibodies against A. actinomycetemcomitans or P. gingivalis in periodontitis-stable patients were higher than those in patients with active periodontitis. The results suggest that elevated levels of IgG antibody against A. actinomycetemcomitans and P. gingivalis have a detectable protective effect against periodontal infections with these microorganisms.

Environmental influences on Actinobacillus actinomycetemcomitans biofilm formation

Fresh clinical isolates of the periodontal pathogen Actinobacillus actinomycetemcomitans have an adherent, rough colony morphology that transforms into a minimally adherent, smooth colony phenotype during successive in vitro passage. The objectives of this study were: (1) to compare biofilm formation of the rough (RVs) and smooth variants (SVs) of several strains of A. actinomycetemcomitans grown under various environmental conditions and (2) to examine the dynamics of biofilm formation. A microtitre plate biofilm assay was used to evaluate biofilm formation of strains grown in broth with modified salt concentration and pH, and to evaluate the effect of pre-conditioning films. Scanning electron microscopy (SEM) was used to monitor microscopic changes in morphology. Dynamics of biofilm formation were measured in a flowcell monitored by confocal microscopy. The RVs generally produced greater biofilm than the SVs. However, medium-dependent differences in biofilm formation were evident for some rough/smooth pairs. The RVs were more tolerant to changes in salt and pH, and more resistant to chlorhexidine than the SVs. Horse serum virtually eliminated, and saliva significantly reduced, biofilm formation by the SVs in contrast to the RVs. SEM revealed no alteration in morphology with change of environment. In a flowcell, the RVs produced towers of microcolonies anchored by a small contact area, whereas the SVs produced an open architecture of reduced height. After 7 days in a flowcell, the rough to smooth phenotype transition could be demonstrated. In conclusion, strain, growth medium and conditioning film all affect biofilm formation. The RVs produce biofilms of unique architecture that may serve to protect the bacterium from environmental perturbations.

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.