Comparative ultrastructure of leukotoxic and non-leukotoxic strains of Actinobacillus actinomycetemcomitans

Unveiling clinical applications of bacterial extracellular vesicles as natural nanomaterials in disease diagnosis and therapeutics

Bacterial extracellular vesicles (BEVs) are naturally occurring bioactive membrane-bound nanoparticles released by both gram-negative and gram-positive bacterial species, exhibiting a multifaceted role in mediating host-microbe interactions across various physiological conditions. Increasing evidence supports BEVs as essential mediators of cell-to-cell communicaiton, influencing bacterial pathogenicity, disease mechanisms, and modulating the host immune response. However, the extent to which these BEV-mediated actions can be leveraged to predict disease onset, guide treatment strategies, and determine clinical outcomes remains uncertain, particularly in terms of their clinical translation potentials. This review briefly describes BEV biogenesis and their internalisation by recipient cells and summarises methods for isolation and characterization, essential for understanding their composition and cargo. Further, it discusses the potential of biofluid-associated BEVs as biomarkers for various diseases, spanning both cancer and non-cancerous conditions. Following this, we outline the ongoing human clinical trials of using BEVs for vaccine development. In addition to disease diagnostics, this review explores the emerging research of using natural or engineered BEVs as smart nanomaterials for applications in anti-cancer therapy and bone regeneration. This discussion extends to key factors for unlocking the clinical potential of BEVs, such as standardization of BEV isolation and characterisation, as well as other hurdles in translating these findings to the clinical setting. We propose that addressing these hurdles through collaborative research efforts and well-designed clinical trials holds the key to fully harnessing the clinical potential of BEVs. As this field advances, this review suggests that BEV-based nanomedicine has the potential to revolutionize disease management, paving the way for innovative diagnosis, therapeutics, and personalized medicine approaches. STATEMENT OF SIGNIFICANCE: Extracellular vesicles (EVs) from both host cells and bacteria serve as multifunctional biomaterials and are emerging in the fields of biomedicine, bioengineering, and biomaterials. However, the majority of current studies focus on host-derived EVs, leaving a gap in comprehensive research on bacteria-derived EVs (BEVs). Although BEVs offer an attractive option as nanomaterials for drug delivery systems, their unique nanostructure and easy-to-modify functions make them a potential method for disease diagnosis and treatment as well as vaccine development. Our work among the pioneering studies investigating the potential of BEVs as natural nanobiomaterials plays a crucial role in both understanding the development of diseases and therapeutic interventions.

Mycobacterium tuberculosis extracellular vesicles: exploitation for vaccine technology and diagnostic methods

Tuberculosis (TB) is a fatal epidemic disease usually caused by Mycobacterium tuberculosis (Mtb). Pervasive latent infection, multidrug- and extensively drug-resistant tuberculosis (MDR- and XDR-TB), and TB/HIV co-infection make TB a global health problem, which emphasises the design and development of efficient vaccines and diagnostic biomarkers. Extracellular vesicles (EVs) secretion is a conserved phenomenon in all the domains of life. Various cargos such as nucleic acids, toxins, lipoproteins, and enzymes have been recognised in these nano-sized vesicles that may be involved in bacterial physiology and pathogenesis. The intrinsic adjuvant effect, native immunogenic cargo, sensing by host immune cells, circulation in all body fluids, and comprehensive distribution of antigens introduce EVs as a promising tool for designing novel vaccines, diagnostic biomarkers, and drug delivery systems. Genetic engineering of the EV-producing bacteria and the subsequent production of proper EVs could facilitate the development of the EV-based therapeutic applications. Recently, it was demonstrated that thick-walled mycobacteria release EVs, which contain immunodominant cargos such as lipoglycans and lipoproteins. The present article is a comprehensive review on the recent findings of Mtb EVs biology and the exploitation of EVs for the vaccine technology and diagnostic methods.

Extracellular vesicles: An emerging platform in gram-positive bacteria

Extracellular vesicles (EV), also known as membrane vesicles, are produced as an end product of secretion by both pathogenic and non-pathogenic bacteria. Several reports suggest that archaea, gram-negative bacteria, and eukaryotic cells secrete membrane vesicles as a means for cell-free intercellular communication. EVs influence intercellular communication by transferring a myriad of biomolecules including genetic information. Also, EVs have been implicated in many phenomena such as stress response, intercellular competition, lateral gene transfer, and pathogenicity. However, the cellular process of secreting EVs in gram-positive bacteria is less studied. A notion with the thick cell-walled microbes such as gram-positive bacteria is that the EV release is impossible among them. The role of gram-positive EVs in health and diseases is being studied gradually. Being nano-sized, the EVs from gram-positive bacteria carry a diversity of cargo compounds that have a role in bacterial competition, survival, invasion, host immune evasion, and infection. In this review, we summarise the current understanding of the EVs produced by gram-positive bacteria. Also, we discuss the functional aspects of these components while comparing them with gram-negative bacteria.

Outer membrane vesicle-mediated serum protection in Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans belongs to the HACEK group of fastidious Gram-negative organisms, a recognized cause of infective endocarditis. A. actinomycetemcomitans is also implicated in periodontitis, with rapid progress in adolescents. We recently demonstrated that the major outer membrane protein, OmpA1 was critical for serum survival of the A. actinomycetemcomitans serotype a model strain, D7SS, and that the paralogue, OmpA2 could operate as a functional homologue to OmpA1 in mediating serum resistance. In the present work, an essentially serum-sensitive ompA1 ompA2 double mutant A. actinomycetemcomitans strain derivative was exploited to elucidate if A. actinomycetemcomitans OMVs can contribute to bacterial serum resistance. Indeed, supplementation of OMVs resulted in a dose-dependent increase of the survival of the serum-sensitive strain in incubations in 50% normal human serum (NHS). Whereas neither OmpA1 nor OmpA2 was required for the OMV-mediated serum protection, OMVs and LPS from an A. actinomycetemcomitans strain lacking the LPS O-antigen polysaccharide part were significantly impaired in protecting D7SS ompA1 ompA2. Our results using a complement system screen assay support a model where A. actinomycetemcomitans OMVs can act as a decoy, which can trigger complement activation in an LPS-dependent manner, and consume complement components to protect serum-susceptible bacterial cells.

Aggregatibacter actinomycetemcomitans leukotoxin: From mechanism to targeted anti-toxin therapeutics

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium associated with localized aggressive periodontitis, as well as other systemic diseases. This organism produces a number of virulence factors, all of which provide some advantage to the bacterium. Several studies have demonstrated that clinical isolates from diseased patients, particularly those of African descent, frequently belong to specific clones of A. actinomycetemcomitans that produce significantly higher amounts of a protein exotoxin belonging to the repeats-in-toxin (RTX) family, leukotoxin (LtxA), whereas isolates from healthy patients harbor minimally leukotoxic strains. This finding suggests that LtxA might play a key role in A. actinomycetemcomitans pathogenicity. Because of this correlation, much work over the past 30 years has been focused on understanding the mechanisms by which LtxA interacts with and kills host cells. In this article, we review those findings, highlight the remaining open questions, and demonstrate how knowledge of these mechanisms, particularly the toxin's interactions with lymphocyte function-associated antigen-1 (LFA-1) and cholesterol, enables the design of targeted anti-LtxA strategies to prevent/treat disease.

Aggregatibacter actinomycetemcomitans Leukotoxin Is Delivered to Host Cells in an LFA-1-Indepdendent Manner When Associated with Outer Membrane Vesicles

The Gram-negative bacterium, Aggregatibacter actinomycetemcomitans, has been associated with localized aggressive periodontitis (LAP). In particular, highly leukotoxic strains of A. actinomycetemcomitans have been more closely associated with this disease, suggesting that LtxA is a key virulence factor for A. actinomycetemcomitans. LtxA is secreted across both the inner and outer membranes via the Type I secretion system, but has also been found to be enriched within outer membrane vesicles (OMVs), derived from the bacterial outer membrane. We have characterized the association of LtxA with OMVs produced by the highly leukotoxic strain, JP2, and investigated the interaction of these OMVs with host cells to understand how LtxA is delivered to host cells in this OMV-associated form. Our results demonstrated that a significant fraction of the secreted LtxA exists in an OMV-associated form. Furthermore, we have discovered that in this OMV-associated form, the toxin is trafficked to host cells by a cholesterol- and receptor-independent mechanism in contrast to the mechanism by which free LtxA is delivered. Because OMV-associated toxin is trafficked to host cells in an entirely different manner than free toxin, this study highlights the importance of studying both free and OMV-associated forms of LtxA to understand A. actinomycetemcomitans virulence.

Comparative study of pathogenic and non-pathogenic Escherichia coli outer membrane vesicles and prediction of host-interactions with TLR signaling pathways

Objective

The intestine is the major defensive barrier in the body by having more than 60% of the immune cells in the intestinal mucosa. The aim of this study was to evaluate the Toll like receptor (TLR) signaling pathways and immune response profiles, against outer membrane vesicles (OMVs) in pathogenic and non-pathogenic strains of Escherichia coli.

Results

Our results demonstrated that despite inducing inflammatory and regulatory responses to OMVs released by both strains, there is a remarkable difference in the nature and severity of these responses between the two strains. Following the production and release of OMV by the pathogenic strain, the expressions of the pro-inflammatory cytokines were significantly elevated, in comparison to the non-pathogenic strains. Eventually, our findings suggest that OMV released by the pathogen strain might be colonized, causing inflammation, eliminating the tight junctions of epithelial cells and damaging underlying cells, without the presence of IL-17 at the inflammation site. This could have happened to prevent the development of more severe inflammation, which could lead to the inhibition of colonization. The production of IL-10 is also preventing such inflammations. On the other hand, OMV released by non-pathogenic E. coli appears to influence intestinal homeostasis by causing more anti-inflammatory responses and mild inflammation.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3648-3) contains supplementary material, which is available to authorized users.

Comparative Analysis of Membrane Vesicles from Three Piscirickettsia salmonis Isolates Reveals Differences in Vesicle Characteristics

Membrane vesicles (MVs) are spherical particles naturally released from the membrane of Gram-negative bacteria. Bacterial MV production is associated with a range of phenotypes including biofilm formation, horizontal gene transfer, toxin delivery, modulation of host immune responses and virulence. This study reports comparative profiling of MVs from bacterial strains isolated from three widely disperse geographical areas. Mass spectrometry identified 119, 159 and 142 proteins in MVs from three different strains of Piscirickettsia salmonis isolated from salmonids in Chile (LF-89), Norway (NVI 5692) and Canada (NVI 5892), respectively. MV comparison revealed several strain-specific differences related to higher virulence capability for LF-89 MVs, both in vivo and in vitro, and stronger similarities between the NVI 5692 and NVI 5892 MV proteome. The MVs were similar in size and appearance as analyzed by electron microscopy and dynamic light scattering. The MVs from all three strains were internalized by both commercial and primary immune cell cultures, which suggest a potential role of the MVs in the bacterium’s utilization of leukocytes. When MVs were injected into an adult zebrafish infection model, an upregulation of several pro-inflammatory genes were observed in spleen and kidney, indicating a modulating effect on the immune system. The present study is the first comparative analysis of P. salmonis derived MVs, highlighting strain-specific vesicle characteristics. The results further illustrate that the MV proteome from one bacterial strain is not representative of all bacterial strains within one species.

Production of outer membrane vesicles by the plague pathogen Yersinia pestis

Many Gram-negative bacteria produce outer membrane vesicles (OMVs) during cell growth and division, and some bacterial pathogens deliver virulence factors to the host via the release of OMVs during infection. Here we show that Yersinia pestis, the causative agent of the disease plague, produces and releases native OMVs under physiological conditions. These OMVs, approximately 100 nm in diameter, contain multiple virulence-associated outer membrane proteins including the adhesin Ail, the F1 outer fimbrial antigen, and the protease Pla. We found that OMVs released by Y. pestis contain catalytically active Pla that is competent for plasminogen activation and α2-antiplasmin degradation. The abundance of OMV-associated proteins released by Y. pestis is significantly elevated at 37°C compared to 26°C and is increased in response to membrane stress and mutations in RseA, Hfq, and the major Braun lipoprotein (Lpp). In addition, we show that Y. pestis OMVs are able to bind to components of the extracellular matrix such as fibronectin and laminin. These data suggest that Y. pestis may produce OMVs during mammalian infection and we propose that dispersal of Pla via OMV release may influence the outcome of infection through interactions with Pla substrates such as plasminogen and Fas ligand.

Pulmonary inflammation induced by bacteria-free outer membrane vesicles from Pseudomonas aeruginosa

Pseudomonas aeruginosa is often involved in lung diseases such as cystic fibrosis. These bacteria can release outer membrane vesicles (OMVs), which are bilayered proteolipids with diameters of approximately 20 to 250 nm. In vitro, these OMVs activate macrophages and airway epithelial cells. The aim of this study was to determine whether OMVs from P. aeruginosa can induce pulmonary inflammation in vivo and to elucidate the mechanisms involved. Bacteria-free OMVs were isolated from P. aeruginosa cultures. Wild-type, Toll-like receptor (TLR)2 and TLR4 knockout mice were exposed to OMVs by the airway, and inflammation in the lung was assessed using differential counts, histology, and quantification of chemokines and cytokines. The involvement of the TLR2 and TLR4 pathways was studied in human cells using transfection. OMVs given to the mouse lung caused dose- and time-dependent pulmonary cellular inflammation. Furthermore, OMVs increased concentrations of several chemokines and cytokines in the mouse lungs and mouse alveolar macrophages. The inflammatory responses to OMVs were comparable to those of live bacteria and were only partly regulated by the TLR2 and TLR4 pathways, according to studies in knockout mice. This study shows that OMVs from P. aeruginosa cause pulmonary inflammation without live bacteria in vivo. This effect is only partly controlled by TLR2 and TLR4. The role of OMVs in clinical disease warrants further studies because targeting of OMVs in addition to live bacteria may add clinical benefit compared with treating with antibiotics alone.

Role for Mycobacterium tuberculosis membrane vesicles in iron acquisition

Mycobacterium tuberculosis releases membrane vesicles packed with molecules that can modulate the immune response. Because environmental conditions often influence the production and content of bacterial vesicles, this study examined M. tuberculosis microvesicles released under iron limitation, a common condition faced by pathogens inside the host. The findings indicate that M. tuberculosis increases microvesicle production in response to iron restriction and that these microvesicles contain mycobactin, which can serve as an iron donor and supports replication of iron-starved mycobacteria. Consequently, the results revealed a role of microvesicles in iron acquisition in M. tuberculosis, which can be critical for survival in the host.

Localization and characterization of Xylella fastidiosa haemagglutinin adhesins

Xylella fastidiosa is a gram-negative, xylem-inhabiting, plant-pathogenic bacterium responsible for several important diseases including Pierce's disease (PD) of grapevines. The bacteria form biofilms in grapevine xylem that contribute to the occlusion of the xylem vessels. X. fastidiosa haemagglutinin (HA) proteins are large afimbrial adhesins that have been shown to be crucial for biofilm formation. Little is known about the mechanism of X. fastidiosa HA-mediated cell-cell aggregation or the localization of the adhesins on the cell. We generated anti-HA antibodies and show that X. fastidiosa HAs are present in the outer membrane and secreted both as soluble proteins and in membrane vesicles. Furthermore, the HA pre-proteins are processed from the predicted molecular mass of 360 kDa to a mature 220 kDa protein. Based on this information, we are evaluating a novel form of potential resistance against PD by generating HA-expressing transgenic grapevines.

Differential effects of five Aggregatibacter actinomycetemcomitans strains on gingival epithelial cells

INTRODUCTION

We investigated gingival epithelial cell proliferation and expression of interleukin-8 (IL-8) and intercellular adhesion molecule 1 (ICAM-1) in response to Aggregatibacter actinomycetemcomitans serotypes a, b, and c.

METHODS

Human gingival cells (Ca9-22) were cultured in bacterial extracts prepared from five strains of A. actinomycetemcomitans: ATCC 43717 (serotype a); ATCC 29524, ATCC 29522, and ATCC 43718 (all serotype b); and ATCC 43719 (serotype c).

RESULTS

In bacterial extracts of ATCC 29522, cell growth was significantly impaired, while the expression of IL-8 and ICAM-1 was significantly increased. The level of induction in response to the other strains was minimal.

CONCLUSION

Our results indicate that the five strains of A. actinomycetemcomitans have distinct effects on the abilities of human gingival epithelial cells to proliferate and to produce proinflammatory factors.

Bacterial outer membrane vesicles and the host-pathogen interaction

Extracellular secretion of products is the major mechanism by which Gram-negative pathogens communicate with and intoxicate host cells. Vesicles released from the envelope of growing bacteria serve as secretory vehicles for proteins and lipids of Gram-negative bacteria. Vesicle production occurs in infected tissues and is influenced by environmental factors. Vesicles play roles in establishing a colonization niche, carrying and transmitting virulence factors into host cells, and modulating host defense and response. Vesicle-mediated toxin delivery is a potent virulence mechanism exhibited by diverse Gram-negative pathogens. The biochemical and functional properties of pathogen-derived vesicles reveal their potential to critically impact disease.

Relationship ofActinobacillus actinomycetemcomitansSerotype b to Aggressive Periodontitis: Frequency in Pure Cultured Isolates

BACKGROUND

To our knowledge, the association of the five serotypes of Actinobacillus actinomycetemcomitans (A. actinomycetemcomitans) to the new diagnostic classification scheme defined by the American Academy of Periodontology in 1999 has not yet been described. The goal of this study was to characterize the frequencies of the five serotypes of A. actinomycetemcomitans in A. actinomycetemcomitans isolates from various forms of periodontitis using both old and new diagnostic classifications and to determine the relationships between serotype and age and clinical diagnosis.

METHODS

A total of 345 A. actinomycetemcomitans isolates from 115 A. actinomycetemcomitans culture-positive subjects (mean age 38.0 +/- 18.3 years, 59% female) were collected. Based on the new classifications, 33 subjects had aggressive periodontitis and 82 chronic periodontitis. According to old classifications, there were six prepubertal periodontitis (PPP), 12 localized juvenile periodontitis (LJP), 15 post-localized juvenile periodontitis (PLJP), 28 refractory periodontitis (Ref-P), and 54 adult periodontitis (AP) cases. Serotypes of A. actinomycetemcomitans were determined by an indirect immunofluorescence assay using serotype-specific polyclonal antisera to A. actinomycetemcomitans strains ATCC 29523, ATCC 43728, ATCC 33384, IDH 781 and IDH 1705 (serotype a, b, c, d, and e, respectively). Proportions of serotype b were examined between different diagnostic and age groups with a Z-test for proportions.

RESULTS

Most subjects (n = 100, 86.96%) were infected with a single serotype (22 serotype a, 44 serotype b, 30 serotype c, 1 serotype d, and 3 serotype e). There were 11 subjects (9.57%) with two serotypes and two subjects (1.74%) with 3 serotypes. Two individuals had isolates lacking any detectable serotype antigen. Serotype b was the predominant serotype in children under 18 years of age and young adults between 19 to 35 years, although serotype b status was not significantly associated with age. Serotypes d and e were not found in patients under 35 years old. In 62 adult patients, one subject had serotype d and three had serotype e. Serotype b was the most common serotype in aggressive periodontitis (60.61%). The proportion of cases with serotype b was significantly higher in aggressive periodontitis compared to chronic periodontitis (P = 0.031). Other serotypes were not significantly associated with new diagnostic categories. Serotypes d and e were not detected in aggressive periodontitis.

CONCLUSION

The results of this study show that proportions of serotype b of A. actinomycetemcomitans are significantly greater in culture-positive patients with aggressive periodontitis than those with chronic periodontitis.

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.

Peptide methionine sulfoxide reductase (MsrA) is not a major virulence determinant for the oral pathogen Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is an oral pathogen that is a causative agent for periodontal disease as well as other non-oral infections. The chronic inflammation associated with periodontal diseases suggests that the bacterium must be able to neutralize oxygen intermediates to survive in the host tissues. Methionine sulfoxide reductase (MsrA) is an enzyme that has been demonstrated to have a role in protection against oxidative damage and has also been identified to be required for the proper expression or maintenance of functional adhesins on the surface of several pathogenic bacteria. The A. actinomycetemcomitans homologue of msrA has been isolated and a chromosomal insertion mutant constructed by allele replacement mutagenesis. Inactivation of the gene led to a complete loss of enzymic activity toward a synthetic substrate. However, the isogenic mutant was not more sensitive to oxidative stress or less adherent to epithelial cells as compared with the parent strain. These data suggest that this strain of A. actinomycetemcomitans has redundant systems that compensate for the MsrA activities ascribed for other organisms.

Purification of secreted leukotoxin (LtxA) from Actinobacillus actinomycetemcomitans

The RTX (repeats in toxin) family of toxins is important in the pathogenesis of many Gram-negative bacteria. The oral and systemic human pathogen Actinobacillus actinomycetemcomitans produces a member of this family known as leukotoxin (LtxA). Previously, we found that LtxA is secreted into culture supernatants of A. actinomycetemcomitans and that this protein is abundant and relatively pure. Here, we report a large-scale method for the isolation and purification of LtxA from culture supernatants of A. actinomycetemcomitans strain JP2. The purification scheme involves ammonium sulfate precipitation of culture supernatants, dialysis, and ultrafiltration to concentrate LtxA to approximately 10mg/ml. We found that LtxA remained soluble in buffer that contained at least 250mM NaCl. Purified LtxA was >98% pure and the final preparations were active against HL-60 cells. The entire purification protocol can be completed within 2 days. The ability to readily obtain a large amount of purified leukotoxin should accelerate investigations into the structure and biology of this important virulence factor.

Outer membrane-like vesicles secreted by Actinobacillus actinomycetemcomitans are enriched in leukotoxin

Actinobacillus actinomycetemcomitans is associated with early onset periodontal diseases and secretes membranous vesicles that appear to contain several virulence-associated proteins. However, the composition of these vesicles and the process leading to their secretion are not well defined. Electron micrographs of thin sectioned bacterial cells and purified vesicle preparations showed that vesicles are spherical lipid bilayers, 50-100 nm in diameter, that appear to form by budding from the outer membrane of the bacterium. Thin layer chromatography identified the predominant lipid components of vesicles as lipopolysaccharide, phosphatidylethanolamine and cardiolipin, similar to the main lipid constituents of the outer membrane. However, vesicles also contained minor lipids that were not detected in outer membrane samples. The major protein constituents of vesicles co-migrated with proteins in outer membrane extracts of A. actinomycetemcomitans, but the outer membrane preparations possessed polypeptides that were not detected in vesicles. Three vesicle proteins were identified; the heat-modifiable OmpA homologue of A. actinomycetemcomitans, a 28 kDa lipoprotein related to the major outer membrane lipoprotein of Mannheimia haemolytica and leukotoxin. Incubation of leukotoxin-sensitive human HL60 cells with vesicles from A. actinomycetemcomitans strains JP2 and 652 resulted in cell lysis, indicating that vesicle-associated leukotoxin is biologically active. Vesicles from the highly leukotoxic strain JP2 were five- to 10-fold more toxic than vesicles from the minimally leukotoxic 652 strain. Furthermore, the specific leukotoxic activity of JP2 vesicles was approximately four- to five-fold higher than isolated outer membrane preparations from JP2, suggesting that vesicles are enriched in leukotoxin. Together, these results suggest that the formation of A. actinomycetemcomitans vesicles occurs by a process that results in the enrichment of leukotoxin.

Environmental cues and gene expression in Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans

Microorganisms typically adapt to environmental cues by turning on and off the expression of virulence genes which, in turn, allows for optimal growth and survival within different environmental niches. This adaptation strategy includes sensing and responding to changes in nutrients, pH, temperature, oxygen tension, redox potential, microbial flora, and osmolarity. For a bacterium to adhere to, penetrate, replicate in, and colonize host cells, it is critical that virulence genes are expressed during certain periods of the infection process. Thus, throughout the different stages of an infection, different sets of virulence factors are turned on and off in response to different environmental signals, allowing the bacterium to effectively adapt to its varying niche. In this review, we focus on the regulation of virulence gene expression in two pathogens which have been implicated as major etiological agents in adult and juvenile periodontal diseases: Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans. Understanding the mechanisms of virulence gene expression in response to the local environment of the host will provide crucial information in the development of effective treatments targeted at eradication of these periodontal disease pathogens.

Oral flora and pathogenic organisms

Oral microbial flora consist of numerous bacterial taxa, ranging from aerobes through fastidious anaerobes, and fungi, viruses, and protozoa. Many of these bacteria are unique to the oral cavity. The organisms exist in a complex interrelationship that is regulated and maintained by physical and metabolic microbial interactions, and by environmental factors, such as saliva and diet. Many of these organisms are relatively harmless, although others are significant pathogens, producing local and systemic diseases in healthy and compromised individuals.

Bacterial adhesion of Actinobacillus actinomycetemcomitans serotypes to titanium implants: SEM evaluation. A preliminary report

BACKGROUND

In this study, the adherence ability of Actinobacillus actinomycetemcomitans serotypes to titanium implant surfaces was evaluated to demonstrate if any selective adherence occurs according to the serotypes of the microorganism.

METHODS

The study material included 3 reference strains of A. actinomycetemcomitans serotypes a, b, and c (ATCC 29523, ATCC 43718, ATCC 33384) and 2 clinical isolates of A. actinomycetemcomitans serotypes d and e (IDH 781, IDH 1705), together with commercially available titanium blade implants. For each strain, bacterial suspensions with identical concentrations (5 x 10(7) cells/ml) were prepared and 0.5 ml of each was added on to the implant surfaces, which had been precoated with glycine-bovine serum albumin (BSA). After incubation at 37 degrees C for 60 minutes in 5% CO2 in air, the implants with attached bacteria were prepared for scanning electron microscopic (SEM) observations. Bacterial adhesion was quantified on the textured body surfaces of the implants, and results were statistically analyzed with analysis of variance followed by Duncan's test. The surface ultrastructure of the bacterial cells was also evaluated descriptively.

RESULTS

The tested strains adhered to implant surfaces in different quantities. Serotype a (ATCC 29523) showed the highest adherence affinity (statistically significant, P <0.01). When compared with each other, serotypes b, c, and d (ATCC 43718, ATCC 33384, and IDH 781) attached equally well, whereas serotype e (IDH 1705) had a statistically significant low adherence capability.

CONCLUSIONS

It is suggested that in vitro A. actinomycetemcomitans adhesion to implant surfaces is strain dependent.

Virulence factors of Actinobacillus actinomycetemcomitans

A. actinomycetemcomitans has clearly adapted well to its environs; its armamentarium of virulence factors (Table 2) ensures its survival in the oral cavity and enables it to promote disease. Factors that promote A. actinomycetemcomitans colonization and persistence in the oral cavity include adhesins, bacteriocins, invasins and antibiotic resistance. It can interact with and adhere to all components of the oral cavity (the tooth surface, other oral bacteria, epithelial cells or the extracellular matrix). The adherence is mediated by a number of distinct adhesins that are elements of the cell surface (outer membrane proteins, vesicles, fimbriae or amorphous material). A. actinomycetemcomitans enhances its chance of colonization by producing actinobacillin, an antibiotic that is active against both streptococci and Actinomyces, primary colonizers of the tooth surface. The fact that A. actinomycetemcomitans resistance to tetracyclines, a drug often used in the treatment of periodontal disease, is on the rise is an added weapon. Periodontal pathogens or their pathogenic products must be able to pass through the epithelial cell barrier in order to reach and cause destruction to underlying tissues (the gingiva, cementum, periodontal ligament and alveolar bone). A. actinomycetemcomitans is able to elicit its own uptake into epithelial cells and its spread to adjacent cells by usurping normal epithelial cell function. A. actinomycetemcomitans may utilize these remarkable mechanisms for host cell infection and migration to deeper tissues. A. actinomycetemcomitans also orchestrates its own survival by elaborating factors that interfere with the host's defense system (such as factors that kill phagocytes and impair lymphocyte activity, inhibit phagocytosis and phagocyte chemotaxis or interfere with antibody production). Once the organisms are firmly established in the gingiva, the host responds to the bacterial onslaught, especially to the bacterial lipopolysaccharide, by a marked and continual inflammatory response, which results in the destruction of the periodontal tissues. A. actinomycetemcomitans has at least three individual factors that cause bone resorption (lipopolysaccharide, proteolysis-sensitive factor and GroEL), as well as a number of activities (collagenase, fibroblast cytotoxin, etc.) that elicit detrimental effects on connective tissue and the extracellular matrix. It is of considerable interest to know that A. actinomycetemcomitans possesses so many virulence factors but unfortunate that only a few have been extensively studied. If we hope to understand and eradicate this pathogen, it is critical that in-depth investigations into the biochemistry, genetic expression, regulation and mechanisms of action of these factors be initiated.

Oral pathogens: from dental plaque to cardiac disease

Oral bacteria exhibit highly specific adherence mechanisms and as a result they colonize and cause disease principally in the oral cavity. Oral pathogens, however, can produce systemic disease and are known causative agents of infective endocarditis. Recent studies have revealed that periodontal disease per se is also a statistically significant risk factor for cardiovascular disease. A link between the two diseases is the secretion and systemic appearance in periodontitis of pro-inflammatory cytokines capable of eliciting effects associated with atherosclerosis and coronary heart disease.

Models of invasion of enteric and periodontal pathogens into epithelial cells: a comparative analysis

Bacterial invasion of epithelial cells is associated with the initiation of infection by many bacteria. To carry out this action, bacteria have developed remarkable processes and mechanisms that co-opt host cell function and stimulate their own uptake and adaptation to the environment of the host cell. Two general types of invasion processes have been observed. In one type, the pathogens (e.g., Salmonella and Yersinia spp.) remain in the vacuole in which they are internalized and replicate within the vacuole. In the other type, the organism (e.g., Actinobacillus actinomycetemcomitans, Shigella flexneri, and Listeria monocytogenes) is able to escape from the vacuole, replicate in the host cell cytoplasm, and spread to adjacent host cells. The much-studied enteropathogenic bacteria usurp primarily host cell microfilaments for entry. Those organisms which can escape from the vacuole do so by means of hemolytic factors and C type phospholipases. The cell-to-cell spread of these organisms is mediated by microfilaments. The investigation of invasion by periodontopathogens is in its infancy in comparison with that of the enteric pathogens. However, studies to date on two invasive periodontopathogens. A actinomycetemcomitans and Porphyromonas (Bacteroides) gingivalis, reveal that these bacteria have developed invasion strategies and mechanisms similar to those of the enteropathogens. Entry of A. actinomycetemcomitans is mediated by microfilaments, whereas entry of P. gingivalis is mediated by both microfilaments and microtubules. A. actinomycetemcomitans, like Shigella and Listeria, can escape from the vacuole and spread to adjacent cells. However, the spread of A. actinomycetemcomitans is linked to host cell microtubules, not microfilaments. The paradigms presented establish that bacteria which cause chronic infections, such as periodontitis, and bacteria which cause acute diseases, such as dysentery, have developed similar invasion strategies.

Characteristics of Actinobacillus actinomycetemcomitans invasion of and adhesion to cultured epithelial cells

Actinobacillus actinomycetemcomitans (A.a.) is highly implicated in periodontitis. We have developed several in vitro models using the KB oral cell line to examine A.a.-epithelial cell interactions. In support of the use of KB cell line model systems is our finding that A.a. invaded KB and primary gingival cells to the same extent. Invasion is an active event which requires new protein synthesis by both KB and A.a. Like many other intracellular parasites, A.a. invade by receptor-mediated endocytosis. We observed that internalized A.a. were surrounded by foci of actin which had been transported from the periphery of the KB cell. Adhesion of A.a. to KB cells occurred rapidly and stimulated the formation of microvilli. Adhesion is affected by both host factors (saliva, serum, [NaCl]) and culture conditions. Multiple determinants [fimbriae, outer membrane proteins, vesicles, and/or an extracellular amorphous material (ExAmMat)] which are either associated with the A.a. surface or are released into the milieu are involved. We determined that ExAmMat can convey adhesiveness to weakly adherent A.a. and to at least one other oral species (Streptococcus parasanguis).

Pathogenic mechanisms in periodontal disease

Periodontal diseases have been considered as "infections" in which micro-organisms initiate and maintain the destructive inflammatory response. Host-mediated tissue destruction occurs via complement activation and the release of lysosomal enzymes, and connective tissue matrix metalloproteinases. Microbial enzymes may damage connective tissues directly, and, together with toxic metabolites and structural materials, are thought to disrupt the reparative activities of fibroblasts and cells of the immune defenses. The significance and relative contributions of host and microbial factors to the disease process remain unresolved. Environmental changes in the gingival sulcus and periodontal pocket and tissues, the degree of the host response and nutrient availability, concomitant with disease progression, compromise tissue metabolism and repair, and allow for enhanced or de novo expression of microbial virulence factors, such as proteases, which alter microbial pathogenicity. Proteolytic destruction of specific antibodies and complement by both viable and non-viable bacterial cells may retard phagocytic killing and removal of pathogens, thus prolonging the inflammatory response. Bacterial products may indirectly mediate tissue destruction by stimulating release of matrix metalloproteinases or by proteolytically inactivating the specific inhibitors of these enzymes.

Evidence that extracellular components function in adherence of Actinobacillus actinomycetemcomitans to epithelial cells

Extracellular microvesicles and a highly proteinaceous polymer associated with a leukotoxin-producing strain, Actinobacillus actinomycetemcomitans SUNY 75, were shown to increase adherence of other weakly adherent A. actinomycetemcomitans strains to KB epithelial cells.

Ultrastructural localization of alkaline and acid phosphatase activities in dental plaque

Ultrastructural cytohistochemical techniques showed presence of acid and alkaline phosphatases in dental plaque. Both phosphatases had intra- and extramicrobial localization. In the extracellular matrix, phosphatases were associated with small vesicles of bacterial origin, or were freely scattered in the matrix without apparent connection with microbial structures. Intracellularly, alkaline (AlkP) and acid (AcP) phosphatases were observed in Gram-negative and Gram-positive bacteria, showing a different localization. The AlkP was mainly located in the periplasmic space, while AcP had a double preferential localization: along the outer surface of the cell wall and in the periplasmic space. Less frequently an intracellular phosphatase reaction was seen in the cytoplasm.

Electron immunocytochemical localization of Actinobacillus actinomycetemcomitans leukotoxin

The Actinobacillus actinomycetemcomitans leukotoxin was localized in A. actinomycetemcomitans bacteria using an electron immunocytochemical thin-section labeling method. An immuno-dot blot procedure was initially used to ascertain the optimal specimen fixation. This consisted of a periodate-lysine-paraformaldehyde (2%) fixative in a phosphate buffer followed by embedding in LR White. Affinity-purified toxin was used to produce a monospecific polyclonal antibody. The reaction sites were visualized with a colloidal gold-tagged reporter antibody. The leukotoxin was found to be localized either in the cell envelope and/or in membranous vesicles on the outer surface of the bacterial cell. These results support previous observations indicating the same location.

Factors in virulence expression and their role in periodontal disease pathogenesis

The classic progression of the development of periodontitis with its associated formation of an inflammatory lesion is characterized by a highly reproducible microbiological progression of a Gram-positive microbiota to a highly pathogenic Gram-negative one. While this Gram-negative microbiota is estimated to consist of at least 300 different microbial species, it appears to consist of a very limited number of microbial species that are involved in the destruction of periodontal diseases. Among these "putative periodontopathic species" are members of the genera Porphyromonas, Bacteroides, Fusobacterium, Wolinella, Actinobacillus, Capnocytophaga, and Eikenella. While members of the genera Actinomyces and Streptococcus may not be directly involved in the microbial progression, these species do appear to be essential to the construction of the network of microbial species that comprise both the subgingival plaque matrix. The temporal fluctuation (emergence/disappearance) of members of this microbiota from the developing lesion appears to depend upon the physical interaction of the periodontal pocket inhabitants, as well as the utilization of the metabolic end-products of the respective species intimately involved in the disease progression. A concerted action of the end-products of prokaryotic metabolism and the destruction of host tissues through the action of a large number of excreted proteolytic enzymes from several of these periodontopathogens contribute directly to the periodontal disease process.(ABSTRACT TRUNCATED AT 250 WORDS)

A new method of bacterial identification using gold immunolabelling and scanning electron microscopy

This study sought to develop an immunolabelling technique to identify specific bacteria by scanning electron microscopy. Bacterial suspensions were prepared of Actinobacillus actinomycetemcomitans, Porphyromonas (Bacteroides) gingivalis and mixtures of these species with other genera of common oral microorganisms. This method was also used to examine subgingival plaques from 6 subjects with chronic adult periodontitis. Sample preparation consisted of prefixation of the bacterial suspensions with 0.2% glutaraldehyde, incubation with species-specific rabbit antisera and goat anti-rabbit IgG conjugated with colloidal gold particles, postfixation in 2% glutaraldehyde and dehydration in ethanol. Finally, the samples were dried, coated with evaporated carbon and examined by scanning electron microscopy. Pure cultures and artificial mixtures of A. actinomycetemcomitans and Bact. gingivalis were specifically labelled by gold probes as demonstrated by both secondary and back-scattered imaging. These species were also evident in samples of subgingival plaque. The findings indicate that this new technique can be used to identify specific microorganisms both in plaque samples and on the root surface of extracted teeth.

Outer membranous vesicles and leukotoxic activity of Actinobacillus actinomycetemcomitans from subjects with different periodontal status

Strains of A. actinomycetemcomitans (A.a) from juvenile periodontitis patients (JP), adult periodontitis patients (AP), and 14-yr-old healthy children were tested for the correlation between leukotoxin activity and the number of outer membranous vesicles measured in electron micrographs. To determine the potential for connective tissue destruction following the interaction of polymorphonuclear leukocytes (PMN) with the bacteria, the lysosomal release of neutrophil elastase was assessed. The highest potential to kill leukocytes and to release lysosomal elastase from them was observed in the strains isolated from JP patients. No correlation existed between leukotoxic activity and the number of outer membranous vesicles per bacterium when the data from A.a. strains from all sources were combined. Furthermore, no significant differences were found between the numbers of outer membranous vesicles in the three groups tested. The only significant correlation between the number of vesicles and leukotoxicity was found in the A.a. strains derived from the mouths of healthy children.

Biology of asaccharolytic black-pigmented Bacteroides species

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Functional characterization of extracellular vesicles produced by Bacteroides gingivalis

Extracellular vesicles of Bacteroides gingivalis (type strain 33277) were isolated, and some of their biological activities were characterized. The vesicles were obtained from a 2-day culture after ammonium sulfate precipitation, differential centrifugation, and dialysis. When viewed by electron microscopy, vesicles of approximately 50 nm predominated. The results indicated that the enriched vesicle fraction had a high proteolytic activity against collagen, Azocoll, and N-alpha-benzoyl-DL-arginine p-nitroanilide. The polypeptide pattern of the vesicles was similar but not identical to that of the outer membrane. The membrane vesicles could also promote bacterial adherence between homologous cells as well as mediate attachment between two noncoaggregating bacterial species. These vesicles could thus play an important role in periodontal diseases by serving as a vehicle for toxins and various proteolytic enzymes, as well as being involved in adherence.

Precipitating antibody against lipopolysaccharide of Haemophilus actinomycetemcomitans in human serum

Approximately 6% of 50 tested human sera possessed precipitating antibody against lipopolysaccharide from Haemophilus actinomycetemcomitans (Actinobacillus actinomycetemcomitans).

The presence of bacteria within the oral epithelium in periodontal disease. I. A scanning and transmission electron microscopic study

The presence of bacteria within the gingival oral epithelium and adjacent connective tissue in cases of periodontitis and localized juvenile periodontitis have been described using scanning and transmission electron microscopy. The following bacterial morphotypes were identified: cocci, short rods, filaments and few spirochetes in periodontitis and mainly coccobacillary-shaped bacteria in localized juvenile periodontitis. Also Mycoplasma-like structures were identified in the localized juvenile periodontitis cases. Tunnel-like formations at different depths of the oral epithelium contained higher numbers of bacteria than those seen on the adjacent oral surface. Identification of specific bacteria in the oral epithelium may have important pathogenic and therapeutic implications.

Actinobacillus actinomycetemcomitans in human periodontal disease

Recent evidence implicates Actinobacillus actinomycetemcomitans in the etiology of localized juvenile periodontitis. This paper reviews the morphological, biochemical and serological charcteristics of A. actinomycetemcomitans, evidence incriminating it as a periodontopathogen, its importance in human nonoral infections, and virulence factors which may be involved in the pathogenesis of A. actinomycetemcomitans infections. A. actinomycetemcomitans is a non-motile, gram-negative, capnophilic, fermentative coccobacillus which closely resembles several Haemophilus species but which does not require X or V growth factors. The organism has been categorized into 10 biotypes based on the variable fermentation of dextrin, maltose, mannitol, and xylose and into 3 serotypes on the basis of heat stable, cell surface antigens. A. actinomycetemcomitans' primary human ecologic niche is the oral cavity. It is found in dental plaque, in periodontal pockets, and buccal mucosa in up to 36% of the normal population. The organism can apparently seed from these sites to cause severe infections throughout the human body such as brain abscesses and endocarditis. There is a large body of evidence which implicates A. actinomycetemcomitans as an important micro-organism in the etiology of localized juvenile periodontitis including: (1) an increased prevalence of the organism in almost all localized juvenile periodontitis patients and their families compared to other patient groups; (2) the observation that localized juvenile periodontitis patients exhibit elevated antibody levels to A. actinomycetemcomitans in serum, saliva and gingival crevicular fluid; (3) the finding that localized juvenile periodontitis can be successfully treated by eliminating A. actinomycetemcomitans from periodontal pockets; (4) histopathologic investigations showing that A. actinomycetemcomitans invades the gingival connective tissue in localized juvenile periodontitis lesions; (5) the demonstration of several pathogenic products from A. actinomycetemcomitans including factors which may: (a) facilitate its adherence to mucosal surfaces such as capsular polysaccharides; (b) inhibit host defense mechanisms including leukotoxin, a polymorphonuclear leukocyte chemotaxis inhibiting factor, and a lymphocyte suppressing factor (c) cause tissue destruction such as lipopolysaccharide endotoxin, a bone resorption-inducing toxin, acid and alkaline phosphatases, collagenase, a fibroblast inhibiting factor and an epitheliotoxin.(ABSTRACT TRUNCATED AT 400 WORDS)