Differential regulation of the leukotoxin operon in highly leukotoxic and minimally leukotoxic strains of Actinobacillus actinomycetemcomitans

Leukotoxin A Production and Release by JP2 and Non-JP2 Genotype Aggregatibacter actinomycetemcomitans in Relation to Culture Conditions

Aggressive forms of periodontitis, especially in young patients, are often associated with an increased proportion of the Gram-negative bacterium Aggregatibacter actinomycetemcomitans of the microbiota of the affected periodontal sites. One of the virulence factors of A. actinomycetemcomitans is a leukotoxin (LtxA) that induces a pro-inflammatory cell death process in leukocytes. A. actinomycetemcomitans exhibits a large genetic diversity and different genotypes vary in LtxA production capacity. The genotype JP2 is a heavy LtxA producer due to a 530-base pair deletion in the promoter for the toxin genes, and this trait has been associated with an increased pathogenic potential. The present study focused on the production and release of LtxA by different A. actinomycetemcomitans genotypes and serotypes under various growth conditions. Four different strains of this bacterium were cultured in two different culture broths, and the amount of LtxA bound to the bacterial surface or released into the broths was determined. The cultures were examined during the logarithmic and the early stationary phases of growth. The JP2 genotype exhibited the highest LtxA production among the strains tested, and production was not affected by the growth phase. The opposite was observed with the other strains. The composition of the culture broth had no effect on the growth pattern of the tested strains. However, the abundant release of LtxA from the bacterial surface into the culture broth was found in the presence of horse serum. Besides confirming the enhanced leucotoxicity of the JP2 genotype, the study provides new data on LtxA production in the logarithmic and stationary phases of growth and the effect of media composition on the release of the toxin from the bacterial membrane.

Toxin-triggered liposomes for the controlled release of antibiotics to treat infections associated with the gram-negative bacterium, Aggregatibacter actinomycetemcomitans

Antibiotic resistance has become an urgent threat to health care in recent years. The use of drug delivery systems provides advantages over conventional administration of antibiotics and can slow the development of antibiotic resistance. In the current study, we developed a toxin-triggered liposomal antibiotic delivery system, in which the drug release is enabled by the leukotoxin (LtxA) produced by the Gram-negative pathogen, Aggregatibacter actinomycetemcomitans. LtxA has previously been shown to mediate membrane disruption by promoting a lipid phase change in nonlamellar lipids, such as 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-methyl (N-methyl-DOPE). In addition, LtxA has been observed to bind strongly and nearly irreversibly to membranes containing large amounts of cholesterol. Here, we designed a liposomal delivery system composed of N-methyl-DOPE and cholesterol to take advantage of these interactions. Specifically, we hypothesized that liposomes composed of N-methyl-DOPE and cholesterol, encapsulating antibiotics, would be sensitive to LtxA, enabling controlled antibiotic release. We observed that liposomes composed of N-methyl-DOPE were sensitive to the presence of low concentrations of LtxA, and cholesterol increased the extent and kinetics of content release. The liposomes were stable under various storage conditions for at least 7 days. Finally, we showed that antibiotic release occurs selectively in the presence of an LtxA-producing strain of A. actinomycetemcomitans but not in the presence of a non-LtxA-expressing strain. Together, these results demonstrate that the designed liposomal vehicle enables toxin-triggered delivery of antibiotics to LtxA-producing strains of A. actinomycetemcomitans.

Aggregatibacter actinomycetemcomitans and Filifactor alocis: Two exotoxin-producing oral pathogens

Periodontitis is a dysbiotic disease caused by the interplay between the microbial ecosystem present in the disease with the dysregulated host immune response. The disease-associated microbial community is formed by the presence of established oral pathogens like Aggregatibacter actinomycetemcomitans as well as by newly dominant species like Filifactor alocis. These two oral pathogens prevail and grow within the periodontal pocket which highlights their ability to evade the host immune response. This review focuses on the virulence factors and potential pathogenicity of both oral pathogens in periodontitis, accentuating the recent description of F. alocis virulence factors, including the presence of an exotoxin, and comparing them with the defined factors associated with A. actinomycetemcomitans. In the disease setting, possible synergistic and/or mutualistic interactions among both oral pathogens might contribute to disease progression.

The Hunger Games: Aggregatibacter actinomycetemcomitans Exploits Human Neutrophils As an Epinephrine Source for Survival

Aggregatibacter actinomycetemcomitans is a gram-negative facultative anaerobe and an opportunistic oral pathogen, strongly associated with periodontitis and other inflammatory diseases. Periodontitis is a chronic inflammation of the periodontium resulting from the inflammatory response of the host towards the dysbiotic microbial community present at the gingival crevice. Previously, our group identified catecholamines and iron as the signals that activate the QseBC two-component system in A. actinomycetemcomitans, necessary for the organism to acquire iron as a nutrient to survive in the anaerobic environment. However, the source of catecholamines has not been identified. It has been reported that mouse neutrophils can release catecholamines. In periodontitis, large infiltration of neutrophils is found at the subgingival pocket; hence, we wanted to test the hypothesis that A. actinomycetemcomitans exploits human neutrophils as a source for catecholamines. In the present study, we showed that human neutrophils synthesize, store, and release epinephrine, one of the three main types of catecholamines. Human neutrophil challenge with A. actinomycetemcomitans induced exocytosis of neutrophil granule subtypes: secretory vesicles, specific granules, gelatinase granules, and azurophilic granules. In addition, by selectively inhibiting granule exocytosis, we present the first evidence that epinephrine is stored in azurophilic granules. Using QseC mutants, we showed that the periplasmic domain of the QseC sensor kinase is required for the interaction between A. actinomycetemcomitans and epinephrine. Finally, epinephrine-containing supernatants collected from human neutrophils promoted A. actinomycetemcomitans growth and induced the expression of the qseBC operon under anaerobic conditions. Based on our findings, we propose that A. actinomycetemcomitans promotes azurophilic granule exocytosis by neutrophils as an epinephrine source to promote bacterial survival.

JP2 Genotype of Aggregatibacter actinomycetemcomitans in Caucasian Patients: A Presentation of Two Cases

Aggregatibacter actinomycetemcomitans is a key pathogen that has been associated with periodontal disease. Its most important virulence factor is a leukotoxin capable of inactivating immune cells. The JP2 genotype of Aggregatibacter actinomycetemcomitans shows enhanced leukotoxic activity and is mostly present in individuals of North and West African origin with severe periodontitis. In this paper, two cases of Caucasians diagnosed with the JP2 genotype are presented. A 50-year-old female patient had three approximal sites with ≥ 6 mm clinical attachment loss (CAL) and eight sites with probing depth (PD) ≥ 5 mm. Microbiological diagnostics revealed A. actinomycetemcomitans JP2 genotype, but not Porphyromonas gingivalis. This JP2 genotype was highly leukotoxic to monocytic cells. The second case was a 55-year-old female patient with CAL of > 5 mm at all molars and PD of up to 12 mm. A. actinomycetemcomitans JP2 was identified, but not P. gingivalis. Her husband originated from North-Africa. In him, no A. actinomycetemcomitans was detected, but their 17-year-old daughter was diagnosed with periodontitis and was found to be positive for the JP2 genotype. Both patients were successfully treated with adjunctive antibiotics and the JP2 genotype was eliminated. In summary, here, the microbiological diagnosis was key for the treatment with adjunctive antibiotics.

Herpesvirus-bacteria synergistic interaction in periodontitis

The etiopathogenesis of severe periodontitis includes herpesvirus-bacteria coinfection. This article evaluates the pathogenicity of herpesviruses (cytomegalovirus and Epstein-Barr virus) and periodontopathic bacteria (Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis) and coinfection of these infectious agents in the initiation and progression of periodontitis. Cytomegalovirus and A. actinomycetemcomitans/P. gingivalis exercise synergistic pathogenicity in the development of localized ("aggressive") juvenile periodontitis. Cytomegalovirus and Epstein-Barr virus are associated with P. gingivalis in adult types of periodontitis. Periodontal herpesviruses that enter the general circulation may also contribute to disease development in various organ systems. A 2-way interaction is likely to occur between periodontal herpesviruses and periodontopathic bacteria, with herpesviruses promoting bacterial upgrowth, and bacterial factors reactivating latent herpesviruses. Bacterial-induced gingivitis may facilitate herpesvirus colonization of the periodontium, and herpesvirus infections may impede the antibacterial host defense and alter periodontal cells to predispose for bacterial adherence and invasion. Herpesvirus-bacteria synergistic interactions, are likely to comprise an important pathogenic determinant of aggressive periodontitis. However, mechanistic investigations into the molecular and cellular interaction between periodontal herpesviruses and bacteria are still scarce. Herpesvirus-bacteria coinfection studies may yield significant new discoveries of pathogenic determinants, and drug and vaccine targets to minimize or prevent periodontitis and periodontitis-related systemic diseases.

Virulence and Pathogenicity Properties of Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is a periodontal pathogen colonizing the oral cavity of a large proportion of the human population. It is equipped with several potent virulence factors that can cause cell death and induce or evade inflammation. Because of the large genetic diversity within the species, both harmless and highly virulent genotypes of the bacterium have emerged. The oral condition and age, as well as the geographic origin of the individual, influence the risk to be colonized by a virulent genotype of the bacterium. In the present review, the virulence and pathogenicity properties of A. actinomycetemcomitans will be addressed.

Aggregatibacter actinomycetemcomitans Leukotoxin (LtxA; Leukothera®): Mechanisms of Action and Therapeutic Applications

Aggregatibacter actinomycetemcomitans is an oral pathogen that produces the RTX toxin, leukotoxin (LtxA; Leukothera^®). A. actinomycetemcomitans is strongly associated with the development of localized aggressive periodontitis. LtxA acts as a virulence factor for A. actinomycetemcomitans to subvert the host immune response by binding to the β₂ integrin lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) on white blood cells (WBCs), causing cell death. In this paper, we reviewed the state of knowledge on LtxA interaction with WBCs and the subsequent mechanisms of induced cell death. Finally, we touched on the potential therapeutic applications of LtxA (trade name Leukothera^®) toxin therapy for the treatment of hematological malignancies and immune-mediated diseases.

Aggregatibacter actinomycetemcomitans induces detachment and death of human gingival epithelial cells and fibroblasts via elastase release following leukotoxin-dependent neutrophil lysis

Aggregatibacter actinomycetemcomitans is considered to be associated with periodontitis. Leukotoxin (LtxA), which destroys leukocytes in humans, is one of this bacterium's major virulence factors. Amounts of neutrophil elastase (NE), which is normally localized in the cytoplasm of neutrophils, are reportedly increased in the saliva of patients with periodontitis. However, the mechanism by which NE is released from human neutrophils and the role of NE in periodontitis is unclear. In the present study, it was hypothesized that LtxA induces NE release from human neutrophils, which subsequently causes the breakdown of periodontal tissues. LtxA-treatment did not induce significant cytotoxicity against human gingival epithelial cells (HGECs) or human gingival fibroblasts (HGFs). However, it did induce significant cytotoxicity against human neutrophils, leading to NE release. Furthermore, NE and the supernatant from LtxA-treated human neutrophils induced detachment and death of HGECs and HGFs, these effects being inhibited by administration of an NE inhibitor, sivelestat. The present results suggest that LtxA mediates human neutrophil lysis and induces the subsequent release of NE, which eventually results in detachment and death of HGECs and HGFs. Thus, LtxA-induced release of NE could cause breakdown of periodontal tissue and thereby exacerbate periodontitis.

Aggregatibacter (Actinobacillus) actimycetemcomitans leukotoxin and human periodontitis - A historic review with emphasis on JP2

Aggregatibacter (Actinobacillus) actimycetemcomitans (Aa) is a gram-negative bacterium that colonizes the human oral cavity and is causative agent for localized aggressive (juvenile) periodontitis (AgP). In the middle of 1990s, a specific JP2 clone of belonging to the cluster of serotype b strains of Aa with highly leukotoxicity (leukotoxin, LtxA) able to kill human immune cells was isolated. JP2 clone of Aa was strongly associated with in particularly in rapidly progressing forms of aggressive periodontitis. The JP2 clone of Aa is transmitted through close contacts. Therefore, AgP patients need intense monitoring of their periodontal status as the risk for developing severely progressing periodontitis lesions are relatively high. Furthermore, timely periodontal treatment, including periodontal surgery supplemented by the use of antibiotics, is warranted. More importantly, periodontal attachment loss should be prevented by early detection of the JP2 clone of Aa by microbial diagnosis testing and/or preventive means.

Increased leukotoxin production: Characterization of 100 base pairs within the 530 base pair leukotoxin promoter region of Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans leukotoxin (LtxA) is a major virulence factor that kills leukocytes permitting it’s escape from host immune surveillance. A. actinomycetemcomitans strains can produce high or low levels of toxin. Genetic differences reside in the “so called JP2” ltxA promoter region. These hyper-leukotoxin producing strains with the 530 bp deletion have been studied in detail. However, regions contained within the 530 bp deletion that could be responsible for modulation of leukotoxin production have not been defined. Here, we report, for the first time, on regions within the 530 bp that are responsible for high-levels of ltxA expression. We constructed a deletion of 530 bps in a primate isolate of A. actinomycetemcomitans, which produced leukotoxin equivalent to the JP2 strain. We then constructed sequential deletions in regions that span the 530 bps. Results indicated that expression of the ltxA transcript was reduced by a potential transcriptional terminator in promoter region 298 to 397 with a ΔG = −7.9 kcal/mol. We also confirmed previous findings that transcriptional fusion between the orfX region and ltxC increased ltxA expression. In conclusion, we constructed a hyper-leukotoxin producing A. actinomycetemcomitans strain and identified a terminator located in the promoter region extending from 298–397 that alters ltxA expression.

Trends in Susceptibility to Aggressive Periodontal Disease

Aggregatibacter actinomycetemcomitans is a gram-negative microbe involved in periodontitis. Strains with varying degrees of virulence have been identified, in healthy and periodontally compromised individuals alike. Hosts mount differential immune responses to its various serotypes and virulence factors. Studies have explored host immune response in terms of antibody titers, leukocyte responses, and specific inflammatory mediators, questioning the ways in which the infectious microorganism survives. This mini-review will identify the key themes in immune response patterns of individuals both affected by and free from aggressive periodontal disease, thereby using it to understand various forms of periodontitis.

Pathogenicity of the highly leukotoxic JP2 clone of Aggregatibacter actinomycetemcomitans and its geographic dissemination and role in aggressive periodontitis

For decades, Aggregatibacter actinomycetemcomitans has been associated with aggressive forms of periodontitis in adolescents. In the middle of the 1990s, a specific JP2 clone of A. actinomycetemcomitans, belonging to the cluster of serotype b strains of A. actinomycetemcomitans and having a number of other characteristics, was found to be strongly associated with aggressive forms of periodontitis, particularly in North Africa. Although several longitudinal studies still point to the bacterial species, A. actinomycetemcomitans as a risk factor of aggressive periodontitis, it is now also widely accepted that the highly leukotoxic JP2 clone of A. actinomycetemcomitans is implicated in rapidly progressing forms of aggressive periodontitis. The JP2 clone strains are highly prevalent in human populations living in Northern and Western parts of Africa. These strains are also prevalent in geographically widespread populations that have originated from the Northwest Africa. Only sporadic signs of a dissemination of the JP2 clone strains to non-African populations have been found despite Africans living geographically widespread for hundreds of years. It remains an unanswered question if a particular host tropism exists as a possible explanation for the frequent colonization of the Northwest African population with the JP2 clone. Two exotoxins of A. actinomycetemcomitans are known, leukotoxin (LtxA) and cytolethal distending toxin (Cdt). LtxA is able to kill human immune cells, and Cdt can block cell cycle progression in eukaryotic cells and thus induce cell cycle arrest. Whereas the leukotoxin production is enhanced in JP2 clone strains thus increasing the virulence potential of A. actinomycetemcomitans, it has not been possible so far to demonstrate such a role for Cdt. Lines of evidence have led to the understanding of the highly leukotoxic JP2 clone of A. actinomycetemcomitans as an aetiological factor of aggressive periodontitis. Patients, who are colonized with the JP2 clone, are likely to share this clone with several family members because the clone is transmitted through close contacts. This is a challenge to the clinicians. The patients need intense monitoring of their periodontal status as the risk for developing severely progressing periodontal lesions are relatively high. Furthermore, timely periodontal treatment, in some cases including periodontal surgery supplemented by the use of antibiotics, is warranted. Preferably, periodontal attachment loss should be prevented by early detection of the JP2 clone of A. actinomycetemcomitans by microbial diagnostic testing and/or by preventive means.

Mlc is a transcriptional activator with a key role in integrating cyclic AMP receptor protein and integration host factor regulation of leukotoxin RNA synthesis in Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans, a periodontal pathogen, synthesizes leukotoxin (LtxA), a protein that helps the bacterium evade the host immune response. Transcription of the ltxA operon is induced during anaerobic growth. The cyclic AMP (cAMP) receptor protein (CRP) indirectly increases ltxA expression, but the intermediary regulator is unknown. Integration host factor (IHF) binds to and represses the leukotoxin promoter, but neither CRP nor IHF is responsible for the anaerobic induction of ltxA RNA synthesis. Thus, we have undertaken studies to identify other regulators of leukotoxin transcription and to demonstrate how these proteins work together to modulate leukotoxin synthesis. First, analyses of ltxA RNA expression from defined leukotoxin promoter mutations in the chromosome identify positions -69 to -35 as the key control region and indicate that an activator protein modulates leukotoxin transcription. We show that Mlc, which is a repressor in Escherichia coli, functions as a direct transcriptional activator in A. actinomycetemcomitans; an mlc deletion mutant reduces leukotoxin RNA synthesis, and recombinant Mlc protein binds specifically at the -68 to -40 region of the leukotoxin promoter. Furthermore, we show that CRP activates ltxA expression indirectly by increasing the levels of Mlc. Analyses of Δmlc, Δihf, and Δihf Δmlc strains demonstrate that Mlc can increase RNA polymerase (RNAP) activity directly and that IHF represses ltxA RNA synthesis mainly by blocking Mlc binding. Finally, a Δihf Δmlc mutant still induces ltxA during anaerobic growth, indicating that there are additional factors involved in leukotoxin transcriptional regulation. A model for the coordinated regulation of leukotoxin transcription is presented.

Monodisperse and LPS-free Aggregatibacter actinomycetemcomitans leukotoxin: interactions with human β2 integrins and erythrocytes

Aggregatibacter actinomycetemcomitans is a gram-negative, facultatively anaerobic cocco-bacillus and a frequent member of the human oral flora. It produces a leukotoxin, LtxA, belonging to the repeats-in-toxin (RTX) family of bacterial cytotoxins. LtxA efficiently kills neutrophils and mononuclear phagocytes. The known receptor for LtxA on leukocytes is integrin α(L)β(2) (LFA-1 or CD11a/CD18). However, the molecular mechanisms involved in LtxA-mediated cytotoxicity are poorly understood, partly because LtxA has proven difficult to prepare for experiments as free of contaminants and with its native structure. Here, we describe a protocol for the purification of LtxA from bacterial culture supernatant, which does not involve denaturing procedures. The purified LtxA was monodisperse, well folded as judged by the combined use of synchrotron radiation circular dichroism spectroscopy (SRCD) and in silico prediction of the secondary structure content, and free of bacterial lipopolysaccharide. The analysis by SRCD and similarity to a lipase from Pseudomonas with a known three dimensional structure supports the presence of a so-called beta-ladder domain in the C-terminal part of LtxA. LtxA rapidly killed K562 target cells transfected to express β(2) integrin. Cells expressing α(M)β(2) (CD11b/CD18) or α(X)β(2) (CD11c/CD18) were killed as efficiently as cells expressing α(L)β(2). Erythrocytes, which do not express β(2) integrins, were lysed more slowly. In ligand blotting experiments, LtxA bound only to the β(2) chain (CD18). These data support a previous suggestion that CD18 harbors the major binding site for LtxA as well as identifies integrins α(M)β(2) and α(X)β(2) as novel receptors for LtxA.

Tobacco smoke augments Porphyromonas gingivalis-Streptococcus gordonii biofilm formation

Smoking is responsible for the majority of periodontitis cases in the US and smokers are more susceptible than non-smokers to infection by the periodontal pathogen Porphyromonas gingivalis. P. gingivalis colonization of the oral cavity is dependent upon its interaction with other plaque bacteria, including Streptococcus gordonii. Microarray analysis suggested that exposure of P. gingivalis to cigarette smoke extract (CSE) increased the expression of the major fimbrial antigen (FimA), but not the minor fimbrial antigen (Mfa1). Therefore, we hypothesized that CSE promotes P. gingivalis-S. gordonii biofilm formation in a FimA-dependent manner. FimA total protein and cell surface expression were increased upon exposure to CSE whereas Mfa1 was unaffected. CSE exposure did not induce P. gingivalis auto-aggregation but did promote dual species biofilm formation, monitored by microcolony numbers and depth (both, p<0.05). Interestingly, P. gingivalis biofilms grown in the presence of CSE exhibited a lower pro-inflammatory capacity (TNF-α, IL-6) than control biofilms (both, p<0.01). CSE-exposed P. gingivalis bound more strongly to immobilized rGAPDH, the cognate FimA ligand on S. gordonii, than control biofilms (p<0.001) and did so in a dose-dependent manner. Nevertheless, a peptide representing the Mfa1 binding site on S. gordonii, SspB, completely inhibited dual species biofilm formation. Thus, CSE likely augments P. gingivalis biofilm formation by increasing FimA avidity which, in turn, supports initial interspecies interactions and promotes subsequent high affinity Mfa1-SspB interactions driving biofilm growth. CSE induction of P. gingivalis biofilms of limited pro-inflammatory potential may explain the increased persistence of this pathogen in smokers. These findings may also be relevant to other biofilm-induced infectious diseases and conditions.

RTX proteins: a highly diverse family secreted by a common mechanism

Repeats-in-toxin (RTX) exoproteins of Gram-negative bacteria form a steadily growing family of proteins with diverse biological functions. Their common feature is the unique mode of export across the bacterial envelope via the type I secretion system and the characteristic, typically nonapeptide, glycine- and aspartate-rich repeats binding Ca²⁺ ions. In this review, we summarize the current state of knowledge on the organization of rtx loci and on the biological and biochemical activities of therein encoded proteins. Applying several types of bioinformatic screens on the steadily growing set of sequenced bacterial genomes, over 1000 RTX family members were detected, with the biological functions of most of them remaining to be characterized. Activities of the so far characterized RTX family members are then discussed and classified according to functional categories, ranging from the historically first characterized pore-forming RTX leukotoxins, through the large multifunctional enzymatic toxins, bacteriocins, nodulation proteins, surface layer proteins, up to secreted hydrolytic enzymes exhibiting metalloprotease or lipase activities of industrial interest.

Repression of aerobic leukotoxin transcription by integration host factor in Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans has been implicated as the primary etiologic agent in localized aggressive periodontitis. This bacterium produces a leukotoxin which may help the bacterium evade the host immune response. Leukotoxin transcription is induced when A. actinomycetemcomitans is grown anaerobically, as in the periodontal pocket. Previously, a 35 bp oxygen-response-element (ORE) was shown to be responsible for oxygen regulation at the leukotoxin promoter. However, the gene's transcription is not controlled by Fnr or ArcA, the major oxygen regulators in other bacteria. To identify the potentially novel protein(s) that regulate leukotoxin transcription, protein extracts of A. actinomycetemcomitans were tested for ORE binding by mobility shift assays; one ORE-specific binding complex was found. Standard fractionation protocols and protein sequencing identified the ORE binding protein as integration host factor (IHF). DNaseI protection assays showed that the IHF binding site overlaps the first half of the ORE. To assess the effect of IHF on leukotoxin synthesis, an A. actinomycetemcomitans deletion mutant in ihfB was constructed and characterized. Interestingly, leukotoxin RNA and protein synthesis was de-repressed in the ihf mutant, although leukotoxin synthesis in still oxygen-regulated in the mutant cells. Thus, IHF plays a direct role in repressing leukotoxin transcription, but another protein is also involved in regulating leukotoxin expression in response to oxygen.

Serotypes of Aggregatibacter actinomycetemcomitans in patients with different ethnic backgrounds

BACKGROUND

The identification of Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans) serotypes may add some important information to the understanding of the pathogenetic background of severe periodontal infections. This study compared serotypes of A. actinomycetemcomitans in two groups of periodontal patients with different ethnic backgrounds.

METHODS

A total of 194 patients (96 Germans and 98 Koreans) with aggressive or severe chronic periodontitis participated in the study. Microbiologic analysis of pooled samples from subgingival plaque was performed by using a real-time polymerase chain reaction (PCR) test for A. actinomycetemcomitans. In patients who tested positive for A. actinomycetemcomitans, serotypes (a through f) were determined by nucleic acid-based methods.

RESULTS

The prevalence of patients who tested positive for A. actinomycetemcomitans with the real-time PCR was comparable in both groups (Germans: 27.0%; Koreans: 22.2%). In German patients, the serotypes detected most frequently were b (33.3%), c (25.0%), and a (20.8%), whereas in Korean patients, the serotype distribution was different, with serotypes c (61.9%) and d (19.0%) accounting for >80% of the complete serotype spectrum.

CONCLUSION

Even if the percentage of patients who tested positive for A. actinomycetemcomitans was identical in patients with generalized aggressive and severe chronic periodontitis and different ethnic backgrounds, the distribution of A. actinomycetemcomitans serotypes may exhibit marked differences.

Aggregatibacter actinomycetemcomitans leukotoxin: from threat to therapy

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium that colonizes the human oral cavity and is the causative agent for localized aggressive periodontitis (LAP), an aggressive form of periodontal disease that occurs in adolescents. A. actinomycetemcomitans secretes a protein toxin, leukotoxin (LtxA), which helps the bacterium evade the host immune response during infection. LtxA is a membrane-active toxin that specifically targets white blood cells (WBCs). In this review, we discuss recent developments in this field, including the identification and characterization of genes and proteins involved in secretion, regulation of LtxA, biosynthesis, newly described activities of LtxA, and how LtxA may be used as a therapy for the treatment of diseases.

Uncovering metabolic pathways relevant to phenotypic traits of microbial genomes

Identifying the biochemical basis of microbial phenotypes is a main objective of comparative genomics. Here we present a novel method using multivariate machine learning techniques for comparing automatically derived metabolic reconstructions of sequenced genomes on a large scale. Applying our method to 266 genomes directly led to testable hypotheses such as the link between the potential of microorganisms to cause periodontal disease and their ability to degrade histidine, a link also supported by clinical studies.

Induction of Aggregatibacter actinomycetemcomitans leukotoxin expression by IS1301 and orfA

Most Aggregatibacter actinomycetemcomitans strains express relatively low levels of leukotoxin, encoded by the orfA-ltxCABD operon. However, several strains isolated from patients with localized aggressive periodontitis are hyperleukotoxic and transcribe the ltx operon at high levels. These strains possess a copy of IS1301 in the ltx promoter and previous studies have suggested that the presence of the insertion sequence increases ltx transcription by uncoupling a cis-acting negative regulator of ltx expression from the basal elements of the ltx promoter. However, we now report that replacing IS1301 with an equal length of random sequence has little effect on transcriptional activity of the ltx promoter, suggesting that the physical displacement of the negative regulatory element does not contribute to the hyperleukotoxic phenotype of IS1301-containing strains. Instead, we show that a -10-like element upstream of the transposase ORF of IS1301 is required for increased transcriptional activity of the ltx promoter. Site-specific mutation of the -10 sequence, or reversing the orientation of IS1301 relative to the basal ltx promoter elements, reduced transcriptional activity to levels exhibited by the native ltx promoter. However, no increase in transcription was observed when IS1301 was recombinantly inserted into a ltx promoter that contained a truncated copy of orfA, suggesting that an intact orfA may also be required for IS1301-mediated induction of ltxCABD. Therefore, to determine if orfA functions as a regulator of ltx expression, three independent ltx-promoter-lacZ-reporter constructs containing frameshift mutations in orfA were analysed. Each exhibited significantly lower expression of beta-galactosidase than the control reporter with intact orfA. In addition, OrfA protein was shown, by mobility shift electrophoresis, to interact with the ltx promoter at or downstream of the -35 sequence. These results suggest that a potential transposase promoter and the OrfA polypeptide may modulate leukotoxin expression in hyperleukotoxic A. actinomycetemcomitans strains containing IS1301.

Regulation of Aggregatibacter (Actinobacillus) actinomycetemcomitans leukotoxin secretion by iron

The gram-negative oral and systemic pathogen Aggregatibacter (Actinobacillus) actinomycetemcomitans produces a leukotoxin (LtxA) that is a member of the RTX (repeats in toxin) family of secreted bacterial toxins. We have recently shown that LtxA has the ability to lyse erythrocytes, which results in a beta-hemolytic phenotype on Columbia blood agar. To determine if LtxA is regulated by iron, we examined beta-hemolysis under iron-rich and iron-limiting conditions. Beta-hemolysis was suppressed in the presence of FeCl3. In contrast, strong beta-hemolysis occurred in the presence of the iron chelator deferoxamine. We found that secretion of LtxA was completely inhibited by free iron, but expression of ltxA was not regulated by iron. Free chromium, cobalt, and magnesium did not affect LtxA secretion. Other LtxA-associated genes were not regulated by iron. Thus, iron appears to play an important role in the regulation of LtxA secretion in A. actinomycetemcomitans in a manner independent of gene regulation.

Inactivation of DNA adenine methyltransferase alters virulence factors in Actinobacillus actinomycetemcomitans

DNA adenine methyltransferase (DAM) plays critical roles in diverse biological pathways in gram-negative bacteria, and specifically in regulating the expression of virulence genes in several organisms. Actinobacillus actinomycetemcomitans plays an important role in the pathogenesis of juvenile and adult periodontal disease, yet little is known about its mechanisms of gene regulation. DAM is shown here to directly or indirectly affect well-known A. actinomycetemcomitans virulence factors. A mutant A. actinomycetemcomitans strain lacking the dam gene was created by homologous recombination and shows normal growth phenotypes when grown exponentially. This mutant strain has four sixfold increased levels of extracellular leukotoxin, altered cellular levels of leukotoxin, and significant changes in bacterial invasion of KB oral epithelial cells. These results provide a basis for further characterization of regulatory mechanisms that control A. actinomycetemcomitans virulence.

Usefulness of real time PCR for the differentiation and quantification of 652 and JP2 Actinobacillus actinomycetemcomitans genotypes in dental plaque and saliva

BACKGROUND

The aim of our study is to describe a fast molecular method, able to distinguish and quantize the two different genotypes (652 and JP2) of an important periodontal pathogen: Actinobacillus actinomycetemcomitans. The two genotypes show differences in the expression of an important pathogenic factor: the leukotoxin (ltx). In order to evidence this, we performed a real time PCR procedure on the ltx operon, able to recognize Aa clinical isolates with different leukotoxic potentials.

METHODS

The specificity of the method was confirmed in subgingival plaque and saliva specimens collected from eighty-one Italian (Sardinian) subjects with a mean age of 43.9, fifty five (68 %) of whom had various clinical forms of periodontal disease.

RESULTS

This procedure showed a good sensitivity and a high linear dynamic range of quantization (10(7)-10(2) cells/ml) for all genotypes and a good correlation factor (R2 = 0.97-0.98). Compared with traditional cultural methods, this real time PCR procedure is more sensitive; in fact in two subgingival plaque and two positive saliva specimens Aa was only detected with the molecular method.

CONCLUSION

A low number of Sardinian patients was found positive for Aa infections in the oral cavity, (just 10 positive periodontal cases out of 81 and two of these were also saliva positive). The highly leukotoxic JP2 strain was the most representative (60 % of the positive specimens); the samples from periodontal pockets and from saliva showed some ltx genotype for the same patient. Our experience suggests that this approach is suitable for a rapid and complete laboratory diagnosis for Aa infection.

Characterization of leukotoxin from a clinical strain of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a Gram negative pathogen that is the etiologic agent of localized aggressive periodontitis (LAP), a rapidly progressing and severe disease of the oral cavity that affects predominantly adolescents. A. actinomycetemcomitans is also found in extraoral infections including infective endocarditis. As one of its many virulence determinants, A. actinomycetemcomitans produces the RTX (repeats in toxin) exotoxin, leukotoxin (LtxA). LtxA specifically kills leukocytes of humans and Old World Monkeys. All of our current knowledge of A. actinomycetemcomitans LtxA is based on the protein from strain JP2, a nonadherent laboratory isolate. Because laboratory isolates can lose virulence properties, we wished to examine LtxA from a clinical isolate, NJ4500. We show that localization patterns of LtxA do not differ between the strains. Subcellular localization studies with NJ4500 revealed that LtxA localizes to the outer membrane and that the interaction between LtxA and the surface of cells is specific. Surface localized LtxA was not removed with NaCl treatment and protease protection experiments revealed that approximately 10 kDa of LtxA is exposed. We purified secreted LtxA from NJ4500 and found that the specific activity of this toxin was greater than that of secreted LtxA from JP2. For other RTX toxins, fatty acid modification affects toxin activity, and A. actinomycetemcomitans LtxA is predicted to be modified. We show by two-dimensional gel electrophoresis that NJ4500 LtxA is more highly modified than JP2 LtxA, suggesting that the difference in activities could be due to differential modification. Studies of A. actinomycetemcomitans pathogenesis should therefore consider LtxA from clinical isolates.

Anaerobic regulation of Actinobacillus actinomycetemcomitans leukotoxin transcription is ArcA/FnrA-independent and requires a novel promoter element

The periodontal pathogen, Actinobacillus actinomycetemcomitans, produces a 116-kDa leukotoxin that appears to help the bacterium evade the innate host immune response. The expression of leukotoxin is induced when cells are grown anaerobically, a condition found in the subgingival crevice. This regulation most likely occurs at the transcriptional stage since the levels of leukotoxin RNA are induced by hypoxic growth. In order to map the leukotoxin promoter element(s) responsible for oxygen regulation, deletion and linker-scanning mutations were cloned into a transcriptional reporter gene plasmid and then tested in A. actinomycetemcomitans grown aerobically or anaerobically. A 35-bp DNA element, at position -36 to -70, was found to be responsible for the repression of leukotoxin synthesis in aerobically grown A. actinomycetemcomitans. The sequence of this oxygen response element (ORE) does not match the consensus binding sites for known DNA binding proteins, not even Fnr or ArcA which play major roles in oxygen regulation in other bacteria. However, since sequence analysis alone cannot disprove a role for the Fnr or ArcAB pathways in leukotoxin regulation, the genes for the Fnr and ArcA homologues in A. actinomycetemcomitans were identified, mutated by targeted insertional mutagenesis and assessed for loss of oxygen regulation. Deletion of either fnr or arcA altered the expression of numerous A. actinomycetemcomitans proteins, but leukotoxin expression was still repressed by oxygen. These results, coupled with the promoter mutation analyses, lead to the conclusion that A. actinomycetemcomitans employs a novel pathway in the aerobic/anaerobic regulation of leukotoxin synthesis.

Selective induction of human beta-defensin mRNAs by Actinobacillus actinomycetemcomitans in primary and immortalized oral epithelial cells

Human beta-defensin-2, and -3 (hBD-2, -3) are small inducible antimicrobial peptides involved in host defense. Actinobacillus actinomycetemcomitans, a gram-negative facultative anaerobe, is frequently associated with oral disease in humans. A. actinomycetemcomitans, strain JP2, was examined for its ability to modulate hBD-2 and -3 gene expression in normal human oral epithelial cells (NHOECs) and in OKF6/Tert cells, an immortalized cell line derived from human oral epithelial cells. Stimulation of both cell types by live bacteria, at a minimal bacteria/cell ratio of 500 : 1, resulted in increased hBD-3 gene expression. This was not evinced for hBD-2 in either cell type with live bacteria, even at bacteria/cell ratios exceeding 500 : 1. The increased hBD-3 gene expression was dependent upon viable bacteria, and not their lipopolysaccharides (LPS), since heat-killed A. actinomycetemcomitans did not induce hBD-3 transcript expression. The overall similarity between results obtained in OKF6/Tert cells and NHOECs suggest that the OKF6/Tert cell line may be a useful tool in the study of beta-defensin expression in oral epithelium.

Serum-mediated release of leukotoxin from the cell surface of the periodontal pathogen Actinobacillus actinomycetemcomitans

The leukotoxin of the periodontopathogen Actinobacillus actinomycetemcomitans is an important virulence factor that lyses human neutrophils and monocytes and thus, it may enable the bacterium to evade the local host defense. The toxin also induces degranulation of neutrophils and cytokine release in monocytes. To trigger these biological activities, leukotoxin has to be released from the bacterium and diffuse into the periodontal tissues. To date, the conditions found to cause toxin release have been artificial and have included high ion concentration and alkaline conditions. To study the release of the toxin under conditions mimicking the natural environment of the periodontium the ability of human serum to enable leukotoxin release from the bacterial surface was examined. Suspensions of leukotoxic A. actinomycetemcomitans strains were incubated with various concentrations of human serum or serum albumin. The suspensions were centrifuged and the leukotoxin in the supernatants or the cell pellets was detected by gel electrophoresis and immunoblotting. Serum was found to cause the rapid release of leukotoxin from the bacteria in a concentration-dependent manner. Pure albumin exhibited a similar effect. The leukotoxin released was active against human neutrophils. Only a minor proportion of it was associated with membranous vesicles produced by the bacteria. The results indicate that serum, a fluid closely related to the exudate in inflamed periodontal pockets, releases leukotoxin from the cell surface of A. actinomycetemcomitans. The process may enable the diffusion of the toxin from the bacterial biofilm into the surrounding tissues, where it can exert its biological effect.

Actinobacillus actinomycetemcomitans genetic heterogeneity: amplification of JP2-like ltx promoter pattern correlated with specific arbitrarily primed polymerase chain reaction (AP-PCR) genotypes from human but not marmoset Brazilian isolates

Specific clonal types of Actinobacillus actinomycetemcomitans, a major human periodontal pathogen, may be responsible for clinical manifestations and the production of leukotoxin virulence factors. Leukotoxicity is associated with genetic polymorphism at the promoter region of the leukotoxin (lItx) gene. Here, we describe the use of arbitrarily primed polymerase chain reaction (AP-PCR) and ltx promoter PCR to molecularly characterise 35 A. actinomycetemcomitans Brazilian isolates: 21 of human origin and 14 from captive marmosets (Callitrix spp., primates commonly used as animal models for periodontal research). The discriminative capacity of each of 12 arbitrary primers was found to be variable, yielding between 3 and 24 PCR amplitypes. Combination of the results for all primers led to characterisation of 14 genotypes that grouped into four major clusters based on genetic similarity. Clusters 2, 3, and 4 were discriminative to host origin. A correlation with periodontal disease was suggested for strains belonging to clusters 3 and 4. The JP2-like PCR amplification pattern, associated with highly leukotoxic strains, was exclusive to human isolates and present in 29% of human isolates where it occurred in close relationship with AP genotypes L and J (cluster 3).

Leukotoxins of gram-negative bacteria

Leukotoxins are a group of exotoxins that produce their primary toxic effects against leukocytes, especially polymorphonuclear cells (PMNs). Leukotoxins include a variety of chemicals ranging from 9,10-epoxy 12-octadecenoate, a fatty acid derivative secreted by leukocytes themselves, to proteins such as RTX (repeats in toxin). This review focuses on leukotoxins of three species of gram-negative bacteria, Mannheimia (Pasteurella) haemolytica, Actinobacillus actinomycetemcomitans, and Fusobacterium necrophorum.

Beyond the specific plaque hypothesis: are highly leukotoxic strains of Actinobacillus actinomycetemcomitans a paradigm for periodontal pathogenesis?

Actinobacillus actinomycetemcomitans is a facultative anaerobe implicated in a variety of periodontal diseases. Its presence is most closely associated with localized juvenile periodontitis (LIP), although the exact role of the organism in this and other periodontal diseases is not entirely clear. While A. actinomycetemcomitans produces several different putative virulence factors, the most widely studied is the leukotoxin. The leukotoxin selectively kills polymorphonuclear leukocytes and macrophages in vitro, constituting the host's first line of defense. Interestingly, even though all strains of A. actinomycetemcomitans have the genes encoding the leukotoxin, there is variability in leukotoxin expression. Differences in the structure of the promoter region of the leukotoxin gene operon were shown to correlate directly with levels of leukotoxin production. Highly leukotoxic forms appear to exhibit increased pathogenic potential, as evidenced by recent studies that have shown a significant association between the prevalence of such strains and the occurrence of LIP in several different populations. This represents the first demonstration of an association between a particular subset of a pathogenic species and a specific periodontal disease. Early identification of A. actinomycetemcomitans by microbial and genetic assays to evaluate leukotoxicity may enhance the efficacy of preventive and/or therapeutic techniques. Future investigations should continue to evaluate pathogenic variations of additional virulence factors expressed in vivo, not only of A. actinomycetemcomitans, but also of other periodontal bacteria and infectious disease pathogens.

Genes for tight adherence of Actinobacillus actinomycetemcomitans: from plaque to plague to pond scum

The Gram-negative periodontal pathogen Actinobacillus actinomycetemcomitans forms an extremely tenacious biofilm on solid surfaces such as glass, plastic and hydroxyapatite. This characteristic is likely to be important for colonization of the oral cavity and initiation of a potentially devastating form of periodontal disease. Genetic analysis has revealed a cluster of tad genes responsible for tight adherence to surfaces. Evidence indicates that the tad genes are part of a locus encoding a novel secretion system for the assembly and release of long, bundled Flp pili. Remarkably similar tad loci appear in the genomes of a wide variety of Gram-negative and Gram-positive bacteria, including many significant pathogens, and in Archaea. We propose that the tad loci are important for microbial colonization in a variety of environmental niches.

Molecular characterization of the hlyX-like gene of Actinobacillus actinomycetemcomitans Y4

We isolated and characterized a possible regulatory gene, designated actX gene, from Actinobacillus actinomyctemcomitans Y4, which defined the Actinobacillus pleuropneumoniae hlyX-like regulatory gene. DNA sequence analysis for plasmid clone pKM317 containing a 1.6-kb DNA insert indicated an open reading frame encoding a polypeptide of 257 amino acid residues. Analysis of the deduced amino acid sequence showed the presence of five characteristic cysteine residues in the N-terminal region and a putative DNA binding residue in the C-terminal region, indicating that actX might belong to a regulatory gene family. Escherichia coli DH5alpha and a mutant strain JRG1728 transformed by plasmid carrying actX manifested apparent hemolytic activity on sheep blood agar and grew anaerobically, although the original strains did not.

Inhibition of Actinobacillus actinomycetemcomitans leukotoxicity by bacteria from the subgingival flora

Actinobacillus actinomycetemcomitans produces a pore-forming leukotoxin that lyses human polymorphonuclear leukocytes and monocytes. Certain proteolytic bacteria may coexist with A. actinomycetemcomitans in periodontal pockets. We aimed therefore to examine whether oral bacteria can modify the leukotoxicity of A. actinomycetemcomitans. A total of 55 strains representing 45 bacterial species of the subgingival flora were tested. Each strain was incubated with the highly toxic strain of A. actinomycetemcomitans HK 1519 and the leukotoxic activity of the suspension against human polymorphonuclear leukocytes was determined from the activity of the lactate dehydrogenase released upon lysis of the leukocytes. Porphyromonas gingivalis, Prevotella intermedia, Prevotella nigrescens, Prevotella melaninogenica and Prevotella loeschii inhibited the leukotoxicity of A. actinomycetemcomitans cells as well as the activity of leukotoxin purified from the same strain. The bacterial strains without the ability to block leukotoxic activity also failed to destroy pure leukotoxin even after 5 h of incubation. The proteolytic degradation of leukotoxin by P. gingivalis was mainly dependent on the activity of the enzymes R- and K-gingipains. P. intermedia and P. nigrescens also degraded the leukotoxin by enzymes. The results imply a role of the periodontal microflora in modifying the virulence of A. actinomycetemcomitans by destroying its leukotoxin.

Actinobacillus species and their role in animal disease

Actinobacillus species are Gram-negative bacteria responsible for several quite distinct disease conditions of animals. The natural habitat of the organisms is primarily the upper respiratory tract and oral cavity. A. lignieresii is the cause of actinomycosis (wooden tongue) in cattle: a sporadic, insidiously-developing granulomatous infection. In sharp contrast is A. pleuropneumoniae which is responsible for a rapidly spreading often fatal pneumonia, common among intensively reared pigs. Detailed investigation of this organism has provided a much clearer picture of the bacterial factors involved in causing disease. A. equuli similarly causes a potent septicaemia in the neonatal foal; growing apparently unrestricted once infection occurs. Other members of the genus induce characteristic pathogenesis in their preferred host, with one, A. actinomycetemcomitans, being a cause of human periodontal disease. This article reviews recent understanding of the taxonomy and bacteriology of the organisms, and the aetiology, pathogenicity, diagnosis and control of animal disease caused by Actinobacillus species.

A novel insertion sequence increases the expression of leukotoxicity in Actinobacillus actinomycetemcomitans clinical isolates

BACKGROUND

The expression of leukotoxin varies among Actinobacillus actinomycetemcomitans strains and is dependent in part on the structure of the ltx promoter region. Highly leukotoxic strains, characterized by a 530 base pair (bp) deletion within the ltx promoter, have been associated with juvenile periodontitis in the United States and Europe. In the present study, we analyzed the ltx promoter structure to elucidate whether A. actinomycetemcomitans from Japanese periodontitis patients exhibits the highly toxic phenotype.

METHODS

Forty-five A. actinomycetemcomitans strains, including 43 clinical isolates, the highly leukotoxic strain JP2, and a minimally leukotoxic strain 652 were used in the study. The ltx promoter structure was analyzed by polymerase chain reaction (PCR), with oligonucleotide primers focusing the ltx promoter region, and nucleotide sequencing. Leukotoxic activity was determined by trypan blue exclusion. Western blotting assay was performed to detect the level of leukotoxin polypeptide.

RESULTS

A 495 bp PCR product was amplified from JP2, a 1025 bp product from 652 and 41 of the clinical isolates, and a 1926 bp product from the remaining two clinical isolates (AaIS1, AaIS2). Sequencing of the 1926 bp PCR fragment showed that it was similar to that of strain 652 but contained an 886 bp region that was identified as an insertion sequence (IS). Both AaIs strains expressed high levels of leukotoxicity, similar to strain JP2. In addition, a mutant (AaIS-) that had lost the IS element expressed a significantly lower level of leukotoxicity compared with AaIS strains. Furthermore, the levels of leukotoxin polypeptide expressed by these strains were consistent with their whole cell leukotoxicity.

CONCLUSIONS

A. actinomycetemcomitans clinical strains which were isolated from Japanese periodontitis patients do not possess the 530 bp ltx promoter deletion. The results of this study suggest that a high level of leukotoxin expression correlates with the insertion of the transposable DNA element.

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.

Localized sinus inflammation in a rabbit sinusitis model induced by Bacteroides fragilis is accompanied by rigorous immune responses

We evaluated inflammatory and immune responses against Bacteroides fragilis in a rabbit sinusitis model. Bacteroides was inoculated into the left maxillary sinus, and inflammatory (histology, cell number/cytology, lactose dehydrogenase, and apoptosis) and immune responses in the sinus, airway, and peripheral blood (PB) were determined for up to 4 weeks. In the inflamed sinus, the lactose dehydrogenase level was markedly elevated, with neutrophilic infiltration, severe tissue inflammation, and increased apoptosis. Low-grade tissue inflammation was present in the contralateral and sham-operated sinuses, but other parameters remained unchanged, and so did those in the airway and PB in the inoculated rabbits. Serum IgG antibody levels increased rapidly, were highest at 3 weeks, and began to decline at 4 weeks. Cellular immune responses (proliferation and interferon-gamma mRNA expression) against Bacteroides were detected in the PB of all inoculated rabbits. Vigorous immune responses against Bacteroides may have localized but failed to terminate inflammation in the sinus, indicating importance of microenvironmental factors.