Binding of the periodontal pathogen Actinobacillus actinomycetemcomitans to extracellular matrix proteins

The Trimeric Autotransporter Adhesin EmaA and Infective Endocarditis

Infective endocarditis (IE), a disease of the endocardial surface of the heart, is usually of bacterial origin and disproportionally affects individuals with underlying structural heart disease. Although IE is typically associated with Gram-positive bacteria, a minority of cases are caused by a group of Gram-negative species referred to as the HACEK group. These species, classically associated with the oral cavity, consist of bacteria from the genera Haemophilus (excluding Haemophilus influenzae), Aggregatibacter, Cardiobacterium, Eikenella, and Kingella. Aggregatibacter actinomycetemcomitans, a bacterium of the Pasteurellaceae family, is classically associated with Aggressive Periodontitis and is also concomitant with the chronic form of the disease. Bacterial colonization of the oral cavity serves as a reservoir for infection at distal body sites via hematological spreading. A. actinomycetemcomitans adheres to and causes disease at multiple physiologic niches using a diverse array of bacterial cell surface structures, which include both fimbrial and nonfimbrial adhesins. The nonfimbrial adhesin EmaA (extracellular matrix binding protein adhesin A), which displays sequence heterogeneity dependent on the serotype of the bacterium, has been identified as a virulence determinant in the initiation of IE. In this chapter, we will discuss the known biochemical, molecular, and structural aspects of this protein, including its interactions with extracellular matrix components and how this multifunctional adhesin may contribute to the pathogenicity of A. actinomycetemcomitans.

Serotype-Specific Sugars Impact Structure but Not Functions of the Trimeric Autotransporter Adhesin EmaA of Aggregatibacter actinomycetemcomitans

The human oral pathobiont Aggregatibacter actinomycetemcomitans expresses multiple virulence factors, including the trimeric, extracellular matrix protein adhesin A (EmaA). The posttranslational modification of EmaA is proposed to be dependent on the sugars and enzymes associated with O-polysaccharide (O-PS) synthesis of the lipopolysaccharide (LPS). This modification is important for the structure and function of this adhesin. To determine if the composition of the sugars alters structure and/or function, the prototypic 202-kDa protein was expressed in a non-serotype b, emaA mutant strain. The transformed strain displayed EmaA adhesins similar in appearance to the prototypic adhesin as observed by two-dimensional (2D) electron microscopy of whole-mount negatively stained bacterial preparations. Biochemical analysis indicated that the protein monomers were posttranslationally modified. 3D electron tomographic reconstruction and structure analyses of the functional domain revealed three well-defined subdomains (SI, SII, and SIII) with a linker region between SII and SIII. Structural changes were observed in all three subdomains and the linker region of the adhesins synthesized compared with the known structure. These changes, however, did not affect the ability of the strain to bind collagen or form biofilms. The data suggest that changes in the composition of the glycan moiety alter the 3D structure of the molecule without negatively affecting the function(s) associated with this adhesin. IMPORTANCE The human oral pathogen A. actinomycetemcomitans is a causative agent of periodontal and several systemic diseases. EmaA is a trimeric autotransporter protein adhesin important for colonization by this pathobiont in vivo. This adhesin is modified with sugars associated with the O-polysaccharide (O-PS), and the modification is mediated using the enzymes involved in lipopolysaccharide (LPS) biosynthesis. The interaction with collagen is not mediated by the specific binding between the glycans and collagen but is attributed to changes in the final quaternary structure necessary to maintain an active adhesin. In this study, we have determined that the composition of the sugars utilized in the posttranslational modification of this adhesin is exchangeable without compromising functional activities.

The serotype a-EmaA adhesin of Aggregatibacter actinomycetemcomitans does not require O-PS synthesis for collagen binding activity

Aggregatibacter actinomycetemcomitans, a causative agent of periodontitis and non-oral diseases, synthesizes a trimeric extracellular matrix protein adhesin A (EmaA) that mediates collagen binding and biofilm formation. EmaA is found as two molecular forms, which correlate with the serotype of the bacterium. The canonical protein (b-EmaA), associated with serotypes b and c, has a monomeric molecular mass of 202 kDa. The collagen binding activity of b-EmaA is dependent on the presence of O-polysaccharide (O-PS), whereas biofilm activity is independent of O-PS synthesis. The EmaA associated with serotype a strains (a-EmaA) has a monomeric molecular mass of 173 kDa and differs in the amino acid sequence of the functional domain of the protein. In this study, a-emaA was confirmed to encode a protein that forms antenna-like appendages on the surface of the bacterium, which were found to be important for both collagen binding and biofilm formation. In an O-PS-deficient talose biosynthetic (tld) mutant strain, the electrophoretic mobility of the a-EmaA monomers was altered and the amount of membrane-associated EmaA was decreased when compared to the parent strain. The mass of biofilm formed remained unchanged. Interestingly, the collagen binding activity of the mutant strain was similar to the activity associated with the parent strain, which differs from that observed with the canonical b-EmaA isoform. These data suggest that the properties of the a-EmaA isoform are like those of b-EmaA, with the exception that collagen binding activity is independent of the presence or absence of the O-PS.

Infective Endocarditis: A Focus on Oral Microbiota

Infective endocarditis (IE) is an inflammatory disease usually caused by bacteria entering the bloodstream and settling in the heart lining valves or blood vessels. Despite modern antimicrobial and surgical treatments, IE continues to cause substantial morbidity and mortality. Thus, primary prevention and enhanced diagnosis remain the most important strategies to fight this disease. In this regard, it is worth noting that for over 50 years, oral microbiota has been considered one of the significant risk factors for IE. Indeed, among the disparate recommendations from the American heart association and the European Society of Cardiology, there are good oral hygiene and prophylaxis for high-risk patients undergoing dental procedures. Thus, significant interest has grown in the role of oral microbiota and it continues to be a subject of research interest, especially if we consider that antimicrobial treatments can generate drug-resistant mutant bacteria, becoming a severe social problem. This review will describe the current knowledge about the relationship between oral microbiota, dental procedures, and IE. Further, it will discuss current methods used to prevent IE cases that originate from oral pathogens and how these should be focused on improving oral hygiene, which remains the significant persuasible way to prevent bacteremia and systemic disorders.

Candidate genes and gene markers for the resistance to porcine pleuropneumonia

Actinobacillus (A.) pleuropneumoniae is one of the most important respiratory pathogens in global pig production. Antimicrobial treatment and vaccination provide only limited protection, but genetic disease resistance is a very promising alternative for sustainable prophylaxis. Previous studies have discovered multiple QTL that may explain up to 30% of phenotypic variance. Based on these findings, the aim of the present study was to use genomic sequencing to identify genetic markers for resistance to pleuropneumonia in a segregating commercial German Landrace line. 163 pigs were infected with A. pleuropneumoniae Serotype 7 through a standardized aerosol infection method. Phenotypes were accurately defined on a clinical, pathological and microbiological basis. The 58 pigs with the most extreme phenotypes were genotyped by sequencing (next-generation sequencing). SNPs were used in a genome-wide association study. The study identified genome-wide associated SNPs on three chromosomes, two of which were chromosomes of QTL which had been mapped in a recent experiment. Each variant explained up to 20% of the total phenotypic variance. Combined, the three variants explained 52.8% of the variance. The SNPs are located in genes involved in the pathomechanism of pleuropneumonia. This study confirms the genetic background for the host's resistance to pleuropneumonia and indicates a potential role of three candidates on SSC2, SSC12 and SSC15. Favorable gene variants are segregating in commercial populations. Further work is needed to verify the results in a controlled study and to identify the functional QTN.

Interactions between the Trimeric Autotransporter Adhesin EmaA and Collagen Revealed by Three-Dimensional Electron Tomography

Bacterial adhesion to host tissues is considered the first and critical step of microbial infection. The extracellular matrix protein adhesin A (EmaA) is a collagen-binding adhesin of the periodontal pathogen Aggregatibacter actinomycetemcomitans Three 202-kDa EmaA monomers form antenna-like structures on the bacterial surface with the functional domain located at the apical end. The structure of the 30-nm functional domain has been determined by three-dimensional (3D) electron tomography and subvolume averaging. The region exhibits a complex architecture composed of three subdomains (SI to SIII) and a linker between subdomains SII and SIII. However, the molecular interaction between the adhesin receptor complexes has yet to be revealed. This study provides the first detailed 3D structure of reconstituted EmaA/collagen complexes obtained using 3D electron tomography and image processing techniques. The observed interactions of EmaA with collagen were not to whole, intact fibrils, but rather to individual collagen triple helices dissociated from the fibrils. The majority of the contacts with the EmaA functional domain encompassed subdomains SII and SIII and in some cases the tip of the apical domain, involving SI. These data suggest a multipronged mechanism for the interaction of Gram-negative bacteria with collagen.IMPORTANCE Bacterial adhesion is a crucial step for bacterial colonization and infection. In recent years, the number of antibiotic-resistant strains has dramatically increased; therefore, there is a need to search for novel antimicrobial agents. Thus, great efforts are being devoted to develop a clear understanding of the bacterial adhesion mechanism for preventing infections. In host/pathogen interactions, once repulsive forces are overcome, adhesins recognize and tightly bind to specific receptors on the host cell or tissue components. Here, we present the first 3D structure of the interaction between the collagen-binding adhesin EmaA and collagen, which is critical for the development of endocarditis in humans.

A Nonfimbrial Adhesin of Aggregatibacter actinomycetemcomitans Mediates Biofilm Biogenesis

Periodontitis is an inflammatory disease caused by polymicrobial biofilms. The periodontal pathogen Aggregatibacter actinomycetemcomitans displays two proteinaceous surface structures, the fimbriae and the nonfimbrial extracellular matrix binding protein A (EmaA), as observed by electron microscopy. Fimbriae participate in biofilm biogenesis and the EmaA adhesins mediate collagen binding. However, in the absence of fimbriae, A. actinomycetemcomitans still retains the potential to form robust biofilms, suggesting that other surface macromolecules participate in biofilm development. Here, isogenic mutant strains lacking EmaA structures, but still expressing fimbriae, were observed to have reduced biofilm potential. In strains lacking both EmaA and fimbriae, biofilm mass was reduced by 80%. EmaA enhanced biofilm formation in different strains, independent of the fimbriation state or serotype. Confocal microscopy revealed differences in cell density within microcolonies between the EmaA positive and mutant strains. EmaA-mediated biofilm formation was found to be independent of the glycosylation state and the precise three-dimensional conformation of the protein, and thus this function is uncorrelated with collagen binding activity. The data suggest that EmaA is a multifunctional adhesin that utilizes different mechanisms to enhance bacterial binding to collagen and to enhance biofilm formation, both of which are important for A. actinomycetemcomitans colonization and subsequent infection.

Aggregatibacter actinomycetemcomitans Growth in Biofilm versus Planktonic State: Differential Expression of Proteins

Aggregatibacter actinomycetemcomitans (Aa) is a pathogenic bacterium residing in the subgingival plaque biofilm strongly associated with the pathogenesis of periodontitis. The aim of this investigation was to study the protein differential expression of Aa when growing on biofilm compared with planktonic state using proteomic analysis by the 2D-DIGE system. Eighty-seven proteins were differentially expressed during biofilm growth (1.5-fold, p < 0.05), with 13 overexpressed and 37 down-expressed. Those repressed were mainly proteins involved in metabolism, biosynthesis, and transport. The overexpressed proteins were outer membrane proteins (OMPs) and highly immunogenic proteins such as YaeT (OMP), FtsZ, OMP39, OMP18/16, the chaperone GroEL, OMPA, adenylate kinase (Adk), and dihydrolipoamide acetyltransferase. The enrichment fractions of the OMPs from biofilm and planktonic states were obtained, and these proteins were analyzed by Western blotting with human serum from a periodontitis patient and one healthy control. These immunogenic proteins overexpressed in the biofilm may represent candidate virulence factors.

Expression of cellular fibronectin mRNA in adult periodontitis and peri-implantitis: a real-time polymerase chain reaction study

Cellular fibronectin (cFn) is a type of bioactive non-collagen glycoprotein regarded as the main substance used to maintain periodontal attachment. The content of cFn in some specific sites can reflect the progress of periodontitis or peri-implantitis. This study aims to evaluate the expression of cFn messenger RNA (mRNA) in tissues of adult periodontitis and peri-implantitis by real-time fluorescent quantitative polymerase chain reaction (PCR) and to determine its clinical significance. A total of 30 patients were divided into three groups of 10: healthy, adult periodontitis and peri-implantitis. Periodontal tissue biopsies (1 mm×1 mm×1 mm) from each patient were frozen in liquid nitrogen. Total RNA was extracted from these tissues, and the content, purity and integrity were detected. Specific primers were designed according to the sequence, and the mRNA expression levels of cellular fibronectin were detected by real-time PCR. The purity and integrity of the extracted total RNA were both high, and the specificity of amplified genes was very high with no other pollution. The mRNA expression of cFn in the adult periodontitis group (1.526±0.441) was lower than that in the healthy group (3.253±0.736). However, the mRNA expression of cFn in the peri-implantitis group (3.965±0.537) was significantly higher than that in the healthy group. The difference revealed that although both processes were destructive inflammatory reactions in the periodontium, the pathomechanisms were different and the variation started from the transcription level of the cFn gene.

Human pathogens utilize host extracellular matrix proteins laminin and collagen for adhesion and invasion of the host

Laminin (Ln) and collagen are multifunctional glycoproteins that play an important role in cellular morphogenesis, cell signalling, tissue repair and cell migration. These proteins are ubiquitously present in tissues as a part of the basement membrane (BM), constitute a protective layer around blood capillaries and are included in the extracellular matrix (ECM). As a component of BMs, both Lns and collagen(s), thus function as major mechanical containment molecules that protect tissues from pathogens. Invasive pathogens breach the basal lamina and degrade ECM proteins of interstitial spaces and connective tissues using various ECM-degrading proteases or surface-bound plasminogen and matrix metalloproteinases recruited from the host. Most pathogens associated with the respiratory, gastrointestinal, or urogenital tracts, as well as with the central nervous system or the skin, have the capacity to bind and degrade Lns and collagen(s) in order to adhere to and invade host tissues. In this review, we focus on the adaptability of various pathogens to utilize these ECM proteins as enhancers for adhesion to host tissues or as a targets for degradation in order to breach the cellular barriers. The major pathogens discussed are Streptococcus, Staphylococcus, Pseudomonas, Salmonella, Yersinia, Treponema, Mycobacterium, Clostridium, Listeria, Porphyromonas and Haemophilus; Candida, Aspergillus, Pneumocystis, Cryptococcus and Coccidioides; Acanthamoeba, Trypanosoma and Trichomonas; retrovirus and papilloma virus.

Characterization of the secretion pathway of the collagen adhesin EmaA of Aggregatibacter actinomycetemcomitans

The extracellular matrix protein adhesin A (EmaA) surface antennae-like structures of the periodontal pathogen Aggregatibacter actinomycetemcomitans are composed of three identical protein monomers. Recently, we have demonstrated that the protein is synthesized with an extended signal peptide of 56 amino acids necessary for membrane targeting and protein translocation. In this study, EmaA secretion was demonstrated to be reliant on a chaperone-dependent secretion pathway. Deletion of secB partially reduced but did not abolish the amount of EmaA in the membrane. This observation was attributed to an increase in the synthesis of DnaK in the ΔsecB strain. Overexpression of a DnaK substitution mutant (A174T), with diminished activity, in the ΔsecB strain further reduced the amount of EmaA in the membrane. Expression of dnaK A174T in the wild-type strain did not affect the amount of EmaA in the membrane when grown under optimal growth conditions at 37°C. However, EmaA was found to be reduced when this strain was grown at heat-shock temperature. A chromosomal deletion of amino acids 16-39 of the EmaA extended signal peptide, transformed with either the wild-type or dnaK A174T-expressing plasmid, did not affect the amount of EmaA in the membrane. In addition, the level of EmaA in a ΔsecB/emaA(-) double mutant strain expressing EmaAΔ16-39 was unchanged when grown at both temperatures. The data suggest that chaperones are required for the targeting of EmaA to the membrane and a specific region of the signal peptide is necessary for secretion under stress conditions.

The extended signal peptide of the trimeric autotransporter EmaA of Aggregatibacter actinomycetemcomitans modulates secretion

The extracellular matrix protein adhesin A (EmaA) of the Gram-negative bacterium Aggregatibacter actinomycetemcomitans is a fibrillar collagen adhesin belonging to the family of trimeric autotransporters. The protein forms antenna-like structures on the bacterial surface required for collagen adhesion. The 202-kDa protein monomers are proposed to be targeted and translocated across the inner membrane by a long signal peptide composed of 56 amino acids. The predicted signal peptide was functionally active in Escherichia coli and A. actinomycetemcomitans using truncated PhoA and Aae chimeric proteins, respectively. Mutations in the signal peptide were generated and characterized for PhoA activity in E. coli. A. actinomycetemcomitans strains expressing EmaA with the identical mutant signal peptides were assessed for cellular localization, surface expression, and collagen binding activity. All of the mutants impaired some aspect of EmaA structure or function. A signal peptide mutant that promoted alkaline phosphatase secretion did not allow any cell surface presentation of EmaA. A second mutant allowed for cell surface exposure but abolished protein function. A third mutant allowed for the normal localization and function of EmaA at 37°C but impaired localization at elevated temperatures. Likewise, replacement of the long EmaA signal peptide with a typical signal peptide also impaired localization above 37°C. The data suggest that the residues of the EmaA signal peptide are required for protein folding or assembly of this collagen adhesin.

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.

Investigation of the three-dimensional architecture of the collagen adhesin EmaA of Aggregatibacter actinomycetemcomitans by electron tomography

The periodontal pathogen Aggregatibacter actinomycetemcomitans displays on the bacterial surface a nonfimbrial adhesin, EmaA, which is required for collagen binding. In this study, electron tomography was used to characterize the three-dimensional (3D) architecture of this adhesin. The antenna-like surface appendages, corresponding to EmaA, were found to be composed of an ellipsoidal domain capping a rod-like domain that adopts either a straight or a bent conformation at various positions along the length. The most common flexible point along the length of the EmaA appendage was localized 29.4 nm away from the distal end. One-fifth of the appendages were straight and the remaining showed angles distributed between 140 degrees and 170 degrees at this location. Deletion analysis mapped this bend to amino acids 611 to 640 of the protein sequence. The 3D structure of the collagen binding domain of EmaA was generated by alignment and averaging of 9 subvolumes of the adhesin extracted from tomograms. The structure contains three subdomains: a globular structure with a diameter of approximately 5 nm and a cylindrical domain ( approximately 4.4 nm by 5.8 nm) separated by a linker region with a diameter of approximately 3 nm, followed by a cylindrical domain ( approximately 4.6 nm by 6.6 nm). This is the first 3D structure of a trimeric autotransporter protein of A. actinomycetemcomitans.

EmaA, a potential virulence determinant of Aggregatibacter actinomycetemcomitans in infective endocarditis

The gram-negative fastidious human oropharyngeal Aggregatibacter actinomycetemcomitans is implicated in the etiology of infective endocarditis. EmaA, an oligomeric coiled-coil adhesin homologous to YadA of Yersinia enterocolitica, was hypothesized to mediate the interaction of A. actinomycetemcomitans with collagen. Collagen, the most abundant protein in human bodies and the main component of extracellular matrix (ECM), predominates in the supporting tissue of cardiac valves. To extend our earlier studies using purified collagen to determine bacterial binding activities, we developed a tissue model using rabbit cardiac valves to investigate the interaction of A. actinomycetemcomitans with native collagen. The resected mitral valves, with or without removal of the endothelium, were incubated with equivalent numbers of the wild type and the isogenic emaA mutant defective in collagen binding. There was no difference in binding between the wild-type and the mutant strains when the endothelium remained intact. However, the emaA mutant was fivefold less effective than the wild-type strain in colonizing the exposed ECM. A 10-fold increase in the binding of the wild-type strain to ECM was observed compared with the intact endothelium. Similar observations were replicated in an in vivo endocarditis rabbit model; the emaA mutant was 10-fold less effective in the initial infection of the traumatized aortic valve. Colocalization studies indicated that A. actinomycetemcomitans bound to type I collagen. A. actinomycetemcomitans preferentially colonized the ECM and, together with the evidence that EmaA interacts with the native collagen, suggested that the adhesin is likely a potential virulence determinant of the bacterium in the initiation of infective endocarditis.

Functional mapping of an oligomeric autotransporter adhesin of Aggregatibacter actinomycetemcomitans

Extracellular matrix protein adhesin A (EmaA) is a 202-kDa nonfimbrial adhesin, which mediates the adhesion of the oral pathogen Aggregatibacter actinomycetemcomitans to collagen. EmaA oligomers form surface antenna-like protrusions consisting of a long helical rod with an ellipsoidal ending. The functional analysis of in-frame emaA deletion mutants has located the collagen binding activity to the amino terminus of the protein corresponding to amino acids 70 to 386. The level of collagen binding of this deletion mutant was comparable to the emaA mutant strain. Transmission electron microscopy studies indicate that the first 330 amino acids of the mature protein form the ellipsoidal ending of the EmaA protrusions, where the activity resides. Amino acid substitution analysis within this sequence has identified a critical amino acid, which is essential for the formation of the ellipsoidal ending and for collagen binding activity.

Novel surface structures are associated with the adhesion of Actinobacillus actinomycetemcomitans to collagen

Actinobacillus actinomycetemcomitans is a gram-negative, facultative, anaerobic bacterium that colonizes the human oral cavity and the upper respiratory tract. This bacterium is strongly associated with localized aggressive periodontitis and adult periodontitis and is the causative agent for other serious systemic infections. Recently, we have identified a protein, EmaA (extracellular matrix protein adhesin A), that mediates the adhesion of A. actinomycetemcomitans to collagen. The conserved sequence and predicted secondary structure suggest that EmaA is an orthologue of the Yersinia enterocolitica adhesin YadA. Electron microscopy examinations of A. actinomycetemcomitans have identified antenna-like protrusions associated with the surface of the bacterium. These structures are absent on emaA mutant strains and can be restored by transformation of the mutant strain with emaA in trans. The loss of these structures is associated with a decrease in the binding of this bacterium to collagen. The antenna-like structures are composed of a long rod that terminates in an ellipsoidal head region. The analysis of these structures using image processing techniques has provided an initial estimate of the overall dimensions, which suggests that the appendages are oligomeric structures formed by either three or four subunits. Together, the data suggest that emaA is required for the expression of novel appendages on the surface of A. actinomycetemcomitans that mediate the adhesion of the bacterium to collagen.

Identification of Fur-regulated genes in Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is an oral pathogen that causes aggressive periodontitis as well as sometimes life-threatening, extra-oral infections. Iron regulation is thought to be important in the pathogenesis of A. actinomycetemcomitans infections and, consistent with this hypothesis, the fur gene has recently been identified and characterized in A. actinomycetemcomitans. In this study, 14 putatively Fur-regulated genes were identified by Fur titration assay (Furta) in A. actinomycetemcomitans, including afuA, dgt, eno, hemA, tbpA, recO and yfe – some of which are known to be Fur regulated in other species. A fur mutant A. actinomycetemcomitans strain was created by selecting for manganese resistance in order to study the Fur regulon. Comparisons between the fur gene sequences revealed that nucleotide 66 changed from C in the wild-type to T in the mutant strain, changing leucine to isoleucine. The fur mutant strain expressed a nonfunctional Fur protein as determined by Escherichia coli-based ferric uptake assays and Western blotting. It was also more sensitive to acid stress and expressed higher levels of minC than the wild-type strain. minC, which inhibits cell division in other bacterial species and whose regulation by iron has not been previously described, was found to be Fur regulated in A. actinomycetemcomitans by Furta, by gel shift assays, and by RT-qPCR assays for gene expression.

Basement membrane protein and matrix metalloproteinase deregulation in engineered human oral mucosa following infection with Candida albicans

A variety of morphological changes in the basement membrane (BM) are known to occur in inflammatory diseases. Modifications of the BM can be associated with significant changes in protein content. Candida albicans (C. albicans) is normally a commensal organism and is a member of the natural flora of a large number of healthy individuals. However, under certain conditions, C. albicans can invade host tissues, causing inflammation and tissue damage. The aim of this study was to investigate the effect of C. albicans on the expression and production of structural (laminin-5 and type IV collagen) and inflammatory [matrix metalloproteinases (MMPs) and their inhibitors] proteins by human oral epithelial cells. Using engineered normal human oral mucosa infected with 10(5) C. albicans/cm2 for different periods of time, we were able to demonstrate that this yeast promotes significant laminin-5 and type IV collagen gene activation and protein secretion. These effects were accompanied by MMP-2 and MMP-9 gene activation. Interestingly, only the levels of active MMP-9 rose. The increase in MMP levels was paralleled by a decrease in the secretion of type 2 matrix metalloproteinase tissue inhibitors (TIMP-2). Our results demonstrated that C. albicans has a significant effect on tissue structure through BM protein and MMP modulation. This might help C. albicans overcome the mechanical and biological defenses of the tissue and allow it to disseminate, causing severe infections. If C. albicans uses MMPs (mainly MMP-9) to disseminate, inhibition of this protease could be of interest in treating a variety of inflammatory disorders, including oral candidiasis.

Adherence of Actinobacillus pleuropneumoniae to swine-lung collagen

Actinobacillus pleuropneumoniae serotype 1 adhered to immobilized swine-lung collagen. Bacteria bound to collagen type I, III, IV and V. At 5 min incubation, 30 % of bacteria adhered to collagen, reaching saturation in around 90 min. Treatment of bacteria with divalent-metal chelators diminished their attachment to collagen, and Ca(2+) but not Mg(2+) increased it, suggesting Ca(2+) dependence for adherence. Proteolytic enzymes drastically reduced bacterial adherence to collagen, showing that binding involved bacterial surface proteins. Porcine fibrinogen, haemoglobin and gelatin partially reduced collagen adhesion. A 60 kDa outer-membrane protein of A. pleuropneumoniae recognized the swine collagens by overlay. This membrane protein was apparently involved in adhesion to collagen and fibrinogen, but not to fibronectin and laminin. Antibodies against the 60 kDa protein inhibited the adhesion to collagen by 70 %, whereas pig convalescent-phase antibodies inhibited it by only 40 %. Serotypes 1 and 7 were the most adherent to pig collagen (taken as 100 %); serotypes 6 and 11 were the lowest (approximately 50 %), and neither showed the 60 kDa adhesin to biotinylated collagens. By negative staining, cells were observed initially to associate with collagen fibres in a polar manner, and the adhesin was detected on the bacterial surface. The results suggest that swine-lung collagen is an important target for A. pleuropneumoniae colonization and spreading, and that the attachment to this protein could play a relevant role in pathogenesis.

Identification of an extracellular matrix protein adhesin, EmaA, which mediates the adhesion of Actinobacillus actinomycetemcomitans to collagen

Actinobacillus actinomycetemcomitansis an aetiologic agent in the development of periodontal and some systemic diseases in humans. This pathogen localizes to the underlying connective tissue of the oral cavity in individuals with periodontal disease. The adhesion ofA. actinomycetemcomitansto extracellular matrix components of the connective tissue prompted this study to identify gene products mediating the interaction ofA. actinomycetemcomitansto these molecules. A transposon mutagenesis system was optimized for use inA. actinomycetemcomitansand used to generate an insertional mutant library. A total of 2300 individual insertion transposon mutants were screened for changes in the adhesion to collagen and fibronectin. Mutants were identified which exhibited the following phenotypes: a decrease in collagen binding; a decrease in fibronectin binding; a decrease in binding to both proteins; and an increase in binding to both collagen and fibronectin. The identification of mutants defective in adhesion to the individual proteins indicates that distinct adhesins are expressed by this organism. Molecular analysis of these mutants implicated 11 independent loci in protein adhesion. One gene,emaA, is likely to encode a direct mediator of collagen adhesion, based on predicted protein features homologous to the collagen-binding protein YadA ofYersinia enterocolitica. EmaA was localized to the outer membrane, as expected for an adhesin. Reduction in fibronectin adhesion appeared to be influenced by abrogation of proteins involved in molybdenum-cofactor biosynthesis. Several other loci identified as reducing or increasing adhesion to both collagen and fibronectin are suggested to be involved in regulatory cascades that promote or repress expression of collagen and fibronectin adhesins. Collectively, the results support the hypothesis thatA. actinomycetemcomitanshost colonization involves afimbrial adhesins for extracellular matrix proteins, and that the expression of adhesion is modulated by global regulatory mechanisms.

Two epithelial cell invasion-related loci of the oral pathogen Actinobacillus actinomycetemcomitans

Two invasion-related loci, apiA and the two-gene operon apiBC, were isolated from the oral pathogen Actinobacillus actinomycetemcomitans UT32. apiA encodes a 32.5 kDa protein that migrates on SDS-PAGE as a 101 kDa protein as detected by Western blot analysis or silver staining of an outer membrane-enriched fraction of Escherichia coli transformants. E. coli expressing ApiA have a different phenotype than the host vector, in broth and on solid media, and a colony morphology that resembles that of fresh A. actinomycetemcomitans isolates. These E. coli transformants bound to chicken collagen type II, human collagen type II, III, V and fibronectin. apiB and apiC encode proteins of 130.1 and 70.6 kDa, respectively. ApiBC conferred on E. coli a slightly enhanced ability to bind to collagen type III. ApiA- and ApiB-deficient mutants were constructed in A. actinomycetemcomitans. The ApiB-mutant had 4-fold diminished invasion of KB cells; the ApiA-mutant had increased invasion. Both loci were found in all A. actinomycetemcomitans strains, although polymorphism was detected only for apiBC. The deduced sequences of these invasion-related proteins are homologous to members of the YadA adhesin/invasin family.

Molecular pathogenicity of the oral opportunistic pathogen Actinobacillus actinomycetemcomitans

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

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.

Role of collagen in adherence of Streptococcus uberis to bovine mammary epithelial cells

We reported previously that pre-incubation of Streptococcus uberis with collagen induced expression of S. uberis surface proteins. In a subsequent study, we showed that incubation of S. uberis with extracellular matrix proteins, particularly collagen, increased adherence and internalization of S. uberis to mammary epithelial cells. In the present report, the potential mechanism by which S. uberis exploits the presence of collagen to enhance adherence to bovine mammary epithelial cells was evaluated. Adherence assays were conducted with S. uberis pre-treated with and without collagen and co-cultured in medium supplemented with or without collagen. Pre-incubation with collagen followed by co-culture in medium containing collagen up-regulated ligands that enhanced adherence of S. uberis to mammary epithelial cells. Collagen-up-regulated ligand(s) also increased adherence of S. uberis to mammary epithelial cells in the absence of collagen, but adherence was lower than when collagen was present during the adherence assay. Chloramphenicol was added to the culture medium to inhibit bacterial protein synthesis. Adherence decreased significantly in chloramphenicol-treated S. uberis pre-treated or co-cultured in the presence of collagen. These results suggest that S. uberis expresses ligands with affinity for collagen that are up-regulated by collagen. We hypothesize that these ligands increase adherence by using collagen as a bridge between the bacterium and host cell and/or by direct interaction with host cell receptor(s).

Identification of a Treponema denticola OppA homologue that binds host proteins present in the subgingival environment

Proteins secreted or exported by Treponema denticola have been implicated as mediators of specific interactions between the spirochete and subgingival tissues in periodontal diseases. However, limited information is available on the ability of this peptidolytic organism to bind or transport soluble peptides present in the subgingival environment. A prominent 70-kDa protein was isolated from surface extracts of T. denticola ATCC 35405. A clone expressing a portion of the protein was identified in an Escherichia coli expression library of T. denticola DNA. DNA sequence analysis showed that the cloned gene encoded a peptide homologous to OppA, the solute binding protein of an ATP-binding cassette-type peptide transporter involved in peptide uptake and environmental signaling in a wide range of bacteria. Genes encoding OppB, -C, -D, and -F were identified directly downstream of oppA in T. denticola. OppA was present in representative strains of T. denticola and in Treponema vincentii but was not detected in Treponema pectinovorum or Treponema socranskii. Immunogold electron microscopy suggested that OppA was accessible to proteins at the surface of the spirochete. Native OppA bound soluble plasminogen and fibronectin but did not bind to immobilized substrates or epithelial cells. A T. denticola oppA mutant bound reduced amounts of soluble plasminogen, and plasminogen binding to the parent strain was inhibited by the lysine analog epsilon-aminocaproic acid. Binding of soluble host proteins by OppA may be important both for spirochete-host interactions in the subgingival environment and for uptake of peptide nutrients.

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.