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.

SummaryAn adhesin of Streptococcus parasanguis FW213, a primary colonizer of the tooth surface, has been purified from the culture medium by immunoaffinity chromatography. The purified protein has a molecular mass of 200 kDa and stains positively for carbohydrate. The amino-terminal sequence indicated that this protein represented a unique streptococcal surface protein. Immunogold labelling of the bacterium indicated that this protein was associated with fimbriae and designated Fap1 (fimbriae-associated protein). A polymerase chain reaction (PCR) product based on the amino terminus of Fap1 was used to probe an FW213 genomic library. A 9 kb fragment containing the fap1 gene was isolated and 2.5 kb have been sequenced. Generation of fap1 mutants by a single cross-over (Campbell insertion) or a non-polar allelic exchange abolished the expression of Fap1. The inactivation of fap1 resulted in a dramatic reduction in the expression of the long peritrichous fimbriae and adhesion to saliva-coated hydroxylapatite (SHA). Northern blots probed with an internal gene fragment of fap1 hybridized to a 9 kb transcript, which suggests that fap1 is transcribed as a polycistronic message. These data demonstrate that Fap1 is a unique streptococcal adhesin that is involved in the assembly of S. parasanguis FW213 fimbriae and adhesion to SHA.

Actinobacillus actinomycetemcomitans is 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 of A. actinomycetemcomitans to extracellular matrix components of the connective tissue prompted this study to identify gene products mediating the interaction of A. actinomycetemcomitans to these molecules. A transposon mutagenesis system was optimized for use in A. actinomycetemcomitans and 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 of Yersinia 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 that A. actinomycetemcomitans host colonization involves afimbrial adhesins for extracellular matrix proteins, and that the expression of adhesion is modulated by global regulatory mechanisms.

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.

The Fap1 fimbrial adhesin is a glycoprotein: antibodies specific for the glycan moiety block the adhesion of Streptococcus parasanguis in an in vitro tooth model the 'blocking' antibodies are specific for the glycan moiety and that the adhesion of S. parasanguis is mediated by sugar residues associated with Fap1.