It is now well established that a number of bacteria communicate through diffusible signals that may induce and/or regulate a coordinated response by the individual organisms that make up a given population or biofilm. For many of these organisms, it has been suggested that intercellular signaling functions to report population density or to coordinate a response from all cells in a microbial community. Therefore, cell-to-cell communication has been referred to as auto-induction or quorum sensing. The response of bacteria to quorum sensing signals is quite varied and includes, for example, the induction of bioluminescence, the regulation of virulence gene expression, the formation of biofilms, or the induction of horizontal transfer of genetic material. It is also becoming increasingly apparent that some bacteria may communicate via contact-dependent signaling mechanisms, and that the response to direct cell-to-cell contact influences complex behaviors that may contribute to multicellular development or the adaptation to growth in complex biofilms. In the past five to ten years, increased interest and research in the mechanisms of bacterial cell-to-cell communication has revealed surprising complexity both in the signaling processes themselves and in the breadth of the response of recipient cells to the signal molecules. For example, a variety of chemical species, e.g. acyl-homoserine lactones, oligopeptides, furan derivatives (i.e. AI-2), quinolones, butyrolactones, and unsaturated fatty acids are known or have been suggested to function as diffusible signals. Furthermore, some organisms, most notably Pseudomonas aeruginosa and species of Vibrio, have been shown to produce and respond to multiple diffusible signal molecules.

INTRODUCTION

The microbial community that exists in the oral cavity is perhaps the most accessible, complex and pathogenic of the naturally occurring human biofilms. Over 500 different species of bacteria have been identified in the mature biofilm that forms on tooth surfaces (38). This complex community tenaciously adheres to and develops on the acquired salivary pellicle, a conditioning film of salivary proteins and glycoproteins adsorbed to oral tissue surfaces. The initial colonizers of the salivary pellicle are predominantly Gram-positive facultative anaerobes such as the streptococci; these organisms normally exist in commensal harmony with the host. However, as the oral biofilm matures, there is a change in the microbial composition, with an increasing presence of Gram-negative organisms. The two most common oral diseases in humans, dental caries and periodontal disease, arise from populational shifts in the biofilm in response to a variety of host and/or environmental stimuli. This results in over-representation of pathogenic organisms in the biofilm at afflicted sites in the oral cavity. For example, excessive consumption of dietary sucrose favors the overgrowth of highly fermentative acidophilic organisms such as Streptococcus mutans. The acidic local environment generated by these organisms promotes demineralization of the hydroxyapatite matrix of enamel, thus increasing the risk of dental caries. In contrast, periodontal disease is caused by a biofilm that thrives in the subgingival pocket and induces a chronic inflammatory condition that results in the destruction of the connective tissues and bone that support the teeth (23).

Few microbiologists are likely to forget the moment when the sheer scale and diversity of the microbial world became apparent to them. Similarly, it is a sobering thought that, in (or more accurately on) the human body, bacteria outnumber human cells by at least 10 to 1. Fortunately, most of these bacteria behave as good guests should, and they are content to remain on the other side of the physical barriers that separate us from the outside world. It is inevitable that at such a large gathering, some guests, the pathogens, will misbehave, and worse, start a fight. For many years it was thought that the battle between host and pathogen at the mucosal membranes was fought at arm's length. The bacteria lobbed toxins and other noxious agents at the host and the host returned the favor with antibodies. Bacteria that ventured too close were rapidly dispatched by the professional killing machines, the phagocytic cells. Some bacteria, such as Mycobacterium tuberculosis, however, proved inconveniently recalcitrant to intracellular killing and could become permanent guests within macrophages. Nonetheless, despite the appreciation that mitochondria originate from intracellular bacteria, the notion that mucosal pathogens could be intimately involved with nonprofessional phagocytes is relatively new.

We now know that a wide variety of organisms are capable of directing their own entry into epithelial cells and other host nonphagocytic cells. These bacteria engage in a remarkably sophisticated molecular dialogue with host cells in order to manipulate signal transduction pathways and effectuate bacterial entry.

Porphyromonas gingivalis cells are Gram-negative, anaerobic, nonmotile short rods that produce black pigmented colonies on blood agar. The taxonomy of the species dates back to 1921 when Oliver and Wherry isolated an organism from a variety of oral and nonoral sites that they were to designate Bacterium melaninogenicum. This heterogeneous grouping was later subdivided into nonfermenters, weak fermenters, and strong fermenters. After a number of status changes within the genus Bacteroides, asaccharolytic oral isolates were assigned to the taxon P. gingivalis. The primary ecological niche of P. gingivalis is in the subgingival crevice, the gap between the surfaces of the tooth and the gingiva (gum); however, the organism can be found elsewhere in the mouth, including supragingival (above the gum) tooth surfaces, the tongue, tonsils, and buccal (cheek) mucosa. Although the species has been associated with odontogenic abscesses and nonoral infections (discussed later), the primary pathogenic potential of P. gingivalis is in periodontal disease. The periodontal tissues include the gingiva, periodontal ligament, and alveolar bone, and they constitute the supporting tissues of the teeth. Chronic destruction of the periodontium, such as occurs in periodontal diseases, can eventually lead to exfoliation of teeth and is the most common cause of tooth loss in adults. Periodontal diseases vary in severity and age of onset, and P. gingivalis is associated, either alone or in combination with other bacteria, with the most severe manifestations.

INTRODUCTION

The area of contact between the teeth and the gums (gingiva) is an anatomically unique region that comprises mineralized tissue embedded in epithelium and exposed to a microbially abundant environment. The small (1–4mm deep) gap between the surfaces of the tooth and the gingiva is known as the gingival sulcus or crevice. The gingiva is highly vascularized and the crevice is lined with sulcular epithelial cells that differ from oral epithelial cells by exhibiting less keratinization. Apically, sulcular epithelium becomes junctional epithelium that is characterized by a lack of keratinization, limited differentiation and a relatively permeable structure. It is this junctional epithelium that directly interposes between the gingiva and the tooth surface (Fig. 13.1). In destructive periodontal disease there is migration of the junctional epithelium resulting in enlargement of the crevice into a deeper periodontal pocket that contains inflammatory cells such as neutrophils and T-cells. The gingiva itself also contains immune cells including B-cells, T-cells and dendritic cells. The microbiota of the gingival area in both health and disease is complex, with at least 500 species of bacteria present in the gingival crevice. Although many of these have pathogenic potential, the strongest causal associations have been demonstrated between Porphyromonas gingivalis and severe adult periodontitis, and between Actinobacillus actinomycetemcomitans and localized juvenile periodontitis.

Many factors contribute to the maintenance or disruption of the ecological balance in the subgingival area. The immunological status of the host, the relative and absolute numbers of specific organisms or groups of organisms, and environmental parameters such as tobacco use, all play a role in determining gingival health or disease (Socransky and Haffajee, 1992).