Subgingival biofilm formation
Authors: Masae Kuboniwa, Richard J. Lamont
Journal: Periodontology 2000 (2009)

The human body contains numerous distinctive ecosystems that provide a unique environment for colonizing microorganisms.The periodontal pocket is one such microniche.This environment is partially sheltered from the physical shear forces in the oral cavity and contains the hard, nonshedding surfaces of the tooth root along with the shedding surfaces of gingival mucosa.The junctional epithelium, which is attached to the tooth root, is poorly differentiated, lacks keratinization and has relatively wide intercellular spaces.Consequently, junctional epithelium is permeable and allows the migration of polymorphonuclear leukocytes into the periodontal pocket.Furthermore, the tissues in the periodontal pocket are bathed in gingival crevicular fluid, a serum exudate with antioxidant properties.The initial bacterial colonizers attach to the available surfaces, as discussed elsewhere in this volume of Periodontology 2000.Later colonizers attach to the antecedent organisms and assemble into polymicrobial communities.The biofilms on the hard surfaces develop into spatially organized structures that can extend several hundred micrometers from the surface.By contrast, the epithelial surfaces, which are continually being sloughed and replenished, tend to be colonized with monolayers of microorganisms.However, several of the more pathogenic species of bacteria are able to invade the gingival cells and tissues where they can remain viable and thus constitute a nidus of infection.Interspecies adherence interactions help to shape the temporal and spatial development of the complex bacterial consortia in the gingival crevice.Bacteria within these communities encounter high cell densities and, in consequence, community living involves adaptation to higher (and unevenly distributed) levels of metabolic by-products, secondary metabolities and other secreted molecules, and to the sporadic availability of nutrients and oxygen.Bacterial inhabitants of biofilms are known to both collaborate (e.g. through nutritional cross-feeding) and compete (e.g. through production of bacteriocins) as they strive to optimize their adaptation to these environmental constraints.Bacteria can also communicate with one another through a variety of sensing and response systems based on either cell-tocell contact or detection of soluble mediators.The signaling molecules are processed through transcriptional and post-transcriptional networks and they allow bacterial inhabitants of biofilms to coordinate activities at a group or community level.An understanding of the mechanisms of subgingival biofilm formation and development needs, therefore, to accommodate the multiple interspecies interactions that occur in polymicrobial communities. Co-adhesion controls community architectureThe predominant early colonizers of the subgingival plaque biofilms are the Actinomyces species and streptococci (110).A complex microbial community then develops within the space of only a few days (76), and the secondary colonizers tend to be the more pathogenic species such as Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, Fusobacterium nucleatum and Aggregatibacter actinomycetemcomitans.These later colonizers express numerous adhesions that enable attachment to the earlier bacterial inhabitants of the region, often 'choosing' partners that are metabolically compatible.Moreover, a number of the secondary colonizers, in particular F. nucleatum and P. gingivalis, can bind both to early colonizers and to other, later, colonizers (46, 113), thus