Bacterial interactions and successions during plaque development

Studies from the 1960s indicated that increased microbial diversity and a succession in the predominant bacterial species in plaque correlate with the appearance of gingival inflammation and the development of periodontal disease. In the past few years, molecular characterization of the microflora found in various sites of the oral cavity of different subjects has detected around 700 bacterial species or phylotypes (1, 63, 93). Some of these species are considered commensal and a positive feature of our healthy microflora, while others are considered pathogenic. The colonization of pathogenic bacteria is probably dependent upon the interaction of pathogens and commensal organisms. The clinical relevance and periodontal microbial ecology of these bacteria have been presented in an outstanding, comprehensive review (101). The discussion in the present article centers on interactions among bacterial species and how these interactions contribute to the development of plaque and ultimately to the formation of periodontopathogenic communities. Interactions among human oral bacteria are integral to the development of plaque. From the early stages of colonization to the formation of mature supragingival and subgingival plaque, a diverse array of bacterial species colonizes into densely populated communities. Interactions among different bacterial cell types are proposed to drive the maturation of plaque. These interactions occur at several levels, including physical contact, metabolic exchange, small-signal-molecule-mediated communication and exchange of genetic material. A principal feature of human oral bacteria is their ability to interact by coaggregation with other oral bacteria (48). Coaggregation is defined as the specific cell-to-cell recognition that occurs between genetically distinct cell types. Each cell type bears on its surface one or more types of coaggregation mediator, which are called adhesins and receptors (described below). The adhesins and receptors confer a particular set of coaggregation properties. Coaggregation partnerships are central to the development of biodiversity in supragingival and subgingival plaque. Coaggregations effect changes in populations, from low diversity in initial communities of supragingival plaque to high diversity in subgingival plaque. This article will provide an overview of the physical and metabolic interactions that occur among the oral microflora in the context of plaque development.