Surface Recognition among Oral Bacteria: Multigeneric Coaggregations and their Mediators

Nearly all human oral bacteria participate in intergeneric coaggregation, which is defined as cell-to-cell recognition and adherence between bacterial pairs from different genera.’ It is becoming increasingly clear that coaggregation is a critical factor in bacterial colonization and accretion on oral surface~.~.~ Furthermore, the concept of multigeneric coaggregates,6 interacting bacterial networks composed of coaggregating cells of three or more genera, is central to understanding the accumulation of dental plaque on teeth. While the recent studies of initial adherence by bacteria to oral surfaces are briefly covered here, the major focus of this article is the potential role of multigeneric coaggregations in the bacterial ecology of the oral cavity. Their relevance to (1) a lotic or flowing environment; (2) sequential addition6 or accretion of bacteria on teeth; (3) temporal changes in bacterial populations that accompany progressively more severe stages of periodontal disease;’-I9 and (4) interactive cellular arrangements that illustrate bacterial competitions,2’.22 and rosettesU are discussed. Excepting conjugation systems, reports of intergeneric bacteria-bacteria interactions outside of the oral microflora are few, and the adherence properties of the participating cell types have not been de~cribed.~’~’ Some of these appear to involve nutritiona128*3 or predatory”32 relationships. Ecosystems where coaggregation might be expected to play a role are the large cellular aggregations of metabolic consortia found in anaerobic digestors that convert organic matter into CH, and C02.33.y Although direct cell-to-cell contact between these synmphic organisms has not been reported, it has been proposed that metabolic communication within the extracellular biopolymer matrix occurs by interspecies juxtapositioning rather than between separate clusters of the Merent types of cells.3s In contrast, studies of metabolic communication among the obligately anaerobic methanogenic human oral bacteria in dental plaque are just beginning,19’ and their involvement in coaggregation has not been investigated. Because the bacterial composition of subgingival dental plaque is in a constant state of flux, a recurring theme in this review is the apparent relationship of coaggregating partnerships to their temporal appearance in this ecosystem. Subgingival bacteria that are found in the environment of normal healthy gingiva tend to form coaggregating pairs, whereas those bacteria that predominate in more advanced stages of periodontal disease tend not to recognize the early colonizers.’% Instead, they coaggregate with other bacteria