Cell-cell communication in oral microbial communities.

Alex H. Rickard, Gilad Bachrach and David G. Davies
Most of the bacterial species found in the mouth belong to microbial communities, called "biofilms", a feature of which is inter-bacterial communication, mediated by two distinct phenomena. The first is through direct cell-cell contact, which is mediated by specific protein "adhesins" and often, as in the case of inter-species aggregation, by complementary polysaccharide receptors.Intra (auto-) and inter-species (co-) aggregation both promote ordered successional integration of species into the biofilm.The second method of communication invoves cell-cell signalling molecules, which are of two classes; those used for intra-species and those used for inter-species signalling. An example of the former is "quorum sensing", a process in which acyl-homoserine lactones (AHLs) induce members of the same population to produce and release specific enzymes or to initiate biofilm formation. As yet, no oral bacteria have been shown to produce AHLs. However, they can produce small peptides, such as "competence stimulating peptides", which have been shown to mediate intra-species signalling and to promote single-species biofilm formation. A common form of inter-species signalling is mediated by 4, 5-dihydroxy-2, 3-pentanedione (DPD). This spontaneously forms a family of inter-convertible compounds, collectively called "Autoinducer-2" (Al-2). With respect to oral biofilm communities, the present review will focus on the molecular basis of communication and the effects of cell-cell contact and signal molecules on gene expression. A model relating inter-species cell-cell communication and biofilm development is proposed.