July 26, 2006
Nearly 40 years ago, scientists noticed that genetically distinct bacteria isolated from dental plaque often adhere to one another, calling this phenomenon “coaggregation.” Over 1,000 oral bacteria already have been shown to coaggregate, and coaggregations were recently confirmed to be extremely important in the formation of dental plaque biofilms. In addition, coaggregations were shown to bring together two oral species in saliva-fed biofilms in vitro to form a mutualistic interaction: neither species could grow by itself in saliva, but, together, they showed abundant growth (mutualism) in these model biofilms. Exactly how these two bacteria communicate with each other during mutualistic growth is not understood.
In the June issue of the journal Molecular Microbiology, NIDCR scientists and colleagues report they may have solved some of the mystery. In studies of a dual-species oral biofilm formed in free-flowing saliva, they show Actinomyces naeslundii and Streptococcus oralis coaggregating pairs are mutualistic when the latter produces a signaling molecule called autoinducer 2, or AI-2, which scientists suspect may serve as a bacterial Esperanto for interspecies communication. They determined the optimum levels of this signaling molecule in suppressing or promoting mutualistic biofilm formation. The group also raised an intriguing, though speculative, possibility. Both A. naeslundii and S. oralis are commensals, which are bacteria that inhabit the mouth without normally causing disease. “We propose that coaggregation and AI-2 signaling have synergistic effects in the production of protective commensal communities,” the scientists write. “In the absence of the concentration of AI-2 required for optimal commensal community growth, opportunistic bacterial pathogens might infiltrate the community more easily.”