TheluxSgene, present in many bacterial genera, encodes the autoinducer 2 (AI-2) synthase. AI-2 has been implicated in bacterial signaling, and this study investigated its role in biofilm formation byStreptococcusgordonii, an organism that colonizes human tooth enamel within the first few hours after professional cleaning. Northern blotting and primer extension analyses revealed thatS.gordoniiluxSis monocistronic. AI-2 production was dependent on nutritional conditions, and maximum AI-2 induction was detected whenS.gordoniiwas grown in the presence of serum and carbonate. In planktonic cultures, AI-2 production rose sharply during the transition from exponential to stationary phase, and the AI-2 concentration peaked approximately 4 h into stationary phase. AnS.gordoniiluxSmutant that did not produce AI-2 was constructed by homologous recombination. Complementation of the mutant by insertion of an intactluxSgene into the chromosome in tandem with the disrupted gene restored AI-2 production to a level similar to that of the wild-type strain. In planktonic culture, no growth differences were observed between the mutant and wild-type strains when five different media were used. However, when grown for 4 h as biofilms in 25% human saliva under flow, theluxSmutant formed tall microcolonies that differed from those formed by the wild-type and complemented mutant strains. Biofilms of theluxSmutant exhibited finger-like projections of cells that extended into the flow cell lumen. Thus, the inability to produce AI-2 is associated with altered microcolony architecture withinS.gordoniibiofilms formed in saliva during a time frame consistent with initial colonization of freshly cleaned enamel surfaces.

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