Autoinducer 2 Production by Streptococcus gordonii DL1 and the Biofilm Phenotype of a luxS Mutant Are Influenced by Nutritional Conditions

Abstract

ABSTRACT The luxS gene, 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 by Streptococcus gordonii , an organism that colonizes human tooth enamel within the first few hours after professional cleaning. Northern blotting and primer extension analyses revealed that S . gordonii luxS is monocistronic. AI-2 production was dependent on nutritional conditions, and maximum AI-2 induction was detected when S . gordonii was 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. An S . gordonii luxS mutant that did not produce AI-2 was constructed by homologous recombination. Complementation of the mutant by insertion of an intact luxS gene 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, the luxS mutant formed tall microcolonies that differed from those formed by the wild-type and complemented mutant strains. Biofilms of the luxS mutant 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 within S . gordonii biofilms formed in saliva during a time frame consistent with initial colonization of freshly cleaned enamel surfaces.