Autoinducer-2 influences interactions amongst pioneer colonizing streptococci in oral biofilms

Streptococcus gordonii and Streptococcus oralis are among the first bacterial species to colonize clean tooth surfaces. Both produce autoinducer-2 (Al-2): a family of inter-convertible cell-cell signal molecules synthesized by the LuxS enzyme. The overall aim of this work was to determine whether Al-2 alters interspecies interactions between S. gordonii DL1 and S. oralis 34 within dual-species biofilms in flowing human saliva. Based upon Al-2 bioluminescence assays, S. gordonii DL1 produced more Al-2 activity than S. oralis 34 in batch culture, and both were able to remove Al-2 activity from solution. In single-species, saliva-fed flowcell systems, S. oralis 34 formed scant biofilms that were similar to the luxS mutant. Conversely, S. gordonii DL1 formed confluent biofilms while the luxS mutant formed architecturally distinct biofilms that possessed twofold greater biovolume than the wild-type. Supplementing saliva with 0.1-10 nM chemically synthesized Al-2 (csAl-2) restored the S. gordonii DL1 luxS biofilm phenotype to that which was similar to the wild-type; above or below this concentration range, biofilms were architecturally similar to that formed by the luxS mutant. In dual-species biofilms, S. gordonii DL1 was always more abundant than S. oralis 34. Compared with dual-species, wild-type biofilms, the biovolume occupied by S. oralis 34 was reduced by greater than sevenfold when neither species produced Al-2. The addition of 1 nM csAl-2 to the dual-species luxS-luxS mLtant biofilms re-established the biofilm phenotype to resemble that of the wild-type pair. Thus, this work demonstrates that Al-2 can alter the biofilm structure and composition of pioneering oral streptococcal biofilms. This may influence the subsequent succession of other species into oral biofilms and the ecology of dental plaque.