Lactose Triggers Bacillus subtilis Biofilm Formation via Quorum Sensing Pathway

Objectives: Biofilm formation represents the first and main step in the development and progression of the majority of oral disorders. Recent studies have isolated the biofilm forming bacteria Bacillus subtilis from the oral cavity. Biofilm formation by B. subtilis is apparently dependent on quorum sensing (QS) via LuxS cascade (Autoinducer-2 (AI-2)). However, the link between sensing environmental cues and AI-2 induced biofilm formation remains largely unknown. The aim of this study is investigating the role of lactose, the primary sugar in milk, on biofilm formation by Bacillus species and its possible link to QS process.
Methods: Biofilm formation was tested using the chemical defined agar (CDA), with or without lactose, for colony type biofilm, while the confocal laser scanning microscopy (CLSM) was used for biofilm bundle formation analysis as well as for examining the expression of major operons involved in the biofilm formation. In order to test the level of AI-2 production by B. subtilis in response to lactose, reporter strain assay Vibrio harveyi MM77 (AI-2-/-) was used.
Results: We found that lactose highly induced biofilm formation in bundle as well as in colony type biofilms. Furthermore, we observed an increase in AI-2 production by B. subtilis in response to lactose in a dose dependent manner. Finally, we found that expression of eps and tapA operons, which are responsible for extracellular matrix synthesis, was highly up-regulated in response to lactose.
Conclusions: Our results suggest that lactose induce biofilm formation by Bacillus species through the LuxS quorum sensing pathway.