Mixed communities of bacteria in a biofilm can perform multistep transformations that are important in bioremediation and microbial fuel cells, but typically biofilms have not been easy to engineer. Hong et al. report a microfluidic biofilm engineering circuit that is based on a population-responsive quorum-sensing switch. The basis of the system is a primary genetic switch that responds to a diffusible quorum-sensing signaling molecule together with a secondary switch that responds to a chemical inducer. Each of the two gene circuits includes previously identified genes that encode proteins engineered by the group to mediate enhanced biofilm dispersal. Different populations were labeled green (colonizers) and red (dispersers) using the respective fluorescent protein tags, allowing community dynamics to be tracked. A colonizer biofilm was established and overgrown with a disperser biofilm to form a dual-species community. Once this attained sufficient population density, synthesis of the diffusible signal by dispersers resulted in removal of at least 80% of the colonizer community. Subsequent exogenous addition of the chemical inducer removed 100% of the disperser community. The approach opens up the possibility of studies of the dynamics of biofilm formation and dispersal and of antibacterial activity on such structures. (Nat. Comm. 3, doi:10.1038/ncomms1616, published online 3 January 2012)