Diffusion-Sensing versus Quorum Sensing in a Model Biofilm

Abstract

Cell interactions through soluble signaling molecules control differentiation, immune response and other physiologically vital processes in everything from tissue to biofilms. We tested a model of a bacterial biofilm, which uses an autoinducer (AI), N-acyl-L-homoserine lactone (AHL) for signaling, to discriminate quorum-sensing (QS) and diffusion-sensing (DS). The AI induces the transcription of a set of genes that includes the gene-encoding the AI-producing enzyme, promoting a positive feed-back. We creating a synthetic biofilm using a microfluidic network, to convey cells to an assembly area where multiple, time-shared optical tweezers are used to array them. The cells are encapsulated in a 30μm×30μm×45μm volume of hydrogel mimicking an extra-cellular matrix. To extend the size, shape and constituency of the array, we then step to an adjacent location while maintaining registration with the reference array, and repeat the process as illustrated in Figure 1(a). Using this step-and-repeat method, we formed arrays of E. coli engineered to produce AHL, which is functionally linked to a fluorescence reporter. As shown in Fig. 1(d), the threshold to induce AI production and fluorescence depends on the number of cells and the mass-transfer, indicating that QS is a side effect of DS.View Large Image | View Hi-Res Image | Download PowerPoint Slide