The ability to control cellular motility in response to an external signal can facilitate the development of biosensors or micromachines that use microbes for motion. Our goal is to use a synthetic biology approach to build a system where the motility of an Escherichia coli strain is controlled by a cell-cell communication signal produced by a sensor strain. By separating the actuating and sensing components into two strains, we may improve system design through modularity, signal amplification and logic gate-type responses to multiple inputs. Towards this goal, we have designed an E. coli strain where motility is controlled by an acyl-homoserine lactone (AHL) quorum-sensing (QS) signal molecule. We targeted the repressor-based esa QS system to enable tight regulation of a key motility gene, motA. We restored motility in an E. coli motA deletion strain by expressing motA under the control of the esa promoter, which is repressed by the transcriptional regulator (EsaR) in the absence of AHL. In the presence of AHL, EsaR is expected to dissociate from the promoter triggering expression of motA. To assess whether these cells exhibited AHL-dependent motility, we measured the swimming of cells through semi-solid agar plates. We observed that the cells do not swim in the absence of AHL. However, swimming was observed in the presence of micromolar AHL concentrations. Thus, our engineered cells show communication-dependent motility (CoMot). Interestingly, we observed directional swimming of CoMot cells up an AHL gradient. Modelling of the system showed that the kinetics of the directional behavior was similar to that of the enhanced diffusion exhibited by enzyme-based systems in their substrate gradient. In our system, the robust transcriptional control enabled an enhanced swimming up an AHL gradient, thereby allowing for directional control of motility in E. coli without manipulation of chemotaxis. By manipulating parts of the QS system we have engineered CoMot variants that exhibit increased sensitivity to AHL and can swim down an AHL gradient. Further, we have demonstrated that CoMot cells respond to AHL produced by a second strain of E. coli in which AHL production is triggered by the recognition of a small molecule. Variants of this sensing strain when combined with CoMot variants enable a modular biosensing framework. This ability to precisely control, tune and steer bacterial motility broadens the application of bacteria in actuation and sensing systems.

Engineering and modeling of a modular, motility-based biosensor system in Escherichia coli

253rd National Meeting (2017)

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