ACS Synth. Biol. http://dx.doi.org/10.1021/acssynbio.6b00079 (2016)

The presence of Vibrio cholerae—the causative agent of cholera—is currently detected by culture of the bacterium, a slow process, or through the use of molecular methods such as PCR, which can be expensive. To make the detection of V. cholerae faster, easier and less expensive, Holowko et al. engineered a strain of Escherichia coli that provides a simple fluorescence-based readout in response to exposure to V. cholerae. The presence of the quorum-sensing molecule CAI-1, which is uniquely produced by V. cholerae, triggers a dephosphorylation cascade that culminates in repression of transcription from the Qrr4 promoter, which controls expression of an RNA-targeting sequence. When Qrr4 is activated (no CAI-1 present), a CRISPRi system prevents expression of GFP by targeting dCas9 to the GFP promoter. However, when Qrr4 is repressed, dCas9 targeting is prevented, allowing expression of the GFP reporter. Following optimization of the system guided by in silico modeling, the engineered E. coli strain produced an easily quantifiable fluorescence readout. This system provides a blueprint for the future development of strains that could detect quorum-sensing molecules from other bacteria. Replacing the GFP readout with production of antibacterial compounds could also turn the sensor into a treatment modality.