Fig. 1.

Biosensors enable real-time monitoring of biological chemical production. A metabolic pathway converts a low-cost starting material such as glucose (green triangle) to a desired product (green square) through a series of enzymatic reactions. A biosensor (blue ellipse) regulates the production of a fluorescent reporter by preventing transcription in the absence of the correct inducing molecule. The fluorescent response of the biosensor is proportional to the amount of product produced by the cell because the starting material and intermediates do not activate the biosensor. Each cell reports its rate of chemical production through its fluorescence intensity. Cells with high intensity, and consequently highly productive metabolic pathways, are easily identified.