Each engineered cell (blue capsule) has genes (green box with arrow) for a light sensing protein (green oval)

The light sensor is activated by passing the beam from a laser pointer over the petri plate

The light sensor controls another gene (red box with arrow), for the protein agarase (red circles). Once activated by the laser pointer, the light sensor activates the agarase gene.

Agarase breaks down the jello-like matrix that the cells rest on. When many cells produce agarase a depression becomes visible on the petri plate.