Frontiers Of Biology

A sensing array of radically coupled genetic ‘biopixels’

Through synthetic biology, scientists have managed to engineer novel biosensing mechanisms into bacteria, which allows the bacteria to respond to predetermined environmental stimulus. While synthetic biology seeks to engineer novel mechanisms into individual cells, practical applications of the future may require the synchronous function of multiple cells on a macroscopic scale. In “A sensing array of radically coupled genetic ‘biopixels”, researchers built a device that uses the joint action of millions of bacterial cells to create a functional oscillator that can respond to environmental conditions. An oscillator is a device that has a predictable frequency. In this case, the frequency is a function predictable glowing patterns by the bacteria. By designing a mechanism that allowed the frequency to respond to certain environmental stimuli, the researchers hoped to design a device that could relay information about the environment to a human user.

In order to create a frequency­modulated biosensor, an LCD­like microfluidic array was created to facilitate bacterial communication. The scientists used ‘biopixels,’ which are thousands of small oscillating colonies wired together, in this microfluidic array. Each cell was engineered with a synthetic oscillatory mechanism that showed periodic fluorescence. In order to synchronize the bacterial oscillatory periods, two cell­cell communication mechanisms were utilized. Quorum sensing is a short range communication mechanism that functions via bacterial chemicals released into the solution. This mechanism alone would have been too slow to diffuse rapidly throughout the whole chamber, so a second mechanism that used rapid gaseous communication via hydrogen peroxide was also used. These mechanism were engineered to affect the oscillatory period, and the 2.5 millions cells within the chamber began to fluoresce in a synchronous manner, showing extremely consistent oscillation.

Having the ability to generate consistently detectable and predictable oscillations, the scientists attempted to use the circuit to create an arsenic­sensing macroscopic biosensor. By adding small amounts of arsenite, the repression of the AHL synthase, which mediates intracolony synchronization, is relieved which consequently allows additional luxI, a quorum­sensing machinery, to be transcribed. By varying the arsenite concentration, the levels of luxI were changed resulting in obvious changes to the oscillatory period. Additionally, by modifying the size, number and arrangement of biopixels in the device, the output waveforms were significantly altered. For instance, the scientists constructed a device that contained an array of 416 traps in which trap separation distance was increased. In these larger device experiments, the bacteria glowed in a predictable alternating pattern, which the researchers called anti­phase synchronization.

In order to demonstrate that this biosensing technique could be applied in real world, the researchers scaled up the device, and used an inexpensive LED to apply the light needed to stimulate GFP, and used a device that was 24mm x 12mm, a form­factor applicable to hand­held devices. A device like this that measures oscillatory periods is inherently less prone to error than previous devices that used amplitude because measuring amplitude requires precise calibration of sensors. This technology may be used in future devices that measure environmental conditions, and the conceptual bacterial sensing used could also be applied to scale up biological switches and logic gates into multicellular systems.