Building Sensors using Self-organising Bacteria

Bacteria have become builders of useful devices, say Researchers in America, who programmed them with a synthetic gene circuit and supplied them gold nanoparticles.

Bacteria have become builders of useful devices, say Researchers in America, who programmed them with a synthetic gene circuit and supplied them gold nanoparticles. Image Credit: Duke University

The inspiration for this research was Nature, which is full of examples of combinations organic and inorganic compounds to yield better materials. Molluscs, for instance, combine calcium carbonate with tiny amounts of organic compounds resulting in a shell three times stronger than calcium carbonate alone.

Such biological fabrication uses raw materials and energy efficiently, and harnessing such construction abilities in bacteria could have many advantages over current manufacturing processes. In this synthetic system, tweaking growth instructions to create different shapes and patterns could be much cheaper and faster than casting new dies or molds.

Nature is a master of fabricating structured materials consisting of living and non-living components. But it is extraordinarily difficult to program nature to create self-organized patterns. This work, however, is a proof-of-principle that it is not impossible.

Lingchong You, Paul Ruffin Scarborough Associate Professor of Engineering, Duke University

You and his colleagues were able to direct bacteria with a synthetic gene circuit to self-assemble, building working pressure sensors which they used to tap out Morse code.

"This technology allows us to grow a functional device from a single cell," You said. "Fundamentally, it is no different from programming a cell to grow an entire tree."

The genetic circuit is like a biological set of instructions embedded into the DNA of the bacterium. The directions tell the bacteria to produce a protein called T7 RNA polymerase (T7RNAP), which triggers its own expression in a positive feedback loop. It also produces AHL, a small molecule that can diffuse into the environment like a messenger.

As the cells proliferate, the concentration of AHL hits a critical concentration threshold, triggering the production of two more proteins: T7 lysozyme, which inhibits the production of T7RNAP; and curli, which acts as a biological Velcro that can latch onto inorganic compounds.

The interaction of these feedback loops causes the bacterial colony to grow in a dome-shape until it runs out of nutrients. The bacterium on the outside of the dome produces the biological Velcro, which grabs onto gold nanoparticles supplied by the Researchers, forming a shell about the size of a freckle.

The Researchers changed the size and shape of the dome by controlling the properties of the porous membrane it grows on: varying the size of the pores or how much the membrane repels water affects how many nutrients are passed to the cells, altering their growth pattern.

We're demonstrating one way of fabricating a 3-D structure based entirely on the principal of self-organization. That 3-D structure is then used as a scaffold to generate a device with well-defined physical properties. This approach is inspired by nature, and because nature doesn't do this on its own, we've manipulated nature to do it for us.

Stefan Zauscher, Sternberg Family Professor of Mechanical Engineering & Materials Science, Duke University

To demonstrate how the system could be used to manufacture working devices, the Researchers used their hybrid organic/inorganic structures as pressure sensors. They grew identical arrays of domes on two substrate surfaces and sandwiched them together so that each dome was positioned directly across from its counterpart on the other substrate.

Each dome was linked to an LED light bulb through copper wiring. When pressure was applied to the sandwich, the domes pressed into each another. This caused a deformation which resulted in an increase in conductivity and the corresponding LED light bulbs to brighten depending on the amount of pressure being applied.

In this experiment we're primarily focused on the pressure sensors, but the number of directions this could be taken in is vast. We could use biologically responsive materials to create living circuits. Or if we could keep the bacteria alive, you could imagine making materials that could heal themselves and respond to environmental changes.

Will (Yangxiaolu) Cao, Postdoctoral Associate, You's laboratory

But it doesn’t stop there: the Researchers – who published their work in Nature Biotechnology – are already planning the next step, says You,

"Another aspect we're interested in pursuing is how to generate much more complex patterns. Bacteria can create complex branching patterns, we just don't know how to make them do that ourselves yet."

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