Bacterial Biofilms That Can Incorporate Nonliving Materials & Emit Light Or Conduct Electricity

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Bacterial cells can be manipulated into creating biofilms that incorporate nonliving materials, such as quantum dots and/or gold nanoparticles, according to new research from MIT.

These new “living materials” — which were inspired by complex matrix-like natural materials, such as bone — can be created so that they can perform functions such as emitting light and/or conducting electricity.

These sorts of materials could one day be used to create/design complex devices like solar cells, or self-healing materials, according to the researchers behind the work.

“Our idea is to put the living and the nonliving worlds together to make hybrid materials that have living cells in them and are functional,” states Timothy Lu, an assistant professor of electrical engineering and biological engineering. “It’s an interesting way of thinking about materials synthesis, which is very different from what people do now, which is usually a top-down approach.”

The press release from MIT provides the technical details of the work:

Lu and his colleagues chose to work with the bacterium E. coli because it naturally produces biofilms that contain so-called “curli fibers” — amyloid proteins that help E. coli attach to surfaces. Each curli fiber is made from a repeating chain of identical protein subunits called CsgA, which can be modified by adding protein fragments called peptides. These peptides can capture nonliving materials such as gold nanoparticles, incorporating them into the biofilms.

By programming cells to produce different types of curli fibers under certain conditions, the researchers were able to control the biofilms’ properties and create gold nanowires, conducting biofilms, and films studded with quantum dots, or tiny crystals that exhibit quantum mechanical properties. They also engineered the cells so they could communicate with each other and change the composition of the biofilm over time.

First, the MIT team disabled the bacterial cells’ natural ability to produce CsgA, then replaced it with an engineered genetic circuit that produces CsgA but only under certain conditions — specifically, when a molecule called AHL is present. This puts control of curli fiber production in the hands of the researchers, who can adjust the amount of AHL in the cells’ environment. When AHL is present, the cells secrete CsgA, which forms curli fibers that coalesce into a biofilm, coating the surface where the bacteria are growing.

The researchers then engineered E. coli cells to produce CsgA tagged with peptides composed of clusters of the amino acid histidine, but only when a molecule called aTc is present. The two types of engineered cells can be grown together in a colony, allowing researchers to control the material composition of the biofilm by varying the amounts of AHL and aTc in the environment. If both are present, the film will contain a mix of tagged and untagged fibers. If gold nanoparticles are added to the environment, the histidine tags will grab onto them, creating rows of gold nanowires, and a network that conducts electricity.

The researchers also showed that the cells in the biofilm are able to coordinate with each other to control the composition. They created cells that produce untagged CsgA and also AHL, which then stimulates other cells to start producing histidine-tagged CsgA.

“It’s a really simple system but what happens over time is you get curli that’s increasingly labeled by gold particles. It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time,” Lu explains. “Ultimately, we hope to emulate how natural systems, like bone, form. No one tells bone what to do, but it generates a material in response to environmental signals.”

To add quantum dots to the curli fibers, the researchers engineered cells that produce curli fibers along with a different peptide tag, called SpyTag, which binds to quantum dots that are coated with SpyCatcher, a protein that is SpyTag’s partner. These cells can be grown along with the bacteria that produce histidine-tagged fibers, resulting in a material that contains both quantum dots and gold nanoparticles.

The researchers note that hybrid materials such as these could potentially be of great use in the energy field — particularly with regard to battery technology, or solar cell technology. Biofuel production is another option. As well as a number of options in the medical fields.

A research paper describing the new work was published in the March 23rd issue of the journal Nature Materials.