March 24, 2014
MIT Engineers Use Bacterial Biofilms To Create Lifelike Materials
redOrbit Staff & Wire Reports - Your Universe Online
By drawing inspiration from the way in which minerals, living cells and other substances combine to form bone, a team of MIT engineers have created a type of “living material” out of bacterial cells, according to research appearing in Sunday’s edition of the journal Nature Materials.Lead author Allen Chen, an MIT-Harvard MD-PhD student, and his colleagues coaxed those cells to produce biofilms capable of incorporating nonliving materials such as gold nanoparticles and quantum dots. They combined the ability of living cells to respond to their environments and produce complex biological molecules with the benefits of nonliving materials, such as adding functions like electrical conductivity or light emissions.
Chen and his colleagues created new materials that give a glimpse into the potential benefits of this method, which could ultimately be used to create complex devices such as enhanced solar cells, materials that can heal themselves, or improved diagnostic sensors, explained senior author Timothy Lu.
“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,” said Lu, an assistant professor of electrical engineering and biological engineering at MIT. “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 study authors used E. coli in order to conduct their research, since it is known to naturally produce biofilms containing “curli fibers” – amyloid proteins which help the bacteria attach itself to surfaces. Those curli fibers are made out of a repeating chain of identical protein subunits, CsgA, which can be modified by adding protein fragments known as peptides, which in turn can capture nonliving material and incorporate it into the biolfilms.
“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,” MIT's Anne Trafton explained. “They also engineered the cells so they could communicate with each other and change the composition of the biofilm over time.”
To start, Lu’s team switched off the bacterial cells’ ability to produce CsgA and replaced it with an engineered genetic circuit that only produce the protein subunits when a molecule known as AHL is present. This allows them to control the production of curli fibers by adjusting 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,” Trafton said. “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.”
When both AHL and aTc are present, the biofilm will contain a mix of both tagged and untagged fibers. If gold nanoparticles are added to the environment, the histidine tags will latch onto them and create rows of gold nanowires and a network that conducts electricity.
The researchers also demonstrated that the cells can communicate with each other to control the actual composition of the biofilm. They designed cells that produce both untagged CsgA and AHL, which then serves as a catalyst to encourage other cells to begin production of 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,” said Lu. “It shows that indeed you can make cells that talk to each other and they can change the composition of the material over time. 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.”
The study – which was funded by the Office of Naval Research, the Army Research Office, the National Science Foundation, the Hertz Foundation, the Department of Defense, the National Institutes of Health, and the Presidential Early Career Award for Scientists and Engineers – could have several real-world applications.
For example, Lu said that they could use it in energy applications such as solar cells and batteries, or in diagnostic devices and scaffolds for use in tissue engineering. Furthermore, the researchers have expressed interest in coating the biofilms with enzymes capable of catalyzing cellulose breakdown, which would allow the substance to be used to convert agricultural waste into biofuels.