Biofilms are living factories. These slicks of bacteria can harm humans when they cause stubborn infections. Yet biofilms have other properties, remarkable ones, such as their ability to respond to their environments and to exhibit heterogeneity across nearly all measurable parameters—chemical, structural, electrical, and physiological—that synthetic biologists such as Tim Lu ’10 would like to exploit.
Lu, an associate professor of biological engineering and electrical engineering at MIT, wants to integrate the best of the living and nonliving worlds into biofilms. His team of researchers has been moving in that direction and has recently successfully combined biofilms, composed of the bacteria Escherichia coli, with gold nanoparticles and with quantum dots. Gold nanoparticles allow the cells to conduct electricity while quantum dots allow them to emit light. The work is reported in the May issue of Nature Materials.
Biofilms assemble themselves into multicellular materials that grow, sense their environments, and adapt to them. These characteristics suited Lu and his team; they wanted cells that could organize materials on a small scale, a large scale, and all scales in between. The scientists also wanted to control when these biological foundries would begin producing materials. And they wanted cells that naturally communicate with one another, so that the cells could send and receive messages about the functional materials being formed. Biofilms satisfied these spatial, temporal, human, and autonomous control specs.
With these specifications in mind, Lu recruited E. coli. These bacteria have chains of amyloid proteins, known as curli fibers, that can grow long and thin over scales that range from the nanometer to several microns.
When the scientists tagged the curli fibers with bits of protein called peptides, the fibers became versatile enough to grab onto gold particles, transforming themselves in electricity-conducting nanowires. The scientists also grew quantum dots, which are light-emitting nanocrystals, by capturing the zinc and sulfur that had been fed to the bacteria. Zinc and sulfur are minerals typically found in semiconductor materials.
For now, adding such chemicals to biofilms is the way Lu’s team controls the process. The next goal is to use light to activate different circuits that switch individual genes on and off.
“The systems we have are nowhere near as complex as natural biology can be,” says Lu. “Our fantasy is to grow something like bone or enamel. This is a first step to show it’s possible.”
Image: John Soares