Scientists Engineer Bacteria to Create Living Photographs
The photos were created by projecting light on “biological film” — billions of genetically engineered E. coli growing in dishes of agar, a standard jello-like growth medium for bacteria. The work is published in the Nov. 24, 2005 issue of Nature, devoted entirely to the emerging field of synthetic biology. The new field focuses on identifying genes that control key traits and then engineering microbes to activate the genes in novel combinations to create useful tools for medicine and technology. The students produced the innovative bacterial images and a bacterial camera as part of MIT’s intercollegiate Genetically Engineered Machine (iGEM) competition. The
project won “best part” for the genetically engineered light receptor developed by UCSF graduate students working with Chris Voigt, Ph.D., assistant professor of pharmaceutical chemistry at UCSF and a leader in synthetic biology. “Our living photographs are a somewhat playful example of how devices quite useful to technology and medicine can be created in the new field of synthetic biology,” Voigt said. “Essentially, we engineer the bug to give it new capabilities by combining different gene-based skills that it or other organisms already have.” The bacterial photograph could allow material to be printed with incredibly high precision, Voigt said. “We estimate that the resolution of these photographs is about 100 Megapixels, or about ten times better than high-resolution printers. The difference is that we can print gene expression.” Anselm Levskaya, a UCSF biophysics graduate student, led the effort to “build the bug,” while the University of Texas team figured out how to create the photographs. Levskaya is the Nature paper’s lead author and Voigt the senior author. “With the growing number of sequenced microbes, we can search through nature’s large trove of tools to find ones that fit the job,” Levskaya said. “In our case, searching for light-sensing domains led us to use a photosynthetic bacterium.” The students produced ghostlike, living photos of many things, including themselves and their advisors. Like pixels on a computer screen switching between white and black, each bacterium either produced black pigment or didn’t, based on whether it was growing in a dark or light place in the dish. The resulting images are a collection of all the bacteria responding to the pattern of light. E. coli live in the dark confines of the human gut and wouldn’t normally sense light, so the students had to engineer the unicellular machines to work as a photo-capturing surface. Levskaya and Voigt first engineered the bacteria to sense light by adding a light receptor protein from a photosynthetic blue-green algae to the E. coli cell surface. The microbe’s metabolism was modified to produce a chemical that gave the protein sensor the ability to see light in its new microbial host. The light sensor was also genetically modified so that light turns off a gene that ultimately controls the production of a colored compound. To create the actual photographs, the Texas students optimized pigments and growth media, and used a unique light projector largely designed and built by Aaron Chevalier, a physics undergraduate. They added a chemical compound to the agar so that those bacteria that are in the dark produce black pigment and those that are in the light do not. The device projects the pattern of light, such as an image of a person, onto the dish of bacteria growing at body temperature in an incubator. After about 12 to15 hours of exposure (the time it takes for a bacterial population to grow and fill-up the Petri dish), the light projector is removed. What’s left is a permanent living photograph. The biological technologies these students are building could be applied in a variety of ways beyond making photos. For example some genes produce plastics or precipitate metals. By activating these genes in the light, high-resolution materials could be printed. In addition, the activation of genes can differentiate cells, so one day this strategy could be used to build tissues based on patterns of light, the scientist-engineers say. The students are following up the living photograph with another innovation — bacteria that can find and create a line around the edges of an image, a process that requires the bacteria to communicate with each other.