Anyone who has ever smelled E. coli bacteria knows that they smell bad. Putridly bad. So, a group of student bioengineers at MIT set out to sweeten the scent of this commonly used lab bacteria. The team constructed its creation from a collection of biological "parts"--bits of DNA that, when inserted into living organisms, can make the organisms glow, detect light, and perform a number of other unusual functions. The team will showcase its sweet-smelling bug this weekend at the International Genetically Engineered Machine competition (iGEM) at MIT, along with 37 other student groups from around the world.

While the projects are executed largely by undergraduate students (with guidance from faculty and graduate-student advisors), the designs represent some of the most complex biologically engineered machines to date--and they promise to further the field of synthetic biology, a newly emerging discipline that views living systems from an engineering point of view.

The MIT team, for example, tosses out wacky applications for its technology: minty-fresh foot fungus or baker's yeast that smells of bananas. But its real goal is the construction of functional biological parts. "The key idea here is to develop a library of composable parts which we think of in the same way as Lego blocks," says Tom Knight, an engineer at MIT who cofounded the competition with MIT bioengineer Drew Endy. (Both advise the MIT team.) "These parts can be assembled into more-complex pieces, which in many cases are functional when inserted into living cells."

To create the scented bacteria, the students looked for different genes that convert chemicals naturally made by bacteria into chemical precursors of aromatic compounds, as well as genes that convert the precursors to the aromatics themselves -- methyl salicylate, commonly known as oil of wintergreen, and isoamyl acetate, a component of the ripe-banana smell. The genes were then hooked up to genetic controllers, known as promoters, which determine when and where that gene is turned on. A gene from a plant, for example, might be controlled by a promoter from bacteria.

The various DNA components, collected from fellow scientists and from a genetic repository housed at MIT, were then embedded in a circular string of DNA and inserted into bacteria. The end result is a new strain of E. coli that smells of mint and bananas. The team also eliminated the gene responsible for E. coli's natural stink.