“Disco bacteria” could churn out drugs and useful chemicals

THE central idea of synthetic biology is that living cells can be programmed in the same way that computers can, in order to make them do things and produce compounds that their natural counterparts do not. As with computers, though, scientists need a way to control their creations. To date, that has been done with chemical signals. In a paper published in Nature Chemical Biology, Christopher Voigt, a biologist at the Massachusetts Institute of Technology, describes an alternative. Instead of chemicals, he and his colleagues demonstrate how to control customised cells with coloured light.

Engineering cells to respond to light is not a new idea. The general approach is called optogenetics, and it has become a popular technique for controlling nerve cells in neuroscience. But Dr Voigt is not interested in nerve cells. In 2005 he altered four genes in a strain of Escherichia coli bacteria, which gave the creatures the ability to respond to light and darkness by becoming lighter or darker in colour themselves. The state of the art has advanced since then. This time Dr Voigt wanted to see if he could give his bacterial charges—which normally live in the human gut, and therefore rarely see light of any sort—the ability to sense colour.

That meant tinkering with 18 different genes containing more than 46,000 base pairs of DNA. These changes persuaded the cells to build three different kinds of light sensors that are similar in structure to the photoreceptors found in plants and algae. These sensors allow their owners to track things like the time of day and the seasons, and to plan their reproduction accordingly. One sensor, for instance, harvested from a type of algae, switches on when struck by light with a wavelength of 535 nanometres (which humans would perceive as green) and then off again when exposed to light of a longer, redder wavelength. Other sensors responded to light at the blue and red ends of the visible spectrum.

Those photoreceptors, in turn, control the expression of certain bacterial genes. For demonstration purposes, Dr Voigt’s prototype E. coli have been designed to produce enzymes that cause the bacteria to become the same colour as the light being shone upon them. So, if a plate of the bacteria is struck by a beam of green light, the plate becomes green. If a plate has an image composed of red, green and blue projected upon it, the plate will reproduce the projection (see picture).

Such artistry is the tip of the iceberg: the light sensors could be used to control genes that make all sorts of different chemicals. Scientists have toyed with the idea of using vats of genetically altered bacteria to produce things like artificial sweeteners or drugs. But getting the chemistry right requires that the bugs execute several chemical reactions in exactly the right sequence. Doing that with chemicals is expensive, and fiddly, since the chemicals take time to circulate around the vat. Dr Voigt imagines instead vats fitted with coloured lights that wink on and off in sequence. It would look, he jokes, a bit like a bacterial disco. Such single-celled revellers could yet be the future of chemical manufacturing.