Strains of microbes like yeast and E. coli can be engineered to produce useful chemicals and fuels, and can produce more fuel more efficiently by working together. The problem is that when grown together in co-cultures, the fastest-growing strain often outcompetes the others, causing the community to break down and stop chemical production. Now, Princeton researchers have discovered a new way to stabilize co-cultures of microbes using light. By engineering the faster-growing strain to respond to light, the researchers can control its growth using light pulses, stabilizing the community as a whole and optimizing it for chemical production.