 Did you know that over half of the cells in our bodies are not human cells? Many of our organs are heavily colonized with microbes, particularly our GI tracts. Scientists have known for a while that our gastrointestinal microbiome is critical to maintain our overall health. Now, however, researchers have found an entirely new function for these bacteria, living medicine. To understand how some scientists turned bacteria into medicine, we first have to understand the disease they were trying to treat, a genetic disorder called phenylketonuria, PKU. PKU is caused by excessive levels of phenylalanine. Phenylalanine is an amino acid, one of the building blocks of proteins. But high levels of this compound in the bloodstream can lead to numerous health problems, including severe brain damage. For most of us, however, this is no problem because we have a specialized protein that breaks down excess phenylalanine into non-harmful components. PKU patients, however, do not have this protein. Because phenylalanine is found in protein-rich foods, people with PKU have to follow a carefully controlled diet for their entire lives to prevent serious health issues. There is an FDA-approved medicine on the market, but it is not widely effective. To find a better option, scientists turned to synthetic biology and the reservoir of the miniature digestion machines already found in our bodies. Bacteria. Synthetic biology is the ability to reprogram living organisms to carry out functions they are not normally able to perform. By introducing new genes or changing how existing genes are turned on and off. In this case, scientists wanted to make gut bacteria digest and break down phenylalanine. So the scientists introduced two new genes into the bacteria. The first would allow it to vacuum up any phenylalanine in its vicinity. And the second would break down the molecule into non-harmful components. The scientists first tested out their approach in mice and monkeys. And it worked! Next, they tested the same approach in healthy humans. Excitingly, the bacteria concoction worked in humans as it had done in the monkeys, indicating that the two introduced genes seemed to function normally even in the human GI tract. Next, scientists wanted to determine if this treatment has a therapeutic effect in PKU patients. There is currently a phase 1 clinical trial looking into just that. If successful, this could be the first ever synthetic biology based drug to be approved by the FDA. Using bacteria as living medicine is particularly appealing because they are common members of our bodies. So the side effects of such a treatment may be fairly mild. In fact, we all give our GI tracts constant new doses of bacteria every time we eat cultured foods, like yogurt. As with many new treatments, however, there are potential risks. In this case, introducing new genes into a bacterium may give it an environmental survival advantage, which could crowd out other beneficial microbes. Moving forward, it will be important for scientists to monitor how engineered bacteria might influence the overall ecology of the gut microbiome. This, however, is only the beginning for using synthetic biology to treat disease. The potential for cancer therapeutics is particularly exciting because many available cancer medicines are unsuccessful due to their inability to penetrate tumors. Tumors, however, are thought to contain their own microbiomes, so scientists could engineer bacteria naturally found in tumors to kill cancer cells. This is the true promise and potential of synthetic biology based medicines. The ability to re-engineer beneficial microbes to treat devastating diseases.