By feeding mice a genetically modified version of E. coli, a bacterium that naturally lives in guts, scientists were able to prevent them from gaining as much weight as mice not given the treatment. The microbes were made to express a substance called NAPE, which mice and humans convert into a hormone when food is digested.
This hormone then moves through the bloodstream to the brain, and reduces appetite. The idea is that mice with this new bug in their guts thought they were eating more than they were, without any apparent ill effects. In the study, published in The Journal of Clinical Investigation, mice given the microbe and un-dosed animals were both put on a high-fat diet.
The animals given the bacteria (in their drinking water) ate less and gained less weight, and also showed less insulin resistance, a marker for diabetes. The finding raises the possibility that these bacteria, which basically amount to an engineered probiotic, could be imported into humans to do the same thing. That's obviously a ways off and hypothetical at this point, since mice are not humans.