It’s a frightening — and increasingly common — problem. A patient seeks treatment for a particular ailment in a hospital and develops an entirely different disease: a bacterial infection that is resistant to antibiotics. Researchers have worked to come up with better and more powerful antibiotics, but even an antibiotic such as vancomycin, which used to be the last line of defense against bacterial infections, has been beaten by bacteria. Harvard researchers came up with a way to treat a bacterium called Enterococcus faecalis, which has recently emerged as a major source of infection in hospitals in the United States and other developed countries. By replacing one of its genes, the research team was able to render the bacterium susceptible to vancomycin once again. Now that it’s worked in the lab, the researchers are working to develop new gene therapies that can be used in humans.