Doctors have identified a powerful bacteria that cannot be killed with antibiotics. The bacteria, called a superbug, was found in a woman from Pennsylvania. Tom Frieden is the head of the U.S. Centers for Disease Control and Prevention. He says this new bug is resistant to every antibiotic available, including colistin. Colistin is used only as a last measure, as the drug can cause organ damage. The woman is infected with a rare kind of E. coli bacteria known as CRE. The version she has includes a gene that is resistant to colistin. This is its first appearance in the United States. Researchers who studied the case in Pennsylvania wrote about their findings in Antimicrobial Agents and Chemotherapy, a publication of the American Society for Microbiology. They wrote that the case could signal the development of “a truly pan-drug resistant bacteria.” Health officials consider CRE, “nightmare bacteria.” They can kill up to 50 percent of infected patients.