WEDNESDAY, March 4 (HealthDay News) -- American researchers say they've engineered a virus that disables the defense systems of bacteria to enhance the effectiveness of antibiotics.

The scientists said this approach could help prevent bacteria from becoming resistant to antibiotics and kill bacteria that have already become antibiotic resistant.

The Massachusetts Institute of Technology and Boston University scientists engineered existing bacteriophages (viruses that infect bacteria) to attack the SOS system -- a bacterial DNA repair system that bacteria use when they're exposed to antibiotics that damage DNA -- and other gene networks in bacteria.

The survival rate for mice infected with bacteria and treated with engineered bacteriophages and antibiotics was 80 percent, compared with 50 percent for mice treated with natural bacteriophages and antibiotics, 20 percent for mice treated with antibiotics alone, and 10 percent for untreated mice.

The study was published in the March 2 online issue of the journal Proceedings of the National Academy of Sciences.

"This work lays the groundwork for the development of a library of bacteriophages, each designed to attack different bacterial targets," lead author Timothy Lu, an M.D. candidate in the Harvard-MIT Division of Health Sciences and Technology, said in an MIT news release.

Antibiotic-resistant bacteria are a serious and growing health threat. For example, methicillin-resistant Staphylococcus aureus (MRSA) causes about 94,000 infections and contributes to 19,000 deaths each year in the United States, according to the U.S. Centers for Disease Control and Prevention.

New drugs are needed to combat such superbugs, but few new antibiotics have been developed in recent decades.

"There are a lot of targets to go after, but people haven't been able to find the drugs," Lu said.