Viruses that target bacteria could help give antibiotics a boost ... Biologists have engineered viruses to weaken the bacteria they infect, leaving the bugs more vulnerable to antibiotics. With more bacteria becoming resistant to the most commonly used antibiotics, the viral approach could extend the useful lifetime of these drugs. The notion of fighting infection by harnessing the viruses that infect and kill bacteria dates back nearly a century. Doctors in the former Soviet Union routinely prescribed a cocktail of such viruses — called 'bacteriophages' or just 'phages'. But the treatment never caught on in the West, where it was largely abandoned when antibiotics emerged on the scene. Since that time, researchers have become trapped in an accelerating arms race to develop new drugs against antibiotic-resistant bacteria, leading some to turn to the alternative approach of 'phage therapy'. Several companies are now developing such therapies, and in 2006, the US Food and Drug Administration approved a bacteriophage that could be sprayed onto luncheon meats to kill the bacterium Listeria monocytogenes. This bacterium causes listeriosis — a rare but sometimes fatal infection that can be particularly dangerous for those with a weak immune system, as well as pregnant women and their unborn babies.

Because bacteriophages generally parasitize only closely related bacteria, it is assumed that phage-mediated genetic exchange occurs primarily within species. Here we report that staphylococcal pathogenenicity islands, containing superantigen genes, and other mobile elements transferred to Listeria monocytogenes at the same high frequencies as they transfer within Staphylococcus aureus. Several staphylococcal phages transduced L. monocytogenes but could not form plaques. In an experiment modeling phage therapy for bovine mastitis, we observed pathogenicity island transfer between S. aureus and L. monocytogenes in raw milk. Thus, phages may participate in a far more expansive network of genetic information exchange among bacteria of different species than originally thought, with important implications for the evolution of human pathogens.