A British teenager has become the first person in the world to have a drug-resistant bacterial infection treated by genetically engineered viruses. Isabelle Holdaway, 17, was given just a 1% chance of survival after a double lung transplant to treat her cystic fibrosis left her with an intractable bacterial infection that could not be treated with antibiotics. Her arms, legs and buttocks had numerous big, black, festering lesions where the bacteria were pushing up through her skin. She finally ended up in intensive care after her liver began to fail. Every previous patient in Isabelle’s situation died – some within a year, despite aggressive treatment. Desperate for a solution, Isabelle’s mother researched alternative treatments online and came across phage therapy. It’s not new; doctors have been using it for nearly a century, but its use has been eclipsed by antibiotics because they are much easier to use. Isabelle’s care team at Great Ormond Street Hospital contacted Prof Graham Hatfull at the Howard Hughes Medical Institute, in the US, who had the world's largest collection of phages (approximately 15,000). Hatfull and his team identified three potential phages that would be effective in tackling Isabelle’s bacterial infection and genetically modified two of them to make them more effective. Isabelle was injected with the cocktail of phages twice daily and they were also applied to the lesions on her skin. Within just six weeks, a liver scan showed that the infection had essentially disappeared. Phage therapy involves injecting bacteria-killing viruses into a patient’s body which track down, infect and ultimately kill bacteria. The phages hijack the bacterial cell and turn it into a phage factory until the viruses burst out of the bacteria killing it in the process. While Isabelle’s fatal infection has not been completely cured, it is under control and she is beginning to lead a normal life. She still has two infusions of phages every day and is currently waiting for a fourth phage to be added to the mix, which will hopefully clear the infection completely.