A new study has identified a genetic scar left in the genomes of bacteria as they become resistant to antibiotic treatment. This breakthrough will allow scientists to better predict which strains of pneumonia will become highly resistant in the future, giving them time to put control measures in place to help save patients’ lives. Pneumonia is a very serious infection and the third leading cause of death in the UK population. Often these infections are caused by a bacterium called Streptococcus pneumoniae (S. pneumoniae). Antibiotics are given to patients to kill the bacteria; however, the bacteria are finding ways to become resistant and this resistance threatens patient treatment in the long term. The Sheffield team discovered mutations called pde1 act as an evolutionary gateway through which the S. pneumoniae cells start to become resistant to antibiotics. This research is a major step forward in understanding how antibiotic resistance occurs and how we might be able to predict it. If we understand the emergence of antibiotic resistance, we can predict what groups of bacterial strains are becoming more dangerous. This will give us time to put control measures in place to stop their spread, saving patients’ lives.