A study of samples from 92 patients with cystic fibrosis (CF) has revealed that certain bacteriophages - viruses that infect bacteria - worsen the severity of bacterial infections associated with the disorder. The results indicate that these viruses could be a major contributor to poorer clinical outcomes in CF, as well as a potential target for new therapies aimed at easing infections in patients. One of the most common causes of chronic bacterial infections in patients with CF is Pseudomonas aeruginosa, a bacterial species that is well-adapted to the human airways and often resists antibiotic treatment. Researchers have observed that filamentous bacteriophages (Pf phages) can exacerbate the severity of P. aeruginosa infections in animal models, but it is unclear whether Pf phages play a role in human disease. Seeking further insight, Elizabeth Burgener and colleagues analyzed bacterial genomes in sputum samples from two groups of patients with CF, and discovered that nine of 34 patients in the first group and 21 of 58 patients in the second group consistently harbored Pf phages. In both cohorts, patients were typically older than those without Pf phages, and in the larger, second cohort, patients were more likely to suffer from chronic P. aeruginosa infections and poorer lung function. Furthermore, phage-carrying P. aeruginosa from the second patient group were more resistant to the standard antibiotics aztreonam, amikacin and meropenem. The authors found the crystalline structure of the phages captured these antibiotics and prevented their diffusion, potentially explaining their decreased effectiveness. As such, future studies should investigate additional mechanisms of antibiotic resistance. The authors say that if the findings are validated in larger studies and independent cohorts, a diagnostic test for the presence of Pf phage may have future implications for clinical care.