In a collaboration led by researchers at Harvard Medical School and the Broad Institute, scientists sequenced the genome of the Gram-negative bacterium Pseudomonas aeruginosa and then compared two strains isolated from cystic fibrosis, describing their findings in this PNAS paper. A hallmark of this pathogen is its ability to survive in many different host environments, and the researchers found that this results from having a "mosaic genome" composed of a core and regions where whole sections are variable. This plasticity, the scientists write, is a large evolutionary force in molding bacterial genomes.