Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Confocal laser scanning microscopy images of a Pseudomonas aeruginosa PAO1 flow cell biofilm (green) after treatment with colistin and staining of dead cells with propidium iodide (red). Colistin, surprisingly, kills the inner part of the biofilm, leaving the surface cell layers unaffected. Colistin-induced modification of the lipopolysaccharide seems to be responsible for this difference in susceptibility. Total three-dimensional views are shown together with image-processed views of “mushroom” colonies where the top has been cut off. (See related article on page 28.)