Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): The attaching and effacing mouse pathogen Citrobacter rodentium heavily colonizes the luminal surfaces of superficial epithelial cells in the distal colon of an infected mouse. Localization of these bacteria was visualized by staining for C. rodentium lipopolysaccharide (green) to identify the host cell populations exposed to and potentially activated by the lipopolysaccharide, since Toll-like receptor 4 mediates much of the inflammatory response during C. rodentium infection. Host cells were outlined by staining for filamentous actin with phalloidin (red). Host cell nuclei were stained with 4`,6-diamidino-2-phenylindole (blue). (See related article on page 2522.)