Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Severe necrotizing colitis in an FVB/NTac mouse experimentally inoculated with Citrobacter rodentium. Lesions include necroulcerative colitis with reactive crypt hyperplasia and dysplasia and multifocal fibrinonecrotic sequestra extending into the lumen. Without fluid therapy intervention, the infection in FVB/NTac mice is fatal. (Photo courtesy of Arlin B. Rogers, Massachusetts Institute of Technology, Cambridge, MA.) (See related article on page 3271.)