Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Streptococcus mutans, the principal causative agent of human dental caries, grown as a biofilm for 48 h on an abiotic surface in the presence of sucrose. As shown, the biofilm architecture includes distinct water channels that form a diffusion barrier at the tooth surface which, when altered, can promote cariogenesis. (Photo courtesy of Lindsay Newlon [Middlebury College] and Michele Von Turkovitch [University of Vermont]). (See related article on page 5033.)