Cover photograph (Copyright © 2008, American Society for Microbiology. All Rights Reserved.): Colored scanning electron micrograph of an acidophilic bacterial biofilm. This sample was collected from a 75-m-deep borehole located at the Kempton Mine complex in Maryland. Image courtesy of Graeme Bowles, Philip Piccoli, Philip Candela, Michael Seufer, and Steven K. Lower. (See related article on page 309.)