Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Composite fluorescence in situ hybridization image showing Rikenella-like bacteria (red) and nuclei (blue) of their host, the medicinal leach (Hirudo verbena). The medicinal leech houses two bacterial symbionts, Aeromonas veronii and Rikenella-like bacteria, in the crop of its digestive tract. The Rikenella-like bacteria colonize the epithelium of the leech crop. Erythrocytes present in the lumen of the crop exhibit red autofluorescence in this photograph. (See related article on page 1984.)