Cover photograph (Copyright © 2007, American Society for Microbiology. All Rights Reserved.): Depicted are two medicinal leeches, each weighing approximately 0.5 g. The pigmentation of the animals is characteristic for Hirudo verbana even though they are commonly sold as Hirudo medicinalis. The digestive tract of the leech harbors two dominant beneficial symbionts, Aeromonas veronii and a Rikenella-like bacterium. This simplicity allows the investigation of bacterium-bacterium and bacterium-host interactions. (The photo was taken by Michele Maltz, University of Connecticut.) (See related article on page 6763.)