Cover photograph (Copyright © 2006, American Society for Microbiology. All Rights Reserved.): Actinobacillus actinomycetemcomitans is the etiologic agent of localized aggressive periodontitis and can also cause infective endocarditis and abscess formation. Clinical isolates of A. actinomycetemcomitans exhibit rough colony morphology, autoaggregate in broth, and form extremely tenacious biofilms. These phenotypes are conferred by a 14-gene tad locus that encodes the molecular apparatus for biogenesis of adhesive Flp pili, which are critical for colonization and pathogenesis. Shown is a composite of an A. actinomycetemcomitans strain CU1000 colony, exhibiting the representative rough, star-shaped morphology. The image was captured at a magnification of ×630 following incubation on AAGM agar for 72 h at 37°C in 10% CO2. (Image produced by Mladen Tomich and Brenda Perez.) (See related article on page 6361.)