Cover photograph (Copyright © 2000, American Society for Microbiology. All rights reserved.): Clinical isolate of Actinobacillus actinomycetemcomitans, the agent of severe juvenile periodontitis, forms a biofilm (left), whereas mutants defective in any of seven "tight adherence" genes (tadABCDEFG) do not (right). Related open reading frames have been found in the genomes of a wide variety of Bacteria (e.g., Yersinia pestis, Pasteurella multocida, and Pseudomonas aeruginosa) and Archaea. The tad genes are likely to be important for microbial colonization. (Photograph by Scott Kachlany and David Figurski.) (See related article on page 6169.)