Cover photograph (Copyright © 2003, American Society for Microbiology. All Rights Reserved.): Polyhedral organelles involved in coenzyme B₁₂-dependent 1,2-propanediol degradation by Salmonella enterica. The organelles are approximately 100 to 150 nm in cross section. Although their exact structure is uncertain, they are thought to consist of a protein shell composed of seven polypeptides encasing four enzymes: coenzyme B₁₂-dependent diol dehydratase, propionaldehyde dehydrogenase, diol dehydratase reactivation factor, and cob(I)alamin adenosyltransferase. As yet, no associated lipids have been found. Their proposed function is to sequester propionaldehyde, an intermediate of 1,2-propanediol degradation that is potentially toxic. Based primarily on bioinformatic studies, organelles having related protein shells are thought to be encoded by 30 of 209 bacterial genomes. (See related article on page 5086.)