Cellulose is an abundant biopolymer made of strands of β-1,4-linked glucose units. It provides mechanical strength to plants and biofilms made by bacteria. A team led by Lynette Cegelski of Stanford University and Regine Hengge of Humboldt University of Berlin now reports that the cellulose made by bacteria is a chemically modified version (Science 2018, DOI: 10.1126/science.aao4096). Using solid-state nuclear magnetic resonance spectroscopy to analyze the structure of the cellulosic material, the researchers determined that it contains a phosphoethanolamine group (shown in blue) attached to the C6 position of a glucose ring in the repeating unit. The modification had not been observed before because standard HCl hydrolysis degrades the modification, leaving only glucose, glucose-6-phosphate, and ethanolamine. The researchers also identified the genes behind the modification. They propose that a protein known as BcsG acts as an enzyme that installs phosphoethanolamine and that two other proteins in the same cluster play roles. By making mutant bacteria that can’t make phosphoethanolamine-modified cellulose, the researchers find that the modified cellulose is necessary for Escherichia coli bacteria to form biofilms with the correct architecture. The team hopes to use the biosynthetic machinery to make bioengineered cellulosic materials with other types of modifications.