Typically, synthetic leather is petroleum based, but materials scientists searching for an ecologically sustainable alternative have used various bacteria that secrete linear chains of glucose, forming a dense mesh of cellulose called a pellicle, which is leatherlike except in color. The standard process for dyeing leather generates substantial wastewater and other undesirable byproducts, so adopting such a regimen would run counter to the ecological promise of the pellicle approach. To address this, Kenneth T. Walker and colleagues worked to modify Komagataeibacter rhaeticus bacteria to produce a pellicle with embedded pigmentation cells, thereby allowing the pellicle to “dye” itself from the inside.