A. Putrebactin, the natural siderophore of S. putrefaciens with putrescine (diaminobutane, red) incorporated. B. When ornithine decarboxylase is inhibited, preventing the production of putrescine, then S. putrefaciens makes desferrioxamine, using cadaverine (diaminopentane, green) instead. C. When ornithine decarboxylase is inhibited and an external source of diaminobutene (blue) is added to the culture, then an unsaturated form of putrebactin is formed.