Polyamine is a small organic-polycation exists in all living organism, being implicated in wide variety of biological reaction, and is essential for proper growth of the organisms. It generally exists as free form but in Selenomonas ruminantium, an strictly anaerobic Gram-negative bacterium, cadaverine (NH_3^+・(CH_2)_5・NH_3^+) covalently links to peptidoglycan as a pivotal constituent for maintaining the envelope integrity through the interaction with outer membrane protein Mep45. Cytoplasmic biosynthesis of cadaverine occurs totally in a eukaryotic-like manner rather than in a conventional way of bacteria. Lysine/ornithine decarboxylase (LDC/ODC), a pyridoxal-5' -phosphate (Vitamin B_6)-dependent enzyme responsible for cadaverine synthesis, displays significant homology to eukaryotic ODC but not to the bacterial LDC. Its activity is tightly regulated by the antizyme-mediated proteolysis, a regulatory process generally found in eukaryotes. These findings represent the biological diversity of this bacterium beyond the pre-exist knowledge related to the polyamine-physiology, cell envelope-architecture, and the regulatory system for the enzyme.