The first enzyme leading to putrescine and thus polyamine synthesis. H(+) + L-ornithine = CO2 + putrescine Optimum pH is 7.0. Amine and polyamine biosynthesis; putrescine biosynthesis via L-ornithine pathway; putrescine from L-ornithine: step 1/1. Induced at low environmental pH (at protein level). Part of the speFL-speF-potE operon (PubMed:1939141). Expression induced by ornithine (at protein level) (PubMed:32094585) (Probable). Belongs to the Orn/Lys/Arg decarboxylase class-I family.