Catalyzes the decarboxylation of carboxynorspermidine and carboxyspermidine. Essential for biofilm formation. carboxynorspermidine + H(+) = CO2 + norspermidine carboxyspermidine + H(+) = CO2 + spermidine Homodimer. Abolishes sym-norspermidine and spermidine accumulation and results in substantially increased accumulation of diaminopropane, whereas putrescine levels remain unaffected. Disappearance of norspermidine and accumulation of carboxynorspermidine. 50-60% reduction in growth rate of planktonic cells and severely reduced biofilm formation, which can be rescued by exogenously supplied sym-norspermidine but not spermidine. Not required for colonizing the small intestine in infant mouse model of infection. Belongs to the Orn/Lys/Arg decarboxylase class-II family. NspC subfamily.