Spermidine-binding protein probably required for its uptake into cells. Binds spermidine with high affinity (KD=14.3 nM). Does not bind putrescine, cadaverine or spermine. Spermidine binding induces large inter-domain conformational changes. Implicated in induction of type 3 secretion systems (T3SS), which play a role in virulence. Significant decrease in T3SS induction. Belongs to the bacterial solute-binding protein PotD/PotF family.