In natural environments bacteria often adopt a biofilm-growth mode. PotD is a spermidine/putrescine-binding periplasmic protein belonging to polyamine transport system and we have examined its role during biofilm formation and for planktonic growth in Escherichia coli BL21(DE3) strains that either over-express PotD (PotD+), or under-express it (PotDi) and also in a control strain with vector pET26b(+) (PotD0). The three strains displayed similar growth in planktonic growth-mode, but over expression of PotD protein greatly stimulated the formation of biofilms, while less biofilm formed by strain PotDi in comparison to strain PotD0. The expressions of five genes, recA, sfiA, groEL, groES, and gyrA, were increasingly expressed in PotD+ biofilm cells. Thus, PotD is likely to change the rate of polyamine synthesis, which stimulates the expression of SOS genes and biofilm formation.