After we confirmed that PEG connected to hydroxyapatite via dopamine, human dentine blocks were used to study the inhibition effect of this polydopamine-induced-PEG coating on dentine against a cariogenic biofilm formation. The CFU and SEM results corroborated that dopamine-induced-PEG had inhibited the growth of a cariogenic biofilm on the dentine surface. Rinsing with distilled water did not notably reduce the surface density after the polydopamine-induced-PEG treatment. This suggested covalent binding between the polydopamine and PEG, which made the coating surface more stable.