The initial adhesion takes place predominantly in the secondary minimum of the DLVO free energy curve. The strength of adsorption depends on different physicochemical forces like ionic, dipolar, hydrogen bonds or hydrophobic interactions. The secondary minimum does not usually reach large negative values

and particles captured in this minimum generally show reversible adhesion. In this case, there is a separation distance between the adhering bacteria and a thin water film remains present between the interacting surfaces. However, if a bacterium can reach the primary mini-mum, short-range interaction forces become effective and irreversible adhesion occur.

Irreversible adhesion can occur due to specific bacterial characteristics such as appendages, cell surface structures or polymers. However, it is not clear if bacteria first adhere reversibly and then produce ECP or if bacteria first produce ECP and then adhere irrever-sibly.