Specific Molecular Recognition and Nonspecific Contributions to Bacterial Interaction Forces

Bacteria adhere to surfaces by virtue of their interaction forces with a substratum surface. A few decades ago, a paper on bacterial adhesion to surfaces typically would either com- mence with the statement (15) "bacterial adhesion to surfaces is mediated by highly specific, stereo-chemical interactions be- tween complementary components on the interacting surfaces" or (16, 24) "bacterial adhesion is mediated by a complicated interplay between attractive Lifshitz-Van der Waals forces and repulsive or attractive electrostatic and acid-base forces, orig- inating from the interacting surfaces." Generally, the "specific" approach was favored by microbiologists and biochemists, while physico-chemists usually took a "nonspecific" approach. The two approaches were reconciled with each other (7, 8) by the realization that both interactions originate from the same, fundamental physico-chemical forces (Lifshitz-Van der Waals, electrostatic, and acid-base interaction) (37). Nonspe- cific, Lifshitz-Van der Waals interactions operate over longer distances (several tens of nanometers) and originate from all atoms in the interacting entities. The summation of the rela- tively weak pairwise interactions between all atoms in an ad- hering bacterium and a substratum yields the final interaction force, similar to the origin of the gravitational force of the earth. Specific interactions, making up for molecular recogni- tion between ligand and receptor molecules, operate over spa- tially well-confined stereochemical regions, established for in- stance by interactions between acid, electron-accepting and basic, electron-donating groups or oppositely charged do- mains, at close approach (up to several nanometers). Characterization of the bacterial cell and substratum sur- faces in terms of their zeta potentials and surface free energies (from measured contact angles with liquids) offers the possi- bility to calculate the electrostatic and Lifshitz-Van der Waals contributions to the interaction force between two entities in an approach called the DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory (5, 16). In the so-called "extended DLVO" theory (38), acid-base interaction forces are ac- counted for in addition to Lifshitz-Van der Waals and electro- static forces. Application of physico-chemical theories toward explaining bacterial adhesion to surfaces has not always been