The adhesion of bacteria to solid surfaces was studied using a physicochemical approach. Adhesion to negatively charged polystyrene was found to be reversible and could be described quantitatively using the DLVO theory for colloidal stability, i.e., in terms of Van der Waals and electrostatic interactions. The influence of the latter was assessed by varying the electrolyte strength. Adhesion increased with increasing electrolyte strength. The adhesion Gibbs energy for a bacterium and a negatively charged polystyrene surface was estimated from adhesion isotherms and was found to be 2-3 kT per cell. This low value corresponds to an adhesion in the secondary minimum of interaction as described by the DLVO theory. The consequences of these findings for adhesion in the natural environment are discussed.