The interaction of cationic liposomes with the skin-associated bacterium Staphylococcus epidermidis: effects of ionic strength and temperature.

Cationic liposomes have been prepared from dipalmitoylphosphatidylcholine (DPPC), cholesterol (Chol) and stearylamine (SA). These phospholipid vesicles were exposed to adsorbed biofilms of the skin-associated bacteria Staphylococcus epidermidis, to which they showed a strong affinity. The interaction (as assessed by the apparent monolayer coverage of the biofilms by liposomes) was described in terms of a Langmuir adsorption isotherm which enabled determination of the maximum theoretical coverage of the bacterial surface and association/dissociation constants. The interaction was shown to be dependent on the ionic strength of the surrounding medium; on increasing the ionic strength the biofilm-vesicle dissociation constant decreased. This suggested that the adsorption was mediated by electrostatic effects. The adsorption of the vesicles was examined at various temperatures, enabling determination of thermodynamic parameters for the interaction. The adsorbed state of the liposomes was energetically favoured and the interaction was enthalpy driven. The Gibbs energies of adsorption were in a range from -15 to -19 kJ mol-1 and the enthalpies of adsorption from -26 to -22 kJ mol-1. Studies using cell populations of different hydrophobicity showed that the hydrophobic character of the bacterial cells also had an effect on the adsorption of the vesicles to the biofilm.