Bacteria have complex parietal structures in direct contact with the medium,which constitute a zone of exchanges with their local environments. Recent studies on physicochemistry of the bacterial interfaces have highlighted that physical properties (viscoelasticity, hydrophobicity) and electrokinetic ones (charge density, hydrodynamic permeability) are crucial determinants in bacterial adhesion processes (1). In this context, the physicochemical surface properties (charge and hydrophobicity) of the human pathogenic bacteria, Legionella pneumophila sg1 (Lp1) were investigated to explore the initial interaction step during host cells-bacteria interactions. The bio-interfaces of different clinical L. pneumophila strains and also four mutants deficient in some parietal structures (pili, lipopolysaccharides) were characterized at the micrometer scale using microelectrophoresis and at nanometer scale by atomic force microscopy (AFM) (2). The first allowed to measure electrophoretic mobilities (electrostatic interactions) and the second to quantify local hydrophobicity and specific adhesion interactions of surface biopolymers. The impact of different ionic strenghts (1 to 100 mM) at neutral pH of the medium on the physicochemical properties and infection capacity were investigated. Our results showed that initial long-range repulsive forces could possibly play an important role in defining the adhesion/infection process. The LPS nature of the Lp1 strains contributed to their negative electrical charge and consequently to their electrophoretic mobilities. Complementary informations, particularly the cell surface imaging (presence of flagella and pili) and mapping the hydrophobic domains of cells, were performed with AFM on the Lp1 strains with contrasted mobility patterns. These data should lay the molecular basis of the L. pneumophila adhesion mechanisms to host cells.