Ica-status of clinical Staphylococcus epidermidis strains affects adhesion and aggregation: a thermodynamic analysis

Staphylococcus epidermidis is a major nosocomial pathogen associated with infections of indwelling medical devices. One important virulence factor of these organisms is their ability to adhere to devices and form biofilms. In this study, we evaluated the effect of the ica operon on cell surface hydrophobicity, thermodynamics of adhesion, and biofilm formation for seven S. epidermidis strains. The surface free energy parameters of the bacterial cell surface and the substratum were determined by contact angle measurement. Biofilm formation was assayed using crystal violet staining. Results showed that ica-positive strains demonstrated a higher hydrophobic characteristic than ica-negative strains, suggesting that the ica-operon seems to determine the cell surface hydrophobicity of S. epidermidis. Interaction of ica-positive strains with a tissue-culture treated polystyrene surface was energetically favourable (ΔG^Tot < 0), in contrast to ica-negative strains (ΔG^Tot > 0). The interfacial free energy of aggregation of S. epidermidis was lower for ica-positive than for ica-negative strains. Our study suggests that, in addition to biofilm formation, adhesion and aggregation of clinical S. epidermidis is stimulated in ica-positive strains by influencing the thermodynamics of interaction.