Quantification of the adhesion free energy between bacteria and hydrophobic and hydrophilic substrata

In this paper, the bacterial adhesion to hydrophilic glass and hydrophobic indium tin oxide (ITO) coated glass surfaces was investigated. The aim of this work was to examine the effect of the hydrophobicity of two bacterial strains (Pseudomonas stutzeri PS, and Staphylococcus epidermidis SE) and substrata surfaces on the adhesion mechanism. The hydrophobicity of the surfaces of the bacterial strains as well as of the surfaces of the tested materials was characterized using the contact angle measurement via sessile drop technique. The water contact angle measurements of the intrinsic cell hydrophobicity showed that the PS strain is more hydrophobic (θ = 40°) than SE strain (θ = 21°). For both tested materials, this angle was 12° for glass surface and 84° for ITO-coated glass surface. Based on the thermodynamic approach, the Lifshitz van der Waals interaction free energy, the hydrophobic interaction free energy and the total interaction free energy between the bacteria and the substratum through the suspending medium were quantified. In order to verify the thermodynamic approach predictions of the bacterial adhesion on each substratum, adhesion tests were carried out for each bacteria/substratum combination. The results revealed that for both materials surfaces, the adhesion energy of the hydrophobic PS strain (26.1 mJ/m² for glass and − 3.8 mJ/m² for ITO-coated glass) is higher than that of SE strain (31.8 mJ/m² for glass and 14.3 mJ/m² for ITO-coated glass). For both bacterial strains, the effect of the hydrophobicity property of the substratum seems more important in the initial adhesion step. However, the second step of adhesion involves the biological approach, since a discrepancy was found between physicochemical theoretical approach and adhesion tests for SE strain.