Bacterial adhesion to silica sand as related to Gibbs energy variations

Bacterial adhesion to silica sand was related to variations in system Gibbs energy ΔGadh. Two typical Gram-positive bacterial strains of Streptococcus mitis and Lactobacillus casei were used as the model bacteria in this research. Impacts of solution chemistry and goethite coating of silica sand on bacterial adhesion were also explored. S. mitis and L. casei had negative ΔGadh with both uncoated and goethite-coated silica sand, demonstrating their adhesion potentials to these substrate. After goethite coating, ΔGadh decreased (negatively increased) for both S. mitis and L. casei. In the presence of rhamnolipid biosurfactant, ΔGadh increased (negatively decreased) in answer to the increase of the rhamnolipid biosurfactant concentration. Bacterial percentage adhesion to silica sand corresponded to ΔGadh. This study demonstrated that bacterial adhesion to substrate could be explained in terms of bacterial, substratum and intervening medium physicochemical surface properties, which can be independently determined based on contact angle measurements.