3.2.2 Hydrophobicity and charge
(1) Hydrophilic and hydrophobic surfaces
Bacteria prefer to adhere to surfaces with higher hydrophobicity[65,77-78,81-84,93]. This is mainly because of the excellent adhesion between bacteria and hydrophobic surfaces. Surface hydrophobicity can facilitate landing and bonding as bacteria approach the surface by reducing bacterial movement through collisions[94]. During the reversible adhesion phase, Lactobacillus Plantarum interacts with hydrophobic end-alkyl surfaces faster and more intensely than hydrophilic end-hydroxy surfaces[74].
However, some studies have shown that certain hydrophilic surfaces also facilitate bacterial adhesion [79-81,85-86,95-96]. Microscopic observation revealed that fewer Pseudomonas aeruginosa and Staphylococcus aureus adhered to hydrophobic silicone surfaces than to hydrophilic surfaces [85]. Kriegel and Ducker found that liquid films on hydrophobic surfaces evaporated more rapidly, causing adherent bacteria to leave the material’s surface[97]. Both hydrophilic and hydrophobic surfaces are suitable for the adhesion of gentamicin-resistant Pseudomonas aeruginosa, where the hydrophilic surface promotes the formation of microcolonies, and the hydrophobic surface encouragess the production of EPS [98].
(2) Superhydrophobic and superhydrophobic surfaces
Many studies have shown that superhydrophobic[87-88] and superhydrophobic[87-91,99] surfaces have the good antibacterial ability and are not conducive to bacterial adhesion. The hydrated layer on the surface of super hydrophilic TiO2 coatings has good antimicrobial adhesion properties. However, fluids in the environment can reduce the hydrated layer’s thickness and decrease its antimicrobial efficiency[87].
On superhydrophobic surfaces, tiny air bubbles in the nanostructure can reduce the contact area of bacteria (Pseudomonas aeruginosa) with the material, thereby reducing their adhesion and preventing adhesion[92,100]. However, under static incubation conditions, S. aureus can successfully colonize the superhydrophobic surface’s three-phase interface (air, liquid and solid)[75-76,100-101], as the superhydrophobicity of the material is sub-stable and the liquid eventually replaces these tiny air bubbles. The superhydrophobic surface can further improve the antimicrobial efficiency by binding to silver nanoparticles[103].
(3) Electric charge
Bacteria are more likely to adhere to positively charged material surfaces because most bacteria have negatively charged cell walls[104-105]. The effect of charge repulsion between it and bacteria on bacterial adhesion increases with the hydrophilicity of the material surface for negatively charged material surfaces[103], so antimicrobial efficiency can be improved by modifying the positive or negative charge of the material surface. However, it was also found that many bacteria adhering to positively charged surfaces showed little growth [106], possibly because of strong, attractive electrostatic interaction forces that inhibit the elongation and division required for bacterial growth and reproduction [107].