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].