Fig. 5 Number of adherent bacteria about roughness[52]
(1) Nano-roughness
When the roughness is minor (Ra:0.23-6.13 nm), smooth surfaces under static conditions are more conducive to bacterial adhesion; as the surface roughness decreases, the more bacteria adhere to the surface[53-56]. This phenomenon is associated with cellular metabolic activity, such as an increase in bacterial size and an increase in EPS production on smooth surfaces[54-55]. For example, the adhesion ofStaphylococcus aureus decreases with increasing nanostructure size. This is because the macromolecules on the bacterial cell wall can only bind the top region of the material surface, and the top part decreases with increasing nanostructure size [57], as shown in Figure 6(a).
When the roughness is relatively large (Ra:6-30 nm), the rough surface is more conducive to bacterial adhesion. The number of bacteria adhering to the surface increases as the surface roughness increases[58-60]. For example, oral restorative materials with greater roughness and surface energy can increase the adhesion of S. aureus [61]. A saliva coating alters this causal relation, and the layer may change the surface roughness[12].
(2) Submicron and micron roughness
Many researchers have reported that bacterial adhesion positively correlates with the submicron or micron-scale roughness[62-67]. The adhesion force between bacteria and surfaces with submicron scale roughness was enhanced as the roughness increased until the necessary roughness was reached[68-70]. Bacteria attached to rougher surfaces have more obvious deformation. More considerable deformation of bacteria could increase the contact area and the adhesion force. If the roughness of the character is so high, the bacteria on the bacterial probe could only touch the protruding parts of the surface [63, 70], as shown in Figure 6 (b). Higher roughness didn’t further promote the initial adhesion in static culture conditions. At the same time, the deep valleys on the rough surface could trap bacteria and protect them from the sheer force of the washing procedure, as shown in Figures 6 (b) and 6 (d).
Compared with Ra, peak density (Spd) also has a significant effect on bacterial adhesion, especially at lower roughness[73]. Siegismund et al. combined the XDLVO theory with the surface element integration method to compare the impact of Ra and S PD on bacterial adhesion on rough titanium surfaces[70], as shown in Figure 6. The calculation result revealed that when the average roughness was low (Ra < 70 nm), the interaction energy had a negative correlation with the peak density; when the average roughness was higher (Ra > 150 nm), the interaction energy only depended on Ra.