In the present study, sandblasting (with 50 or 250 μm aluminum trioxide) resulted in significant increases of R _a on titanium and ceramic surfaces. These R _a values were higher than those for commercially available implant abutments (observed to range from 0.10 to 0.30 μm) [35]. According to the classification by Albrektsson and Wennerberg, smooth ceramic and titanium materials and the medium ceramic material were classified as “smooth” (R _a  < 0.5 μm), the medium titanium material as “minimally rough” (R _a 0.5–1.0 μm), the rough ceramic material as “moderately rough” (R _a 1.1–2.0 μm), and the rough titanium material as “rough” (R _a  > 2.0 μm) [36]. Although titanium and zirconia had the same treatment, polishing and sandblasting resulted in significantly higher R _a values on the titanium specimens than on the zirconia specimens. For titanium, Bollen et al. and Quirynen et al. evaluated a threshold R _a of 0.2 μm; below this threshold, a change in roughness did not significantly affect the quantity of plaque accumulation [27, 37]. The medium and rough surfaces in the present study showed R _a values above the threshold of 0.2 μm; therefore, a correlation between R _a and bacterial adhesion should be expected. Surprisingly, in the present study, surface roughness (R _a ) did not influence the quantity of adhering S. epidermidis, neither on titanium nor on zirconia. For S. sanguinis, such correlation was observed for zirconia but not for titanium. A possible explanation for this phenomenon can be found in the AFM observations. On closer examination (9 μm² scan areas, see Fig. 2b) and from a bacterial point of view (a single cell measures approximately 1 μm in diameter), no significant differences in surface profile or morphology could be found between all surfaces tested (except for rough titanium). From a microscopic or an AFM viewpoint, most surfaces are rough no matter how fine the finish; therefore, all types of surfaces provide adequate adhesion conditions for microbial accumulation [1]. The large-scale surface irregularities (>30 μm) on the sandblasted titanium and zirconia specimens, which were observed during the examination of the 900 μm² scan areas (Fig. 2a) and which were indicated by high R _a values, did not influence bacterial adhesion in the present semi-static experimental setup. However, these irregularities will probably increase microbial adhesion in an in vivo testing with supragingival exposition of specimens, when the influence of intraoral shear forces becomes apparent [25, 28, 29]. In contrast, the small grooves (measuring approximately 0.5 μm in diameter) on smooth zirconia surfaces in AFM may possibly explain the enhanced potential to adhere bacteria in contrast to totally plane titanium surfaces, because initial microbial colonization has been shown to start from very small—and not from large-scale—pits and gullies [25, 26, 38, 39]. In summary, characterizing the influence of surface morphology on initial bacterial adhesion (in the semi-static and static environment such as the peri-implant) by surface roughness values such as R _a alone is rather inadequate because of the requirement of an additional three-dimensional analysis of the microstructure. These observations were confirmed by Barbour et al. who observed different bacterial coverage on surfaces of the same roughness but different detailed surface morphology [40]. The different adhesion properties of S. epidermidis and S. sanguinis with regard to the influence of surface morphology may result from morphologic differences between the bacterial species. Accordingly, Barbour et al. observed that Actinomyces naeslundi adhere better to smooth surfaces than to rough surfaces, whereas Streptococcus mutans prefer rough substrata [40]. In addition, Taylor et al. could not clearly relate surface roughness of PMMA surfaces to the amount of adhering S. epidermidis, which supports the results of the present study [41].