## 5 Discussion
Most often, these are the surface characteristics of the materials, such as microtopography, surface roughness, surface free energy and chemical characteristics [35]. The adhesion of microorganisms on the surface of 3Y-TZP, after the routine procedure of ultrasonic scaling and brushing was evaluated using _S. aureus_, _S. sanguinis_ and _C. albicans_, which are known to be initial colonizers among the microorganisms who compose dental plaque [36]. Therefore, these three representative microorganisms were chosen for this study. The results of some studies have shown that glazed surfaces of monolithic zirconia samples have a greater surface roughness, and that was why they tend to accumulate more biofilm [37]. _S. aureus_ showed the highest adhesion on the veneered and polished zirconia samples after ultrasonic scaling, while _S. sanguus_ mostly adhered to the zirconia surfaces after ultrasonic scaling. It was obvious that the defects created after the action of ultrasonic scaling favored the retention of microorganisms. In a mutual comparison of microorganisms, _Streptococcus_ showed a greather growth, independent of the type of the surface. Also, in support of the previous study, a greater colonization of microorganisms was detected on the glazed surfaces in comparison to the polished surfaces [37]. _C. albicans_ adhered more to the vennered zirconia samples, but there was no difference in fungal adhesion between the polished and glazed sample groups, which was confirmed by the results [25; 38]. Surface microtopography plays an important role in the mechanism of the adhesion of microorganisms. Microtopographic changes of the surface properties (at the micro or nano level) can occur as a consequence of the applied technique of processing of the material surface and can affect the inhibition or development of a biofilm in relation to the same material processed differently [39]. In this study, however, the highest bacterial adhesion was on the zirconia sample surfaces, with most drastic defects after ultrasonic scaling, which could be explained by the depth and rough edges of the defects that retained microorganisms. One of the most important factors in the formation of an oral biofilm is surface roughness, which is correlated with bacterial adhesion. The surface finishing protocols (grinding directions, pressure and water coolants), as well as treatment during the routine maintenance of oral hygiene procedures, can affect the roughness of the surfaces of zirconia prosthetic restorations [40]. In the treatment of periodontal disease, patients are offered modern surgical procedures; after which, it is recommended to make prosthetic ceramic restorations in order to maintain the achieved therapeutic results. Furthermore, patients are scheduled for regular hygiene procedures that include the use of ultrasound, brushes and abrasive paste. This experiment has shown that these routine hygiene procedures should be carried out very carefully, as they can damage the surface of the prosthetic restoration. The importance of this study is that SEM technology provided extensive data of the surface properties of zirconia samples obtained by CAD/CAM and veeenering with different finishing procedures while simulating routine professional procedures for maintaining oral hygiene in daily dental practice. Finally, it should be pointed out that the results of this in vitro study were based on data of viable microorganism counts (microbial growth), which might show uneven results due to the general problem of achieving the reproduction of microorganisms in microbial tests and the reliable determination of cell viability. The estimation of the cell count by CFU, as used in this experiment, usually underestimates the number of viable cells present in a sample, as clumps of microbial cells can be miscounted as single colonies. This can pose a problem for statistical analyses. This study did not simulate all the relevant conditions, such as the influence of saliva, rinsing effects or changes in the pH.