**4. Conclusions**

In this paper, two types of experiments were carried out in order to evaluate the antibacterial properties of a semi-transparent coating incorporating photocatalytic TiO2 particles. Experimentation focused on the measurement of: (a) the photoinduced antibacterial activity of TiO2 coating, and (b) its resistance to the formation of bacterial biofilm under accelerated growth conditions.

The tested coating showed significant effects in terms of antibacterial activity and resistance to biofilm formation under low irradiation (2.5 W/m2). The antibacterial activity was induced by photocatalysis. SEM analyses confirmed the lethal activity of the coating on bacterial cells, even in favorable conditions of biofilm formation considered as a 'worst case' in comparison to the main conditions of use.

The Epifluorescence approach carried out to evaluate biofilm formation was suitable for TiO2-coated cementitious supports. Fluorescence observations provided good pictures of the colonization patterns on the surface of samples. This work can be seen as a preliminary study exploring the potential of epifluorescence microscopy in a field in which it is little exploited: microbial contamination of building materials. Further tests are necessary to validate the methodology, e.g., quantitative measurements by chemical or molecular methods. Subsequently, the methodology could be used in studies on microbial growth and spread on coatings and/or on building materials.

Regarding the coating, further research should be undertaken to explore the optimal formulation, the distribution of TiO2 particles and the application of the coating to building materials. More work should also be carried out to evaluate the antimicrobial properties towards gram positive bacteria, including *Staphylococcus aureus* (also recommended by JIS Z 2801) and molds. The molds will be chosen among the most detected in indoor environments such as *Penicillium*, *Aspergillus*, and *Cladosporium* species.

**Acknowledgments:** The authors would like to thank Université Toulouse III—Paul Sabatier for its financial support.

**Author Contributions:** T.V. conceived and designed the experiments with the help of C.R.; T.V. performed the experiments; T.V. and B.E. performed the Epifluorescence observations and the corresponding analysis; T.V., A.B. and C.R. analyzed the data; T.V. wrote the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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