Resazurin assay as a suitable method for testing the antimicrobial activity of photocatalytic surfaces

Abstract

The antimicrobial activity of different types of photocatalytic surfaces (titania porous and nonporous phthalocyanine films) was tested using a resazurin assay. The gram-negative bacteria Escherichia coli was chosen as the model microorganism. A bacteria inoculum with a concentration of 1.75 × 106 CFU/ml was applied to hydrogel carriers and these were subsequently placed on the study surfaces. Half of the samples were kept in the dark and the other half were illuminated. After 4 h, the carriers were transferred to vials, resazurin added and the number of viable cells in the mixture counted. For nonporous surfaces, the effect of the addition of a biocide was investigated. The authors observed that as the amount of biocide in the layer increased, the proportion of viable cells in the samples kept in the dark decreased. These results are in good agreement with general assumptions. For the titania surfaces, the antimicrobial activity (characterized by ΔR) obtained through resazurin assay was compared to that obtained through the use of standard techniques. The final activity was very similar (the ΔR from the resazurin assay was 1.54 and the standard test gave a result of 1.56). The results confirm the reliability of the proposed method, and the use of carriers give us the possibility to test much larger installed surfaces without a need to cause damage to them.