Quantitative evaluation of antifungal activity of metallic oxide powders (MgO, CaO and ZnO) by an indirect conductimetric assay

Aims: To evaluate antifungal activities of MgO, CaO and ZnO powders quantitatively by indirect conductimetric assay. Methods and Results: Candida albicans NBRC1060, Saccharomyces cerevisiae NBRC1950, Aspergillus niger NBRC4067 and Rhizopus stolonifer NBRC4781 were used as test micro-organisms. The indirect conductimetric assay, in which the change in electrical conductivity of an alkaline solution (NaOH) is produced by absorption of CO2 from microbial metabolism, could offer a simple and rapid evaluation of the antifungal activity within 24-48 h. The conductivity curves obtained for MgO, CaO and ZnO were analysed using the growth inhibition kinetic model proposed by Takahashi for calorimetric evaluation, and the kinetic parameters and minimum inhibitory concentration ([I](100)) could be determined. MgO and CaO powders exhibited the antimicrobial activities against all fungi used in this study and showed little differences between types of fungi. However, although ZnO powder inhibited fungal growth, the values of [I](100) were over 100 mg ml(-1). Conclusions: Although a common method for evaluating antifungal activity requires over 5-7 days, the indirect assay could provide a rapid and quantitative evaluation of antifungal activity within approx. 2 days, and MgO and CaO were found to have antifungal activities. Significance and Impact of the Study: The indirect assay can be applicable for simple and rapid evaluation of the antimicrobial activity of insoluble or slightly soluble materials with high turbidity such as antibacterial ceramic powders. Moreover, these materials can be useful for controlling fungi in food processing and the environment.