Comparison of methods for assessing temporal variation of growth of fungi on building materials.

Fungal growth on indoor surfaces can decay building materials and release hazardous substances that affect indoor air quality. Despite the numerous methods available for growth determination, there is no commonly accepted standard. The goal of this study was to compare five different assay methods for the measurement of fungal growth: cultivation, MS-based determination of ergosterol, beta-N-acetylhexosaminidase activity, quantitative PCR and microscopic spore counting. Three fungal species (Aspergillus puulaauensis, Cladosporium montecillanum and Penicillium polonicum) were grown on three different building materials (two types of acoustic board and wood). Fungal load was determined at different time points. Results from all of the methods, except the spore count, showed good correlation between each other (r=0.6-0.8). Results obtained with the cultivation method had the highest variability among replicate samples (65 %), making it the least reproducible in repeated measurements. However, it also displayed the highest variability in incubation times (149 %), indicating its suitability for detecting transient changes in the physiological state of cells. Similar to the cultivation method, quantitative PCR correlated well with the other methods and had high variability in incubation times but had lower variability among replicate samples. Ergosterol and beta-N-acetylhexosaminidase enzyme activity seemed to be the methods least dependent on the physiological state of the cells. Varying growth dynamics were observed for different species over time with the different assay methods. Each one of the tests provides a different perspective on fungal quantification due to its specific responses to the various stages of fungal growth.