Since current standard laboratory methods are time-consuming macro-assays that rely on subjective
visual ratings of mould growth, rapid and quantitative laboratory methods are needed to screen potential
mould inhibitors for use in and on cellulose-based products. A colorimetric micro-assay has been
developed that uses XTT tetrazolium salt to enzymatically assess metabolic activity in mould spores,
saving significant time and resources (i.e. wood specimens). Minimal inhibitory concentration (MIC90) of
isothiazolinone, a known mould inhibitor, in the XTT assay was the same or within a two-fold dilution of
the MIC90 for two methods currently used to determine mould resistance of cellulose-based products. An
ATP assay corroborated XTT assay findings; isothiazolinone appeared to be fungicidal rather than
fungistatic to the spores of fungi used in this study. After 24 h exposure to the chemical, spores remained
inactive indefinitely. The XTT assay would be a useful tool for screening mould inhibitors for numerous
applications in addition to those of the forest products industry.