Ergosterol-modified silica enables physicochemical screening of sterol-associated metabolites from actinomycetes

Ergosterol is the principal sterol of fungal membranes and a key target of polyene antifungal agents. However, the relationship between sterol association and antifungal activity remains incompletely understood. In this study, metabolites produced by actinomycetes were investigated using an ergosterol-modified silica resin (ES-silica) as a physicochemical screening platform for sterol-associated compounds. Large-scale screening of 859 actinomycete culture broths resulted in the tentative annotation of 91 ES-silica-retained metabolites, including previously reported compounds and 27 previously unreported candidate compounds, demonstrating the broad chemical diversity accessible through ES-silica-based enrichment. Analysis of ES-silica-retained compounds showed that nitrogen-containing metabolites accounted for approximately 78% of the detected compounds under the screening conditions used. Among the metabolites prioritized by ES-silica screening, dipyrimicin A and B were identified as previously reported compounds, whereas dietziamide C was isolated as a previously unreported bis-tetramic acid derivative for which the planar structure was determined. Comparative analyses of these metabolites suggested that retention on ES-silica does not necessarily correlate with antimicrobial activity under the conditions examined. Notably, dipyrimicin B and dietziamide C were retained on ES-silica but showed no detectable antimicrobial activity in the paper disk assay, whereas dipyrimicin A exhibited antifungal activity without comparable resin retention. These findings suggest that ES-silica retention alone is insufficient to predict antifungal activity and highlight the utility of ES-silica screening as a complementary approach to bioactivity-guided discovery.