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β-thujaplicin, fungal pathogen, Scedosporium apiospermum


Background: β-thujaplicin (β-Th), also known as hinokitiol, naturally occurs in cedar mulch, is found in personal care products and has in vitro antitumor activities. It is antibacterial and antifungal, but has not been tested on soil. Scedosporium apiospermum (Sce) is an emerging “extremophile” fungal pathogen found in built outdoor environments.

Purpose: Pilot β-Th as “natural” soil antimicrobial or for isolation of extremophiles, and to explore β-Th resistance as selective advantage to Sce in mulched landscape.

Methods: A variety of outdoor and indoor environments were used for 2 sets of 24 paired soil samples. Soil/H20 slurry (0.1 ml) was spread on Sabouraud dextrose agar with titrated β-Th levels of 0, 25, 250 and 500 mg/L at 20° C. Fungal and bacterial growth was semiquantitated with 4-point Likert scale. Wilcoxon signed rank test was used for comparison. A local soil Sce isolate was tested on each β-Th concentration.

Results: There was no significant inhibition of total bacterial growth at β-Th 250 mg/L (mean 1.7/4) or 500 mg/L (mean 1.7) compared to plain Sabouraud dextrose agar (mean 1.6). Purple bacteria seemed to be selected for by β-Th. Fungal inhibition was essentially complete, similar, and significantly different from no β-Th (mean 3.4/4) at levels of 250 (mean 0.1) and 500 mg/L (mean 0.0). There was no significant fungal inhibition at 25 mg/L (mean 3.2, second set samples). Similarly, Sce was completely inhibited at 250 and 500 mg/L, but not inhibited at 25 mg/L.

Conclusion: In vitro, β-thujaplicin profoundly, but nonselectively, inhibits fungal growth in soil samples at moderately high levels. It does not appear likely that this Scedosporium apiospermum strain employs β-Th resistance for selective advantage in cedar mulched landscaping.