Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Biology

Supervisor

Topp, Edward

2nd Supervisor

Henry, Hugh

Co-Supervisor

Abstract

Miconazole and clotrimazole are environmentally-persistent drugs that are entrained into crop soils through the application of biosolids. There is concern that environmental exposure to such azole antifungals, which inhibit fungal growth by disrupting the production of the fungal cell membrane component ergosterol, promotes resistance in clinically or agriculturally relevant fungi. Thus, either environmentally-relevant or excessive levels of these drugs were applied to microplots over ten years and compared with drug-free plots. Overall, ergosterol quantification, plates counts, and identification of >250 fungal isolates showed lower fungal counts and species richness in plots receiving excessive drug amounts. In addition, fungi from treated plots did not show increased resistance to a panel of medical and agricultural azole drugs in disk diffusion assays. Altogether, while the highly contaminated soils showed lower fungal counts, lower species richness, and fewer isolates highly-susceptible to miconazole, increased resistance to azoles was not evident at environmentally relevant concentrations.

Summary for Lay Audience

Products sold to treat fungal infections –such as those of the hair, skin, nails, and urogenital tract– include drugs that specifically kill or slow the growth of fungi. These products tend to be washed down the shower, sink, or toilet and make their way into the sewage system. Following sewage treatment, some traces of drugs remain, and two medical antifungal drugs that particularly accumulate and persist in the environment, possibly due to their use in over-the-counter products, are clotrimazole and miconazole. Although not used to treat systemic infections, these drugs, which are chemically defined as azoles, are chemically related to azoles used to treat life-threatening, systemic fungal infections and agricultural azoles used to control crop diseases in agriculture. This reliance on azole products has raised concerns over azole contamination of the environment, resulting from the application of biosolid fertilizer derived from sewage treatment products onto soils. More specifically, there is concern that azole contamination of soils can shift the soil fungal community's population structure and lead to the selection of azole-resistant phenotypes.

To address these concerns, from 2010 to 2020, amounts of clotrimazole and miconazole typical to those found in biosolid fertilizer were annually added to 2 m2 plots of soil grown with soybeans. Soil from these microplots, in addition to identical microplots receiving no drugs, were sampled, starting in 2018, for fungal isolation. The resulting fungal libraries were analyzed for changes in composition between the control and treated microplots. In addition, fungal libraries from the treated plots were compared to libraries from the control plots for their susceptibility to clotrimazole, miconazole, medical azoles used for systemic infections, and agricultural azoles used for crop protection.

Overall, it was found that the azole-treated soils showed lower fungal counts compared to the untreated soils and the soils treated with excess clotrimazole and miconazole showed lower species richness. On the other hand, there was no clear evidence that the fungal community recovered from the azole-contaminated soils were less susceptible to azoles than fungi recovered from the control plots. Nonetheless, several species with medical and agricultural significance were recovered from the contaminated soils, highlighting the importance of monitoring soil fungi for increased azole resistance.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.

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