P100 Potent inhibition of dermatophyte fungi by Australian native jarrah honey

Poster session 1, September 21, 2022, 12:30 PM - 1:30 PM

 

Objectives

Honey has been used as a remedy for multiple ailments, and the antibacterial activity of many different floral honeys has been extensively explored. The capacity of honey to inhibit fungi is much less well understood. Here we investigate the inhibition of dermatophyte species by native Australian jarrah honey.

Methods

Jarrah honey was sourced from beekeepers and commercial suppliers. Artificial honey, made from glucose (22.9%), fructose (20.7%), and sucrose (1.6%), was used to control for osmolarity. Hydrogen peroxide production by honey was assessed using horseradish peroxidase (HRP)/ο‐dianisidine colorimetric test. Dermatophytes included Microsporum canis, M. nanum, Nannizzia gypsea, Trichophyton interdigitale, T. rubrum, and T. tonsurans. Minimum inhibitory concentrations (MICs) and minimum fungicidal concentrations (MFCs) for honey were assessed using CLSI methods. Fluorescent and scanning electron microscopy were used to visualize the effect of honey on fungal conidia and hyphae.

Results

Jarrah honey inhibited all of the dermatophyte species with MICs ranging from 1.5-3.5% w/v, and MFCs from 2-5% w/v. No antifungal activity was seen with the artificial honey indicating this was not due to osmolarity. Microscopy revealed honey treatment prevented the germination of conidia and caused hyphae to bulge and collapse. While the inhibitory action of jarrah honey was greatly reduced by the addition of catalase suggesting hydrogen peroxide production was responsible for inhibition and killing, microscopy revealed hyphae were still damaged suggesting there are agents within honey that augment antifungal activity. REDOX fluorophores failed to detect internal oxidative stress within hyphae, indicating that damage likely occurs on the hyphal surface.

Conclusion

Jarrah honey is a non-toxic agent that may have utility in the treatment of superficial fungal infections caused by dermatophyte fungal species.

Inhibition of Dermatophyte Fungi by Australian Jarrah Honey

Superficial dermatophyte infections, commonly known as tineas, are the most prevalent fungal ailment and are increasing in incidence, leading to an interest in alternative treatments. Many floral honeys possess antimicrobial activity due to high sugar, low pH, and the production of hydrogen peroxide (H2O2) from the activity of the bee-derived enzyme glucose oxidase. Australian jarrah (Eucalyptus marginata) honey produces particularly high levels of H2O2 and has been found to be potently antifungal. This study characterized the activity of jarrah honey on fungal dermatophyte species. Jarrah honey inhibited dermatophytes with minimum inhibitory concentrations (MICs) of 1.5-3.5% (w/v), which increased to ≥25% (w/v) when catalase was added. Microscopic analysis found jarrah honey inhibited the germination of Trichophyton rubrum conidia and scanning electron microscopy of mature T. rubrum hyphae after honey treatment revealed bulging and collapsed regions. When treated hyphae were stained using REDOX fluorophores these did not detect any internal oxidative stress, suggesting jarrah honey acts largely on the hyphal surface. Although H2O2 appears critical for the antifungal activity of jarrah honey and its action on fungal cells, these effects persisted when H2O2 was eliminated and could not be replicated using synthetic honey spiked with H2O2, indicating jarrah honey contains agents that augment antifungal activity.

Factors affecting the production and measurement of hydrogen peroxide in honey samples

Many Australian native honeys possess significant antimicrobial properties due to the production of hydrogen peroxide (H₂O₂) by glucose oxidase, an enzyme derived from the honeybee. The level of H₂O₂ produced in different honey samples is highly variable, and factors governing its production and stability are not well understood. In this study, highly active Australian honeys that had been stored for >10 years lost up to 54 % of their antibacterial activity, although almost all retained sufficient activity to be considered potentially therapeutically useful. We used a simple colourimetric assay to quantify H₂O₂ production. Although we found a significant correlation between H₂O₂ production and antibacterial activity across diverse honey samples, variation in H₂O₂ only explained 47 % of the variation observed in activity, limiting the assay as a screening tool and highlighting the complexity of the relationship between H₂O₂ and the killing power of honey. To further examine this, we tested whether H₂O₂ detection in honey was being inhibited by pigmented compounds and if H₂O₂ might be directly degraded in some honey samples. We found no correlation between H₂O₂ detection and honey colour. Some honey samples rapidly lost endogenous and spiked H₂O₂, suggesting that components in honey, such as catalase or antioxidant polyphenols, may degrade or quench H₂O₂. Despite this rapid loss of H₂O₂, these honeys had significant peroxide-based antibacterial activity, indicating a complex relationship between H₂O₂ and other honey components that may act synergistically to augment activity.

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