Atividade antifúngica de óleos essenciais de diferentes cultivares de Humulus lupulus sobre espécies de Candida não albicans

Abstract

The increasing incidence of infections caused by Candida non-albicans species, coupled with resistance to conventional antifungal agents, highlights the importance of seeking effective nat- ural alternatives. Among explored options, essential oils obtained from commercial varieties of Humulus lupulus: Pahto, Lupomax Citra, Lupomax Mosaic, and Columbus, were tested against strains Candida glabrata ATCC 90030 and Candida krusei ATCC 6258 to evaluate antifungal activity. Testing was conducted using the microdilution technique in Sabouraud Dextrose broth, employing serial dilutions in 96-well plates, with nystatin as a positive control. Additionally, for the determination of the Minimum Fungicidal Concentration (MFC), concentrations equiv- alent to the Minimum Inhibitory Concentration (MIC), MICx2, and MICx4 were used, allowing the classification of the oils as fungicidal or fungistatic based on the MFC/MIC ratio. Investi- gations also assessed the involvement of fungal cell wall and plasma membrane as possible targets, using sorbitol and exogenous ergosterol, respectively, alongside caspofungin and nys- tatin as positive controls. Essential oils from the Pahto, Lupomax Citra, and Lupomax Mosaic varieties showed identical MIC values of 250 µg/mL, whereas the Columbus variety stood out with a MIC of 250 µg/mL for C. glabrata and 125 µg/mL for C. krusei. These results indicate moderate bioactivity for the first three hop varieties, while Columbus essential oil exhibited moderate bioactivity against C. glabrata and good bioactivity against C. krusei. The fungicidal nature of all tested oils was confirmed by the MFC/MIC ratio. Moreover, investigation into the fungal cell wall showed no significant change in MIC values in the presence of sorbitol, indi- cating no action on the fungal cell wall. Conversely, assessment of the plasma membrane showed elevated MIC values in the presence of exogenous ergosterol, especially for Pahto and Columbus oils against C. glabrata, and for all four hop varieties against C. krusei, suggesting plasma membrane involvement. Essential oils from Pahto, Lupomax Citra, and Lupomax Mo- saic varieties demonstrated moderate antifungal activity against both C. glabrata and C. krusei, while Columbus oil exhibited moderate activity against C. glabrata and strong activity against C. krusei. The plasma membrane of C. krusei was identified as the primary target for all studied essential oils, whereas for C. glabrata, only Pahto and Columbus oils showed effective plasma membrane action. Further studies are necessary to deepen the understanding of the action mech- anisms of essential oils derived from hop varieties.